DISTURBANCE UNITS DESCRIPTIONS

Introduction

There has been a steady increase in the use of our knowledge of natural disturbance dynamics as a basis for forest management policy directed towards maintaining biological diversity (Booth et al. 1993, Biodiversity Guidebook… 1995). The underlying assumption is that the biota of a forest is adapted to the conditions created by natural disturbances and thus should cope more easily with the ecological changes associated with forest management activities if the pattern and structure created resemble those of natural disturbance (Hunter 1993, Swanson et al. 1993, Bunnell 1995, DeLong and Tanner 1996, Bergeron and Harvey 1997, Angelstam 1998, DeLong and Kessler 2000).

For a variety of reasons, past forest management policies and guidelines have been directed towards setting somewhat arbitrary limits. These limits were often related to maximizing timber volume or creating conditions that favoured certain organisms (e.g., ungulates). Limits were often stated for things such as patch size, species composition, stand density, non-forested area and soil disturbance. Although well meaning and easily administered, these practices result in patterns bearing little relationship to those created by natural disturbance dynamics. Studies of natural disturbance in the boreal forest have demonstrated large ranges in disturbance patch size (Eberhart and Woodward 1987, DeLong and Tanner 1996), tree density (DeLong and Kessler 2000), and volume of coarse woody debris (CWD) (Clark et al. 1998, DeLong and Kessler 2000)

The Biodiversity Guidebook (1995) was the first attempt in British Columbia to present guidance for forest management based on the natural disturbance template. Specific guidance for seral stage distribution, patch size, wildlife tree patch amount, and spatial arrangement and more general guidance on species composition and stand structure were included in this guide. Since the completion of the Biodiversity Guidebook, more information on natural disturbance dynamics has become available. Within the Prince George Forest Region a number of studies have investigated particular aspects of natural disturbance (e.g., DeLong 1998, DeLong and Kessler 2000, Lewis and Lindgren 2000, Rogeau 2001). This document is meant to present updated guidance for the Prince George Forest Region based on this new information. Unlike the Biodiversity Guidebook, this document presents goals based on the “range of natural variability” and does not present any numbers that represent a compromise between biodiversity and timber management. Thus this document presents guidance based on “the best available information” that would result in the least possible differences between harvesting and natural disturbance.

Instead of adopting the Natural Disturbance Types (NDT’s) presented in the Biodiversity Guidebook (1995) this document presents information for 9 Natural Disturbance Units (NDU’s) (Figure 1). These units are felt to better separate areas based on differences in disturbance processes, stand development, and temporal and spatial landscape pattern. In the drawing of the boundaries of the NDU’s, Landscape Unit boundaries were used whenever possible. This avoids the problem of having very small areas within a planning unit with different guidance than the rest of the unit.

Figure 1. Natural Disturbance Units of the Prince George Forest Region.

Information presented for each Natural Disturbance Unit (NDU) includes: background information on location, climate and vegetation; discussion of natural disturbance ecology; discussion of the major effects of forest management on natural patterns; and recommended practices based on the natural disturbance template. Table 1 provides a breakdown of biogeoclimatic units found within each of the NDU’s.

Table 1. List of biogeoclimatic units within each of the Natural Disturbance Units

Natural Disturbance Unit	Biogeoclimatic Units¹
Boreal Plains - Alluvial	BWBSmw2
Boreal Plains - Upland	BWBSmw1, BWBSmw2, (BWBSwk1, BWBSwk2, SWBmk)
Boreal Foothills - Valley	BWBSwk1, (BWBSmw1)
Boreal foothills - Mountain	ESSFmv2, (ESSFwk2, ESSFwc3)
Wet Mountain	SBSvk, ESSFwk2, ESSFwc3, (SBSwk1, SBSwk2, ESSFmv2)
Wet Trench - Valley	ICHwk3, ICHvk2, SBSvk, (ICHwk2, SBSwk1)
Wet Trench - Mountain	ESSFwk1, ESSFwk2, ESSFwc3, (ESSFmm)
Moist Trench – Valley	ICHmm, SBSdh, (ICHwk1, SBSvk)
Moist Trench - Mountain	ESSFmm, (ESSFwc2)
McGregor Plateau	SBSwk1, (ESSFwk2, SBSwk2)
Moist Interior - Plateau	SBSdk, SBSdw2, SBSdw3, SBSmc2, SBSmc3, SBSmk1, SBSmw, SBSwk3a, (SBPSdc,SBPSmc, SBSdw1, SBSmh, SBSwk1)
Moist Interior - Mountain	ESSFmv1, ESSFmv3, (ESSFwk1)
Omineca - Valley	BWBSdk1, BWBSwk2, SBSmk1, SBSmk2, SBSwk2, SBSwk3, (ICHmc1, SBSdk, SBSmc2, SWBmk)
Omineca - Mountain	ESSFmc2, ESSFmv3, ESSFmv4, (ESSFwv, SWBmk)
Northern Boreal Mountains	BWBSdk1, BWBSdk2, SWBmk, (ESSFmc, ESSFmv4, BWBSmw2, BWBSwk2, BWBSwk3)

¹ Units in brackets cover a minor (i.e., <5%) portion of the Natural Disturbance Unit.

Moist Interior

Location, Climate, and Vegetation

This unit occupies the gently rolling terrain and broad mountain peaks of the Fraser Plateau and the Fraser Basin Ecoregions. This NDU is found over a wide geographic range, from 53^° - 55^°N latitude and 122^° - 125^°W longitude. The elevation range of this NDU is 600 – 1800m but most of it is between 700 – 1200m.

The climate of this unit is continental, and is characterized by seasonal extremes of temperature; severe, snowy winters; relatively warm, moist, and short summers; and moderate annual precipitation (Meidinger et. al. 1991). Excluding the Boreal Plains, this is drier than the other NDU’s in the region. It is intermediate in temperature between the colder montane and northern units, and warmer trench units. Mean annual temperature (MAT) for moist of this unit ranges from 0.6 to 3.7°C. Average temperature is below 0°C for 4-5 months of the year, and above 10°C for 2-5 months. Mean annual precipitation (MAP) data from long-term stations ranges from 481-727 mm, of which perhaps 25-50% is snow. Higher elevation mountains in the unit will likely have a more severe climate (i.e., lower temperatures, more precipitation, more snow) but no data is available for these areas.

Upland coniferous forests dominate the Moist Interior - Plateau landscape. Hybrid white spruce (Picea engelmannii x glauca) and subalpine fir are the dominant climax tree species. Lodgepole pine is very common in mature forests throughout the unit and both lodgepole pine and trembling aspen pioneer the extensive seral stands. Paper birch is another pioneer tree, often on moist, rich sites. Douglas fir is usually a long-lived seral species, occurring most abundantly on dry and warm sites in the southeastern part of this unit. Black spruce also occurs in climax upland forest in combination with lodgepole pine on sites with restricted rooting. Forests at higher elevations (>1100m) will have a higher subalpine fir component.

Alluvial forests of black cottonwood, often with a minor component of spruce, occur to a limited extent on active floodplains of the major streams and rivers. Wetlands are common and dot the landscape in poorly drained, postglacial depressions or river ox-bows.

Wetland community types include Carex (sedge) marshes, shrub fens of Betula glandulosa (scrub birch), B. pumila (swamp birch), and Salix spp. (willows), treed fens and swamps with black and hybrid white spruce, and black spruce — Sphagnum bogs. Acidic, nutrient-poor bogs are less common than the richer wetland types (marshes, fens, and swamps). Tamarack occurs in a number of wetlands south of the Nechako River.

Natural grassland and shrub-steppe are uncommon in this NDU, occurring on some warm, dry sites scattered in the major valleys.

Natural Disturbance Ecology

Fire and mountain pine beetle (Dendroctonus ponderosae) are the key stand-replacement disturbance agents operating in this unit. The disturbance rate[1][1] for the plateau and mountain portions of this unit are estimated to be 0.75 - 1.25 % [2][2] and 0.48 % of the total forested area/yr respectively (DeLong 1998). The fire cycles assigned to these unit are 100 and 200 respectively, based on work conducted by Andison (1996) and DeLong (1988). Table 2 shows the amount of forests of different age that would be associated with this fire cycle. Large wildfires (> 1,000 ha) dominated the landscape and were regenerated quickly by dense lodgepole pine and/or trembling aspen resulting in large patches of relatively even-aged forests (Table 2). Minor amounts of young white and/or black spruce forest could be found in wetter patches within the fire boundaries often adjacent to unburned mature forest not burned by the fire. Young Douglas fir stands can be found on drier ridges near larger Douglas-fir veterans that have escaped the fire. Small areas where fire was intense may regenerate to willow or alder (Delong, unpublished data). Stand ages rarely exceeded 200 years, except in the more mountainous areas, but relatively large patches (>100 ha) of older forest (140 – 180 yrs) could be found scattered across the landscape (Andison 1996, DeLong and Tanner 1996). Although patches of old forest (> 140 yrs) likely always occurred in the landscape their position would have moved around the landscape over time. Within the boundaries of the fires 3-15% of total area of the fire can be composed of unburned mature forest remnants (DeLong and Tanner 1996). These mature forest remnants are distributed throughout all landscape positions including flat lodgepole pine stands (DeLong and Tanner 1996). Very little remnant structure exists outside of these patches. Data from DeLong and Tanner (1996) indicate that there was <1 live remnant tree/ha outside of remnant patches. More live remnant trees likely occur in areas with a higher component of Douglas-fir due to their increased ability to survive fire, but this has not been tested.

During stand development, increasing amounts of white spruce, black spruce, and subalpine fir will occur in stands originally dominated by lodgepole pine or trembling aspen. This increase occurs more rapidly and these species become a more dominant portion of the canopy on wetter sites. Douglas fir will be co-dominant where established with lodgepole pine. Post-fire stands are very dense except on the wettest sites and then self thin over time (Table 3). Density of snags > 7.5 cm dbh generally exceeds 100 sph and is highest in mature stands due the effect of self-thinning (Table 3). Larger diameter trees and snags (>15 cm dbh) are most abundant in stands exceeding 140 yrs of age but do occur in stands of all ages (Table 3).

Coarse woody debris (CWD) volume ranges considerably in response to the time since the last fire, age of the stand at time of the fire, and number of times it has burned (Table 4). Fires may burn over the same area 2 –3 times within a short period of time (<50 yrs) leaving very little dead wood on the ground. CWD volumes are highest in young stands due to large amount of wood that is left over from the previous stand (Table 3). Very little of the main stem wood of mature live trees is actually consumed by fire and

Table 2. Estimates of statistics relating to temporal and spatial pattern of natural disturbance in the Natural Disturbance Units of the Prince George Forest Region.

Natural Disturbance Unit	Stand Replacement Disturbance Cyclel	Range in proportion of total forested area				Patch Size (% of total disturbance area)				Disturbance type (% of disturbance area)
Natural Disturbance Unit	Stand Replacement Disturbance Cyclel	>250 yrs	>140 yrs	>100 yrs	<40 yrs	>1000	100 -1000	51 - 100	<50	Stand Replacement	Gap Replacement
Boreal Plains - Alluvial	200	0.23 - 0.36	0.44 - 0.57	0.61 - 0.66	0.18 - 0.24	0	0	40	60	80	20
Boreal Plains – Upland	100	0.06 - 0.12	0.20 - 0.29	0.34 - 0.44	0.29 - 0.47	70	20	5	5	98	2
Boreal Foothills – Valley	120	0.08 - 0.17	0.25 - 0.36	0.33 - 0.51	0.29 - 0.42	40	30	10	20	90	10
Boreal Foothills - Mountain	150	0.15 - 0.25	0.37 - 0.46	0.48 - 0.60	0.19 - 0.34	40	30	10	20	80	20
Wet Mountain	900	0.74 - 0.80	0.84 - 0.88	0.88 - 0.92	0.04 - 0.06	10	60	10	20	40	60
Wet Trench - Valley	600	0.63 -0.72	0.78 - 0.84	0.82 -0.89	0.05 - 0.10	10	60	10	20	60	40
Wet Trench - Mountain	800	0.70 - 0.77	0.82 - 0.86	0.88 - 0.89	0.04 - 0.07	10	60	10	20	40	60
Moist Trench – Valley	150	0.15 - 0.25	0.37 - 0.46	0.48 - 0.60	0.19 - 0.34	70	20	5	5	90	10
Moist Trench - Mountain	300	0.39 - 0.50	0.60 - 0.69	0.68 - 0.76	0.14 - 0.21	60	30	5	5	70	30
McGregor Plateau	220	0.26 - 0.39	0.49 - 0.60	0.58 - 0.71	0.16 - 0.24	40	45	5	10	90	10
Moist Interior - Plateau	100	0.06 - 0.12	0.20 - 0.29	0.34 - 0.44	0.29 - 0.47	70	20	5	5	98	2
Moist Interior - Mountain	200	0.23 - 0.36	0.44 - 0.57	0.61 - 0.66	0.18 - 0.24	40	30	10	20	70	30
Omineca - Valley	120	0.08 - 0.17	0.25 - 0.36	0.33 - 0.51	0.29 - 0.42	60	30	5	5	95	5
Omineca - Mountain	300	0.39 - 0.50	0.60 - 0.69	0.68 - 0.76	0.14 - 0.21	40	30	10	10	70	30
Northern Boreal Mountains	180	0.20 - 0.35	0.41 - 0.51	0.53 - 0.64	0.19 - 0.32	60	30	5	5	70	30

Table 3. Means and standard deviations of selected stand characteristics for young matrix forest (40 – 70 yrs old), mature matrix forest (70 –140 yrs old), remnant patches and old matrix forest (> 140 yrs old) for mesic sites within the SBSmk1 (adapted from DeLong and Kessler 2000).

Stand characteristics			Stand type
	Young	Mature		Remnant	Old
Tree density	2597(471)	1910 (780)		1165 (394)	984 (263)
Snag density	268 (198)	460 (193)		158 (78)	170 (72)
>15-cm-d.b.h. tree density	408 (273)	860 (189)		693 (171)	698 (215)
>25-cm-d.b.h. tree density	9 (11)¹	59 (55)¹		221 (90)	334 (99)
>15-cm-d.b.h. snag density	12 (17)	59 (52)		73 (45)	126 (54)
> 25-cm-d.b.h. snag density	3 (5)¹	2 (5)¹		15 (17)	31 (27)
# of cavities/ha	13 (18)	9 (11)		30 (34)	18 (20)
Average tree d.b.h. (cm)	11.3 (1.2)	15.4 (1.9)		17.7 (3.7)	20.8 (3.3)

Table 4. Summary statistics for CWD volume for young matrix, mature matrix, remnant patch, and old matrix forest categories. (n = 10) (from DeLong and Kessler 2000).

		Total		< 17.5 d.b.h.			>17.5 d.b.h.
Category	Mean	Range	SD	Mean	SD	Mean		SD
Young matrix	261.8	5.6 - 590.3	201.3	76	54.7	188.5		177.3
Remnant patch	229.2	33 - 393.4	116.4	104.8	46.7	124.2		86.5
Mature matrix	174.4	23.4 - 283.3	90.5	71.8	37.5	84.1		74.6
Old matrix	192.6	38.6 - 286	78.7	82.5	37.6	112.8		61.5

standing snags are mostly down after 40 years, thus most of the standing live tree biomass ends up as CWD.

Fire control and harvesting pattern are likely the 2 factors most affecting the natural landscape pattern and processes in this NDU.

Effective fire control over the past 40 - 50 yrs has slowed the natural disturbance rate from 0.8 to 0.008% of the total forested area/yr. This has had the compound effect of increasing the amount of old forest in more remote areas where harvesting was not occurring (e.g., south end of Vanderhoof District) and reducing young forest established by fire. Increasing old forest that is the most susceptible to MPB and decreasing the amount of large patches of young forest that is least susceptible to MPB in some remote areas has likely exacerbated the current MPB infestation.

Some organisms appear to be heavily dependent on fire killed forests. Hutto (1995), in a study of bird communities following stand-replacement fires in the Rocky Mountains of Montana, found that black-backed woodpeckers (Picoides arcticus) were generally restricted in their habitat distribution to standing dead forests created by stand-replacement fires. Four talks at the recent “Disturbance Dynamics In Boreal Forests” conference in Finland dealt with fungi and insects that were either fire obligates or heavily favoured by fire. These organisms require the burned dead trees found after fire and occur at much reduced numbers after forest salvage operations (Stepnisky, unpublished data).

While wildfire creates disturbances of all sizes and the landscape is dominated by large disturbances, forest management has generally been directed to achieve mid-sized patches (40 – 100 ha) (DeLong and Tanner 1996). Larger harvest patches often occur due to management of beetle or windthrow. Dispersed harvest of mid-sized patches is both very unnatural but also creates fragmentation and a porous landscape for spread of pests such as MPB.

Currently, disturbance rates associated with harvesting are similar to those previously associated with wildfire. However, harvesting removes old forest at a faster rate then wildfire because harvesting concentrates on stands > 100 years of age whereas wildfire is relatively unselective as to the age of stand it will burn (Van Wagner 1978, Johnson 1992).

Dense stands of lodgepole pine were typical after wildfire. The lowest stocking level found for young natural stands (50 yrs old) on in the SBSmk1 was 2224 sph > 7.5 dbh (Table 3) (DeLong 1998). Managed stands vary considerably in density depending on whether natural or artificial regeneration is used and the rate of ingress from naturally regenerated stems. MoF records for young managed stands in the SBSmk1 indicate stocking levels of 500 - 21 000 sph (average 3475 sph). Certain practices such as low impact site preparation, which limits mineral soil exposure, in combination with modest stocking levels (<1600 sph) may result in some stands being outside the natural range of variability in stocking level but this remains to be examined.

Recommended Practices

Since forests with “old forest characteristics”, on the plateau portion of this NDU, typically ranged from 120 – 200 years, it seems appropriate to have a system of rotating old forest reserves between these ages. This would insure stands with “old forest characteristics” exist but they are not unnaturally old and more susceptible to pest infestation. Large patches (> 100 ha) of old forest should be identified and recruited such that replacement areas > 120 years old are available to replace areas > 150 years of age that would be harvested. Recruitment areas should be preferentially selected in the following order: 1) unsalvaged wildfires, 2) partially salvaged wildfires, 3) large blocks designed to approximate wildfire that have mostly been regenerated naturally, 4) large blocks (> 100 ha) designed to approximate wildfire that have mostly been regenerated artificially, 5) large blocks (> 100 ha) that were not designed specifically to approximate wildfire and 6) small to medium sized blocks (< 100 ha). Fixed reserves may be more appropriate in the mountain portions of the NDU but may be augmented with some level of floating reserve. Table 2 contains estimates of seral stage distribution based on the natural range of variability.

Young natural forest

Some proportion of wildfires should be left unsalvaged to provide habitat (e.g., burned snags) that cannot be provided by young managed stands.

Patch size

Since medium sized patches (50 – 100 ha) are rare in the natural landscape and small patches are still naturally created by small fires, windthrow, root disease the emphasis should be on creating larger patches (> 100 ha). Larger patches should be created by aggregating recent blocks in areas previously harvested and/or by designing new large blocks in unharvested areas. Patch size distribution should follow that of wildfire shown in Table 2as closely as possible given social, logistic, or demonstrated ecological constraints. Design of blocks should follow guidance provided in DeLong (2000).

Stocking and stand structure

Stand density in young circumesic stands (< 40 yrs old) should generally kept at total stocking levels of > 2000 sph to approximate dense natural stands. More open patchy stocking (i.e., < 1000 sph) on hygric sites are recommended. Even-aged stands over most of the landscape would approximate the natural pattern.

Wet Mountain

Location, Climate, and Vegetation

This unit occupies the valleys and slopes of the Rocky Mountains west of the continental divide and between 54^° - 56^°N latitude.

The climate of this unit is continental, and is characterized by seasonal extremes of temperature; severe, very snowy winters; and cool, very wet, and short summers. This is the wettest of the NDU’s in the region. The temperature regime varies in a gradient from valley to mountain top. MAT of the lower elevation SBSvk is 2.6°C and 0.3°C for the higher elevation ESSFwk2. MAP data from long-term stations is 1250 and 1537mm respectively for the SBSvk and ESSFwk2. Annual snowfall often reaches 6 to 9 m depending on elevation.

Old upland coniferous forests dominate the Wet Mountain natural landscape. Hybrid white spruce and subalpine fir are the dominant climax tree species. Lodgepole pine is limited to some wetlands and Douglas-fir to a few dry rocky ridges. Paper birch occurs as a pioneer tree in scattered recently disturbed areas. Some cottonwood occurs along the floodplains of the larger rivers. Black spruce occurs in wetlands that occupy some of the broader valley bottoms. Sitka alder (Alnus viridis ssp. sinuata) occurs commonly on slopes throughout the unit on avalanche tracks and in swales.

Natural Disturbance Ecology

Stand-replacement disturbance events occur at irregular intervals with as much as 1000 years between such events on any site. The stand replacement disturbance rate[3][3] for this unit is estimated to be only 0.1 % [4][4] of the total forested area/yr (DeLong 1998). The stand replacement disturbance cycle assigned to this unit is 900, based on work conducted by Hawkes et. al. and DeLong (1988). Table 2 shows the amount of forests of different age classes that would be associated with this disturbance cycle. Fire sizes are generally smaller than for other NDU’s with only 10% of the total area in patches > 1000 ha but there is still 60% in the 100 – 1000 ha patch size (Table 2).

In the absence of stand replacement disturbance, stands are affected by damaging agents that operate in older stands, so called matrix disturbance agents (Lewis and Lindgren 2000). The agents most commonly associated with older trees in this NDU are spruce beetle (Dendroctonus ponderosae), western balsam bark beetle (Dryocoetes confusus), tomentosus root disease (Inonotus tomentosus), and stem decays such as Indian paint fungus (Echinodontium tinctorium). These agents alter stand species composition and horizontal and vertical structure by causing tree mortality either on their own or in combination with other damaging agents (e.g., wind, disease). Spruce beetle may cause severe mortality at regular intervals leading to a shift in species composition to subalpine fir and release of suppressed trees (Lewis and Lindgren 2000). In the absence of lodgepole pine over most of this unit, stands attain stocking slowly even after wildfire resulting in open multi-aged early to mid successional stands (DeLong et al. 1998). The long stand replacement disturbance rate, damaging agents causing selective mortality and slow regeneration result in open multi-aged stands dominating the landscape.

Natural stands, of any age, generally do not exceed 1000 sph > 7.5 cm dbh and density of the main canopy is generally < 400 sph (DeLong et al. 1998) (Table 5). Spruce tends to out live subalpine fir in this unit so they comprise the majority of the largest stems in older stands. Subalpine fir is more abundant as elevation increases due to their greater ability to survive in the severe high elevation environment. Density of snags > 7.5 cm dbh is highest in young (<70 yrs old) stands and generally exceeds 80 sph in most stands (Table 5). The number of snags increases with elevation such that stands in the ESSFwk2/wc3 have almost twice as many snags as equivalent stands in the SBSvk (Table 5).

Coarse woody debris (CWD) volumes show little variation with stand age but decreases with elevation in correspondence to decreases in live tree volume (DeLong et al. 1998). Average CWD volume is generally 150 – 250 m³/ha for stands in the SBSvk and 100 – 200 m³/ha for stands in the ESSFwk2/wc3 (DeLong et al. 1998).

Arboreal lichen abundance is high in older forests especially in the ESSFwk2/wc3. Although young stands (<70 yrs old) have lower amounts of arboreal lichen there appears to be no clear differences between mature (70 – 140 yrs) and old (>140 yrs) stands especially within the ESSFwk2/wc3 (DeLong et al. 1998).

Harvesting and reforestation practices are likely the 2 factors most affecting the natural landscape pattern, stand composition and structure and associated processes in this NDU.

Clearcut harvesting has been quite extensive in portions of this NDU resulting in more area in early seral stands and less area in older forest than would have existed in the natural landscape for at least the last 500 years. Lodgepole pine has been planted in some areas within this NDU on sites where there is no present evidence of it having occurred. Current practices favouring the planting of spruce over subalpine fir at higher elevations will likely lead to stands with a higher proportion of spruce in managed stands as compared to natural stands. Current reforestation standards for stocking will result in stands being more densely stocked and more even-aged than natural stands. The potential impacts of the conversion of a landscape dominated by older more open stands to a landscape with a high proportion of denser younger even-aged stands are uncertain. Lewis and Lindgren (2000) hyphothesize that a transition to more homogenous stands could result in significant pest outbreaks, specifically of white pine weevil (Pissodes strobi) and tomentosus root rot.

Table 5. Mean values and standard deviation (in brackets) for selected stand characteristics in young (0-70 yrs), mature (71-140 yrs) and old (>140 yrs) stands for the SBSvk and ESSFwk2/wc3 (n=4 for SBSvk, n=5 for ESSFwk2/wc3 except where noted) (Adapted from DeLong et al. 1998).

Stand Attributes

Sub-boreal

Subalpine

Young

Mature

Old

Young

Mature

Old

Tree density (stems/ha)

644¹

811

(175)

617

(188)

342

(617)²

542

(177)

558

(311)

Spruce density (stems/ha)

400¹

475

(241)

158

(56)

1044¹

11¹

133

(103)

Subalpine fir density (stems/ha)

244¹

333

(136)

455

(175)

33.3

(15.7)²

540

(181)

424

(344)

Main canopy density (stems/ha)

244¹

319

(72)

128

(58)

173

(332)²

182

(112)

158

(64)

Snags <15 cm dbh (stems/ha)

(80)

(63)

(29)

(97)

(6)

(23)

Snags 15-25 cm dbh (stems/ha)

(52)

(14)

(11)

222

(177)

(23)

Snags >25 cm dbh (stems/ha)

103

(83)

(32)

(29)

253

(129)

158

(150)

(58)

Snag density (stems/ha)

258

(138)

(91)

(46)

440

(428)

220

(176)

122

(73)

Snag basal area (m²/ha)

10.6

(16.2)

7.4

(9.6)

12.3

(1.1)

25.8

(19.9)

17.7

(14.8)

8.0

(6.7)

Mean dbh main canopy (cm)

24.2¹

32.1

(8.2)

49.5

(6.3)

22.5

(4.1)

27.5

(7.9)

38.8

(12.6)

Mean dbh spruce (cm)

15.7¹

27.9

(9.3)

42.9

(8.9)

18.9¹

19.7¹

41.6

(4.1)

Mean dbh subalpine fir (cm)

21.7¹

17.0

(7.4)

21.6

(3.5)

16.7

(4.0)²

20.7

(4.6)

18.3

(3.3)

CWD volume <15cm diam. (m³/ha)

16.4

(5.7)

13.3

(22.5)

9.0

(3.6)

9.7

(9.5)

4.7

(3.0)

8.1

(5.8)

CWD volume 15-25cm diam. (m³/ha)

41.0

(19.6)

58.7

(70.6)

58.5

(6.9)

29.8

(16.3)

31.6

(22.6)

39.2

(26.0)

CWD volume >25cm diam. (m³/ha)

132.4

(56.8)

175.7

(99.0)

183.9

(54.2)

72.9

(29.6)

117.8

(55.9)

157.6

(178.5)

Total CWD volume (m³/ha)

189.8

(67.5)

264.9

(65.9)

251.3

(50.1)

111.6

(24.2)

154.2

(48.6)

204.9

(203.6)

² based on 4 plots, for 3 of the plots trees over 7.5cm dbh were from the pre-disturbance cohort.

Recommended Practices

Since forests with “old forest characteristics” dominated the landscape in this NDU, it seems appropriate to have old forest reserves or forest > 100 yrs old well distributed throughout all watersheds. A high degree of connectivity between these old forest patches should also be managed for since there was always a high degree of connectivity of old forest in the natural landscape. Since differences between mature and old forests appear to be limited based on available data some flexibility in the current age criterion for “old growth forest” should be considered. Table 2 contains estimates of seral stage distribution based on the natural range of variability.

Young natural forest

Some proportion of areas disturbed by natural disturbance agents (e.g., wildfires, pests, wind) should be left unsalvaged to provide habitat (e.g., burned snags) that cannot be provided by young managed stands.

Patch size

The patch size for clearcut harvesting should follow that of wildfire shown in Table 2as closely as possible given social, logistic, or demonstrated ecological constraints. Design of blocks should follow guidance provided in DeLong (2000).

Silviculture system

Some form of partial cutting that approximate the effects of spruce beetle attack would seem appropriate in order to maintain the type of stand structure most common in the natural landscape. Some balance between this system and clearcut with reserves would seem most appropriate.

Stocking and stand structure

Appropriate measures need to be developed to achieve open patchy multi-storied stands over most of the landscape.

Location, Climate, and Vegetation

This unit occupies the gently rolling terrain of the Taiga Plains and Boreal Plains Ecoprovinces. This NDU is found over a wide geographic range, from 54^° - 60^°N latitude and 119^° - 123^°W longitude. The unit generally occurs from the valley bottoms to 900 - 1100 m elevation, below the Boreal Foothills NDU in the south and Northern Rockies NDU in the north.

The northern continental climate of this unit is characterized by frequent outbreaks of arctic air masses resulting in long, very cold winters and short summers. However due to long day length in the summer forest productivity is similar to areas further south. This is driest NDU in the region, with MAP ranging from 330 – 570mm, with 35-55% falling as snow. It is the coldest lower elevation NDU. MAT ranges from –2.9 to 2.0°C. The ground freezes for a large part of the year, and discontinuous permafrost is common in the northeastern parts of this NDU.

Upland climax forests are dominated by hybrid white spruce and/or black spruce depending on topographic position and time since last stand replacement disturbance. It is theorized that in the absence of a stand replacement event, stand productivity will drop and proportion of black spruce will increase in response to reductions in soil temperatures and nutrient cycling due to the build-up of the forest floor. Trembling aspen and to a lesser extent lodgepole pine and paper birch dominate young stands. Mixed forests of trembling aspen and white spruce are very common except in areas where the spruce seed source has been removed due to land clearing.

Wetlands are very common and diverse especially in the northern portion of this NDU. There are 7 forested and 9 non-forested wetland types recognized within this NDU. Black spruce or tamarack dominates the forested wetlands. Non-forested wetlands are most commonly dominated by speckled alder, swamp birch, Alaska paper birch (Betula neoalaskana), willows, sedges, or buckbean (Menyanthes trifoliata).

Alluvial forests of black cottonwood, often with a minor component of spruce, are common along the floodplains of the larger rivers. Natural grassland and shrub-steppe occur on steep, south-facing slopes above some of the major rivers such as the Peace.

Natural Disturbance Ecology

Fire is the key stand-replacement disturbance agent operating in this unit with the exception of the broad alluvial terraces adjacent to the larger rivers. The disturbance rate[5][5] for the non-alluvial portions of this unit is estimated to be about 1% [6][6] of the total forested area/yr and has been assigned a fire cycle of 100 years. Table 2 shows the amount of forests of different age that would be associated with this fire cycle. Large wildfires (> 1,000 ha) dominated the landscape and upland sites were regenerated quickly by dense trembling aspen, trembling aspen and spruce or lodgepole pine resulting in large patches of relatively even-aged forests (Table 2). Black spruce, tamarack, Alaska paper birch and white spruce regenerated the wetland areas after fire. These stands tend to be very open and fill in over time except where they are verging on upland areas, in which case they can be denser. Small areas where fire was intense may regenerate to willow or alder (DeLong, unpublished data). Tomentosus root disease is felt to be a key disturbance agent affecting white spruce and in some localized areas may cause conversion from spruce dominated stands to aspen dominated stands over the course of 20-40 years (pers. comm. Richard Reich, Prince George MoF, Regional Pathologist). Eastern spruce budworm (Choristoneura fumiferana) may also cause significant mortality of mature or immature spruce and lead to conversion of mixed to more pure aspen stands. Stand ages rarely exceeded 200 years but relatively large patches (>100 ha) of older forest (140 – 180 yrs) could be found scattered across the landscape (Don Rosen, unpublished data??). Although patches of old forest (> 140 yrs) likely always occurred in the landscape their position would have moved around the landscape over time. Within the boundaries of the fires 3-15% of total area of the fire can be composed of unburned mature forest remnants (Eberhart and Woodward 1987).

During stand development, increasing amounts of white spruce and black spruce will occur in stands originally dominated by trembling aspen or lodgepole pine. This increase occurs more rapidly and these species become a more dominant portion of the canopy on wetter sites. Post-fire stands are very dense except on the wettest sites and they self thin over time. Density of snags for aspen mixedwood stands is likely similar to that reported in Lee et al. (1995). They report snag densities (±S.E.M.) of 33.0±6.8, 73.1±11.3, and 66.2±9.1 snags/ha (³10 cm dbh) for young (20-30 years), mature (50-65 years) and old (120+ years) stands, respectively.

CWD volumes of aspen mixedwood stands are likely similar to that reported in Lee et al. (1995). They report total CWD volumes (±S.E.M.) of 108.8±5.1, 109.1±7.6, and 124.3±7.1 m³/ha for young (20-30 years), mature (50-65 years) and old (120+ years) stands, respectively.

There is a lack of information on disturbance cycles and patch sizes with respect to flooding along the major rivers in this NDU. We have temporarily assigned a stand replacement disturbance cycle of 200 years to the alluvial portions of this NDU (Table 2). Disturbance patches are thought to be < 100 ha in size (Table 2). There is a typical flood plain succession from willow to cottonwood on the lower benches and from cottonwood to spruce on the higher benches.

Fire control and harvesting pattern are likely the 2 factors most affecting the natural landscape pattern and processes in this NDU.

Effective fire control over the past 40 - 50 yrs has slowed the natural disturbance rate (ANY DATA??). This has had the compound effect of increasing the amount of old forest in more remote areas where harvesting has not occurred and reducing young forest established by fire.

While wildfire creates disturbances of all sizes and the landscape is dominated by large disturbances, forest management has generally been directed to achieve mid-sized patches (40 – 100 ha) (DeLong and Tanner 1996). Larger harvest patches often occur due to management of windthrow. Dispersed harvest of mid-sized patches is both very unnatural but also creates fragmentation and a porous landscape for spread of pests that attack older trees.

Land clearing for agriculture, range, and forestry have removed mature spruce forest from many areas resulting in a loss of natural spruce regeneration. This along with silvicultural practices aimed at regenerating spruce or aspen, but not both, has lead to a shift in natural tree species distribution patterns.

Well site clearing and clearing of seismic lines associated with the oil and gas industry has resulted in fragmentation of the landscape and human access into remote areas of this NDU.

Concentrated harvesting along the alluvial benches of the major rivers has almost certainly reduced the amount of old forest in these areas.

Recommended Practices

Since forests with “old forest characteristics” typically ranged from 120 – 200 years in this NDU, it seems appropriate to have a system of rotating old forest reserves between these ages. This would insure stands with “old forest characteristics” exist but they are not unnaturally old and more susceptible to pest infestation. Large patches (> 100 ha) of old forest should be identified and recruited such that replacement areas > 120 years old are available to replace areas > 150 years of age that would be harvested. Recruitment areas should be preferentially selected in the following order: 1) unsalvaged natural disturbances, 2) partially salvaged natural disturbances, 3) large blocks designed to approximate wildfire that have mostly been regenerated naturally, 4) large blocks (> 100 ha) designed to approximate wildfire that have mostly been regenerated artificially, 5) large blocks (> 100 ha) that were not designed specifically to approximate wildfire and 6) small to medium sized blocks (< 100 ha). Table 2 contains estimates of seral stage distribution based on the natural range of variability.

Young natural forest

Some proportion of natural disturbances should be left unsalvaged to provide habitat (e.g., burned snags) that cannot be provided by young managed stands.

Patch size

Since medium sized patches (50 – 100 ha) are rare in the natural landscape and small patches are still naturally created by small fires, windthrow, and root disease, the emphasis should be on creating larger patches (> 100 ha). Larger patches should be created by aggregating recent blocks in areas previously harvested and/or by designing new large blocks in unharvested areas. Patch size distribution should emulate that of wildfire (Table 2) as closely as possible given social, logistic, or demonstrated ecological constraints. Design of blocks should follow guidance provided in DeLong (2000).

Stocking and stand structure

Location, Climate, and Vegetation

This unit occupies the valleys and mountain slopes on the lee side of the Rocky Mountains in the Hart Foothills and Peace Foothills Ecosections. This NDU occurs in a relatively narrow band, from 54^° - 56^°N latitude. The unit occurs from the valley bottoms to alpine peaks, above the Boreal Plains NDU to the east and adjacent to the Wet Mountain NDU to the west.

The climate of this unit is characterized by frequent outbreaks of arctic air masses resulting in long, cold, snowy winters and short summers. This unit is intermediate in precipitation, but one of the coldest NDU’s. MAP ranges from 560 – 780mm MAT from –0.3 to 0.4°C.

Upland climax forests are dominated by hybrid white spruce and/or subalpine fir. Subalpine fir increases with elevation. Lodgepole pine and to a lesser extent trembling aspen and paper birch dominate young stands. Black spruce occurs sporadically along with lodgepole pine on upland sites.

Wetlands are uncommon but do occur scattered along the broader valleys and in flatter terrain in the mountains.

Alluvial forests of black cottonwood, often with a minor component of spruce, are found sporadically along the floodplains of the larger rivers.

Natural Disturbance Ecology

Fire is the key stand-replacement disturbance agent operating in this unit. The fire cycle assigned to this unit is 120 and 150 respectively for the valley (BWBSmw1, SBSwk2) and mountain (BWBSdk1, ESSFmv2) portions. Table 2 shows the amount of forests of different age that would be associated with these fire cycles. Large wildfires (> 1,000 ha) dominated the landscape and on upland sites were generally regenerated quickly by dense lodgepole pine resulting in large patches of relatively even-aged forests (Table 2). Black spruce and white spruce regenerated the wetland areas after fire. Stand ages occasionally exceed 200 years and relatively large patches (>100 ha) of older forest (120 – 180 yrs) could be found scattered across the landscape (Don Rosen, unpublished data). Although patches of old forest (> 140 yrs) likely always occurred in the landscape at any one time period old forest may be rare in any particular watershed.

During stand development, increasing amounts of white spruce and subalpine fir will occur in stands originally dominated by lodgepole pine. This increase occurs more rapidly and these species become a more dominant portion of the canopy on wetter sites and at higher elevations. Post-fire stands are very dense except on the wettest sites and they self thin over time. No data for snags and CWD is currently available for this unit.

Fire control and harvesting pattern are likely the 2 factors most affecting the natural landscape pattern and processes in this NDU.

Effective fire control over the past 40 - 50 yrs has slowed the natural disturbance rate (ANY DATA TO SUGGEST THIS??). This has had the compound effect of increasing the amount of old forest in more remote areas where harvesting has not occurred and reducing young forest established by fire.

While wildfire creates disturbances of all sizes and the landscape is dominated by large disturbances, forest management has generally been directed to achieve mid-sized patches (40 – 100 ha) (DeLong and Tanner 1996). Larger harvest patches often occur due to management of windthrow. Dispersed harvest of mid-sized patches is both unnatural but also creates fragmentation and a porous landscape for spread of pests that attack older trees.

Recommended Practices

Young natural forest

Some proportion of natural disturbances should be left unsalvaged to provide habitat (e.g., burned snags) that cannot be provided by young managed stands.

Patch size

Stocking and stand structure

Wet Trench

Location, Climate, and Vegetation

This unit occupies the Rocky Mountain Trench from approximately Purden Lake to McBride and valleys and slopes of the Rocky Mountains east to the Continental Divide, and Caribou Mountains to their western extent.

Cool very snowy winters; and warm and very wet summers characterize the climate of this unit. This NDU is second to the Wet Mountain NDU in terms of annual precipitation. The temperature regime varies in a gradient from valley to mountain top. MAT of the lower elevation ICH is 3.3 – 4.8°C and –0.1 - 0.3°C for the higher elevation ESSF. MAP data from long-term stations is 840 for the ICH and 1044 - 1537mm for the ESSF. Annual snowfall often reaches 4 to 8 m depending on elevation.

Old upland coniferous forests dominate the Wet Trench natural landscape. Hybrid white spruce and subalpine fir are the dominant climax tree species in cold air drainage areas and at elevations above about 1200m whereas Western red cedar (Thuja plicata) and western hemlock (Tsuga heterophylla) are the dominant climax species on the warmer slopes. Lodgepole pine is limited to some wetlands and the occasional rocky ridge and Douglas-fir as an sporadic element on drier sites. Paper birch and aspen occur as pioneer trees in scattered recently disturbed areas. Some cottonwood occurs along the floodplains of the larger rivers. Black spruce occurs in wetlands that occupy the broader valley bottoms. Sitka alder occurs commonly on slopes at higher elevations throughout the unit, on avalanche tracks and in swales.

Natural Disturbance Ecology

Stand-replacement disturbance events occur at irregular intervals with as much as 1000 years between such events on any site. The stand replacement disturbance cycle assigned to this unit is 600 for the ICH and 800 for the ESSF. Fire cycle would have likely varied considerably depending on site conditions. It is felt that some toe slope sites in the ICH may have not had a stand replacement fire for 1000 years or more. Western hemlock looper (Lambdina fiscellaria ssp. lugubrosa) can cause significant tree mortality leading to stand replacement. Outbreaks covering 10’s of thousands of hectares have been recorded (Taylor unpublished, Parfett et al. 1995). Table 2 shows the amount of forests of different age classes that would be associated with the assigned stand replacement disturbance cycle. Patch size distribution of fire has not been determined but is thought to be similar to that of the Wet Mountain NDU (Table 2).

Young natural stands in the ICH portion of the NDU may be dominated by western hemlock, spruce, aspen or black cottonwood depending on landscape position and disturbance history and are generally dense to very dense (> 5 000 sph). Over time, most stands increase in proportion of cedar with the exception of some of the driest sites, which remain hemlock dominated. Young stands in the ESSF portion of the NDU are generally dominated by spruce or subalpine fir but may occasionally be dominated by lodgepole pine or aspen, especially at lower elevations. Spruce tend to out live subalpine fir so they comprise the majority of the largest stems in older stands. Subalpine fir is more abundant as elevation increases due to their greater ability to survive in the severe high elevation environment. Based on a recent study by Harrison and DeLong (2000) snag density (> 7.5 dbh) is 145 ± 97 SD stems/ha in the ICHwk3 and 128 ± 74 SD stems/ha for the ESSFwk1 in older stands.

A recent study by Harrison and DeLong (2000) indicates CWD volumes in older stands range from 144 – 658 m³/ha for the ICHwk3 and 60 – 425 m³/ha for the ESSFwk1. Means were 255 and 243 m³/ha respectively.

Arboreal lichen abundance is high in older forests in the ESSF. The study by Harrison and DeLong (2000) found greater numbers of trees with high abundance of arboreal lichen in stands > 200 years old vs stands < 200 years old.

The very oldest forests within the ICH portion of this unit support some globally rare arboreal lichen assemblages. Work done by Trevor Goward indicate that the oldest (>300 yrs) forests support more species and a greater abundance of these species.

Harvesting rate and reforestation practices are likely the 2 factors most affecting the natural landscape pattern, stand composition and structure and associated processes in this NDU.

Lodgepole pine has been planted in some areas within this NDU on sites where there is no present evidence of it having occurred. In the ICH portion of this NDU, current practices of favouring spruce over hemlock or cedar will lead to stands with a higher proportion of spruce in managed stands as compared to natural stands. In the ESSF portion, the same is true for spruce vs subalpine fir (i.e., current practices will lead to stands with a higher proportion of spruce). The transition to more spruce within the ICH may result in an increase in white pine weevil (Pissodes strobi) attack of spruce.

Recommended Practices

Since forests with “old forest characteristics” dominated the landscape in this NDU, it seems appropriate to have old forest reserves or forest > 100 yrs old well distributed throughout all watersheds. A high degree of connectivity between these old forest patches should also be managed for since there was always a high degree of connectivity of old forest in the natural landscape. Since the oldest forests in this NDU appear to have some unique arboreal lichen communities efforts should be made to capture as many of the oldest forests in reserves as is possible. Table 2 contains estimates of seral stage distribution based on the natural range of variability.

Young natural forest

Patch size

Silviculture system

Some form of partial cutting that approximate the effects of hemlock looper attack would seem appropriate in the ICH in order to maintain the type of stand structure associated with this disturbance agent. Some balance between this system and clearcut with reserves would seem most appropriate in the ICH. At higher elevations, use of silvicultural systems that approximate gap disturbance seems appropriate over a significant portion of the landscape (i.e., >50%).

Stocking and stand structure

Appropriate measures need to be developed to ensure that managed stands have a relatively similar species composition across the landscape as natural stands.

Moist Trench

Location, Climate, and Vegetation

This unit occupies the Rocky Mountain Trench from approximately McBride to the southern end of McNaugton Reservoir and valleys and slopes of the Rocky Mountains east to the Continental Divide, and Caribou Mountains to their western extent.

The climate of this unit is characterized by snowy winters and warm and moist summers. This NDU is intermediate to other NDU’s in terms of annual precipitation. The temperature regime varies in a gradient from valley to mountain top. MAT of the lower elevation SBS is 3.1°C, and 4.0°C for the ICH. There is no long-term data for the ESSF. MAP data from long-term stations is 568mm for the SBS, 712mm for the ICH and 816mm for the ESSF. Annual snowfall often reaches 2 to 4 m depending on elevation.

Mature to old upland coniferous forests dominate the Moist Trench natural landscape. Hybrid white spruce and subalpine fir are the dominant climax tree species in cold air drainage areas and at elevations above about 1200m whereas western red cedar and western hemlock are the dominant climax species on the warmer slopes. Lodgepole pine occurs on drier poorer sites, in younger stands on most sites, and in some wetlands. Douglas-fir occurs as a long lived seral species form the valley bottom to mid elevations, generally on mesic and drier sties. Western white pine (Pinus monticola) occurs in pockets at lower elevations south of Valemount and whitebark pine (Pinus albicaulis) occurs sporadically at high elevations throughout. Paper birch and aspen occur as pioneer trees in recently disturbed areas and in particular on warm slopes. Some cottonwood occurs along the floodplains of the larger rivers. Black spruce occurs in wetlands that occupy the broader valley bottoms.

Natural Disturbance Ecology

The stand replacement disturbance cycle assigned to this unit is 150 for the SBS and ICH and 300 for the ESSF. Fire cycle would have likely varied depending on site conditions. Table 2 shows the amount of forests of different age classes that would be associated with the assigned stand replacement disturbance cycle. Patch size distribution for fire is shown it Table 2.

Young natural stands in the ICH portion of this NDU may be dominated by western hemlock, lodgepole pine, spruce, aspen, paper birch, or black cottonwood depending on landscape position and disturbance history. Over time, most stands in the ICH increase in proportion of cedar with the exception of some of the driest sites, which remain hemlock and/or Douglas fir dominated. Young stands in the ESSF portion of the NDU are generally dominated by subalpine fir, lodgepole pine, or spruce but may occasionally be dominated by aspen or paper birch, especially at lower elevations. Spruce tend to out live subalpine fir so they comprise the majority of the largest stems in older stands. Subalpine fir is more abundant as elevation increases due to their greater ability to survive in the severe high elevation environment. Young natural stands in the SBS portion of this NDU are generally dominated by lodgepole pine or trembling aspen and occasionally by spruce or Douglas-fir. Over time, spruce and Douglas-fir dominate. The study by Harrison and DeLong (2000) indicates snag densities (>7.5 dbh) of 193±252 stems/ha for stands in the ICHmm.

A recent study by Harrison and DeLong (2000) indicates CWD volumes in the range of 26 – 557 m³/ha for older stands in the ICHmm and 47 – 753 m³/ha for older stands in the ESSFmm. Means were 290 and 280 m³/ha respectively.

Fire control and harvesting pattern are likely the 2 factors most affecting the natural landscape pattern and processes in this NDU.

Effective fire control over the past 40 - 50 yrs has likely slowed the natural disturbance rate in this NDU. This has had the compound effect of increasing the amount of old forest in more remote areas where harvesting was not occurring (e.g., south end of McNaughton Reservoir) and reducing young forest established by fire. Increasing old forest that is the most susceptible to agents such as MPB and decreasing the amount of large patches of young forest that is least susceptible to these agents in more remote areas has likely exacerbated the current pest problems in this NDU.

Currently, disturbance rates associated with harvesting are higher than those previously associated with wildfire especially at higher elevation. This in combination with the fact that wildfire disturbs all ages of forest whereas harvesting concentrates on older forest has likely resulted in an overall decrease in older forest in this NDU.

Recommended Practices

Since forests with “old forest characteristics” were common in the landscape in this NDU, it seems appropriate to have old forest reserves or forest > 100 yrs old well distributed across the landscape. Since, large wildfires may have resulted in some watersheds with less old forest than others it seems appropriate to allow variability in the amount of old forest retained in different watersheds as long as over the whole NDU the seral stage targets indicated in Table 2 are met throughout the whole NDU. A moderate degree of connectivity between these old forest patches should also be managed for since there was always a fair degree of connectivity of old forest in the natural landscape especially at higher elevations.

Young natural forest

Patch size

Silviculture system

Although not studied, the large proportion of Douglas-fir in most of the lower elevation drier ecosystems likely resulted in lots of residual structure being maintained during fire. Variable amounts of retention within clearcuts and different forms of partial cutting should be used to create a wide range of retention of residual tree cover over the landscape at lower elevations. At higher elevations, use of silvicultural systems that approximate gap disturbance seems appropriate over a significant portion of the landscape (i.e., 30%).

Stocking and stand structure

Natural stands within this NDU contain a wide range of tree species especially during the first 100 years of succession. Appropriate measures need to be developed to ensure that managed stands have a relatively similar species composition across the landscape as natural stands.

Omineca

Location, Climate, and Vegetation

This unit occupies the valleys and mountains of the Omineca Mountain Ecoregion and the northern portion of the Central Canadian Rocky Mountains Ecoregion. This NDU occurs in a broad band, from approximately 55^° - 57^°N latitude. The unit occurs north of the Moist Interior NDU and south of the Northern Rockies NDU.

The climate of this unit is characterized by long, cold, snowy winters and short moist summers. Outbreaks of cold arctic air are common throughout the winter. This unit is intermediate in precipitation, but one of the coldest NDU’s. MAP ranges from 418 – 692mm and MAT from –0.3 to 1.2°C for lower elevation forests. Climate data is not available for upper elevation forests, but the climate is thought to be similar to that for the upper elevation forests of the Boreal Foothills NDU, which has a MAP of 780mm and a MAT of –0.3°C.

Upland climax forests are dominated by hybrid white spruce and/or subalpine fir. Subalpine fir dominance increases with elevation. Lodgepole pine and to a lesser extent trembling aspen and paper birch dominate young stands. Black spruce occurs sporadically along with lodgepole pine on upland sites.

Wetlands are locally common along the broader valleys and in flatter terrain in the mountains.

Alluvial forests of black cottonwood, often with a minor component of spruce, are found along the floodplains of the larger rivers.

Natural Disturbance Ecology

Fire is the key stand-replacement disturbance agent operating in this unit. Mountain pine beetle may be a significant stand replacement disturbance agent in localized areas. The stand replacement disturbance cycle assigned to this unit is 120 and 300 respectively for the valley (SBSmk2, BWBSdk1, BWBSwk2) and mountain (ESSFmv3, ESSFmv4) portions. However, fire cycle likely varied considerably based on breadth of valley, valley orientation, and slope aspect (Rogeau 2001). Table 2 shows the amount of forests of different age that would be associated with these fire cycles. Large wildfires (> 1,000 ha) dominated the landscape and on upland sites were generally regenerated quickly by dense lodgepole pine resulting in large patches of relatively even-aged forests (Table 2). Black spruce and white spruce regenerated the wetland areas after fire. Stand ages often exceed 200 years, especially at higher elevations, and relatively large patches (>100 ha) of older forest (120 – 180 yrs) could be found scattered across the landscape. Although patches of old forest (> 140 yrs) likely always occurred in the landscape at any one time period, old forest may be rare in any particular watershed.

Western balsam bark beetle and 2-year-cycle budworm (Choristoneura biennis) cause significant within stand mortality in spruce and subalpine fir leading stands. Western balsam bark beetle is most prevalent at higher elevations where it attacks mature subalpine fir. The 2-year-cycle budworm attacks both subalpine fir and spruce at all elevations, with highest mortality occurring in co-dominant or suppressed stems. Dendro-ecological analysis by Zhang and Alfaro (??) indicate a 32-year cycle of outbreak recurrence within the Omineca NDU.

During stand development, increasing amounts of white spruce and subalpine fir will occur in stands originally dominated by lodgepole pine. This increase occurs more rapidly and these species become a more dominant portion of the canopy on wetter sites and at higher elevations. Post-fire stands are very dense except on the wettest sites and they self thin over time. Preliminary data indicate CWD volumes of 76 ± 39 SD m³/ha for low elevation forests and 80 ± 71 SD m³/ha for high elevation forests. No data for snags is currently available for this unit.

Fire control and harvesting pattern are likely the 2 factors most affecting the natural landscape pattern and processes in this NDU.

Effective fire control over the past 40 - 50 yrs has slowed the natural disturbance rate by reducing the number of large fires (Rogeau 2001). This has had the compound effect of increasing the amount of old forest in more remote areas where harvesting has not occurred and reducing young forest established by fire. These 2 factors may have increased mountain pine beetle attack in remote areas where harvesting rate has been low.

While wildfire creates disturbances of all sizes and the landscape is dominated by large disturbances, forest management has generally been directed to achieve mid-sized patches (40 – 100 ha) (DeLong and Tanner 1996). Larger harvest patches often occur due to management of windthrow. Dispersed harvest of mid-sized patches is both unnatural but also creates fragmentation and a porous landscape for spread of pests that attack older trees.

Recommended Practices

Since forests with “old forest characteristics” were typically dispersed unevenly across the NDU (i.e., rare in some watersheds but abundant in others) it seems appropriate that old forest targets could be met over multiple watersheds rather than in each watershed. Table 2 contains estimates of seral stage distribution based on the natural range of variability. Based on work by Rogeau (2001) it may be appropriate to allocate more old forest to certain valleys and/or slope aspects.

Young natural forest

Some proportion of natural disturbances should be left unsalvaged to provide habitat (e.g., burned snags) that cannot be provided by young managed stands.

Patch size

Stocking and stand structure

Stand density in young circumesic stands (< 40 yrs old) should generally be kept at total stocking levels of > 2000 sph to approximate dense natural stands. More open patchy stocking (i.e., < 1000 sph) on hygric sites are recommended. Even-aged stands over most of the landscape would approximate the natural pattern. In areas of mixed aspen and spruce efforts should be made to grow mixed stands similar to those that would have developed under a natural fire regime. At higher elevations or in areas with a high proportion of spruce and subalpine fir, silviculture systems that manipulate stand structure in a manner similar to that of balsam bark beetle or 2-year-cycle budworm should be considered.

Location, Climate, and Vegetation

This unit occupies the rolling plateau east of the Crooked River, west to Rocky Mountains and between 54° - 55° N latitude. It is situated between the Moist Interior and Wet Mountain NDU’s.

This unit is one of the wetter NDU’s, but intermediate in temperature. MAP averages 899mm and MAT 2.7°C. Mean annual snowfall is in the range of 3m.

Upland climax forests are dominated by hybrid white spruce with varying amounts of subalpine fir. Lodgepole pine and to a lesser extent trembling aspen and paper birch dominate young stands. Douglas fir is a long lived seral species occurring on drier sites particularly in areas of colluvial soils or bedrock control. Black spruce occurs sporadically along with lodgepole pine on upland sites.

Alluvial forests of black cottonwood, often with a minor component of spruce, are found mainly along the Parsnip River.

Natural Disturbance Ecology

Fire is the key stand-replacement disturbance agent operating in this unit. The stand replacement disturbance cycle assigned to this unit is 220. Table 2 shows the amount of forests of different age that would be associated with this fire cycle. Large wildfires (> 1,000 ha) dominated the landscape and on upland sites were generally regenerated quickly by dense lodgepole pine resulting in large patches of relatively even-aged forests (Table 2). Black spruce and white spruce regenerated wetter sites after fire. Young Douglas fir stands can be found on drier ridges where larger Douglas firs have escaped fire. Stand ages often exceed 200 years, and large patches (>100 ha) of older forest (120 – 180 yrs) could be found scattered across the landscape. Although large patches of old forest (> 140 yrs) likely always occurred in the landscape at any one time period, old forest may be rare in any particular area due to the extent of the fires (> 1,000 ha).

Spruce bark beetle, tomentosus root rot and 2-year-cycle budworm (Choristoneura biennis) may occasionally cause significant within stand mortality in spruce and subalpine fir leading stands. Tomentosus root rot is generally restricted to lower elevations (<900m).

Fire control and harvesting pattern are likely the 2 factors most affecting the natural landscape pattern and processes in this NDU.

Effective fire control over the past 40 - 50 yrs has slowed the natural disturbance rate by reducing the number of large fires. This has had the compound effect of increasing the amount of old forest in more remote areas where harvesting has not occurred and reducing young forest established by fire.

While wildfire creates disturbances of all sizes and the landscape is dominated by large disturbances, forest management has generally been directed to achieve mid-sized patches (40 – 100 ha) (DeLong and Tanner 1996). Larger harvest patches often occur due to management of windthrow. Dispersed harvest of mid-sized patches is both unnatural but also creates fragmentation and a porous landscape for spread of pests that attack older trees.

Recommended Practices

Since forests with “old forest characteristics” were typically dispersed unevenly across the NDU (i.e., rare in some areas but abundant in others) it seems appropriate that old forest targets could be met over multiple watersheds rather than in each watershed. Table 2 contains estimates of seral stage distribution based on the natural range of variability.

Young natural forest

Some proportion of natural disturbances should be left unsalvaged to provide habitat (e.g., burned snags) that cannot be provided by young managed stands.

Patch size

Stocking and stand structure

Current species composition and stocking in young managed stands appear to be consistent with natural patterns. Stand density and in young circumesic stands (< 40 yrs old) should generally be kept at total stocking levels of > 2000 sph to approximate dense natural stands. More open patchy stocking (i.e., < 1000 sph) on hygric sites are recommended. Even-aged stands over most of the landscape would approximate the natural pattern.

Northern Boreal Mountains

Location, Climate, and Vegetation

This unit occupies the valleys and mountains of the Northern Boreal Mountains Ecoregion. This NDU occurs in a broad band, from approximately 57^° - 60^°N latitude. The unit occurs north of the Omineca NDU and west of the Boreal Plains NDU.

The climate of this unit is characterized by long, very cold, moderately snowy winters and short moist summers. Outbreaks of cold arctic air are common throughout the winter. This unit is intermediate in precipitation, but one of the coldest NDU’s. MAP from long term stations is 580mm for higher elevations and 460mm for lower elevations and MAT ranges from –1.5 to –2.1°C from the southern extent to northern extent.

Upland climax forests have sparse crown closure and are dominated by hybrid white spruce and/or subalpine fir. Subalpine fir dominance increases with elevation. Lodgepole pine and to a lesser extent trembling aspen dominate young stands. Black spruce occurs along with white spruce or lodgepole pine on upland sites especially on north aspects.

Wetlands are common along the broader valleys and in flatter terrain in the mountains.

Alluvial forests of black cottonwood, often with a minor component of spruce, are found along the floodplains of the larger rivers.

Natural Disturbance Ecology

Fire is the key stand-replacement disturbance agent operating in this unit. Determining fire cycle is difficult in this NDU due to the amount of prescribed fire that has traditionally been used for management of ungulate forage. These large wildlife burns (>1,000 ha) mask the natural fire history of many of valleys. The provisional stand replacement disturbance cycle assigned to this unit is 180. However, fire cycle likely varied considerably based on breadth of valley, valley orientation, and slope aspect (Rogeau 2001). Table 2 shows the amount of forests of different age that would be associated with this fire cycle. Large wildfires (> 1,000 ha) dominated the landscape and on upland sites may be regenerated to lodgepole pine, trembling aspen (low elevations only), willows, or grasses depending on landscape position, site moisture regime, and available seed. Black spruce and white spruce regenerated the wetter areas after fire, except in areas of cold air drainage where willow and bog birch (Betula nana) dominate. Stand ages often exceed 200 years, especially at higher elevations, and relatively large patches (>100 ha) of older forest (120 – 180 yrs) could be found scattered across the landscape. Although patches of old forest (> 140 yrs) likely always occurred in the landscape at any one time period, old forest may have been rare in any particular watershed.

During stand development, increasing amounts of white spruce and subalpine fir will occur in stands originally dominated by lodgepole pine. This increase occurs more rapidly and these species become a more dominant portion of the canopy on wetter sites and at higher elevations. Post-fire stands are very dense except on the wettest sites and they self thin over time. Preliminary data indicate CWD volumes of 76 ± 39 SD m³/ha for low elevation forests and 80 ± 71 SD m³/ha for high elevation forests. No data for snags is currently available for this unit.

Very little forest management has occurred in this NDU due to the low productivity of the forests and long distances to any major population centre. The dominant anthropogenic influence has been broadcast burning to improve ungulate range. This has resulted in more young forest and open range conditions then likely would have been present in the natural landscape. This influence is strongest in the drainages on the lee side of the Rocky Mountains.

Much of the Northern Boreal Mountains NDU is in land designations that will limit or exclude harvesting. The areas that will receive harvesting are generally in

Recommended Practices

Young natural forest

Young natural forest should be relatively abundant in many areas due to the amount of prescribed burning that has occurred in the past without salvage logging. .

Patch size

Patch sizes should be relatively natural in this NDU due to the lack of harvesting and the amount of prescribed fire.

Stocking and stand structure

Stands should be relatively natural throughout most of this NDU due to the lack of effects of forest management activities.

Angelstam, P.K. 1998. Maintaining and restoring biodiversity in European boreal forests by developing natural disturbance regimes. Journal of Vegetation Science 9:593-602.

Bergeron, Y. & Harvey, B. 1997. Basing silviculture on natural ecosystem dynamics: an approach applied to the southern boreal mixedwood forest of Quebec. Forest Ecology and Management 92: 235-242.

Biodiversity Guidebook. 1995. Forest Practices Code of British Columbia. Victoria, B.C. Queens Printer.

Booth, D.L., Boulter, D.W.K., Neave, D.J., Rotherham, A.A. & Welsh, D.A. 1993. Natural forest landscape management: A strategy for Canada. Forestry Canada, Ottawa, Ontario. 16 pp.

Bunnell, F.L. 1995. Forest-dwelling vertebrate faunas and natural fire regimes in British Columbia. Conservation Biology 9:636-644.

Clark, D.F., Kneeshaw, D.D., Burton, P.J. & Antos, J.A. 1998. Coarse woody debris in sub-boreal spruce forests of west-central British Columbia. Canadian Journal of Forest Research 28:284-290.

DeLong, S.C. 1998. Natural disturbance rate and patch size distribution of forests in northern British Columbia: Implications for forest management. Northwest Science 72:35-48.

______ & Tanner, D. 1996. Managing the pattern of forest harvest: lessons from wildfire. Biodiversity and Conservation 5:1191-1205.

______ & Kessler, W.B. 2000. Ecological characteristics of mature forest remnants left by wildfire. Forest Ecology and Management 131:93-106.

Eberhart, K.E. & Woodward, P.M. 1987. Distribution of residual vegetation associated with large fires in Alberta. Canadian Journal of Forest Research 17:1207-1212.

Hawkes, B., Vasbinder, W. and DeLong, C. 1997. Retrospective fire study: Fire regimes in the SBSvk and ESSFwk2/wc3 biogeoclimatic units of northeastern British Columbia. Final report for McGregor Moder Forest Association, Prince George, B.C. (available on web at http://www.mcgregor.bc.ca/).

Hunter, M.L., Jr. 1993. Natural fire regimes as spatial models for managing boreal forests. Biological Conservation 65:115-20.

Hutto, R.L. 1995. Composition of bird communities following stand-replacement fires in Northern Rocky Mountains (U.S.A.) conifer forests. Conservation Biology 9:1041-1058.

Lewis, K.J. and B.S. Lindgren. 2000. A conceptual model of biotic disturbance ecology in the central interior of B.C.: How forest management can turn Dr. Jekyll into Mr. Hyde. Forestry Chronicle 76: 433-443

Parfett, N., I. S. Otvos, and A. V. Sickle. 1995. Historical Western Hemlock Looper Outbreaks in British Columbia: Input and Analysis Using a Geographic Information System. Canadian Forestry Service and British Columbia Ministry of Forests, Victoria.

Rogeau, M-P. 2001. Fire history study Mackenzie TSA, British Columbia. Report for Abitibi Consolidated Ltd. Mackenzie, B.C. 165pp.

Swanson, F.J., Jones, J.A., Wallin, D.O. & Cissel, J.H. 1993. Natural variability - Implications for Ecosystem Management. In: Jensen, M.E. & Bourgeron, P.S. (eds.). Eastside Forest Ecosystem Health Assessment. Volume 2: Ecosystem management: principles and applications. U.S. Forest Service, Oregon. p. 89-104.

[1][1] All disturbance rates are for stand replacement wildfire except where noted.

[2][2] All estimates quoted are for period of the period of 1911-30 as these were deemed to more accurately reflect the true natural disturbance rate.

[3][3] All disturbance rates are for stand replacement wildfire except where noted.

[4][4] All estimates quoted are for period of the period of 1911-30 as these were deemed to more accurately reflect the true natural disturbance rate.

[5][5] All disturbance rates are for stand replacement wildfire except where noted.

[6][6] Estimate based on calculation shown in Introduction.