May 1999

INTRODUCTION

Public criticism of past practices and better understanding of natural systems has resulted in the emergence of new ideas about forest management (Maser 1988; Franklin 1989, Hansen et al. 1991). One such idea is that managed disturbances should be designed to approximate natural disturbance regimes. This suggestion is derived in part from emerging evidence that disturbance has a key ecological role in many forested ecosystems (Zackrisson 1977, Van Wagner 1978, Hessburg et al. 1994). The underlying assumption is that the biota of a forest is adapted to natural disturbances and thus could cope more easily with the ecological changes associated with timber harvest if the patterns created resemble those of natural disturbances (Hunter 1993, Swanson et al. 1993, Bunnell 1995). The Biodiversity Guidebook (1995) which outlines a process for meeting biodiversity objectives as required in the Forest Practices Code of British Columbia Act and Regulations is based on this approach.

Natural disturbances maintain plant and animal diversity over time and space by maintaining structural complexity within stands and by influencing the size, distribution, edge characteristics, and dispersion of stands across the landscape (Zackrisson 1977, Hansen et al. 1991, Hessburg et al. 1994). The size, shape, and location of individual forest patches or stands profoundly affect landscape stability and productivity (Franklin and Forman 1987, Frank and McNaughton 1991). Legacies of natural disturbance such as old large diameter trees, snags and woody debris play a fundamental role in maintaining the long term ecological functioning of the ecosystem (Hansen et. al. 1991, Maser 1992, Franklin 1994, Ammaranthus 1994).

The impacts of forest management appear now to have a greater influence on landscape pattern than those of natural disturbance agents (DeLong and Tanner 1996, Zackrisson 1977). Hence, there is a need to utilize information collected from studies of natural disturbance to assist forest managers in incorporating some design considerations into future harvest openings in order to reduce the functional dissimilarities between harvested and natural openings.

This workbook is intended to provide some considerations for design of larger or aggregate harvest openings based on studies conducted within the Sub-boreal Spruce Zone. In the long-term, developing larger or aggregated harvest areas on a periodic basis will provide several benefits. They will help:

· · maintain large patches of young, mid-seral and old forest through space and time, thereby reducing fragmentation and thus providing greater forest interior conditions;

· · diminish the amount of permanent roads, with associated benefits to wildlife, the productive landbase, and water quality; and

· · facilitate cost-effective timber harvesting.

The design considerations are applicable to harvest openings within all natural disturbance types (NDT's) as set out in the Biodiversity Guidebook (British Columbia Ministry of Forests 1995) but have particular reference to those where stand replacement wildfire was common (i.e., NDT3).

Table 2. Windthrow hazard evaluation using identifiable features from maps,

air photos and field reconnaissance.

Source of Interpretation/ Factors	High Hazard	Moderate Hazard	Lower Hazard
Air Photos/Maps Stand Evenness/Density Stand Composition Topographic Exposure Terrain Evidence of Previous Windthrow Age Since Disturbance	uneven/open pure spruce crests, saddles, narrow valleys parallel to max. wind, lee slopes of hills (see Stathers et. al. 1994) shallow to bedrock or organic moderate to extensive < 5 years or > 100	uneven/dense even/open mixed stands, pine, subalpine fir mid slope edges on windward side of hills or parallel to wind direction lacustrine, fluvial (wetter sites), compact till minor 5-10 years or 50-100 years	even/dense Douglas-fir, deciduous draws running perpendicular to max. winds till, colluvial, fluvial (moist to dry sties) none 10-50 years
Field Reconnaissance Soils	root restricting layer within 40 cm?, humus depth >8 cm, saturated soils in late spring or summer, poorly drained	root restricting layer 40-80 cm deep, effective rooting depth <40 cm, imperfectly to moderately well drained	root restricting layer >80 cm, effective rooting depth >40 cm, well drained
Trees	dominant stems with low taper and no butt flare	dominant stems with moderate taper and butt flare	dominant stems with high taper and large butt flare
Crowns	large dense crowns	moderately dense crowns	small open crowns
Root Rot	many trees affected	few trees affected	no trees affected

Table 3 Average and range in proportion of area of island remnants by size class of

Fire

Size

(ha)

% of total area in island remnants by size class (ha) of remnant

1-2

2-5

5-10

10-20

20-50

>50

<1000

n=4

(17-46)

(12-35)

(27-40)

(0-44)

>1000

n=5

(2-11)

(6-20)

(11-32)

(2-17)

(8-29)

(10-43)

(0-32)

Figure 1. Comparison of wildfire (top) with harvest block showing difference in shape.

Figure 2. Map of amalgamated block showing application of basic windfirm shape. Hatched areas are large wildlife tree patches.

Figure 3. Percent of total area of wildfire occupied by island remnants by size class of wildfire.