Disturbance and Post-harvest Ground Conditions in a Structural Retention Experiment

Charles B. Halpern and Donald McKenzie* College of Forest Resources Box 352100 University of Washington Seattle, WA 98195-2100 chalpern@u.washington.edu *Current address: Pacific Wildland Fire Sciences Lab, USDA Forest Service, 400 N. 34th Street, Suite 201, Seattle, WA 98103.

We provide a brief overview of this work below. For full text see:

Halpern, C.B., and D. McKenzie. 2001. Disturbance and post-harvest ground conditions in a structural retention experiment. Forest Ecology and Management 154:215-225.

Introduction

Soil disturbance is a natural consequence of timber harvest. Disturbance to soil organic layers and compaction, exposure, or displacement of mineral horizons can affect soil nutrient availability or soil-water retention. Ground disturbance and deposition of logging slash can also change vegetation composition by removing existing plants and by affecting future establishment. Soil disturbance can also stimulate germination of the soil seed bank which is typically dominated by ruderal and, in some instances, exotic species. On the other hand, deposition of logging slash may have a positive influence on understory recovery, by providing protection from excessive solar radiation and heating for shade-tolerant herbs, planted trees, and advanced regeneration. Likewise, soil disturbance can enhance natural regeneration of particular tree species that establish preferentially on mineral substrates.

During conventional (i.e. clearcut) logging operations, the distribution and intensity of ground disturbance reflect, in large part, the method by which logs are removed from a site: disturbance is more pervasive with ground-based systems (e.g. skidders) than with suspension cables or helicopter systems. In this paper, we examine patterns of ground disturbance and production of logging slash during structural retention harvests, focusing in particular on how these vary with level and spatial pattern of live-tree retention and with yarding method.

Methods Within each of the six experimental blocks (see Study Areas), cover of logging slash, cover of disturbed soil, depth of slash, and volume of fresh woody debris (decay class 1) were sampled at multiple points within each of four harvest treatments: 15% aggregated retention, 15% dispersed retention, 40% aggregated retention, and 40% dispersed retention (see Experimental Design).

Results

• Accumulations of slash and fresh woody debris (class 1 CWD) were significantly greater at lower levels of retention (Figure 1).
• Cover of slash was significantly greater in aggregated than in dispersed treatments (Figure 1).
• Large differences in ground conditions were observed among experimental blocks (compare block means in Figure 1), reflecting variation in initial forest structure and harvest methods.

Figure 1.  Mean values for disturbance and ground-condition variables by treatment (pattern and level of retention) and block.  Agg = aggregated retention, Disp = dispersed retention; 15% and 40% are percent basal area retained.  See Study Areas for block names and locations.  Yarding methods varied among blocks: B, DP, and LWS = helicopter; PH and WF = shovel loader; CF = suspension cables.  Levels of significance for treatment and block effects are from a two-factor ANOVA.  P values >0.05 are not indicated.

• We observed significantly greater within-treatment (plot-to-plot) variation in slash cover within dispersed treatments than within the harvested portions of aggregated treatments (Figure 2).
• Similar trends, although not significant, were observed for the remaining disturbance and ground condition variables.

Figure 2.  Plot-to-plot variation (coefficient of variation, CV) for disturbance and ground-condition variables by treatment and block.  CV of slash depth showed a significant pattern x treatment interaction (P = 0.013).  See Figure 1 for other details.

Discussion

Our analyses indicate that both the proportion of basal area retained and the pattern in which it is retained can have significant effects on levels of ground disturbance and slash accumulation. Plot-level disturbance (as measured by cover of disturbed soil) and slash accumulation were significantly reduced by greater retention of live trees and by retention in a dispersed fashion. Although the harvested portions of aggregated units experienced greater disturbance, their patches of retained forest remained relatively untouched. Thus, there is a tradeoff with aggregated retention: greater intensity of disturbance concentrated in a smaller portion of the harvest unit. Our analyses also suggest that pattern of retention affects the local variability of slash cover and depth within treatments: greater variation where trees are dispersed -- a pattern consistent with the heterogeneity that would arise from plots containing varying numbers of residual trees.

Our experimental design does not permit us to make statistical inferences about effects of yarding systems. Nonetheless, the relatively large variation among blocks (and the nature of that variation) suggests that, as with traditional silvicultural systems ground-based yarding is likely to yield significantly greater disturbance than is helicopter yarding.

Our companion studies of vegetation suggest that initial responses of the understory to green-tree retention harvests are shaped, in large part, by these patterns of ground disturbance and slash accumulation (see Responses of Understory Plant Communities). Longer-term measurements of vegetation response are needed to understand the degree to which these initial effects of disturbance will persist.