Estimating retention benchmarks for salvage logging to protect biodiversity

Abstract

Forests are increasingly affected by natural disturbances. Subsequent salvage logging, a widespread management practice conducted predominantly to recover economic capital, produces further disturbance and impacts biodiversity worldwide. Hence, naturally disturbed forests are among the most threatened habitats in the world, with consequences for their associated biodiversity. However, there are no evidence-based benchmarks for the proportion of area of naturally disturbed forests to be excluded from salvage logging to conserve biodiversity. We apply a mixed rarefaction/extrapolation approach to a global multi-taxa dataset from disturbed forests, including birds, plants, insects and fungi, to close this gap. We find that 75 ± 7% (mean ± SD) of a naturally disturbed area of a forest needs to be left unlogged to maintain 90% richness of its unique species, whereas retaining 50% of a naturally disturbed forest unlogged maintains 73 ± 12% of its unique species richness. These values do not change with the time elapsed since disturbance but vary considerably among taxonomic groups.

Fig. 1. Hypothetical example of mixed rarefaction/extrapolation.

The example depicts species sampled in naturally disturbed but unlogged plots and naturally disturbed and salvage-logged plots. Unlogged plots (dashed green curve, upper x-axis) had a higher species richness rarefaction curve than salvage-logged plots (dashed orange curve, upper x-axis) across the proportion of sampled area. The solid purple curve represents the species richness for a given mixture of salvage-logged and unlogged plots when a proportion of disturbed area remains unlogged (lower x-axis). The richness of species that are unique to unlogged plots (solid green curve, lower x-axis; used in our analysis) increases with increasing proportion of unlogged disturbed plots in the mixture. Here only a mixture of two rarefaction curves is presented; see ref. for a mixture including both rarefaction and extrapolation curves.

Fig. 2. Location of studies included in the present analysis.

Each study provided species-by-plot abundance matrices for salvage-logged and unlogged, naturally disturbed forest plots. Disturbance types are indicated by different symbol colors (red: wildfires, blue: windstorms, yellow: insect outbreaks; see Supplementary Table 1 for details and references). Background colors indicate major terrestrial ecoregions.

Fig. 3. Response of species richness to different retention levels in salvage logging.

Mean and standard deviation (shading) of richness of species unique to naturally disturbed, unlogged plots that would be maintained under varying portions of naturally disturbed forest excluded from salvage logging. The solid lines (means) are analogous to the solid green line from the hypothetical community in Fig. 1, indicating the mean response of 201 individual species matrices with (a) all data pooled, (b) datasets separated into different disturbance types, and (c) datasets separated into saproxylic and non-saproxylic taxa. Source data are provided as a Source Data file.

Fig. 4. Estimated retention benchmarks for the assessed taxonomic groups.

Distribution (dot histogram) and mean values (white diamonds) with corresponding standard deviation for: (a) the retention area needed to maintain 90% of species unique to unlogged naturally disturbed forest; and (b) the portion of species unique to unlogged naturally disturbed forest that would be maintained if 50% of the disturbed area was salvage logged. Saproxylic groups are marked with an asterisk. The right-hand box-whisker plots depict the median number of species unique to unlogged naturally disturbed plots (see Supplementary Table 1 for details) with lower and upper quartiles (box). Icons with permission from thenounproject.com. Source data are provided as a Source Data file.

Fig. 5. Response of retention benchmarks to time since disturbance.

Distribution (scatterplot) and annual mean values (white diamonds) with corresponding standard deviation across years for: (a) the retention area needed to maintain 90% of species unique to unlogged naturally disturbed forest; and (b) the portion of species unique to unlogged naturally disturbedforest that are maintained if 50% of the disturbed area is salvage logged. Source data are provided as a Source Data file.