Mechanical thinning and controlled burning are used to reduce fuel loads and restore the ecological structure and function of dry, mixed-conifer forests of the western U.S. While these treatments reduce live and dead fuels, their effectiveness in meeting ecological objectives is less clear. We asked whether the responses of forest understory vegetation to experimental thinning and/or prescribed burning are influenced by the spatial scale of measurement and whether these responses differ between lifeforms. We analyzed data from a large-scale fuels-reduction experiment in dry forests of central Washington. The experiment employed a factorial combination of thinning and burning treatments. Richness of two lifeforms (shrubs, herbs) was sampled before and 9-12 years after treatment using a spatially nested design of eight experimental units (10-ha replicates of treatments) each sampled with six plots (1000 m2). Each plot contained 10 shrub subplots (50 m2) and 20 herb quadrats (1 m2). We used PERMANOVA to compare (1) change in mean richness of each lifeform at small, intermediate, and large spatial scales (subplot or quadrat, plot, unit); and (2) variation (standard deviation) in herb richness at two spatial scales: among quadrats within plots and among plots within units.
Results/Conclusions
Responses to thinning and burning varied between lifeforms and with spatial scale. We observed 28 shrub species before treatment and 29 after treatment, while the number of herb species observed increased from 72 to 102. Among shrubs, richness was unaffected by thinning, irrespective of scale, but increased with burning at small and intermediate scales. Among herbs, richness increased with both thinning and burning at all spatial scales, but variability in richness was not affected. Thus, treatments did not result in immigration, or emergence from the seed bank, of new shrub species, but burning increased the reproductive and/or vegetative spread of shrubs present prior to disturbance. On the other hand, numerous herbaceous species dispersed into, or emerged from the seed bank of, both treatments, presumably responding to the redistribution of resources by disturbance. At smaller spatial scales, increases in herbaceous richness may reflect establishment of widely dispersed, disturbance-adapted species and/or localized spread of residual species. Our research shows that not only do ecological responses to thinning and burning vary with spatial scale and lifeform, but analyses at multiple scales can reveal the patterns and mechanisms of response to disturbance.