Plant species diversity on logged versus burned sites in central Alaska

Natural fires and logging are two of the main disturbances affecting upland boreal forest in Alaska. The objectives of this study were to determine whether logged sites differ from burned sites in (1) overall plant species richness, 2) successional trajectories, and (3) species diversity at particular stand structural development stages. We compared plant species diversity on sites burned in natural fires to sites that were logged and not subsequently burned in central Alaska. We sampled 12 logged and 12 burned former upland white spruce (Picea glauca (Moench) Voss) forests in four stand development stages representing stand initiation (stage A), early stem exclusion (stage B), understory reinitiation (stage Q, and mature hardwood (stage D) stages. In this study the dates of disturbance varied from 1990 to 1994 in stage A, 1978 to 1983 in stage B, 1957 to 1965 in stage C, and 1900 to 1920 in stage D plots. All sites were similar in slope, aspect, and soil type. Vascular plants were identified to the species level (except for certain willows) and bryophytes and lichens were identified to the level of presumptive (usually unknown) species within family groups. Organic layer thickness was significantly greater on logged sites compared to burned sites overall and at each stage. Burned sites (all stages combined) supported more species (146) than logged sites (I 11), and more species at each stand development stage. Burned plots in stages A and B supported abundant cover of a few apparent fire specialist species (Ceratodon purpurcus (Hedw.) Brid., Marchantia polymorpha L. and Leptobryum pyriforme (Hedw.) Wils.) that were present in only minor amounts on logged sites. Burned plots exhibited higher species turnover from stage to stage and among all stages than logged plots. Species dominant in burned stage A plots were nearly absent in burned stage C and D plots, while logged stage A dominants, which were common mature forest species, increased in each subsequent stage. We compared floristic similarity between our disturbance plots and mature upland white spruce stands in Bonanza Creek Long-Term Ecological Research ALTER) site. Only five species found in the LTER dataset were not also present in this study, which suggests that nearly all species compositional change in our study area occurs during the first century after disturbance. Logged sites appear to begin and continue succession with a greater share of the original mature forest understory plants, while burned sites initiate succession with more distinctive and specialized plant species. (C) 2002 Published by Elsevier Science B.V.