This presentation focuses on changes in forest structure, annualized tree mortality rates (AMR), and above-ground carbon (C) for a research natural area (RNA) of the grand fir (Abies grandis) zone. The RNA forest has been documented as habitat for endangered raptors, carnivores, and plants. Persistent defoliation by western spruce budworm (Choristoneura occidentalis) occurred in the RNA throughout the 1990s. Accordingly, this study asked: What changes occurred in tree density [trees and regeneration per hectare (ha), tree size [basal area per ha (BA/ha) and quadratic mean diameter (QMD)] and in aboveground carbon and how did changes differ by period (1992-2000=outbreak; 2000-2010=post outbreak)? Was tree mortality in the RNA density-dependent or density-independent? How did the AMR change? To address these questions, data on trees, snags, and downed wood were collected from 21 plots on a permanent sample grid in 1992, 2000, and 2010 and then used to calculate AMR, trees/ha (TPH), regeneration/ha, BA/ha, and QMD for all three samples, for two sample periods (1992-2000 and 2000-2010), and for the duration of the entire study (1992-2010). Aboveground C (metric tons/ha or Mg) was estimated by using the eastern Cascades variant of the forest vegetation simulator. Analysis of changes among re-measurement periods was done with a linear mixed model, with time interval as a fixed, repeat effect and plot as a random effect, by using SAS PROC GLIMMIX.
Results/Conclusions
The RNA forest was comprised of fewer, larger trees after the budworm outbreak than it was at its beginning (TPH (adj p<0.0002) and QMD (adj p <0.0001). For all sizes and species of trees, the AMR of trees from 1992-2010 was 6.6%, but AMR varied within and across diameter classes and by species. Mortality in the largest class of trees (>76.2 cm diameter) was of Pinus ponderosa (ponderosa pine), Larix occidentalis (western larch), and Pseudotsuga menziesii (Douglas-fir), whereas regeneration was primarily grand fir. Regeneration became patchier through time, tree species composition and basal area decreased, and estimated total aboveground C declined from 235 Mg/ha to 197 Mg/ha. Density-independent mortality was likely occurring by 2010, but results were inconclusive. Aboveground C calculated for live trees and down wood/litter in the RNA forest decreased from 1992-2010, in contrast to an increase in aboveground C in the dead tree component. Post-budworm forest structure in the RNA is more likely to favor wildlife species associated with standing dead trees of all sizes than with large, living ponderosa pine, western larch, or Douglas-fir trees.