Native forest pathogens may facilitate persistence of Douglas-fir in old-growth forests of Northwestern Californa

In forests native pathogens accelerate tree decline, increase stand structural heterogeneity, and alter tree community composition. In northern California, white fir (Abies concolor var. lowiana) and Douglas-fir (Pseudotsuga menziesii var. menziesii) are co-occurring tree species that exhibit different degrees of shade tolerance and regenerative abilities following disturbance. In this study we asked whether native forest pathogens are having disproportional impacts on white fir. Pathogens were sampled in old-growth white fir/Douglas-fir stands in northwestern California to assess their roles in tree mortality, gap formation, and regeneration. In three study sites ranging from 4-10 ha, we sampled relative abundances and size class distributions of white fir and Douglas-fir, determined presence of pathogens, and estimated causes of tree mortality. Canopy gaps occurring in the sites were sampled for mode of death of gapmaker trees and, along with an equivalent area of closed-canopy forest, for regeneration of white fir and Douglas-fir. For trees ≥ 10 cm dbh root rot fungi, in combination with windthrow, accounted for significantly higher mortality and gap formation in white fir relative to Douglas-fir. Relative density of Douglas-fir seedlings/saplings was higher in canopy gaps than under closed-canopy forest. This study demonstrates the influence of native pathogens on successional patterns.