Forest of the Pacific Northwest (PNW) have experienced rapid rates of climate warming in recent decades, yet individual forest stands have maintained stable species composition, biomass, and basal area, reflecting limited climate change responses. Recruitment limitation within existing stands may slow climate change responses, yet the degree to which overstory competition influences seedling establishment is currently unknown for several key species. The objective of this study was to determine the effects of overstory competition on establishment of seedlings in mature and old-growth PNW forests by quantifying the effects of fine root density, light availability, and neighboring adult tree density and size on seedling density across a range of environmental conditions and geographic positions. Seedlings were evaluated across five long-term monitoring plots extending from the coast to the crest of the Cascade range in western Oregon. Competition indices were developed based on diameter and density of neighboring adults. Light availability was determined using hemispherical canopy photography, and fine root biomass was quantified using existing fine root biomass maps. Hierarchical Bayesian Poisson missed effects regression models were constructed for each species to quantify the effects of fine root density, light availability, and neighboring adult tree density and size on seedling density.
Results/Conclusions
Preliminary results show that seedling establishment in PNW forests is strongly limited by light availability and fine root biomass, and seedlings of all species are largely restricted to canopy gaps. Our results also show that the effect of overstory competition on seedling establishment is highest in low- and mid-elevation stands and declines toward high elevation range margins. This indicates that seedlings currently benefit from reduced biotic pressure in high elevation stands, and may increasingly benefit from frequent gap formation that may result from increased climate change-driven mortality in low elevation stands. We found limited evidence for differential effects of competition across species. Overall, our results demonstrate that overstory competition is an important mechanism underlying recruitment limitation in PNW forests.