Effects of microsite conditions on seedling recruitment across a climate gradient

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Emily K. Morgan and Paige E. Copenhaver-Parry, Department of Biology, George Fox University, Newberg, OR

Background/Question/Methods
As climate change progresses, long-lived tree species will regenerate under conditions that are increasingly different from those under which current canopy trees established. Because seedling recruitment is a limiting demographic transition in closed-canopy forests, current and future recruitment trends are likely to exert a profound influence on the diversity and distributions of forest communities. Inferring the future of forests thus requires a detailed understanding of the conditions under which seedlings can establish, along with knowledge of the mechanisms that serve to limit recruitment across climate gradients. The objective of this study was to determine whether the effects of microsite conditions on seedling recruitment vary across a climate gradient in naturally regenerated conifer forests in the Pacific Northwest. To address this objective, seedling establishment and microsite conditions including substrate, vascular plant cover, light availability, and fine root biomass were monitored over four years and across five long-term plots that span a substantial climate gradient in western Oregon. We fit hierarchical Bayesian regression models to seedling and microsite data to investigate variation in the strength of microsite effects across sites, and to test the hypothesis that the strength of microsite limitation will vary inversely with climatic stress across the climate gradient.
Results/Conclusions
Our results show that the effects of microsite conditions, as predicted, do vary across the climate gradient. Consistent with our initial hypothesis, microsite effects for both focal species are more consistently significant, of greater magnitude, and largely negative in the least climatically-stressful site, whereas microsite conditions appear to have little influence on recruitment in the most climatically stressful sites, with the degree of climate stress determined by the duration and intensity of summer drought. For Douglas-fir, substrate type, vascular plant cover, and light availability appear to strongly limit recruitment in the moist, productive forests of the Pacific Coast, but have little influence on recruit densities at drier sites in the Cascade Mountains. For western hemlock, substrate type, vascular plant cover, and fine root biomass have limiting effects on recruitment in coastal and high elevation sites, but microsite effects are largely nonsignificant at warm, dry sites. Our results suggest that increasing climatic stress in the context of climate change may weaken microsite limitation in some regions and for some species, yet these effects should be considered within the context of shifting canopy conditions and biotic environments to fully anticipate their impact on future forest composition and distributions.