Climate stress and competition: The one-two punch for forest recruitment in a changing climate

Paige E. Copenhaver-Parry and Joanna W. Gunther, Department of Biology, George Fox University, Newberg, OR, Shelby Byerly, Center for Ecosystem Science and Society, Northern Arizona University, Flagstaff, AZ

Background/Question/Methods
Seedling recruitment is a critical transition in the demography of forests, and is likely to profoundly influence the response of forests to ongoing environmental change. Long-lived tree species will probably regenerate under conditions that are increasingly different from those under which current canopy trees established, providing opportunities for the strength of recruitment limitation and its impact on population dynamics to shift. Understanding spatiotemporal variation in the mechanisms underlying recruitment limitation may clarify the anticipated contribution of recruitment to population dynamics and tree range shifts in a changing climate. In this study, we investigate variation in the limiting effects of subcanopy climate and light availability, as a proxy for aboveground competition, on seedling counts across a large climate gradient in western Oregon, USA. We apply a hierarchical Bayesian model with site-varying coefficients to three years of seedling recruitment data to test the hypothesis that the limiting effects of climate and aboveground competition on recruitment will vary inversely along a climatic stress gradient, as consistent with the stress-gradient hypothesis. We apply our model to two abundant tree species to evaluate the mechanisms underlying recruitment limitation for species with different life histories.
Results/Conclusions
Seedling densities were highest in high-elevation sites positioned near the middle of the climatic stress gradient, and lowest at warm, dry sites at the most stressful end of the gradient. The effects of subcanopy climate on recruitment appear weak across all sites, but increase with climatic stress. In these sites, soil moisture limits recruitment. Climate effects, which were modeled at the level of individual research sites, were associated with relatively high uncertainty, likely reflecting the overriding influence of microscale variation in climate on individual seedlings within a site. For western hemlock – a shade-tolerant, mid-seral species – the effect of aboveground competition varied little across the climate gradient, and was consistently negative. For Douglas-fir – an early-seral, shade-intolerant canopy dominant – competition showed an increasingly negative effect on recruitment in sites located toward the lower end of the climatic stress gradient, but contributed little to interannual variation in recruitment in more climatically stressful sites. These results suggest that increasing climate change-induced stress is likely to increase recruitment limitation for shade-tolerant species subject to spatially consistent aboveground competition, whereas competitive effects on recruitment may weaken for shade-intolerant species. The extent to which increasing climatic stress may compensate for reduced competitive effects will have important implications for forest regeneration in a changing climate.