For tree seedlings, shade tolerance (ST) is defined as low-light survival and is characterized by a conservative growth strategy, with associated low respiration and tissue loss/turnover rates and high allocation to storage, which come at the expense of lower growth potential. Although much is known about ST and related traits conceptually, less studied is the explicit quantification of ST and its relationship to growth, survival, and related traits in environments with other stressors and resource availabilities. Here, synthesized data from two (three- and six-year) field experiments using planted seedlings of multiple northern temperate species to investigate how ST and related traits in mesic, closed canopy forest understories relate to seedling performance and related traits in 1) more open-canopy (e.g harvest gap) mesic environments, and 2) in response to drought over both canopy openness and mesic-xeric site gradients.
Results/Conclusions
Among species in both experiments, measured ST (three- or six-year survival in < 5% light) was only weakly associated with published information on ST rankings. In the six-year experiment, ST was more strongly negatively related to height growth potential in high light than any other growth metric, and, in turn, height growth was positively related to successful escape from shrub-herb competition and browsing deer in mesic harvest gaps. In the three-year experiment, drought in the third growing season may have obscured expected differences in ST among species, with areas with higher canopy tree density facilitating seedling survival rather than increasing low light mortality relative to more open areas for all species. This pattern was especially strong on xeric sites where mortality was highest for all species. In contrast to the lack of ST associated patterns among species, xeric-site-adapted species had higher survival than more mesic-site-adapted species in response to drought on mesic and xeric sites, especially in open microsites, and survival was positively related to carbohydrate storage, root size and rooting depth, with depth positively related to water access. Our results have important implications for understanding forest community dynamics, including responses to climate variation and management efforts aimed at mitigating climate change, such as site, microsite and species choices for assisted migration and/or diversity enrichment plantings.