4. Discussion
This study empirically assessed the transition between climate-limited and competition-dominated tree growth across a large geographic and bioclimatic space. Clear patterns emerged that suggest a key role for altitudinal temperature gradients in regulating the relative importance of sink limitation vs. resource availability as constraints on tree growth and species’ ranges throughout a mid-latitude temperate montane ecoregion. Growth patterns towards the range limits of our two focal species aligned with our understanding of these species as representatives of two primary life-history strategies (sensuGrime 1977), with climatic stress setting the upper altitudinal limit of a shade-tolerant angiosperm (Fagus sylvativa ) and interspecific competition setting the lower distributional limit of a frost-hardy conifer (Picea abies ). These findings reinforce important roles for tradeoffs among primary life history strategies and the stress-gradient hypothesis (Anderegg and HilleRisLambers 2019) as explanations for the ecological gradients that are typically encountered along temperate-to-alpine conditions (Schimper 1898).