Conclusions:
Analysis of growth synchrony patterns using comprehensive tree-ring data continues to provide deep insight into climate sensitivity (e.g., Shestakova et al. 2016, Anderegg and HilleRisLambers 2019, del Rio et al. 2020) and forest productivity (Schurman et al. 2019). In the present study, large sample sizes were necessary to accurately portray the sizable extent of variation among trees, both locally and across large geographic space. Such variability could certainly contribute to the inconsistency reported by some studies in demonstrating the importance of competition as a driver of species ranges (see Anderegg and HilleRisLambers 2019 and references therein). Pairing synchrony assessments with intensified sampling allowed us to robustly link high-level biogeographic patterns to their lower-level mechanisms, namely how individual trees respond to climate and competition.
Growing season temperature is perhaps the most widely recognized climatic driver of forest productivity and species composition globally. In our analysis of a classic temperate-to-alpine forest transition, which likely mimics temperate-to-boreal gradients at even larger scales, we demonstrate how the manifold processes underlying tree growth change along a temperature gradient to alter species composition. Towards colder sites, slow growing trees are less likely to encounter resource limitation and competitive traits are likely maladaptive. As growing conditions improve, only competitively advantaged trees will grow at rates close to potential sink-limited maxima and species that cannot endure resource scarcity will struggle to sustain baseline metabolic requirements. Rather than thinking about competition and climate as direct constraints on tree performance, we highlight the importance of considering growth as an emergent property of both sink limitation and resource limitation, two independent processes with unique constraints. In our study, interspecific variation in tolerance of resource and sink limitation constituted the primary mechanism underlying the range limits of two contrasting life strategies, and will certainly dictate how the distributions of these two species respond to forthcoming climate change.