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.