Competition reduces growth synchrony
The pronounced positive effect of altitude on tree growth synchrony and
its negative effect on basal area increments (BAI) (paired with
altitudinal increases in sensitivity to summer temperature and winter
precipitation; supplementary information 1), strongly suggests increased
sink limitation towards harsher alpine conditions and the alleviation of
climatic constraints at low altitudes. If climatic stress declines, then
other factors necessarily increase in importance as wood synthesis
constraints (Bloom, Chapin & Mooney, 1985). The observed decline in
growth synchrony towards lower altitudes was more rapid for understory
trees compared to overstory trees, especially for F .sylvatica , reflecting how the characteristically lower climatic
sensitivity of competitively inferior trees emerges with improved
growing conditions (Teets et al. 2018, Luo et al. 2020). Similarly, we
detected a positive association between growth synchrony and BAI among
low-altitude trees, implying that trees coupled to climate variation
(i.e. overstory trees) grow at rates closer to their physiological
maximum and that competition inhibits the realization of sink-limited
growth. Observing this disparity in climate sensitivity between
competition classes, combined with the declining climate stress towards
lower altitudes, strongly suggests that competition-driven resource
limitation replaces sink limitation as growing conditions improve.