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.