Hotter drought and trade-off between fast and slow growth strategies as major drivers of tree-ring growth variability of global conifers

We found: a) growth variability was mainly affected by warm-induced
drought and increased at lower latitudes. Climate warming in winter could
decrease growth variability, but this effect is by far not enough to
offset the threat of hotter drought; b) there existed a trade-off between
fast- and slow-growing (drought tolerance) strategies for global conifer
species, and abiotic and stand factors affected growth variability via
functional traits. Contrary to common conjecture, species with higher
drought tolerance revealed higher growth variability due to their
occupation of more xeric sites, and may also because higher investment in
drought tolerance leads to less investment remaining for growth; c) older
trees revealed higher growth variability due to their more conservative
growth strategy, while at large scales taller trees showed lower growth
variability due to occupying more productive sites; and d) moderate N
deposition could reduce growth variability by leading conifers to adopt a
more fast-growing strategy (e.g., in Asia), but long-term and excessive N
deposition led to increased growth variability (e.g., in North America and
Europe). Our results suggest that coniferous forests in water-limited
regions should be more vulnerable to hotter drought, and the ‘fast-slow’
growth strategies may be key in regulating the effects of various abiotic
and stand factors on ecosystem stability. Moreover, future hotter drought
and N deposition will severely threaten conifer growth, especially for old
trees and conifers at lower latitudes.