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

Abstract Temporal growth variability is an important indicator of ecosystem function under climate change. However, we still lack a unified understanding of how climate conditions, climate change (trends and variability), nitrogen (N) deposition, functional traits and stand factors together affect radial growth variability. Using global conifer tree‐ring records (123 species from 1780 sites) during 1970–2010 to calculate growth variability, we assessed how abiotic and stand factors affect growth variability directly and indirectly via functional traits with boosted regression tree and structural equation models, and examined the differences among continents (North America, Asia and Europe). 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). Synthesis . 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.