Diverse responses of forest growth to drought time-scales in the Northern Hemisphere

Aim To identify the main spatiotemporal patterns of tree growth responses to different time-scales of drought at a hemispheric scale using a climate drought index and tree-ring records, and to determine whether those patterns are driven by different climate and forest features. Location Northern Hemisphere. Methods We used a large-scale dendrochronological data set of tree-ring width series from 1657 sites and a time-dependent drought index which incorporates information on precipitation and temperature variability (standardized precipitation-evapotranspiration index, SPEI). Correlation analysis was used to quantify how tree growth responds to different drought time-scales. Variation in the correlations was summarized using principal components analysis (PCA) and the contribution of the various environmental factors was estimated using predictive discriminant analysis (PDA). Results The period between the water shortage and the impact on tree growth differs noticeably among forest types and tree families. There is a gradient in the response of growth to drought including: (1) forests that do not respond to drought, such as those located in cold and very humid areas; (2) forests located in semi-arid areas characterized by responses to long-term droughts; (3) forests that respond to medium-to long-term droughts subjected to subhumid conditions; and (4) forests that dominate humid sites and respond to short-term droughts. Main conclusion Forests that experience semi-arid and subhumid conditions tend to respond over longer time-scales than those located in more humid areas. The characteristic time-scale at which forest growth mainly responds to drought is a proxy for drought vulnerability, reflecting the trees' ability to cope with water deficits of different durations and severities.