Drought effects on carbon cycling The response of forest ecosystems to drought is increasingly important in the context of a warming climate. Anderegg et al. studied a tree-ring database of 1338 forest sites from around the globe. They found that forests exhibit a drought “legacy effect” with 3 to 4 years' reduced growth following drought. During this postdrought delay, forests will be less able to act as a sink for carbon. Incorporating forest legacy effects into Earth system models will provide more accurate predictions of the effects of drought on the global carbon cycle. Science , this issue p. 528

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Drought, a recurring phenomenon with major impacts on both human and natural systems, is the most widespread climatic extreme that negatively affects the land carbon sink. Although twentieth-century trends in drought regimes are ambiguous, across many regions more frequent and severe droughts are expected in the twenty-first century. Recovery time-how long an ecosystem requires to revert to its pre-drought functional state-is a critical metric of drought impact. Yet the factors influencing drought recovery and its spatiotemporal patterns at the global scale are largely unknown. Here we analyse three independent datasets of gross primary productivity and show that, across diverse ecosystems, drought recovery times are strongly associated with climate and carbon cycle dynamics, with biodiversity and CO fertilization as secondary factors. Our analysis also provides two key insights into the spatiotemporal patterns of drought recovery time: first, that recovery is longest in the tropics and high northern latitudes (both vulnerable areas of Earth's climate system) and second, that drought impacts (assessed using the area of ecosystems actively recovering and time to recovery) have increased over the twentieth century. If droughts become more frequent, as expected, the time between droughts may become shorter than drought recovery time, leading to permanently damaged ecosystems and widespread degradation of the land carbon sink.

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The R package treeclim helps perform numerical calibration of proxy-climate relationships, with an emphasis on tree-ring chronologies. The package provides a unified, fast, and public-domain compilation of established methods while adding novel functionality not implemented in other software. treeclim includes static and moving bootstrapped response and correlation functions, seasonal correlation analysis, a test for spurious temporal changes in proxy-climate relations, and the evaluation of reconstruction skills. The stationary bootstrap method has been incorporated into the program as a 'blocks of blocks' resampling scheme. Applications of treeclim include the calibration of proxy timeseries used in paleoclimatology, forest ecology, and environmental monitoring.