The frequency of severe droughts is increasing in many regions around the world as a result of climate change [1][2][3] . Droughts alter the structure and function of forests 4,5 . Site-and region-specific studies suggest that large trees, which play keystone roles in forests 6 and can be disproportionately important to ecosystem carbon storage 7 and hydrology 8 , exhibit greater sensitivity to drought than small trees 4,5,9,10 . Here, we synthesize data on tree growth and mortality collected during 40 drought events in forests worldwide to see whether this size-dependent sensitivity to drought holds more widely. We find that droughts consistently had a more detrimental impact on the growth and mortality rates of larger trees. Moreover, drought-related mortality increased with tree size in 65% of the droughts examined, especially when community-wide mortality was high or when bark beetles were present. The more pronounced drought sensitivity of larger trees could be underpinned by greater inherent vulnerability to hydraulic stress 11-14 , the higher radiation and evaporative demand experienced by exposed crowns 4,15 , and the tendency for bark beetles to preferentially attack larger trees 16 . We suggest that future droughts will have a more detrimental impact on the growth and mortality of larger trees, potentially exacerbating feedbacks to climate change.Climate change has been linked to water deficits in many parts of the world, and future climate projections suggest that droughts are likely to increase in severity because of changes in the timing and magnitude of precipitation and rising temperature 1,2,14,17 . Across a wide range of biomes, drought leads to changes in forest composition, structure, productivity and climate interactions 5,18,19 . Drought has many important consequences for forest communities, as species composition and dominance are shaped by water availability and can change rapidly in response to drought 19,20 . Drought-induced forest decline results in climate feedbacks including reduced CO 2 uptake, reduced carbon stocks, increased albedo and decreased evapotranspiration 21 . The impact of drought on forest structure and function depends on which trees are most adversely affected; greater mortality of small trees may modify future forest succession whereas mortality of large trees causes disproportionate losses of carbon and ecosystem function 5-7 . It has not been clear whether large or small trees would suffer more under drought stress. Several studies have documented a greater impact of drought on large trees at a single site 4,9,10 , and a synthesis of data from the humid lowland tropics revealed a tendency for greater drought-related mortality increases in large trees 5 , but these patterns have never been systematically reviewed for forests worldwide.Here, we perform a meta-analysis of data from 40 drought events at 38 forest locations worldwide, ranging from semi-arid woodlands to tropical rainforests, to address whether trees of different size (seedlings excluded) respond differe...

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Global change is impacting forests worldwide, threatening biodiversity and ecosystem services including climate regulation. Understanding how forests respond is critical to forest conservation and climate protection. This review describes an international network of 59 long-term forest dynamics research sites (CTFS-ForestGEO) useful for characterizing forest responses to global change. Within very large plots (median size 25 ha), all stems ≥1 cm diameter are identified to species, mapped, and regularly recensused according to standardized protocols. CTFS-ForestGEO spans 25°S-61°N latitude, is generally representative of the range of bioclimatic, edaphic, and topographic conditions experienced by forests worldwide, and is the only forest monitoring network that applies a standardized protocol to each of the world's major forest biomes. Supplementary standardized measurements at subsets of the sites provide additional information on plants, animals, and ecosystem and environmental variables. CTFS-ForestGEO sites are experiencing multifaceted anthropogenic global change pressures including warming (average 0.61°C), changes in precipitation (up to AE30% change), atmospheric deposition of nitrogen and sulfur compounds (up to 3.8 g N m À2 yr À1 and 3.1 g S m À2 yr À1), and forest fragmentation in the surrounding landscape (up to 88% reduced tree cover within 5 km). The broad suite of measurements made at CTFS-ForestGEO sites makes it possible to investigate the complex ways in which global change is impacting forest dynamics. Ongoing research across the CTFSForestGEO network is yielding insights into how and why the forests are changing, and continued monitoring will provide vital contributions to understanding worldwide forest diversity and dynamics in an era of global change.