Larger trees suffer most during drought in forests worldwide

Abstract

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...

Occurrence of homobaric and heterobaric leaves in two forest types of southern Brazil

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Low Tree-Growth Elasticity of Forest Biomass Indicated by an Individual-Based Model

“…Although periodic droughts, warming, and the increasing evaporative demand of the atmosphere are clearly implicated in many reports of increasing tree mortality [26,[66][67][68], considerable uncertainty exists in the magnitude of interactions between external hydrological conditions and competition and between external hydrological conditions, tree size, and age [26,32,[69][70][71][72][73]. If one assumes that global change factors have increased tree growth, then a proportion of recent evidence indicating increasing time trends in tree mortality may reflect negative feedback responses to growth enhancement.…”

Detecting the fingerprint of drought across Europe’s forests: do carbon isotope ratios and stem growth rates tell similar stories?

“…Because the extent to which trees are affected by drought is known to be size-dependent (e.g., Zang et al 2012;Bennett et al 2015;Grote et al 2016), directly relating growth and isotope values without accounting for differences in the size distribution of trees used to calculate them could affect results. To address this issue, we developed a statistical model that explicitly accounts for the effect of tree size on growth, and then used the model to estimate the stem growth of a tree of comparable size to that sampled for isotopes for each of the 26 study populations (see below for details).…”