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 change1-3. Droughts alter the structure and function of forests4,5. Site- and region-specific studies suggest that large trees, which play keystone roles in forests6 and can be disproportionately important to ecosystem carbon storage7 and hydrology8, exhibit greater sensitivity to drought than small trees4,5,9,10. 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 stress11-14, the higher radiation and evaporative demand experienced by exposed crowns4,15, and the tendency for bark beetles to preferentially attack larger trees16. 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.