[MCN] In 40 periods of drought, big trees first to die

Lance Olsen lance at wildrockies.org
Tue Sep 29 17:45:34 EDT 2015

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Pragmatism. Meridian Books, 1955

Nature Plants Published online:28 September 2015

Larger trees suffer most during drought in forests worldwide
Amy C. Bennett, Nathan G. McDowell, Craig D. 
Allen & Kristina J. Anderson-Teixeira

Abstract (bold emphasis added for readers' quick convenience)

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, 12, 13, 
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 

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