[MCN] Tree physiology: Drought and death in forests

Lance Olsen lance at wildrockies.org
Tue Apr 19 09:47:41 EDT 2016

PNAS Early Edition doi: 10.1073/pnas.1525678113

Meta-analysis reveals that hydraulic traits explain cross-species 
patterns of drought-induced tree mortality across the globe
William R. L. Anderegg, Tamir Klein, Megan Bartlett, Lawren Sack, 
Adam F. A. Pellegrini, Brendan Choat, and Steven Jansen


Drought-induced tree mortality has been observed globally and is 
expected to increase under climate change scenarios, with large 
potential consequences for the terrestrial carbon sink. Predicting 
mortality across species is crucial for assessing the effects of 
climate extremes on forest community biodiversity, composition, and 
carbon sequestration. However, the physiological traits associated 
with elevated risk of mortality in diverse ecosystems remain unknown, 
although these traits could greatly improve understanding and 
prediction of tree mortality in forests. We performed a meta-analysis 
on species' mortality rates across 475 species from 33 studies around 
the globe to assess which traits determine a species' mortality risk. 
We found that species-specific mortality anomalies from community 
mortality rate in a given drought were associated with plant 
hydraulic traits. Across all species, mortality was best predicted by 
a low hydraulic safety margin-the difference between typical minimum 
xylem water potential and that causing xylem dysfunction-and xylem 
vulnerability to embolism. Angiosperms and gymnosperms experienced 
roughly equal mortality risks. Our results provide broad support for 
the hypothesis that hydraulic traits capture key mechanisms 
determining tree death and highlight that physiological traits can 
improve vegetation model prediction of tree mortality during climate 

The global climate change agreement brokered in Paris in December by 
195 nations will come into effect two years earlier than originally 
planned, the top United Nations climate diplomat predicted.

Even so, she said the deal had come "10 years too late," and that the 
world is now at "two minutes to midnight."

"What can be said with some assurance is that there is a unique and 
nearly ubiquitous compound, with the empirical formula 
H(2960)O(1480)C(1480)N(16)P(1.8)S, called living matter.  Its 
synthesis, on an oxidized and uncarboxylated earth, is the most 
intricate feat of chemical engineering ever performed - and the most 
delicate operation that people have ever tampered with."

Edward S. Deevey, Jr.  Mineral Cycles,
Scientific American, September 1970.


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