[MCN] How much drought can a forest take?

[Lance Olsen]

How much drought can a forest take?
Aerial tree mortality surveys show patterns of tree death during 
extreme drought
UNIVERSITY OF CALIFORNIA - DAVIS
PUBLIC RELEASE: 19-JAN-2017

Why do some trees die in a drought and others don't? And how can we 
predict where trees are most likely to die in future droughts?

Scientists from the University of California, Davis, and colleagues 
examined those questions in a study published in the journal Ecology 
Letters <<http://onlinelibrary.wiley.com/doi/10.1111/ele.12711/full>>.

Using climate data and aerial tree mortality surveys conducted by the 
U.S. Forest Service during four years (2012-2015) of extreme drought 
in California, they found that when a drought hits the region, trees 
growing in areas that are already dry are most susceptible.

The research also showed that the effects of drought on forests can 
take years to surface, suggesting that such effects may linger even 
after the drought has ended.

The study said that trees in the driest and densest forests are the 
most at risk of dying in an extreme drought. In California, that 
makes crowded stands of trees in the Southern Sierra Nevada the most 
vulnerable in the state.

The concept is simple: Trees in dense forests are like multiple 
straws competing for the same glass of water. In wet climate 
conditions, that competition goes largely unnoticed. But when it's 
dry, few are able to quench their thirst, setting the stage for mass 
mortality.

"Our analysis found out how much drought a tree can take," said UC 
Davis Ph.D. student Derek Young, who co-led the study with Jens 
Stevens, a UC Davis postdoctoral researcher during the study who is 
currently at UC Berkeley, and Mason Earles, a postdoctoral researcher 
at Yale University. "If forest managers want to get the biggest bang 
for their buck in reducing forest vulnerability to drought, this 
study suggests they should focus on the densest stands in the driest 
areas. And when we reestablish forests burned by severe wildfire in 
these areas, we should plant at lower densities from the beginning."

Tree mortality in the Sierra Nevada in 2015 was the worst in recorded 
history. The U.S. Forest Service aerial tree mortality surveys in 
2015 estimated 29 million trees in California had died after four 
years of extreme drought.

Though the drought began in 2012, major effects on trees did not 
appear immediately. While some trees died every year, mortality 
spiked only in the fourth year of extreme drought.

In a blog post he wrote on the subject in May 2016, Young noted: 
"This observation highlights the fact that tree mortality can take 
several years to respond to drought. Such a delayed response is often 
observed in studies of drought stress, and the existence of this 
delayed response hints that we are likely to observe high mortality 
well into 2016 and potentially beyond, especially in Southern 
California."

Indeed, surveys conducted by the U.S. Forest Service in 2016 
estimated an additional 62 million trees died that year.
-- 
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"A surprising result is the high proportion of species responding to 
recent, relatively mild climate change (global average warming of 0.6 
C)."

Camille Parmesan. "Ecological and Evolutionary Responses to Recent 
Climate Change." Annual Review of Ecol. Evol. & Systematics  2006. 
37:637-69
=--------------------------------------------------------------------------------------------------------=
"We contend that traditional approaches to forest conservation and 
management will be inadequate given the predicted scale of 
social-economic and biophysical changes in the 21st century."

Forest Ecology and Management Accepted 7 October 2015
Review and synthesis
Achievable future conditions as a framework for guiding forest 
conservation and management
S.W. Golladay, K.L. Martin, J.M. Vose, D.N. Wear, A.P. Covich, R.J. 
Hobbs, K.D. Klepzig, G.E. Likens, R.J. Naiman, A.W. Shearer