[MCN] Forest change after historic SW US drought

[Lance Olsen]

Global Change Biology October 2015

Woodland recovery following drought-induced tree 
mortality across an environmental stress gradient
Miranda D. Redmond, Neil S. Cobb, Michael J. Clifford, Nichole N. Barger

Abstract (Bold added)
http://onlinelibrary.wiley.com/doi/10.1111/gcb.12976/full

Recent droughts and increasing temperatures have 
resulted in extensive tree mortality across the 
globe. Understanding the environmental controls 
on tree regeneration following these drought 
events will allow for better predictions of how 
these ecosystems may shift under a warmer, drier 
climate. Within the widely distributed 
piñon-juniper woodlands of the southwestern USA, 
a multiyear drought in 2002-2004 resulted in 
extensive adult piñon mortality and shifted adult 
woodland composition to a juniper-dominated, more 
savannah-type ecosystem. Here, we used pre- 
(1998-2001) and 10-year post- (2014) drought 
stand structure data of individually mapped trees 
at 42 sites to assess the effects of this drought 
on tree regeneration across a gradient of 
environmental stress. We found declines in piñon 
juvenile densities since the multiyear drought 
due to limited new recruitment and high (>50%) 
juvenile mortality. This is in contrast to 
juniper juvenile densities, which increased over 
this time period. Across the landscape, piñon 
recruitment was positively associated with live 
adult piñon densities and soil available water 
capacity, likely due to their respective effects 
on seed and water availability. Juvenile piñon 
survival was strongly facilitated by certain 
types of nurse trees and shrubs. These nurse 
plants also moderated the effects of 
environmental stress on piñon survival: Survival 
of interspace piñon juveniles was positively 
associated with soil available water capacity, 
whereas survival of nursed piñon juveniles was 
negatively associated with perennial grass cover. 
Thus, nurse plants had a greater facilitative 
effect on survival at sites with higher soil 
available water capacity and perennial grass 
cover. Notably, mean annual climatic water 
deficit and elevation were not associated with 
piñon recruitment or survival across the 
landscape. Our findings reveal a clear shift in 
successional trajectories toward a more 
juniper-dominated woodland and highlight the 
importance of incorporating biotic interactions 
and soil properties into species distribution 
modeling approaches.
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"The growth in CO2 emissions closely follows the 
growth in Gross Domestic Product (GDP) corrected 
for improvements in energy efficiency."

P. Friedlingstein, et al. "Update on CO2 emissions."
Nature Geoscience. Published online: 21 November 2010
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"An impressive study across all of northern 
Russia from 1953-2002 showed a shift in tree 
allometries. In areas where summer temperatures 
and precipitation have both increased, a general 
increase in biomass (up 9%) is primarily a result 
of increased greenery (33% more carbon in leaves 
and needles), rather than woody parts (roots and 
stem). In areas that have experienced warming and 
drying trends, greenery has decreased, and both 
roots and stems have increased (Lapenis et al. 
2005)."

"  ...  there is little theoretical or 
experimental support to suggest that climate 
warming will cause absolute climatic tolerances 
of a species to evolve sufficiently to allow it 
to conserve its geographic distribution."

Camille Parmesan. "Ecological and Evolutionary 
Responses to Recent Climate Change." Annual 
Review of Ecol. Evol. & Systematics  2006. 
37:637-69


















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