[MCN] A mistaken generalization about forest dieoffs

[Lance Olsen]

Forest Ecology and Management  Volume 359, 1 January 2016, Pages 19-32

Do denser forests have greater risk of tree mortality: A remote 
sensing analysis of density-dependent forest mortality
K. Jane Van Gunst, Peter J. Weisberg, Jian Yang, Yuanchao Fan

Keywords
Forest disturbance; Landsat; Forest health; Drought; Sierra Nevada; 
Tree mortality

Highlights
*25 years of Landsat data were used to relate forest mortality to 
stocking level.
*Stocking and mortality were positively related in xeric pine forests.
*Stocking and mortality were negatively related in red fir forests 
during wet years.
*Density reduction will yield disparate results depending on forest 
type and climate.

Abstract- bold added
http://www.sciencedirect.com/science/article/pii/S0378112715005277

In forests of the western United States, high levels of forest 
stocking attributed to fire exclusion and settlement-era logging 
practices have coincided with periods of extensive forest mortality 
associated with extreme drought and forest pest outbreaks. This has 
led to the widespread but largely untested generalization that 
increased stand density causes increased levels of forest mortality, 
even for uneven-aged, multi-species, mature stands that are not 
actively undergoing self-thinning. We used archival Landsat imagery 
to examine: (1) variations in forest mortality among forest types and 
climate periods from 1985 to 2010 in the mixed conifer forest of the 
Lake Tahoe Basin, (2) relationships between stand density and 
mortality in the subsequent year, and (3) the influence of 
environmental variables on forest mortality. We found that positive 
density dependent mortality, where increased stand density is 
associated with increased probability of mortality, is associated 
more with lower elevation forests and drier climate periods. In mid- 
to upper-elevation forests, increased density was more often 
associated with decreased probability of mortality, especially during 
wetter periods. Results showed highest tree mortality during a 
drought in the beginning of our 25-year time series, but not in a 
subsequent drought period of similar severity. Our study also found 
increased risk of tree mortality on north-facing slopes across all 
forests and all climate periods, even when stand density was 
controlled for. There is a nuanced relationship between stocking 
level, forest mortality and drought effects, suggesting that no 
single density-reduction forest management strategy will increase 
forest resilience under all climate periods and in all forest types.

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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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