PARENT SESSION
Tuesday, August 8, 1:30-5:00 pm
COS 45 - Forest ecology II: population dynamics and regeneration
Sultana, Mezzanine Level, Cook Convention Center
Presiders: D Chojnacky and C Paine

Differential lag effects of drought on tree mortality in subalpine forests of the Rocky Mountains.

Bigler, Christof^{*,1, 2}, Gavin, Daniel³, Gunning, Charles², Veblen, Thomas², ¹ ETH Zurich, Zurich, Switzerland² University of Colorado, Boulder, CO³ University of Vermont, Burlington, VT

ABSTRACT- Extreme climatic events are key factors in inducing changes in forest ecosystems through the promotion of severe disturbances such as fire, blowdown and insect outbreaks that cause widespread tree mortality. However, little is known about drought as a direct mortality factor in high-elevation forests, and predictions about drought-mortality relationships remain poor. In a subalpine forest landscape in the Rocky Mountains of northern Colorado (USA), we quantified lag effects of drought on tree mortality of Engelmann spruce (Picea engelmannii), subalpine fir (Abies lasiocarpa), and lodgepole pine (Pinus contorta). Bivariate event date analysis was used to relate death dates of 172 crossdated dead trees to drought events based on annual and early-season droughts at different temporal scales. Following annual droughts, spruce showed a small, but significant increase in mortality up to 8 years, whereas fir showed a more consistent but shorter 3-year period of increased mortality. Following early-season droughts, spruce and fir mortality was elevated over 6 years and 12 years, respectively. Mortality of pine was generally not sensitive to drought. Spruce and pine trees above 3200 m and fir trees above 3050 m were more susceptible to drought than trees growing at lower elevations. Steeper slopes and shallow soils intensified drought effects for spruce and fir, as did flat terrain for spruce and deep soils for fir. Large or dominant spruce trees as well as spruce trees older than 350 years and fir trees older than 175 years were more susceptible to drought than smaller or younger trees. Reduced pre-drought growth rates increased the likelihood of spruce or fir trees being killed by drought. Our results show significant differences among subalpine tree species and sites in their lagged tree mortality response to extreme drought.

Key words: tree mortality, dendroecology, subalpine forest