Multi-year impacts of severe drought on mortality losses of trembling aspen biomass in the western Canadian boreal region

Background/Question/Methods

There is growing evidence that climate change is already leading to pervasive, drought-related increases in tree mortality in many regions around the world, including the North American boreal forest.  One of the challenges is distinguishing between changes in tree mortality caused by 1) age-related changes in stand dynamics and 2) changes in climatic factors such as drought.  Since 2000, we have been monitoring changes in stem mortality within pure trembling aspen (Populus tremuloides) stands at 150 plots across west-central Canada.  In 2001-2002, a severe, subcontinental drought led to widespread increases in aspen mortality, both in our study region and elsewhere in western North America.  Drought severity and extent was quantified using a simple climate moisture index (CMI), and the drought’s impacts were assessed using plot-based measurements and tree-ring analysis. Mortality was calculated using two methods: as the percentage of stems dying each year; and as the percentage of stand-level stem biomass dying each year.  The latter was estimated using biomass equations as a function of stem diameter and height.  Relative biomass of dying aspen (RBD) was calculated as the mean biomass of recently dead aspen divided by the mean biomass of living aspen. 

Results/Conclusions

For the study region as a whole, we recorded a 3-fold increase in biomass mortality losses, from 1.3% in 2000-2001 to 4.0% in 2005-2006.  The analysis showed high variation in mortality among sites that was significantly related to variation in drought severity (CMI) in 2001-2002.  Despite the return to moist conditions in some areas, mortality remained high throughout the subsequent decade (2002-2012), leading to a chronic, regional-scale collapse in net biomass increment.  In addition to its effects on mortality, the drought led to a change in the size distribution of dying trees.  In the moister sites where drought effects were minimal, mortality affected mainly small, suppressed aspen (RBD range from ca. 0.3 – 0.5), reflecting the normal process of self-thinning that arises from competition during stand development.  In contrast, the more severely drought-affected sites showed a shift toward mortality of larger size classes of trees (RBD range 0.7 – 1.0).  Thus, the RBD may provide a useful alternative measure for assessing drought-related impacts on mortality of aspen and other species.  In general, reporting of mortality as percentage annual loss of biomass rather than percentage loss of stem number is likely to provide a more reliable measure of climate-related impacts from plot-based measurements.