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Increases in mortality of trembling aspen ( Populus tremuloides Michx.) have been recorded across large areas of western North America following recent periods of exceptionally severe drought. The resultant increase in standing, dead tree biomass represents a significant potential source of carbon emissions to the atmosphere, but the timing of emissions is partially driven by dead-wood dynamics which include the fall down and breakage of dead aspen stems. The rate at which dead trees fall to the ground also strongly influences the period over which forest dieback episodes can be detected by aerial surveys or satellite remote sensing observations. Over a 12-year period (2000-2012), we monitored the annual status of 1010 aspen trees that died during and following a severe regional drought within 25 study areas across west-central Canada. Observations of stem fall down and breakage (snapping) were used to estimate woody biomass transfer from standing to downed dead wood as a function of years since tree death. For the region as a whole, we estimated that >80% of standing dead aspen biomass had fallen after 10 years. Overall, the rate of fall down was minimal during the year following stem death, but thereafter fall rates followed a negative exponential equation with k = 0.20 per year. However, there was high between-site variation in the rate of fall down ( k = 0.08-0.37 per year). The analysis showed that fall down rates were positively correlated with stand age, site windiness, and the incidence of decay fungi ( Phellinus tremulae (Bond.) Bond. and Boris.) and wood-boring insects. These factors are thus likely to influence the rate of carbon emissions from dead trees following periods of climate-related forest die-off episodes. [ABSTRACT FROM AUTHOR]

Drought-related dieback of aspen-dominated woodland has been a persistent and possibly increasing phenomenon over recent decades in the southern boreal forests of western Canada. The Integrated BIosphere Simulator (IBIS) dynamic vegetation model was modified for Canadian ecosystems (hence “Can-IBIS”) and used to simulate effects of water stress on the woody biomass of aspen-dominated stands in the boreal mixedwood regions of Saskatchewan and Alberta. The modified model was evaluated using eddy-covariance measurements of CO 2 and water vapor fluxes made at a forested site and a grassland site located in the study region. The tested model captured 74% of the variation in biomass growth trajectories at 13 boreal and 12 parkland field study sites; the mean difference between simulated and observed values was approximately 1100 g C m −2 . Under the combined influences of climatic variation and increasing atmospheric CO 2 from 2000 to 2008, simulated values of net biomass growth were 544 and 240 g C m −2 for the boreal and parkland study sites, respectively. Drought-induced biomass losses at the drier sites (in both boreal and parkland regions) were simulated to be 100–350 g C m −2 , corresponding to an annual modeled mortality rate of 5–7% during severe drought years. These results were consistent with field measurements and other statistical studies. Changes in biomass over the nine-year period varied with geographical location and corresponded to spatial variation in monthly values of the self-calibrated Palmer Drought Severity Index. We conclude that Can-IBIS can be used to investigate annual impacts of water stress on woody biomass growth, although cumulative physiological effects of multi-year droughts on tree mortality would benefit from improved simulation of subgrid-scale (soil texture-driven) processes. In particular, two areas for further development are: (1) calibration based on the results of soil surveys at fine spatial/temporal scales; and (2) biophysical experiments to refine the representation of water stress constraints on biomass turnover. [ABSTRACT FROM AUTHOR]