Canadian Forest Service Publications
Factors affecting fall down rates of dead aspen (Populus tremuloides) biomass following severe drought in west-central Canada. 2014. Hogg, E.H.; Michaelian, M. Global Change Biology 21(5):1968-1979.
Issued by: Northern Forestry Centre
Catalog ID: 35910
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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.
Plain Language Summary
In our research, we are studying how drought affects the carbon cycle of aspen (poplar) forests across western Canada. We found that a severe drought in 2001-2002 led to a high death rate of aspen trees in this region. We recorded millions of tonnes of dead aspen biomass that represent a large potential source of carbon emissions to the atmosphere. These emissions are minimal in standing dead trees, but after the trees fall to the ground they rot quickly and release carbon dioxide. In this study, we observed a total of 1010 dead aspen trees every year during 2000-2012 to see how quickly they break and fall to the ground. We also calculated how much dead biomass and carbon was transferred to the soil surface each year through fall down of dead aspen crowns and snags. We found that dead aspen fell more rapidly than expected, with over 80% falling within 10 years of when the trees died. We also found that dead aspen fall down more quickly in older forests that have stem decay by a fungus called Phellinus. These results will be used to estimate how droughts may affect carbon cycling of Canada’s forests under a changing climate.