Using unmanned aerial vehicles to assess spatial and temporal variation in canopy tree mortality

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Mark C. Vanderwel, Adam Sprott, Danielle Graff and Tanvir Ahmed Shovon, Dept. of Biology, University of Regina, Regina, SK, Canada, Tanvir Ahmed Shovon, Renewable Resources, University of Alberta, Edmonton, AB, Canada

Background/Question/Methods
Advances in unmanned aerial vehicle (UAV) technology have opened new opportunities for measuring canopy tree mortality processes through high-resolution aerial imagery. UAVs can easily survey areas several hectares in size, and can provide individual-level data on the live/dead status of several hundred canopy trees in a single flight. Here, we carried out annual UAV surveys over 49 forest stands in western Canada to quantify spatial and temporal variation in canopy tree mortality rates following a 2017 summer drought. We segmented and classified individual tree crowns from orthophotos derived from UAV imagery, and analyzed how mortality varied by tree species, among stands, and over time.
Results/Conclusions
We identified more than 45 000 crown segments in our UAV surveys, about 6% of which represented dead trees. Mortality varied considerably among stands, with the proportion of dead trees present ranging from 1% to more than 30%. The mortality rate of trembling aspen (Populus tremuloides) was about twice that of two conifer species (white spruce, Picea glauca; lodgepole pine, Pinus contorta). Despite historically low summer precipitation since 2017, no major increases in mortality were apparent over the following years. As UAV technology becomes more widely used by ecologists, it will enable a better understanding of how canopy tree mortality rates vary across space and time, and their sensitivity to environmental change.