Abstract
Bark beetles cause significant tree mortality in western North America. The United States Forest Service coordinates annual insect and disease surveys (IDS) by observers in airplanes to map and quantify the tree mortality caused by beetles. The subjective nature of these surveys means that accuracy evaluation is important for characterizing uncertainty. Furthermore, the metric reported for quantifying tree mortality recently changed (2012–2018 depending in region) from killed trees per acre to percent tree mortality within damage polygons, posing challenges for linking older and newer records. Here we evaluated IDS severity estimates in a beetle-affected forest in northern Idaho, USA using fine-resolution satellite imagery, which permitted greater areal coverage than field data. We first used well-established methods to map beetle-caused tree mortality in two WorldView-2 (WV2) images with a high accuracy relative to field observations. Trees-per-acre measurements within collocated IDS polygons were then converted to percent mortality using three methods and evaluated with the WV2 maps. The overall accuracies for the three methods ranged from 35–38% (for methods that used five percent-mortality classes) and 49–56% (three classes). When IDS and WV2 estimates of mortality severity that were within ±15% of each other were considered accurate, overall accuracies were 71–78%. Within the aerial survey damage polygons, the total mortality area tended to be overestimated relative to WV2. WV2 imagery identified ~50% more mortality across the study region compared with the IDS methods, with most of the difference occurring where damage was low severity or in wilderness areas. Severity of Douglas-fir beetle-caused tree mortality was estimated the most accurately, whereas severity of mountain pine beetle-caused tree mortality was estimated the least accurately. Future studies that control for temporal ambiguity between IDS and satellite imagery, as well as IDS spatial error, might provide better assessments of IDS severity accuracy. Our study increases the usefulness of the rich aerial survey database by providing estimates of uncertainty in the IDS database of tree mortality severity.
Highlights
Forest insects and diseases affect millions of hectares across Canada and the United States each year [1,2,3]
Mortality agents identified by hosts and bark beetle gallery patterns in the field included the Douglas-fir beetle, fir engraver, mountain pine beetle, and spruce beetle, as well as two mortality agents not identified in sampled Insect and disease surveys (IDS)
We found that the FHP and Forest Health Assessment and Applied Sciences Team (FHAAST) methods yielded the highest accuracies
Summary
Forest insects and diseases affect millions of hectares across Canada and the United States each year [1,2,3]. Insect and disease surveys (IDS) provide valuable information about the extent and severity of insect-caused tree mortality and damage, and comprise the only national scale, annual data set documenting mortality, damage agents, and host tree species impacted. IDS surveys are conducted annually by the United States Forest Service (USFS) across most forested public land in the conterminous. Trained surveyors collect observations of recent forest mortality by drawing polygons on maps and recording damage attribute information that includes beetle and host species and a measure of damage severity (tree mortality in the case of bark beetles) from fixed-wing aircraft and helicopters. Annual trends in tree mortality and damage inferred from IDS data are especially valuable to forest managers and researchers for monitoring and anticipating future tree mortality [4]. Studying long-term trends in relation to other factors can reveal causes and patterns of mortality that are not apparent with annual datasets [5,6,7]
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