Abstract
Disturbance is a critical ecological process in forested systems, and disturbance maps are important for understanding forest dynamics. Landsat data are a key remote sensing dataset for monitoring forest disturbance and there recently has been major growth in the development of disturbance mapping algorithms. Many of these algorithms take advantage of the high temporal data volume to mine subtle signals in Landsat time series, but as those signals become subtler, they are more likely to be mixed with noise in Landsat data. This study examines the similarity among seven different algorithms in their ability to map the full range of magnitudes of forest disturbance over six different Landsat scenes distributed across the conterminous US. The maps agreed very well in terms of the amount of undisturbed forest over time; however, for the ~30% of forest mapped as disturbed in a given year by at least one algorithm, there was little agreement about which pixels were affected. Algorithms that targeted higher-magnitude disturbances exhibited higher omission errors but lower commission errors than those targeting a broader range of disturbance magnitudes. These results suggest that a user of any given forest disturbance map should understand the map’s strengths and weaknesses (in terms of omission and commission error rates), with respect to the disturbance targets of interest.
Highlights
Forest ecosystems are strongly influenced by disturbance processes that interact with the climate and other environmental factors to alter their state and function
Some interesting and important general questions arise in the context of in-depth comparative analyses among disturbance map products: How much agreement is there among the forest disturbance maps derived from these algorithms? Is agreement a function of spectral change magnitude associated with apparent disturbances? How sensitive are the algorithms to the spectral change magnitudes associated with a full range of disturbance severities? With these broad questions in mind, we addressed the following specific questions: 1
Across the six study scenes, aggregate disturbance rates derived from the disturbance map sets were highly variable over time, as expressed by the different algorithms and the reference data (Figure 2, top)
Summary
Forest ecosystems are strongly influenced by disturbance processes that interact with the climate and other environmental factors to alter their state and function. In Europe, increases in insect, wind, and fire disturbances, first noted in the 20th century, have continued into the 21st century and are thought to be strong contributors to a recently observed saturation of the forest carbon sink [1]. Disturbances can range from high-impact, discrete events (such as fires, windstorms, and deforestation, all of which dramatically and sometimes permanently alter forest conditions [2]) to subtler and often gradual and chronic processes that require many years to unfold (such as prolonged droughts that result in partial reduction in canopy cover and increased insect activity [3,4]). High-impact disturbances can have immediate and profound effects on forest function, especially locally, they are generally far less common than subtler disturbances that accumulate over time and space, often having significant aggregate effects on forest function [5,6].
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