Detecting and Tracking Shifts in National Vegetation Composition,Including Donors and Recipients, Across the MODIS Era
Abstract:Forested landscapes are ecologically and economically important, and understanding their dynamics is important for land--management decision making. Forest ecosystems are also under stress, and may be changing due to interannual variability and long term change in climate, natural and anthropogenic disturbance, including human use and management. Detecting and tracking shifts in vegetation is important for land--management, conservation planning, monitoring recovery, managing and monitoring forest structure and composition, maintaining species and habitat diversity and many other purposes.
We used MODIS NDVI to create phenological ecoregions, or "phenoregions" having similar annual phenology using a unsupervised clustering method over the period 2000--2012. These statistically derived phenoregions were reclassified to National Land Cover Database (NLCD) classes using the "Mapcurves" algorithm. Interannual transitions in phenologically defined classes are indicator of disturbance and recovery. Because the area within the CONUS is fixed, land cover area changes are a zero-sum game. Changes in one land cover class must be accompained by compensating changes in other classes.
We demonstrate a full-circle national-scale accounting system which can track not only area changes in land cover classes, but can show which other compensatory land cover class area changes accompanied them. Area changes in the vegetation distributions, as well as compensatory gains, losses, and trades in area of other land cover types, were mapped and tracked annually during 2000--2012 period at MODIS resolution. The types or labels of the classes used in the accounting can easily be changed to sets of land cover types that maximize the utility of the tracking. For any particular "focus" land cover type, results show which other land covers were donors or recipients of area changes, showing ecologists and land managers alike what vegetation types were given up or gained to offset particular increases or losses.