Jonathan Wang PhD Defense

1:00 pm on Tuesday, May 28, 2019
2:00 pm on Tuesday, May 28, 2019
CAS 132
“Interactions among Land Cover, Disturbance, and Productivity across Arctic-Boreal Ecosystems of Northwestern North America from Remote Sensing” // Arctic and boreal ecosystems are experiencing enhanced carbon cycling that correlates with trends in the normalized difference vegetation index (NDVI), a widely used remotely sensed proxy for vegetation productivity. Meanwhile, a variety of processes are extensively altering arctic-boreal land cover, complicating the relationship between NDVI and productivity. Because high-quality information on land cover is lacking, understanding of relationships among boreal and arctic greenness trends, productivity, and land cover change is incomplete. Multidecadal time series of moderate resolution (30 m) surface reflectance data and high resolution (<4 m) imagery were used to map annual cover and quantify changes in plant functional type over the study domain of NASA’s Arctic-Boreal Vulnerability Experiment. Results identify two primary modes of ecosystem transformation that are consistent with increased biome-wide productivity across northwestern North America: (1) in the boreal biome, simultaneous decreases in evergreen forest area and increases in deciduous forest area caused by fire and harvest; and (2) climate change-induced expansion of arctic herbaceous vegetation and woody shrubs. Over a quarter of NDVI trends were associated with land cover change. Relative to locations with stable land cover, areas of land cover change were twice as likely to exhibit statistically significant trends in Landsat-derived NDVI. The highest magnitude trends were concentrated in areas of forest disturbance and regrowth and shrub expansion, while undisturbed land showed subtler, widespread greening trends. Based Orbiting Carbon Observatory-2 data, sun-induced fluorescence, another proxy for photosynthesis, reflected expected relationships among land cover, time since disturbance, and productivity that were not captured in NDVI data. In contrast with NDVI, time series of above ground biomass provide physically-based measures of productivity in boreal forests. Using land cover, Landsat reflectance, and ICESat lidar data, aboveground biomass was mapped annually across the boreal biome. Most forests showed steadily increasing biomass, with wildfires imposing substantial interannual variability and harvest imposing steady biomass losses that varied by ecoregion. Despite these disturbances, boreal forests are overall a net sink. This dissertation provides new information on how land cover and productivity are changing across arctic-boreal northwestern North America andreveals insights regarding the interpretation of remote sensing observations in these biomes.