Krakauer, N.Y.; Lakhankar, T.; Anadón, J.D. Mapping and Attributing Normalized Difference Vegetation Index Trends for Nepal. Remote Sens.2017, 9, 986.
Krakauer, N.Y.; Lakhankar, T.; Anadón, J.D. Mapping and Attributing Normalized Difference Vegetation Index Trends for Nepal. Remote Sens. 2017, 9, 986.
Global change is affecting vegetation cover and processes through multiple pathways. Long time series of surface land surface properties derived from satellite remote sensing offer a unique abilities to observe these changes, particularly in areas with complex topography and limited research infrastructure. Here, we focus on Nepal, a biodiversity hotspot where vegetation productivity is limited by moisture availability (dominated by a summer monsoon) at lower elevations and by temperature at high elevations. We analyze normalized difference vegetation index (NDVI) from 1981 to 2015 semimonthly, at 8 km spatial resolution. We use a random forest (RF) of regression trees to generate a statistical model of NDVI as a function of elevation, land use, CO2 level, temperature, and precipitation. We find that NDVI has increased over the studied period, particularly at low and middle elevations and during fall (post-monsoon). We infer from the fitted RF model that the NDVI linear trend is primarily due to CO2 level (or another environmental parameter that is changing quasi-linearly), and not primarily to temperature or precipitation trends. On the other hand, interannual fluctuation in NDVI is more correlated with temperature and precipitation. RF accurately fits the available data and shows promise for estimating trends and testing hypotheses about their causes.
random forest; regression tree; carbon fertilization; land cover change; climate change
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