Remote sensing of browning trends in the Alaskan boreal forest

Abstract

Vegetation health can be monitored using a time series of remotely sensed images by calculating the Normalized Difference Vegetation Index (NDVI). We assessed temporal trends throughout an NDVI time series with three sensors: Advanced Very High Resolution Radiometer (AVHRR), the Moderate Resolution Imaging Spectroradiometer (MODIS) and Landsat Thematic Mapper/Enhanced Thematic Mapper Plus (TM/ETM+). There has been debate over the reliability of AVHRR sensor NDVI data in the circumboreal region. Therefore, the purpose of this paper was to first use MODIS and Landsat TM/ETM+ data and assess declining trends within twelve Landsat scene footprints across boreal Alaska and second use Landsat TM/ETM+ data to assess NDVI trends at a stand-level in eastern boreal Alaska. For the first objective, there were significant (p-value <0.05) declining trends in eastern boreal Alaska and no significant trends in the western region due to an east-west climate gradient. For the second objective, there were significant declining trends scattered across our two research areas. It was determined that many factors need to be included when determining where declining stands in NDVI are located such as site climate, site landscape position and other unique site conditions.