Phenology and gross primary production of open oak savanna and annual grassland under the Mediterranean climate in California

Abstract:

Phenology and gross primary production (GPP) are sensitive to climate variation over time. In this study we investigated changes in phenology and GPP of open oak savanna and annual grassland in California during 2000-2013. Two CO2 eddy flux tower sites (Tonzi Ranch and Raira Ranch), located in the lower foothills of the Sierra Nevada Mountains, California, were selected in this study, representing open oak savanna and annual grassland under the Mediterranean climate. Three vegetation indices (NDVI, EVI and LSWI) derived from Moderate Resolution Imaging Spectroradiometer (MODIS) were evaluated for their biophysical performance to (1) delineate phenological dynamics (e.g., starting and ending dates of plant growing season) during 2000-2013 and (2) influence GPP dynamics of these two sites, based on field data in 2001-2007. The Vegetation Photosynthesis Model (VPM) was also run to simulate GPP, driven by MODIS-based vegetation indices (EVI and LSWI) and NCEP/NARR climate data (air temperature and radiation). The resultant GPP estimates (GPPvpm) were compared with estimated GPP from the flux tower sites (GPPec) in 2001-2007. Furthermore, to evaluate sensitivity of VPM to climate data, we also ran the model with (1) in-situ climate data (temperature and radiation) from the flux tower in 2001-2007, and (2) temperature data from MODIS land surface temperature (LST) and radiation data from NCEP/NARR. The results show that VPM predicts reasonably well the seasonal dynamics and interannual variation of GPP of open oak savanna and annual grassland under various climate conditions (e.g., wet, normal and drought years). This synthesis study clearly demonstrates that integrating long-term CO2 eddy flux tower site data and satellite images will improve our understanding of the sensitivity of phenology and GPP to climate, soil moisture and vegetation over time.