Canopy and climate controls of gross primary production of Mediterranean-type deciduous and evergreen oak savannas

Understanding the interactions of climate, vegetation growth, and gross primary production (GPP) is critical for accurate estimation of GPP over years. The eco-physiological response of two functional savannas (deciduous and evergreen) to temporal variations in biophysical factors under similar Mediterranean climate is still unclear. In this study, we compared dynamics of major climatic variables, eddy covariance (EC) tower-based GPP (GPPEC), and vegetation indices (VIs: normalized difference vegetation index (NDVI), enhanced vegetation index (EVI), and land surface water index (LSWI)) over the last decade in a deciduous savanna (Tonzi Ranch in California, USA) and an evergreen savanna (Las Majadas del Tietar in Caceres, Spain) under the Mediterranean climate. We also examined the relationships among VIs, GPPEC, and major climatic variables in dry, normal, and wet hydrological years. Seasonal dynamics of climatic variables and GPPEC were similar, but seasonal dynamics of VIs differed at two savanna sites. Both savannas' VIs and GPPEC had similar responses to air temperature. The evergreen savanna showed larger variations in VIs and GPPEC with respect to changes in annual precipitation than did the deciduous savanna. We simulated GPP of these two savanna sites using a light-use efficiency based Vegetation Photosynthesis Model (VPM). The modeled GPP (GPPvpm) at both savanna sites agreed well with the seasonal and interannual dynamics of GPPEC over the study period (slopes of 0.83-1.15 and R2 values of 0.91-0.97). The LSWI-based water scalar parameter in VPM helped to accurately estimate GPP under dry, normal, and wet years. The results of this study help better understanding the eco-physiological response of evergreen and deciduous savannas, and also suggest the potential of VPM to simulate the interannual variations of GPP in different Mediterranean-type savannas through the integration of vegetation indices and climate data.