Department of Earth System Science

We analyzed a 5-year record of the CO₂ and energy exchange, Aboveground Net Primary Production (ANPP), maximum Leaf Area Index (LAI_max), and Enhanced Vegetation Index (EVI) for a Typha marsh in Southern California. The marsh was a net source of carbon over the study, despite high rates of ANPP. Interannual Net Ecosystem Production (NEP) variability was the largest that has been reported for any terrestrial ecosystem and was attributed to changes in maximum photosynthetic rates (GEE_max). The variation in energy and mass exchange was coupled between years; years with higher than average rates of carbon uptake were associated with lower than average sensible heat fluxes. Remotely sensed measures of surface greenness (EVI) were closely related to GEE_max variation, providing further evidence of interannual variability. We were unable to attribute the fluctuations in GEE_max to the direct effects of weather on ecosystem physiology, or to interannual variation in LAI_max. GEE did not vary systematically with air temperature or the presence of standing water in the marsh; GEE_max did not vary with LAI_max between years. Rather, interannual variation in carbon exchange at the SJFM resulted from shifts in the marsh's production efficiency (the rates of gross or net CO₂ exchange per LAI) that were not caused by changes in the weather. Our findings challenge the assumptions that interannual variation of land-atmosphere exchange is universally caused by the direct effect of weather on ecosystem physiology, and that an ecosystem's physiological response to the physical environment is consistent from year-to-year.