Drought tolerant and carbon uptake resulting from year-to-year nitrogen storage in deciduous oak savanna

We studied the ecophysiological processes underlying interannual variability in carbon uptake on a California deciduous oak savanna from 2001 to 2006.  During this period, the ecosystem experienced dry, normal, and wet years.  Eddy-covariance method was used for measuring net carbon exchange of ecosystem (NEE) and partitioning NEE to gross primary productivity (GPP) and ecosystem respiration (Reco).  The savanna functioned as a carbon sink; its annual NEE ranged between ‑145 ± 47 and ‑31 ± 65 gC m^-2 yr^-1.  Extremely high nitrogen concentrations, up to ~3.5%, were found in oak and grass leaves during the dry year, and the extreme foliage nitrogen maintained longer than other years.  It indicated that remaining high nitrogen concentration in photosynthesis tissues (e.g., leaf) might be one of important strategies of the plants to maintain necessary rates of carbon assimilation during a profoundly dry year.  Because water deficits limited nitrogen uptake during the dry spring, leaf obtained nitrogen primarily from nitrogen stored in the tree.  Water deficit limited nitrogen uptake of the tree during the late season, and consequently deficiency of nitrogen storage influenced leaf nitrogen in the year following the dry year.  Our results provided an explanation why year-to-year variations in water supplies (i.e., precipitation) significantly controlled on interannual variability in carbon uptake of the savanna ecosystems.