Aboveground net primary production response of semi‐arid shrublands to chronic experimental dry‐season N input

Urban chaparral and coastal sage scrub (CSS) shrublands of southern Californian have been exposed to high‐levels of dry‐atmospheric nitrogen (N) deposition for decades. The productivity of these semi‐arid shrublands is reportedly N limited, thus N exposure is hypothesized to cause an increase in aboveground net primary production (NPP). To test this hypothesis the aboveground biomass and litter production of a post‐fire chaparral and a mature CSS stand were measured over an 8 year period in plots exposed to either ambient or elevated N input (ambient + 50 kg N ha −1 y −1 ). Results support this hypothesis; however, trends in NPP varied over time or were affected by rainfall. For post‐fire chaparral, NPP was initially inhibited by N input, but over time NPP increased consistently in plots exposed to added N. For CSS, the effect of N exposure was positively correlated with annual rainfall and N exposure caused NPP to increase significantly when rainfall exceeded approximately 450 mm year −1 . Thus, while N enrichment may increase the NPP of these semi‐arid shrublands, temporal patterns may take years to emerge or be dependent on water availability. These results highlight the importance of long‐term experiments and the potential for interactions between water and nutrients in semi‐arid ecosystems.