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

Background/Question/Methods: 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 kgN ha^-1 y^-1).  

Results/Conclusions: 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.