Anthropogenically-driven global environmental change leads to responses in terrestrial ecosystems, and the nature of these responses likely depend upon the combination of environmental factors that shifts, as well as upon plant community composition.  Field plots within a California grassland at Jasper Ridge, USA, were exposed for 9 years to enhanced levels of CO2, nitrogen, heat, and precipitation, singly and in all possible combinations.  Throughout the experiment, we observed consistent greater NPP in response to added nitrogen.  We have also observed responses that indicate interactions among global change factors influence NPP.  For example, in 2006 NPP increased in response to CO2 added to plant communities receiving greater heat and precipitation.  In 2004, plant communities exposed to CO2 in combination with nitrogen exhibited lower NPP then plant communities receiving N amendments alone, a pattern also observed in earlier years of this study.  Further, not all plant groups responded the same way.  Vicia sativa, the dominant nitrogen fixer in these field plots, increased in response to CO2 in the absence of other global change factors, and in the presence of added precipitation, both in 2005 and in 2006.  Many of the patterns we observed indicate that terrestrial ecosystems equally likely to exert positive or negative feedbacks on global environmental change.