Background/Question/Methods Interactive effects of multiple global change factors on ecosystem processes are complex. It is relatively expensive to explore those interactions in manipulative experiments. We conducted a modelling analysis to identify potentially important interactions and to stimulate hypothesis formulation for experimental research. Four models were used to quantify interactive effects of climate warming (T), altered precipitation amounts (doubled [DP] and halved [HP]) and seasonality (SP, moving precipitation in July and August to January and February to create summer drought), and elevated [CO₂] (C) on net primary production (NPP), heterotrophic respiration (R_h), net ecosystem production (NEP), transpiration, and runoff. We examined those responses in seven ecosystems, including forests, grasslands, and heathlands in different climate zones.

Results/Conclusions The modelling analysis showed that none of the three-way interactions among T, C, and altered precipitation was substantial for either carbon or water processes, nor consistent among the seven ecosystems. However, two-way interactive effects on NPP, R_h, and NEP were generally positive (i.e., amplification of one factor’s effect by the other factor) between T and C or between T and DP. A negative interaction (i.e. depression of one factor’s effect by the other factor) occurred for simulated NPP between T and HP. The interactive effects on runoff were positive between T and HP. Four pairs of two-way interactive effects on plant transpiration were positive and two pairs negative. In addition, wet sites generally had smaller relative changes in NPP, R_h, runoff, and transpiration but larger absolute changes in NEP than dry sites in response to the treatments. The modelling results suggest new hypotheses to be tested in multifactor global change experiments. Likewise, more experimental evidence is needed for further improvement of ecosystem models in order to adequately simulate complex interactive processes.