Changes in precipitation, more than increasing temperatures, may shift plant productivity and diversity in Mongolia steppe

Background/Question/Methods

Global climate change will continue to involve changes in both temperature and precipitation.  Therefore, a better understanding of how plant communities will respond to multiple, concurrent global change factors is of increasing importance.  We conducted a climate change experiment in Northern Mongolia, an area already experiencing large shifts in temperature and precipitation. We manipulated climate using open top chambers (OTCs) at two locations on a south-facing slope in the Mongolian steppe.  We examined how plant community productivity and diversity responded to the warmer, drier conditions inside the chamber at both locations (Exp. 1). At the upper, drier location we also simulated additional precipitation in a full factorial experiment with the OTC treatments (Exp. 2).  This design enabled us to separate the effects of warmed temperatures and simultaneous soil drying.  For statistical analyses, we examined effects of experimental treatments in two separate experiments (statistical models), one with OTCs and controls crossed with slope locations and a second with only the upper slope OTCs and controls crossed with an added precipitation treatment.  After four years of treatments, we assessed total plant biomass and diversity (e^H’) using percent cover surveys and a biomass harvest at the end of the growing season.

Results/Conclusions

Response to the OTC differed between the two locations on the same south-facing slope. In Experiment 1, we found a significant location x warming interaction such that the OTC treatment had a significantly more extreme negative effect on diversity (as calculated using percent cover data) in the upper, drier location (p<0.05). The upper, drier location was less productive and more diverse than the moister, lower slope location, measured either using percent cover data or harvested biomass (all p<0.05). In Experiment 2, added precipitation at least partially counteracted the negative effects of the OTC on plant diversity. There was a marginal interactive effect of OTC and precipitation treatment on diversity (p=0.09), where adding precipitation erased the significantly negative effect of the OTC (p<0.05).

These diversity results suggest that much of the negative impact of experimental warming treatments on plant productivity and diversity may be driven by reduced soil moisture and not by increased temperature alone.  Changes in precipitation patterns are likely to have large consequences for vegetation, particularly in high latitude, semi-arid steppe, where the growing season is short.