Site-dependent versus regionally consistent effects of increased temperature and precipitation on plant community composition, productivity, and soil nutrient availability in restored Pacific Northwest prairies

Background/Question/Methods

Pacific Northwest prairies are considered critically imperiled and substantial efforts to restore native plant communities to these ecosystems are underway.  However, how these communities will respond to the indirect and direct effects of future climate change is largely unknown.  Climate change will also likely change a myriad of ecosystem functions, but it is unclear to what extent these effects will be site-specific versus regionally consistent.  We set up an experiment to determine how climate change will affect (i) plant community composition and the relative success of native versus exotic plant species, above- and belowground net primary production (NPP), and nutrient availability, and (ii) if these effects are regionally consistent.  We embedded a manipulative climate change study of warming by 3°C and increased precipitation by 20% within a 520-km latitudinal climate gradient across three sites in western Oregon and Washington, USA.  At each site, plots were restored by mowing, raking, and herbicide application followed by the sowing of the same 33 native grass and forb species in each plot.  Climate treatments were initiated in 2010.  Plant community composition, NPP, and soil nutrient data were measured in 2011.        

Results/Conclusions

Exotic cover was greatest at the southern site (89%) and least at the northern site (53%).  Experimental warming decreased the proportion of native species cover at both the southern and central site while warming increased native cover at the northern site.  Precipitation effects were only significant for the central site where increased precipitation decreased native cover.  Warming increased nitrogen availability in all but the northern site, which also had the lowest nitrogen availability.  The ratio of above- to belowground NPP was greatest at the southern site (2.1) and lowest at the nutrient-poor northern site (0.74) and was not affected by the climate treatments.  Experimental warming had a consistent positive effect on both above- and belowground NPP (by 13% and 47%, respectively). Thus, despite the large site differences in allocation of productivity above- and belowground, total NPP was similar among sites and had a similar positive response to warming (+30%).  We found that differences in plant community composition, above- and belowground NPP, and soil nutrient availability were much greater among sites than among climate treatments, and several site-treatment interactions occurred.  Our numerous warming effects and their dependence on site highlight the importance of conducting manipulative climate experiments across a range of environmental conditions.