Precipitation change alters community composition of a semi-arid grassland

Background/Question/Methods

Shifts in precipitation patterns resulting from global climate change are expected to affect composition and ecosystem function of plant communities. A very relevant, and largely unaddressed, research question is: how will the presence of non-native species interact with climate change to alter ecosystem structure, function, and services? This study addresses the effects of changes in the amount and timing of precipitation on species composition and ecosystem services of a grassland community in Colorado, USA. In spring 2010, a manipulative study was established to examine how changes in precipitation patterns may affect the abundance of historically-dominant (native) and recently-arrived (non-native) plant species, and the resulting impacts on soil moisture, nutrients, and primary productivity. We created 3 precipitation treatments based on climate model predictions for Colorado: winter-wet, winter-wet / summer-wet, and winter-wet / summer dry. In addition, a winter-dry / summer-wet treatment was added to simulate historical conditions. To assess ecosystem productivity under different precipitation scenarios, peak above-ground biomass was measured in each plot at the end of the growing season. To see if changes in precipitation would affect abundance of native and non-native plants,  percent cover and diversity of plant species was measured in spring and summer 2010 (pre-treatment), and 2011.

Results/Conclusions

Peak above-ground biomass, an indicator of ecosystem productivity, did not differ between treatments in late summer 2011, after one year of precipitation manipulations. However, by April 2011, cover of non-native winter-annual grasses was greater in winter-wet treatments than in the control or winter-dry treatment (p < 0.05). In August 2011, cover of native plants was greatest in summer-wet treatments and lowest in the summer-dry treatment (p < 0.05). In general, these results support the hypothesis that increased winter precipitation benefits non-native winter-annual plants, whereas increased summer precipitation benefits later-growing native plants. Cover measurements revealed Bromus tectorum to be the most abundant winter-annual at the study site. Increased cover of B. tectorum in early June 2011 correlated with decreased volumetric soil water and species richness per plot in late June 2011 (p < 0.0001). These data provide preliminary evidence that winter-annuals at the study site may decrease available soil resources, and impact later-growing native plants. If yearly precipitation shifts to a more winter-wet pattern, the abundance of non-native winter-annual species could increase in Colorado grasslands and result in loss of native plant biodiversity. However, overall impacts on ecosystem productivity may be modest.