Managed grassland resiliency to climate change: Shifting species composition buffers climate change effects on plant production and forage quality

Background/Question/Methods:

Managed grasslands cover significant acreage in the eastern half of the U.S., provide a variety of ecosystem services, and are dominated by non-native grass species which may or may not respond to climate change similarly to native species. It is important to understand how plant species composition, plant production, and forage quality will respond to predicted warming and alterations in precipitation because managed grasslands can improve environmental quality and provide the forage base for animal production in the region. We hypothesized that warming would cause species shifts (increased relative abundance of C₄ vs. C₃ species) and would reduce plant production and forage quality, but increases in precipitation would ameliorate these effects.  We tested this hypothesis using a field-based, climate manipulation, located in central Kentucky.  In a mixed species pasture, we established five replicate plots of four climate treatments: an ambient control, increased temperature (+3^oC day and night, year-round), increased precipitation (+30% of the long-term mean annual, applied during the growing season on wet days), and the combination of increased temperature and precipitation. Treatments were applied for five consecutive years (2009-2013), and species composition, plant biomass, and forage quality were measured seasonally in all years.

Results/Conclusions:

Warming significantly increased the relative abundance of C₄ grasses, especially in summer and fall. However, in the spring, C₃ grasses remained the dominant plant functional type in all plots. Initially, warming significantly reduced plant production compared to the other treatments (by 51% in the summer of 2009), but this response was reversed by fall and in several subsequent springs (increased temperature plots had +121 and +62% greater production than the other treatments, in spring 2011 and 2013, respectively).  Climate treatment effects on production varied by year, but when harvested biomass was summed across the five-year experiment, no significant effect was observed.  Effects of climate treatments on forage quality metrics (% crude protein and lignin) also varied by year and season, but were less dramatic than effects on plant production and species composition.  Increased temperature and precipitation did not substantially alter plant production or forage quality, but did promote C₄ vs. C₃ grass dominance.  This result illustrates the potential of species compositional changes to buffer ecosystem response to climate change, and suggests that although dominant plants will differ, managed grasslands of the eastern U.S. will continue, from an animal forage production perspective, to function similarly to today.