Quantifying impacts of drought and grazing on ecosystem multifunctionality through response and effect trait relationships

Meghan T. Hayden, Julie E. Larson, Katharine N. Suding, Thomas Merchant and Laura E. Dee, Department of Ecology and Evolutionary Biology, University of Colorado, Boulder, Boulder, CO

Background/Question/Methods
Increases in the intensity and frequency of droughts with climate change are likely to alter the ability of grasslands to sustain multiple ecosystem functions. Grasslands are commonly managed via livestock grazing, both to help maintain native species’ diversity and to provide humans with predictable food sources. However, grazing under an increasingly arid climate may compound the consequences of grazing for functions, such as carbon storage, nutrient cycling, and water regulation. Prior studies focus on species’ responses to drought and grazing, stopping short of linking compositional changes to consequences for multiple ecosystem functioning. Here, we examine the relationship between responses of plant communities to disturbance and subsequent effects on ecosystem multifunctionality using a functional trait-based approach. Specifically, we assess 1) which traits predict community response to drought and grazing, 2) which traits predict outcomes for ecosystem functioning, and 3) whether changes in community-level metrics of response and effect traits due to treatment are coupled, or correlated. We use species abundance and trait data measured in a natural grassland community subject to a multiyear drought and grazing manipulation in Boulder, Colorado, USA. We subsequently measure indicators of key ecosystem functions, including water and nutrient cycling, plant productivity and habitat quality, to understand how the coupling of response and effect traits determines the sensitivity of multiple ecosystem functions to drought, grazing, and their combined effect.
Results/Conclusions
Preliminary results reveal that annual grasses increased in abundance with grazing and drought manipulations, with the greatest increases resulting from drought. This increase in annual grass abundance, relative to perennials, corresponds with a community shift to a higher average Specific Root Length (SRL) and a lower average Root Mass Ratio (RMR). We found no community-mediated shift in average Specific Leaf Area (SLA) or Leaf Dry Matter Content (LDMC) with the increase in annual grasses. Thus, response traits SRL and RMR appear to predict species persistence with drought and grazing, suggesting selection for species with long, efficient root systems. Measures of ecosystem function will be completed this spring, and will allow us to explicitly relate these community-level trait changes to consequences for ecosystem functioning. Because of the change in community SRL and RMR averages, we might predict changes belowground functions, such as soil retention and water regulation. Changes to these ecosystem functions would suggest that the aforementioned response traits also serve as effect traits, and that these traits dictate the sensitivity of grassland functioning to grazing and drought.