Arid ecosystems comprise 35% of land in the western United States and 41% globally. Multiple climate models predict a 4-6°C temperature increase for the Colorado Plateau and while the precipitation models contain a greater degree of uncertainty, they predict a slight increase in winter precipitation and a 20% decrease in summer precipitation. In a large field experiment near Moab, Utah, we are testing the effects of experimental warming and changes in precipitation on plant phenology, physiology, nutrient status, and soil processes. To examine the responses of plants and soil, we set up 40 2 x 2.5 m plots and randomly applied the following treatments: (1) increased temperature, (2) increased temperature and summer precipitation, (3) increased summer precipitation, and (4) control. Plots were warmed with infrared lamps to +2°C for 2 years and based on more recent climate projections, were doubled to increase the warming treatment to +4°C in January 2009. We altered summer precipitation by adding small watering events every two weeks throughout the summer. Our study focuses on 4 plant species that are commonly found on the Colorado Plateau: Hilaria jamesii, Stipa hymenoides, Bromus tectorum, and Atriplex confertifolia. Plant phenology was assessed at weekly intervals and water stress and overall plant health was assessed by measuring photochemical and non-photochemical quenching, steady state fluorescence, dark-adapted potential, and effective quantum field. To determine plant N status, we measured leaf chlorophyll and epidermal polyphenol contents. Growth, biomass, and reproductive output were all measured in the field and using allometric relationships. We also measured soil plant-available nitrogen and phosphorus, soil microbial biomass, biological soil crust cover classes, and soil nitrification rates.
In response to warming, we observed both plant and soil changes after two years of warming. In particular, A. confertifolia and S. hymenoides reached multiple stages of phenological development earlier in warmed plots, and soil nutrient status was altered. We found differences in steady state fluorescence, leaf chlorophyll concentrations, and epidermal polyphenol contents between treatments, indicating that plants are responding to heat and water stress. Additionally, soil watering led to a dramatic moss die off in watered plotsm resulting in a biogeochemical cascade affecting multiple aspects of soil nutrient cycling. While the data presented here represent the first few years of an on-going experiment, the results suggest that moderate changes to climate will strongly affect the plant and soil environments of the Colorado Plateau.