The Effect of the Foresummer Drought on Carbon Exchange in Subalpine Meadows
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Climate in subalpine meadows of the Rocky Mountains can be characterized by an early (foresummer) drought that occurs after snowmelt (May) and lasts until the start of the summer monsoon season (July). Climate change models predict an increase in the length and severity of this dry period due to earlier snowmelt dates, rising air temperatures, and shifts in the start and/or intensity of the North American monsoon. However, it is unknown how changes in the severity of this early season dry period will affect ecosystem carbon exchange. To address the importance of early season drought, we combined a watering manipulation with 11 years of ecosystem carbon exchange data across an elevational gradient at the Rocky Mountain Biological Laboratory in Gothic, Colorado. Long-term trends reveal that earlier snowmelt dates lead to a decrease in net ecosystem productivity (NEP), in part because of the positive effect on early growing season drought conditions. Manipulating the strength of the foresummer drought by watering revealed that the timing of growing season precipitation is more important than the total amount for determining cumulative NEP. The strength of the foresummer drought did not significantly impact ecosystem respiration rates, but plants that experienced a strong foresummer drought exhibited more water stress, and lower instantaneous rates of NEP, even during the rainy season. Our results highlight the central role of the foresummer drought in determining rates of carbon exchange throughout the growing season, and the potential for an increasingly negative balance of carbon in subalpine meadows under future climate change.
Keywordsclimate change sualpine NEP foresummer drought elevation gradient watering manipulation
We would like to thank Vanessa Buzzard, Colby Sides, and William Driscoll for help in the field during the summer of 2012. Further, we would like to thank all previous Enquist lab field assistants for collecting data along the elevation gradient including A Kerkhoff, P Gaube, M Wilson Colner, L Crumbacher, J Stegen, R Sleith, R Poore, T Potter, B Chaszar, M Smith, N Prohaska, and B Blonder. LS was supported by NSF funding to BJE and AH was supported by an NSF GFRP fellowship. CAL was supported by an EPA STAR Fellowship during data collection and is currently supported by NSF award EPS-0904155 to Maine EPSCoR at the University of Maine and the Senator George J. Mitchell Center. BJE was supported by funding from an NSF CAREER and an NSF Macrosystems award. In addition, funding from the Aspen Center for Environmental Science helped support BJE. We would also like to thank Amy Iler for comments that significantly strengthened this manuscript and the staff of RMBL including Ian Billick and Jennie Reithel and for helping to facilitate this study. Lastly, Billy Bar allowed us to use his weather station data and observations.
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