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Title: Seasonality of soil moisture mediates responses of ecosystem phenology to elevated CO2 and warming in a semi-arid grassland

Author
item ZELIKOVA, TAMARA - University Of Wyoming
item WILLIAMS, DAVID - University Of Wyoming
item HOENIGMAN, RHONDA - University Of Wyoming
item Blumenthal, Dana
item MORGAN, JACK - Retired ARS Employee
item PENDALL, ELISE - Western Sydney University

Submitted to: Journal of Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/18/2015
Publication Date: 8/7/2015
Citation: Zelikova, T., Williams, D., Hoenigman, R., Blumenthal, D.M., Morgan, J., Pendall, E. 2015. Seasonality of soil moisture mediates responses of ecosystem phenology to elevated CO2 and warming in a semi-arid grassland. Journal of Ecology. 103:1119-1130.

Interpretive Summary: Concurrent changes in temperature, atmospheric CO2, and precipitation regimes may be especially important in water-limited ecosystems. Such ecosystems include the semi-arid grasslands of western North America which provide critical ecosystem services, including carbon storage, livestock grazing, and wildlife habitat. Here we show that experimental warming of mixed-grass prairie enhanced plant cover and greenness early in the growing season but had a negative effect during the middle of the summer. In contrast, elevated CO2 had negative or neutral effects on greenness early in the growing season, and increased greenness in the middle of the growing season, especially in dry years. Together elevated CO2 and warming may often increase greenness, particularly early in the season. These global changes are therefore likely to alter carbon balance and increase early-season forage availability in semi-arid grassland.

Technical Abstract: Concurrent changes in temperature, atmospheric CO2, and precipitation regimes are altering ecosystems globally, and may be especially important in water-limited ecosystems. Such ecosystems include the semi-arid grasslands of western North America which provide critical ecosystem services, including carbon storage, livestock grazing, and wildlife habitat. To assess how the individual and combined effects of warming and elevated CO2 impact the cover of C3 and C4 plants, plant community phenology and greenness over a 7 year period, we used time series repeat photography and a novel means of quantifying greenness in digital photographs. We found that the combined effects of warming and elevated CO2 on plant cover and greenness were largely positive, but their singular effects were seasonally variable. Warming enhanced plant cover and greenness early in the growing season but had a negative effect during the middle of the summer, offsetting the early season positive effects. When accumulated over an entire growing season, the effect of warming was largely neutral. The isolated effects of elevated CO2 on plant cover and greenness were less seasonally strong, but were often neutral or negative early in the growing season and positive in the middle, especially in dry years. The overall effect of elevated CO2 dampened over time, but warming continued to affect plant cover and plot greenness throughout the six-year warming experiment. Over and above the singular and combined effects of warming and elevated CO2, interannual variation in precipitation had larger effect on greenness, with 2-3 times greater greenness accumulated in wet years than in dry years. Vegetation uptake of fossil fuel emissions is expected to offset a third of the global output and even small changes in vegetation cover and greenness in response to warming and rising atmospheric CO2 concentrations, such as those we report here, may have large consequences for future carbon uptake. However, our results suggest that vegetation responses to these global change factors may strongly depend on background precipitation and concurrent changes in precipitation regimes.