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United States Department of Agriculture

Agricultural Research Service

Research Project: HYDROLOGIC PROCESSES, SCALE, CLIMATE VARIABILITY, AND WATER RESOURCES FOR SEMIARID WATERSHED MANAGEMENT

Location: Southwest Watershed Research

Title: Temperature and precipitation controls over leaf- and ecosystem-level CO2 flux along a woody plant encroachment gradient

Authors
item Barron-Gafford, G.A. -
item Scott, Russell
item Jenerette, G.D. -
item Hamerlynck, Erik
item Huxman, T.E. -

Submitted to: Global Change Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 24, 2011
Publication Date: March 15, 2012
Citation: Barron-Gafford, G., Scott, R.L., Jenerette, G., Hamerlynck, E.P., Huxman, T. 2012. Temperature and precipitation controls over leaf- and ecosystem-level CO2 flux along a woody plant encroachment gradient. Global Change Biology. 18;1389-1400. doi:10.5194/bg-9-1007-2012.

Interpretive Summary: The worldwide phenomenon of the conversion of many historic grasslands to shrublands and savannas has the potential to alter how ecosystems will respond to changes in global climate because the dominant plants, either grasses and woody plants, have different responses to these changes. We used a combination of leaf and ecosystem-level meteorological measurements to quantify the temperature sensitivity of carbon dioxide exchange in a riparian sacaton grassland and a mesquite woodland across seasonal periods of differing precipitation input in southeastern Arizona. We found that the sensitivity of net ecosystem plant productivity to precipitation was greater in the grassland, but that the woodland was more productive for almost all of the temperature ranges experienced by both ecosystems in both the rainy and dry periods. By maintaining plant function across a wider range of temperatures during periods of limited precipitation, woody plants were more productive, while grass productivity was limited to a narrower temperature range. This higher capacity for assimilation may have significant implications for an ecosystem’s response to projected climate change scenarios of higher atmospheric temperatures and more variable precipitation, particularly as semiarid regions worldwide experience a conversion of dominance from grasses to shrubs.

Technical Abstract: Conversion of grasslands to woodlands may alter the sensitivity of CO2 exchange of both the dominant plants and the entire ecosystem to variation in air temperature and precipitation. We used a combination of leaf-level gas exchange experimentation and ecosystem-level eddy covariance monitoring techniques to quantify the temperature sensitivity of a riparian sacaton grassland and a mesquite woodland across seasonal periods of differing precipitation input in southeastern Arizona, USA. Maximum rates of net CO2 uptake were estimated from a Lorentzian peak function fitted to net uptake plotted against air temperature, with optimum temperature being that at which maximum uptake occurred. The convexity of the temperature response function was quantified by the range of temperatures over which a leaf or an ecosystem assimilated 50% (O50) and 75% of maximum net CO2 uptake. With the exception of the monsoon, when leaf-level O50 was equally high among C3 woody shrubs and C4 grasses in both ecosystems, shrub O50 were as much as three times that in grasses pre-monsoon and twice as great post-monsoon. Sensitivity of ecosystem-scale net ecosystem productivity (NEP) to precipitation was greater in the grassland, as monsoon onset yielded a 104% increase in maximum NEP (NEPmax), in contrast to +57% in average NEPmax of the woodland ecosystem. Also, values of woodland NEP were greater for almost all of the temperature range experienced by both ecosystems in all seasonal periods. By maintaining physiological function across a wider range of temperatures during periods of limited precipitation, woody plants assimilated larger amounts of carbon, while grass function was limited to a narrower temperature range. This higher capacity for assimilation may have significant implications for an ecosystem’s response to projected climate change scenarios of higher atmospheric temperatures and more variable precipitation, particularly as semiarid regions worldwide experience a conversion of dominance from C4 grasses to C3 shrubs.

Last Modified: 4/20/2014
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