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

item BARRON-GAFFORD, G.A. - University Of Arizona
item Scott, Russell - Russ
item JENERETTE, G.D. - University Of Arizona
item Hamerlynck, Erik
item HUXMAN, T.E. - University Of Arizona

Submitted to: Global Change Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/24/2011
Publication Date: 3/15/2012
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.

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 individual plants and entire ecosystems to air temperature and precipitation. We combined leaf-level gas exchange and ecosystem-level eddy covariance measurements to quantify the effects of plant temperature sensitivity and ecosystem temperature responses within a grassland and mesquite woodland across seasonal precipitation periods. In so doing, we were able to estimate the role of moisture availability on ecosystem temperature sensitivity under large-scale vegetative shifts. Optimum temperatures (Topt) for net photosynthetic assimilation (A) and net ecosystem productivity (NEP) were estimated from a function fitted to A and NEP plotted against air temperature. The convexities of these temperature responses were quantified by the range of temperatures over which a leaf or an ecosystem assimilated 50% of maximum NEP (O50). Under dry pre- and postmonsoon conditions, leaf-level O50 in C3 shrubs were two-to-three times that of C4 grasses, but under moist monsoon conditions, leaf-level O50 was similar between growth forms. At the ecosystems-scale, grassland NEP was more sensitive to precipitation, as evidenced by a 104% increase in maximum NEP at monsoon onset, compared to a 57% increase in the woodland. Also, woodland NEP was greater across all temperatures experienced by both ecosystems in all seasons. By maintaining physiological function across a wider temperature range during water-limited periods, woody plants assimilated larger amounts of carbon. This higher carbon-assimilation capacity may have significant implications for ecosystem responses to projected climate change scenarios of higher temperatures and more variable precipitation, particularly as semiarid regions experience conversions from C4 grasses to C3 shrubs. As regional carbon models, CLM 4.0, are now able to incorporate functional type and photosynthetic pathway differences, this work highlights the need for a better integration of the interactive effects of growth form/functional type and photosynthetic pathway on water resource acquisition and temperature sensitivity.