Physiographic position modulates the influence of temperature and precipitation as controls over leaf and ecosystem level CO2 flux in shrubland ecosystems

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

Submitted to: American Geophysical Union
Publication Type: Abstract Only
Publication Acceptance Date: 9/30/2010
Publication Date: 12/13/2010
Citation: Barron-Gafford, G., Scott, R.L., Jenerette, G., Hamerlynck, E.P., Huxman, T. 2010. Physiographic position modulates the influence of temperature and precipitation as controls over leaf and ecosystem level CO2 flux in shrubland ecosystems. American Geophysical Union 2010 Fall Meeting, [Abstract]. AGU, San Francisco, Calif., 13-17 Dec.

Interpretive Summary: Conversion of semiarid grasslands to shrublands 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 and upland shrubland 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% and 75% of maximum net CO2 uptake. We quantified the temperature response of both the dominant vegetative components within both semiarid shrublands of differing physiographic position and the ecosystems themselves to examine how temperature sensitivity varies with access to stable groundwater. By repeatedly measuring CO2 uptake across a wide range of temperatures and estimating soil respiration, we quantified the temperature sensitivity of these systems, computed changes in those responses due to periods of precipitation input, and estimated the role of component fluxes in driving ecosystem-scale responses. We found that having a connectivity to stable groundwater sources decoupled leaf-and ecosystem-scale temperature sensitivity relative to comparable sites lacking such access. Access to groundwater not only resulted in the temperature sensitivity of the riparian shrubland being nearly half that of the upland throughout all seasonal periods, but also actual rates of net ecosystem productivity (NEP) being 1.5X greater when precipitation was relatively abundant and five times greater when it was not. Maxima rates of NEP were nine times more responsive to the onset of the monsoon in the upland than in the riparian shrubland, emphasizing that a weaker coupling to groundwater resulted in a stronger connectivity between ecosystem performance and precipitation input. Similarly, the temperature sensitivity of leaf-level photosynthesis was about 1/3rd that in the drier shrubland than the riparian site during periods of little to no precipitation. At the end of the year, the ecosystem with access to groundwater was a strong sink for CO2 from the atmosphere, while the site without stable water was a nearly neutral source of CO2. This reduced net CO2 assimilation in drier shrubland likely contributed to the relative diminutive stature and sparse coverage of the upland landscape relative to the riparian shrubland, underscoring the influence of connectivity to stable groundwater on ecosystem structure.

Technical Abstract: Conversion of semiarid grasslands to shrublands 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 and upland shrubland 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% and 75% of maximum net CO2 uptake. We quantified the temperature response of both the dominant vegetative components within both semiarid shrublands of differing physiographic position and the ecosystems themselves to examine how temperature sensitivity varies with access to stable groundwater. By repeatedly measuring CO2 uptake across a wide range of temperatures and estimating soil respiration, we quantified the temperature sensitivity of these systems, computed changes in those responses due to periods of precipitation input, and estimated the role of component fluxes in driving ecosystem-scale responses. We found that having a connectivity to stable groundwater sources decoupled leaf-and ecosystem-scale temperature sensitivity relative to comparable sites lacking such access. Access to groundwater not only resulted in the temperature sensitivity of the riparian shrubland being nearly half that of the upland throughout all seasonal periods, but also actual rates of net ecosystem productivity (NEP) being 1.5X greater when precipitation was relatively abundant and five times greater when it was not. Maxima rates of NEP were nine times more responsive to the onset of the monsoon in the upland than in the riparian shrubland, emphasizing that a weaker coupling to groundwater resulted in a stronger connectivity between ecosystem performance and precipitation input. Similarly, the temperature sensitivity of leaf-level photosynthesis was about 1/3rd that in the drier shrubland than the riparian site during periods of little to no precipitation. At the end of the year, the ecosystem with access to groundwater was a strong sink for CO2 from the atmosphere, while the site without stable water was a nearly neutral source of CO2. This reduced net CO2 assimilation in drier shrubland likely contributed to the relative diminutive stature and sparse coverage of the upland landscape relative to the riparian shrubland, underscoring the influence of connectivity to stable groundwater on ecosystem structure.