Landscape and environmental controls over leaf and ecosystem carbon dioxide fluxes under woody plant expansion

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

Submitted to: Journal of Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/12/2013
Publication Date: 10/17/2013
Citation: Barron-Gafford, G., Scott, R.L., Jenerette, G., Hamerlynck, E.P., Huxman, T. 2013. Landscape and environmental controls over leaf and ecosystem carbon dioxide fluxes under woody plant expansion. Journal of Ecology. 101:1471-1483. https://doi.org/10.1111/1365-2745.12161.
DOI: https://doi.org/10.1111/1365-2745.12161

Interpretive Summary: The conversion of many historic grasslands to shrublands and savannas worldwide has the potential to alter how ecosystems will respond to global climate change because the dominant plants, either grasses and woody plants, have different responses to these changes. We explored how temperature and precipitation control carbon fluxes within a pair of semiarid shrublands in Arizona that had undergone woody plant expansion. We found significant differences in the functioning of the two plant functional types, in that the shrubs were able to consistently conduct photosynthesis across a broader temperature range than co-occurring grasses during dry periods, but with grasses outperforming shrubs during the wetter monsoon season. Landscape position modulated these temperature sensitivities, as the range of functional temperatures and maximum rates of photosynthesis were two to three times greater within the riparian shrubland in dry times. Given projections of more variable precipitation and increased temperatures, differences in physiological activity within these growth-forms are likely to drive patterns of ecosystem carbon exchange. As access to more stable groundwater declines with decreased precipitation input, these differential patterns of temperature sensitivity among growth-forms dependent on connectivity to groundwater will only become more important in determining whether these ecosystem will absorb more or less atmospheric carbon dioxide in the future.

Technical Abstract: Many regions of the globe are experiencing a simultaneous change in the dominant plant functional type and regional climatology. We explored how atmospheric temperature and precipitation input control leaf- and ecosystem scale carbon fluxes within a pair of semiarid shrublands that had undergone woody plant expansion. Through a combination of leaf-level measurements on individual bunchgrasses and mesquites shrubs and ecosystem-scale monitoring using eddy covariance techniques, we estimated rates of net carbon dioxide (CO2) flux, CO2 flux temperature sensitivity, and the responsiveness of these parameters to seasonal rains and periods of soil dry-down. We found significant differences in physiological acclimation between the two plant functional types, in that the shrubs were able to consistently conduct photosynthesis across a broader temperature range than co-occurring grasses during dry periods, but with grasses outperforming mesquites during the wetter monsoon season. Landscape position modulated these temperature sensitivities, as the range of functional temperatures and maximum rates of photosynthesis were two to three times greater within the riparian shrubland in dry times. Also, it was unexpected that ecosystem-scale CO2 uptake within both shrublands would become most temperature sensitive within the monsoon, when mesquites and grasses had their broadest range of function. This is likely explained by the changing contributions of component photosynthetic fluxes, in that the more temperature sensitive grasses, which had higher maximal rates of photosynthesis, became a larger component of the ecosystem flux. Given projections of more variable precipitation and increased temperatures, differences in physiological activity within these growth-forms are likely to drive patterns of ecosystem-scale CO2 flux. As access to stable sub-surface water declines with decreased precipitation input, these differential patterns of temperature sensitivity among growth-forms dependent on connectivity to groundwater will only become more important in determining ecosystem carbon source/sink status.