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Title: When vegetation change alters ecosystem water availability

item Scott, Russell - Russ
item HUXMAN, T.E. - University Of California
item BARRON GAFFORD, B. - University Of Arizona
item JENERETTE, G.D. - University Of California
item YOUNG, J.M. - University Of Alaska
item Hamerlynck, Erik

Submitted to: Global Change Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/27/2013
Publication Date: 6/5/2014
Citation: Scott, R.L., Huxman, T., Barron Gafford, B., Jenerette, G., Young, J., Hamerlynck, E.P. 2014. When vegetation change alters ecosystem water availability. Global Change Biology. 20:2198-2210.

Interpretive Summary: The vegetation composition of many ecosystems around the world and in the U.S. is rapidly changing. The causes of these rapid changes are intimately connected with climate change and management practices, but the consequences of these shifts in composition and structure on the fundamental ecosystem services of water and carbon cycling are not well understood. For this study, we use multi-year hydrological and meteorological data to evaluate how soil water accessibility affects the magnitude and variability of the biosphere-atmosphere exchanges of water and carbon dioxide amongst three proximate ecosystems that are representative of varying degrees of mesquite tree invasion. We found that groundwater access increased in ecosystems with greater amounts of trees rather than grasses, and that groundwater accessibility led to significant changes in magnitude and variability in ecosystem water and carbon cycling. Even as depth to groundwater increased from grassland to shrubland to woodland, the woodland had the highest evapotranspiration in excess of precipitation and the grassland was lowest. How the greater density of trees affected the amount of carbon taken in by an ecosystem was more complex. Woodland carbon sequestration was the largest but surprisingly similar to the less mature and dense shrubland, and both systems took in much more carbon than the grassland. Vegetation change in areas where the accessibility to deeper soil water increases will likely increase carbon sequestration but at the expense of higher water use.

Technical Abstract: The combined effects of vegetation and climate change on biosphere-atmosphere water vapor (H2O) and carbon dioxide (CO2) exchanges are expected to vary depending, in part, on how biotic activity is controlled by and alters water availability. This is particularly important when a change in ecosystem composition alters the fractional covers of bare soil, grass, and woody plants so as to influence the accessibility of shallower versus deeper soil water pools. To study this, we compared five years of eddy covariance measurements of H2O and CO2 fluxes over a riparian grassland, shrubland, and woodland. In comparison with the surrounding upland region, groundwater access at the riparian sites increased net carbon uptake (NEP) and evapotranspiration (ET), which were sustained over more of the year. Among the sites, the grassland used less of the stable groundwater resource, and increasing woody plant density decoupled NEP and ET from incident precipitation (P), resulting in greater exchange rates that were less variable year-to-year. Despite similar gross patterns, how groundwater accessibility affected NEP was more complex than ET. The grassland had higher respiration (Reco) costs. Thus, while it had similar ET and gross carbon uptake (GEP) to the shrubland, grassland NEP was substantially less. Also, grassland carbon fluxes were more variable due to occasional flooding at the site, which both stimulated and inhibited NEP depending upon phenology. Woodland NEP was large, but surprisingly similar to the less mature, sparse shrubland, even while having much greater GEP. Woodland Reco was greater than the shrubland and responded strongly and positively to P, which resulted in a surprising negative NEP response to P. This is likely due to the large accumulation of carbon aboveground and in the surface soil. These long-term observations support the strong role that water accessibility can play when determining the consequences of ecosystem vegetation change.