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Title: Water-use-efficiency of annual-dominated and bunchgrass-dominated savanna intercanopy space

item Hamerlynck, Erik
item Scott, Russell - Russ
item Cavanaugh, Michelle
item BARRON GAFFORD, B. - University Of Arizona

Submitted to: Ecohydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/23/2013
Publication Date: 10/22/2013
Citation: Hamerlynck, E.P., Scott, R.L., Cavanaugh, M.L., Barron Gafford, B. 2013. Water-use-efficiency of annual-dominated and bunchgrass-dominated savanna intercanopy space. Ecohydrology. 7:1208-1215.

Interpretive Summary: Ecosystem water use efficiency (WUEe), the ratio between net ecosystem carbon dioxide exchange (NEE) and evapotranspiration (ET) is an important functional feature of terrestrial ecosystems. In savanna ecosystems, which are dynamic shifting mosaics of grass and tree dominated patches, annual or perennial bunchgrasses can dominate tree intercanopy spaces. These growth forms have very different physical structures, life cycles, and physiological characteristics, all of which could strongly affect savanna WUEe. We tracked soil moisture content (q25cm), ET and plant transpiration (T), and the constituent fluxes of NEE, ecosystem respiration (Reco) and gross ecosystem photosynthesis (GEP) and plant community water use efficiency (WUEc = GEP/T) to see how the controls to NEE and WUEe changed under these contrasting growth forms. We specifically expected annual plot WUEe to be lower than in bunchgrass plots, mainly because of lower WUEc. WUEe was indeed lower in annual dominated plots, but WUEc was the same. WUEe in annual plot was lower because 1) these plots had higher soil evaporation contributions to ET, since T was similar to bunchgrasses, and 2) Reco was higher in annual plots, even though GEP was similar to bunchgrass plots, resulting in lower NEE. These findings show that differences in plant community structure dramatically altered the basic controls to ecosystem water and carbon fluxes in intercanopy savanna.

Technical Abstract: In semi-arid savannas, dominance of intercanopy space by annual or perennial grasses may alter partitioning of ecosystem water and carbon fluxes and affect ecosystem water use efficiency (WUEe), the ratio of net ecosystem carbon dioxide exchange (NEE) to evapotranspiration (ET). To establish if these contrasting growth habits changed controls to WUEe, we tracked volumetric soil moisture ('25cm), ET and transpiration (T), NEE and its constituent ecosystem respiration (Reco) and gross ecosystem photosynthesis (GEP) fluxes, and community water use efficiency (WUEc = GEP : T) in annual-dominated and bunchgrassdominated plots in a southern Arizona, United States, savanna. Annual and bunchgrass plots had similar '25cm, ET, and T, suggesting the similarity in ET was due to higher soil evaporation in annual plots. Seasonal NEE was delayed and lower in annual plots compared with that in bunchgrass plots, owing to higher Reco in annual plots. Transpiration, GEP, and Reco in both vegetation types increased following late-season rain, indicating similar late-season phenological constraint. WUEe was lower in annual plots, but with similar WUEc between plot types. These results suggest that differences in annual plant biomass allocation and plot-level leaf area distribution increased proportional soil evaporation and aboveground Reco contributions, reducing plotlevel WUEe, not lowering plant WUE typical of arid-land annuals. Lower plot-level WUEe suggests that any increase in annual plant dominance would increase interannual variation of productivity in savanna intercanopy spaces, which could enhance the negative effects of predicted higher temperatures, greater aridity, and larger and more widely spaced storms on arid-land watershed processes.