ARS | Publication request: Ecosystem water use efficiency in an arid shrubland in Chile under natural and afforested conditions

Submitted to: Ecological Society of America (ESA)
Publication Type: Abstract Only
Publication Acceptance Date: May 5, 2011
Publication Date: August 7, 2011
Citation: Perez-Quezada, J.F., Jara, P.M., Fuster, R., Franck, N., Snyder, K.A., Johnson, D.A. 2011. Ecosystem water use efficiency in an arid shrubland in Chile under natural and afforested conditions [abstract]. In: 96th Ecological Society of America Annual Meeting, Austin, Texas. 29437.

Technical Abstract: Quantification of ecosystem water-use efficiency (WUEe) in natural and managed ecosystems is important to address uncertainties related to the effects of global change and alterations in land-use. This is especially important in arid ecosystems, where the impact of these changes is expected to be high. We continuously measured net ecosystem production (NEP) and evapotranspiration (ET) in a shrubland in the Coquimbo Region of Chile, during 28 months (May 2008-October 2010), using a Bowen ratio energy balance tower. A second tower operated for two months in 2009 on an adjacent shrubland site that was planted in 2005 with Acacia saligna (a fast-growing, non-native legume tree species). Prior to planting, the soil was ripped and mounds of soil were formed to increase soil depth and improve water retention. WUEe was defined as NEP/ET (mmol CO2 mol-1 H2O), considering only daytime data. The results showed that although annual precipitation is very low (average 138 mm), the natural site is a net sink of carbon most of the year, reaching maximum positive carbon fluxes (towards the ecosystem) of 1600 mmol CO2 m-2 d-1 (70 g CO2 m-2 d-1). ET reached a maximum of 210 mol H2O m-2 d-1 (3.8 mm d-1). There was a lag between the times when maximum CO2 and H2O fluxes were observed. Precipitation occurred only during the winter, creating high water availability in the soil, which could not be used very effectively because some species did not have active leaves, or temperatures were below optimum for plant gas exchange. This resulted in maximum ET values in early September (end of winter), whereas NEP reached it’s maximum near the end of December (early-summer). WUEe followed the response of NEP, which was similar to patterns observed in other studies where high WUEe coincided with periods of high water stress. WUEe on the natural shrubland site reached a maximum of 17 mmol CO2 mol-1 H2O, which is high compared to values observed in other studies. During the comparison period, WUEe reached a maximum of 15 mmol CO2 mol-1 H2O on the natural shrubland site, which was much higher than the 5 mmol CO2 mol-1 H2O observed on the afforested site. This was due to a higher NEP and slightly lower ET on the natural shrubland site. Lower NEP on the afforested site was due to lower shrub cover caused during the soil preparation process prior to planting A. saligna.