Agricultural and Forest Meteorology 268 (2019) 269–278 Contents lists available at ScienceDirect Agricultural and Forest Meteorology journal homepage: www.elsevier.com/locate/agrformet Native shrubland and managed buffelgrass savanna in drylands: Implications for ecosystem carbon and water fluxes T ⁎ César Hinojo-Hinojoa, Alejandro E. Castellanosa, , Travis Huxmanb, Julio C. Rodriguezc, Rodrigo Vargasd, José R. Romo-Leóna, Joel A. Biedermane a Departamento de Investigaciones Científicas y Tecnológicas, Universidad de Sonora, Hermosillo, Sonora 83000, México b Ecology and Evolutionary Biology, Center for Environmental Biology, University of California-Irvine, Irvine, CA 92629, USA c Departamento de Agricultura y Ganadería, Universidad de Sonora, Hermosillo, Sonora 83000, México d Department of Plant and Soil Sciences, University of Delaware, Newark, Delaware 19716, USA e Southwest Watershed Research Center, Agricultural Research Service, Tucson, AZ 85719, USA ARTICLE INFO ABSTRACT Keywords: Land cover and land-use change (LCLUC) between woody- and grass-dominated ecosystems in drylands comprise ff Bu elgrass one of the largest uncertainties in the land CO2 sink. This is especially true for the widespread transition from Cenchrus ciliaris shrublands to grasslands/savannas caused by the establishment of exotic C4 grass species for grazing or through Eddy covariance biological invasion of these species, where information about its impacts on ecosystem CO2 fluxes is limited. For Land cover change studying this, we used three years of eddy covariance measurements of net ecosystem production (NEP), gross Sonoran Desert primary production (GPP), ecosystem respiration (R ) and evapotranspiration (ET) over a Sonoran Desert Carbon and water fluxes eco ff Grass encroachment shrubland and an adjacent grazing savanna of bu elgrass (Cenchrus ciliaris L.), established 35 years ago. At monthly, seasonal and annual time scales, we assessed whether between-site differences in CO2 fluxes were related to differences in ecosystem water use, measured as the fraction ET to precipitation, water use efficiency (WUE, i.e. the ratio between GPP and ET) and/or to the relation between Reco and GPP. Although the savanna had higher WUE than the shrubland, its summer NEP was limited by water use, due to limitations in leaf area index and likely rooting patterns. Conversely, the savanna had higher NEP than the shrubland during fall to spring due to increased WUE; possibly due to activity of buffelgrass and remaining woody species using (summer) water from deeper soil layers. However, differences across these seasons compensated each other at the inter-annual scale, and both sites had comparable net carbon sinks over the three-year study period. Further studies are needed to understand and reduce the uncertainty associated with carbon and water fluxes associated with LCLUC in dryland ecosystems. 1. Introduction studied than drylands, LCLUC typically results in net CO2 emissions to the atmosphere due to biomass loss and decomposition (Le Quéré et al. Dryland ecosystems have a key role in the trend and interannual 2016). Changes in the net carbon balance in drylands following LCLUC variability of the global land carbon sink (Ahlstrom et al., 2015; Poulter are still unclear and represent one of the largest uncertainties in the et al., 2014). These globally distributed ecosystems have large temporal global carbon balance (Hayes et al., 2018; King et al., 2007). and spatial variability in net ecosystem production (NEP), and conse- Woody shrubs and grasses are dominant in drylands (Safriel et al., quently in CO2 uptake by gross primary production (GPP) and emis- 2005). The transitions from woody- to grass-dominated ecosystems and sions to the atmosphere by ecosystem respiration (Reco)(Ahlstrom vice versa are recognized as the most widespread LCLUC across dry- et al., 2015; Biederman et al., 2016, 2017). Furthermore, extensive lands (Barger et al., 2011; Marshall et al., 2012; Sala and Maestre, land-cover and land-use change (LCLUC) has been documented across 2014). The gradual change over decades from grass- to woody-domi- drylands (Safriel et al., 2005), which in turn directly influence the nated ecosystems is referred as woody encroachment, which is regu- magnitude of these fluxes (Le Quéré et al., 2016; Baldocchi, 2008). In lated by interactions among climate, vegetation, and management temperate forest ecosystems, which have traditionally have been more across drylands (Van Auken, 2009). In the opposite direction, a change ⁎ Corresponding author. E-mail address: [email protected] (A.E. Castellanos). https://doi.org/10.1016/j.agrformet.2019.01.030 Received 6 March 2018; Received in revised form 14 November 2018; Accepted 21 January 2019 0168-1923/ © 2019 Elsevier B.V. All rights reserved. C. Hinojo-Hinojo et al. Agricultural and Forest Meteorology 268 (2019) 269–278 from woody- (e.g., shrublands) to grass-dominated ecosystems could modified by land cover: the relation of GPP with ET, and the relation of occur by the anthropogenic introduction of highly productive and Reco with GPP (Biederman et al., 2016). ET/GPP describes how avail- drought-resistant African/Asian C4 grass species to increase forage able water is used for photosynthesis, reflecting the ecosystem water- production (Castellanos et al., 2002, 2010; Marshall et al., 2012; use efficiency. Meanwhile, Reco/GPP reflects how carbon availability Williams and Baruch, 2000). Vegetation structure may be dramatically drives ecosystem respiration, and is sensitive to changes in vegetation, changed within a season through land clearing of native vegetation and carbon pools and allocation caused by LCLUC or disturbances seeding of grass or more gradually through invasion of exotic grasses (Baldocchi, 2008; Biederman et al., 2016). In this context, C4 grasslands (Arriaga et al., 2004; Williams and Baruch, 2000). These bidirectional have higher water-use efficiency than woody-encroached ecosystems LCLUC transitions could have important consequences for regional-to- dominated by C3 plants (Emmerich, 2007; Kurc and Small, 2007; Scott global carbon budgets, but large uncertainties remain (Barger et al., et al., 2014). This advantage is counteracted by an increased Reco/GPP, 2011; Houghton et al., 2012 Jackson et al., 2002; Pacala et al., 2001). which is likely related to the high proportion of root biomass in grasses Woody encroachment has been more extensively studied (Petrie et al., (Scott et al., 2014; Petrie et al., 2015). The balance between these re- 2015; Scott et al., 2006, 2014), and because of the lack of information, lationships further contributes to explain the lower NEP of grasslands larger uncertainties exist for the shrubland-to-grassland transition and when compared to woody-encroached sites. It is unclear how a LCLUC particularly the consequences for whole-ecosystem stocks and annual in the opposite direction (i.e., shrublands converted into C4 grasslands) fluxes (Bradley et al., 2006; Prater et al., 2006). influences NEP in dryland ecosystems. We could expect diminished NEP Woody encroachment increases whole-ecosystem carbon fluxes by reversing the typical results of woody encroachment. However (NEP, Reco, GPP) in drylands because access to deeper soil and exotic grass species have historically been selected for high productivity groundwater sources supports longer growing seasons (Scott et al., under limiting abiotic conditions (Cox et al., 1988) and could lead to a 2006, 2014; Petrie et al., 2015), and because encroaching woody spe- different outcome. cies can physiologically outperform native grass species under a wide Buffelgrass (Cenchrus ciliaris L.) is one of the main C4 exotic grass range of drought conditions (Barron-Gafford et al., 2012; Throop et al., species used for converting xeric shrublands into more productive 2012). Changes are expected in evapotranspiration (ET), the water flux grasslands/savannas for cattle grazing over America and Australia (Cox of available soil moisture (SM) that remains after infiltration from et al., 1988; Marshall et al., 2012). Notably, the Sonoran Desert of precipitation (P), and losses to runoff (R) and drainage (D) beyond the Mexico and the US may be one of the region's most heavily impacted by rooting zone (Fig. 1). ET approximates the available water that drives these practices (Bravo-Peña et al., 2010; Bravo-Peña and Castellanos, carbon fluxes (Biederman et al., 2016), and ET/P represents the fraction 2013; Castellanos et al., 2002, 2010; Marshall et al., 2012). This per- of water used at the ecosystem scale. Two relations are particularly ennial bunchgrass has one of the highest photosynthesis rates important and explain the variation of NEP in drylands, which can be (Castellanos et al., 2010; Larcher, 2014) and can physiologically out- perform dominant C3 woody species from the Sonoran Desert (Castellanos et al., 2010) enabling exotic buffelgrass savannas to be substantial carbon sinks (Hinojo-Hinojo et al., 2016). However, remote sensing estimates suggest that buffelgrass-dominated ecosystems have less above—ground biomass production than Sonoran Desert shrub- lands (Bravo-Peña and Castellanos, 2013; Franklin et al., 2006; Franklin and Molina-Freaner, 2010). Thus, research is needed to contribute to our understanding of shrubland—to—grassland transitions across dry- land ecosystems. In this study we compare three years of CO2 and water fluxes over a shrubland and an adjacent ecosystem converted to exotic buffelgrass savanna ˜35 years ago to address the following questions: