Carbon and Water Fluxes in an Exotic Buffelgrass Savanna

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Abstract

Buffelgrass savanna is becoming widespread in aridland ecosystems around the world following invasion or deliberate land conversion for cattle forage. There is still a gap of information regarding functional and ecohydrological aspects such as carbon, water, and greenhouse gas exchanges in these highly productive novel ecosystems where buffelgrass is an exotic species. We measured net ecosystem CO2 exchange (NEE), ecosystem respiration (Reco), gross primary production (GPP), and evapotranspiration (ET) with eddy covariance techniques over a buffelgrass savanna established for cattle grazing, approximately 30 yr ago within the Sonoran Desert. The savanna was a net carbon sink (NEE − 230 g C/m2/yr) during both a year with above average and one with below-average precipitation (NEE − 84 g C/m2/yr). Water loss through evapotranspiration (ET) was similar to total annual rainfall input. Up to 62% of the annual fixed carbon and 75% of ET occurred during the summer monsoon season, when 72  86% of annual rainfall occurred and buffelgrass was active. ET from summer months explained 73% of variation in NEE, with an average ET of 50 mm H2O/month needed to turn the ecosystem into a net carbon sink during this season. Other seasons in the year, when buffelgrass was dormant, contributed with up to 48% of annual fixed carbon but with higher water use efficiency (− NEE/ET). We discuss the importance of the seasonal variability in Reco, GPP, and ET processes and the phenology of native plant species for the net carbon uptake through the year for this managed novel ecosystem.

Introduction

Aridland ecosystems represent close to 50% of the earth’s surface, and more than half of those have seen changes in vegetation that could impact ecosystem goods and services (Reynolds and Stafford-Smith, 2002). Shifts in land use and cover are the second most important components of global change in arid and semiarid lands worldwide (Lal, 2001, Hassan et al., 2005), following rangeland degradation for cattle production (Walker, 1979, Lambin et al., 2003). Since the middle of the past century, ecosystems dominated by perennial African grasses have been intentionally established in aridlands worldwide in order to increase vegetation cover, forage, and cattle production following arid rangeland degradation (Cox et al., 1988, Williams and Baruch, 2000, Belnap et al., 2012). Understanding what governs the structure and function of these novel, widespread ecosystems is a key management concern.

Buffelgrass (Cenchrus ciliaris L.) was one of those grasses successfully introduced worldwide (Herbel, 1979, Cox et al., 1988). Buffelgrass-dominated ecosystems are intentionally established through land clearing of native vegetation and seeding cover from this grass. This results in savanna-like ecosystems when some native woody plants are maintained for shade or have recolonized the area. Extensive transformations to buffelgrass savannas have happened in the past 3 decades, especially in Australia and North America, where expansive buffelgrass savannas are now common (Arriaga et al., 2004, Miller et al., 2010, Marshall et al., 2012). In northwestern Mexico alone, it is estimated that between 1 and 1.8 million hectares of historically arid/semiarid shrublands and tropical deciduous forests have been transformed to buffelgrass savanna (Burquez et al., 1998, Castellanos-V. et al., 2002, Bravo-Peña and Castellanos, 2013).

Most research on exotic buffelgrass-dominated ecosystems has focused on describing biodiversity implications of its establishment and its characteristics as invasive species (see Marshall et al., 2012). Regarding this, studies have described how buffelgrass accelerates the fire-return intervals (D’Antonio and Vitousek, 1992, Butler and Fairfax, 2003, McDonald and McPherson, 2011), reduces plant species diversity (Saucedo-Monarque et al., 1997), intensifies competition (Eilts and Huxman, 2013), or suppresses regeneration (Morales-Romero and Molina-Freaner, 2008). However, there is still a gap of information regarding functional and ecohydrological aspects such as carbon, water, and greenhouse gas exchanges in these widespread buffelgrass-dominated ecosystems (Wilcox et al., 2012).

Estimates of biomass production in exotic buffelgrass savannas range from < 465 and up to 3,045 kg/ha (Martin-R et al., 1995, Rao et al., 1996). Many factors may be contributing to such variation in productivity, such as management schemes, seed viability and abundance, seedling establishment success, nutrient and water availability and use, and microclimatic conditions (Martín et al., 1999, Castellanos-V. et al., 2002, Celaya-Michel et al., 2015). While biomass production is important in the context of range management and forage production, other production estimates such as whole ecosystem carbon dioxide (CO2) flux and water use efficiency (the amount of carbon fixed per unit of water loss through evapotranspiration [ET]) can provide insight into ecosystem function. In northwestern Mexico and the southwestern United States, whole ecosystem fluxes may be influenced by its biseasonal rainfall pattern, where most annual rainfall occurs in summer months due to monsoon storms and from late fall and winter rains (Brito-Castillo et al., 2010). Given the range of values in the literature and the uncertainty associated with how environmental and biological variables drive productivity in buffelgrass savannas, continuous measurements of ecosystem carbon and water exchange may achieve substantial understanding to guide decision makers, especially where there are implications of carbon sequestration.

The eddy covariance (EC) technique provides a continuous measure of ecosystem productivity by estimating CO2, water (H2O), and energy fluxes between the land-surface and surrounding atmosphere (Baldocchi, 2003) and has been used at multiple ecosystems across the world (Baldocchi et al., 2001). In this study, we document carbon (CO2 net ecosystem exchange [NEE], gross primary productivity, and ecosystem respiration) and water (ET) flux dynamics of an exotic buffelgrass savanna established through the transformation of an arid shrubland within the Sonoran Desert. We used EC technique to answer the following questions: 1) What are the dynamics of CO2 and water fluxes in an exotic buffelgrass savanna? 2) How does the biseasonal rainfall pattern from the Sonoran Desert region drive those CO2 and water fluxes? and 3) How big are interannual seasonal differences in carbon and water fluxes? Data that address these questions will allow us to understand a baseline response of buffelgrass savanna biosphere/atmosphere exchanges in the context of seasonal and interannual environmental variability.

Section snippets

Study Site

Our study was initiated in the summer of 2011, and we include 2 1/2 yr of measurements. The site is located at La Colorada, Sonora in northwestern Mexico, at 28°42.672′N and − 110°32.969′W at an altitude of 398 m, and is part of the Mexican EC network (MexFlux; Vargas et al., 2013) within a plain that extends several kilometers. Mean annual air temperature of 22.7°C and 343.8 mm annual rainfall were obtained from a nearby meteorological station (San José de Pimas, ≈ 10 km). Fall rainfall account

Results

During the study, annual rainfall at the site varied from below average (in 2011 and 2013 with 259 and 253 mm, respectively) to above the historic mean (in 2012 with 420 mm). Before the start of our flux measurements, the site had experienced an extended period of 9 months with no measurable rainfall (from September 2010 to June 2011). A large percentage of annual rainfall, 72  86%, was related to the summer monsoon (July to September; Fig. 2d), which varied from 190 to 200 mm in 2011 and 2013

Discussion and Conclusions

In this paper we document the nature of biosphere/atmosphere CO2 and water exchanges in an arid ecosystem that has been transformed into an exotic buffelgrass (C. ciliaris) savanna for cattle grazing management. While studies have previously documented carbon fluxes over buffelgrass-dominated ecosystems near the native range of this species (Lalrammawia and Paliwal, 2010, Hussain et al., 2015), here we document seasonality and interannual variation on ecosystem fluxes for a site where this

Acknowledgements

We thank R. Vargas (University of Delaware) and two anonymous reviewers for their helpful suggestions to improve previous versions of this manuscript. We thank our colleagues at the Plant Ecophysiology Laboratory for their tireless and unconditional help during the field work. Special thanks to M. Heliu NAU for his support in the initial stages of this project. We thank L. Sierra M. for allowing us to conduct our studies on his property and J. Dueñas for logistic support.

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    No funding source was involved in publishing this paper. C.H.H. and H.C.M. thank CONACYT for support as a scholarship to Posgrado en Biociencias at DICTUS and A.E.C.V. for grant support to this project (CB61865 and INF2012/1-188387). A.E.C.V. benefitted from Universidad de Sonora, PIFI-SEP support to set the eddy flux tower.

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