Shifting carbon pools along a plant cover gradient in woody encroached savannas of central Argentina
Introduction
Land cover changes in terrestrial ecosystems not only generate changes locally, they also affect the global system through changes in energy fluxes and element cycles, including changes in greenhouse gas concentrations and radiation balance (Charlson et al., 2005). Rangelands comprise almost 45% of the global land surface (Bailey, 1996), and changes in their cover can significantly influence global climate (Rotenberg and Yakir, 2010). For the last 150 years, increases in woody plant cover (WPC) have been observed in arid and semiarid regions worldwide (Van Auken, 2000, Asner et al., 2004). Given current trends and potential extent of encroachment, significant shifts may be occurring in the global carbon (C) cycle. For example, Houghton (2007) estimated that 40–70% of the net terrestrial C sink in the continental US corresponds to woody plant encroachment (WPE), especially across semiarid grasslands. However, little is known about the implications of this land cover change on ecosystem C stocks in other regions of the world, especially in the southern hemisphere.
Woody plant encroachment is the process of an increase in woody plant density, and is considered one of the most threatening forms of rangeland degradation (Briggs et al., 2005). Woody encroachment is a widespread process occurring in Australia (Fensham et al., 2005, Robinson et al., 2008), Africa (Roques et al., 2001, Augustine and McNaughton, 2004, Sankaran et al., 2005, Wigley et al., 2009), Europe (Anthelme et al., 2007), North America (Glendening, 1952, Asner et al., 2003) and South America (Cabral et al., 2003, Dumig et al., 2008, Silva and Anand, 2011). Given its worldwide occurrence, it has been suggested to be a consequence of global drivers such as the recent increases in atmospheric CO2 (Harrell and Fuhlendorf, 2002, Wigley et al., 2010, Donohue et al., 2013), but local drivers, such as introduction of cattle and changes in fire regimes, seem to be important as well (Archer et al., 1995, Asner et al., 2004).
Although considerable attention has been given to the causes of woody encroachment, less is known about its consequences on ecosystem functioning. A recent meta-analysis found that it had mixed effects on ecosystem structure and functioning at global scales, and that specific shrub traits influence the functional outcome of encroachment (Eldridge et al., 2011). Despite numerous studies in North America, Australia and Africa, it is not clear if areas with higher woody plant density due to woody plant encroachment function as net carbon sinks or sources. Aboveground C stocks typically increase as a consequence of the replacement of grasses by woody species (Eldridge et al., 2011), but differences emerge when the soil component is included. In the southwestern US, wooded areas in Texas increased the rate of accumulation of soil organic C (SOC) by 100–500% as compared to grasslands (Liao et al., 2006), resulting in a significant increase in landscape-scale ecosystem C stocks (Hibbard et al., 2003). In Arizona, soils under shrubs contained twice as much C as surrounding grass sites (Cable et al., 2009). However, Hughes et al. (2006) found that although aboveground C pools increased substantially with Prosopis tree stand development, no such change was found in surface soil C pools (0–10 cm depth). Along a rainfall gradient in North America, Jackson et al. (2002) found that while drier sites were gaining SOC, wetter sites were losing it. Moreover, the losses of SOC at wetter sites were substantial enough to offset the increase in aboveground C. In Africa, WPE generated a low gain in ecosystem C stocks, suggesting that the process is very slow (Coetsee et al., 2013). This variability demonstrates the site- and scale-specific nature of results and the need to measure different components of the soil C pool to assess the net ecosystem effect of WPE on carbon storage. Studies of C stocks across different levels of shrub and tree density are scarce in South American savannas, which represent ∼20% of the global total area occupied by this biome (Olson et al., 2001).
Most studies assessing C stocks for different levels of woody plant density have been measured at relatively fine scales, ranging from hectares to a few square km. To complement such studies, regional assessments are needed to evaluate the effects of WPE at broad scales. In this study, we assessed C stocks along a woody cover gradient in central Argentina (30,000 km2) by assessing woody and non-woody vegetation, litter and soils. Our objective was to examine regional C patterns in order to increase our understanding of WPE consequences in semiarid savannas and to develop models that could be used by managers to estimate C stocks at the local or regional levels.
Section snippets
Study area
The 30,000 km2 study area is located in the northern section of the Caldenal savannas of central Argentina (Fig. 1). The Caldenal is a semiarid savanna ecosystem dominated by the caldén tree (Prosopis caldenia) with an understory of perennial grasses frequently interspersed with dunes, wetlands and lagoons (Cabrera, 1994). The climate is temperate with mean annual temperature of 15 °C; mean monthly temperature of the hottest and coldest month is 24 °C and 8 °C, respectively (Cano et al., 1980).
Results
Closed forests have on average 12.0 kg m−2 additional C than grasslands, 8.5 kg C m−2 more than shrublands and 4.6 kg C m−2 more than open forests, with the difference between forest types not significant (Fig. 2). The increment in total ecosystem C was positively correlated with SOC, tree C, SOH and fine litter (r > 0.6 in all cases). Of the total C stock increment observed along the wood cover gradient, most of the change was due to increases in SOC and tree C. Structurally, grasslands were
Discussion
In this study, the first of its kind for South American semiarid savannahs, we found that closed woodlands have three times more total ecosystem C than grasslands. Given the widespread occurrence of woody plant encroachment in this region and globally, this could have important implications for the global terrestrial C balance. This gain was ∼12 kg C m−2 from grasslands to dense woodlands, similar to values found in encroached areas of central Brazil (Pellegrini et al., 2014) and South Africa (
Conclusion
We assessed the magnitude of the most important ecosystem C stocks along a woody cover gradient in the Caldenal savannas of central Argentina in a effort to better understand the potential implications of woody plant encroachment on ecosystem C storage. We found a significant increase in total ecosystem C stocks along the woody cover gradient, with four times more C in closed forests than in grasslands. In all cases soil organic C was the largest pool, representing over 60% of the total
Acknowledgements
We thank Rodriguez E., Ganora E., Divan S. Frank Buss M.E., Arab F., Brandoni E., Ramirez L., Baumgaertner, C.L., Trelles L., Bianchini F., Gonzalez-Mazzoni F., Luna Toledo L. and Magallanes C. for their assistance during field work. Villarreal D., Hierro J., Grupo de Estudios Ambientales – UNSL, and the Universidad Nacional de La Pampa provided logistical support during field work campaigns. Soil samples were processed at the Jackson Lab at Duke University. We thank J. Kim, Domec J.C and
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