Description
Tree and shrub encroachment and proliferation in global grasslands and savannas is widely considered to imply a trade-off between ecosystem C and H2O regulating functions, based on the premise that increased C sequestration associated with higher woody biomass ostensibly drive declines in water production through concomitant increases in evapotranspiration (ET), but evidence for this across climatic and environmental gradients is equivocal. To evaluate these claims, we ran a paired eddy covariance experiment in a native semi-arid C4 grassland and adjacent encroaching Vachellia karroo woodland on the eastern seaboard of South Africa and compared ecosystem C and H2O budgets over nearly a full year. Near complete failure of spring/early summer rains resulted in reduced ecosystem physiological activity and C uptake in the early growing season in 2019, but systems recovered following above average rainfall over mid-summer/early autumn in 2020, with both the grassland and woodland achieving net negative C balances. The woodland was more productive than the grassland over the majority of the year, but these marginal C gains were offset by consistently higher respiration effluxes, resulting in the grassland sequestering significantly more C (197. 6 g C m-1) than the woodland (114.5 g C m-1). Differences in water use between the two systems were insignificant, and is likely explained by shallow soils and the absence of subsurface water at the site, which largely negated the competitive advantage in terms of access to water conferred by deeper rooting systems of V. karroo. Ecosystem water use efficiencies (WUEE) were essentially identical across the year, but the grassland was more efficient at the peak of the growing season. Our data suggest that encroachment by V. karroo in semi-arid grasslands may result in significant declines in C sequestration despite higher rates of productivity, but is unlikely to lead to substantial increases in water use where trees do not have access to water in deeper soil horizons on the other. These results support growing arguments to preserve semi-arid grassland ecosystems even when C sequestration is the primary objective of land management interventions.
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Differences in Carbon Sequestration and Water Use Between a Semi-Arid Native Grassland and Encroaching Vachellia karroo Woodland
Tree and shrub encroachment and proliferation in global grasslands and savannas is widely considered to imply a trade-off between ecosystem C and H2O regulating functions, based on the premise that increased C sequestration associated with higher woody biomass ostensibly drive declines in water production through concomitant increases in evapotranspiration (ET), but evidence for this across climatic and environmental gradients is equivocal. To evaluate these claims, we ran a paired eddy covariance experiment in a native semi-arid C4 grassland and adjacent encroaching Vachellia karroo woodland on the eastern seaboard of South Africa and compared ecosystem C and H2O budgets over nearly a full year. Near complete failure of spring/early summer rains resulted in reduced ecosystem physiological activity and C uptake in the early growing season in 2019, but systems recovered following above average rainfall over mid-summer/early autumn in 2020, with both the grassland and woodland achieving net negative C balances. The woodland was more productive than the grassland over the majority of the year, but these marginal C gains were offset by consistently higher respiration effluxes, resulting in the grassland sequestering significantly more C (197. 6 g C m-1) than the woodland (114.5 g C m-1). Differences in water use between the two systems were insignificant, and is likely explained by shallow soils and the absence of subsurface water at the site, which largely negated the competitive advantage in terms of access to water conferred by deeper rooting systems of V. karroo. Ecosystem water use efficiencies (WUEE) were essentially identical across the year, but the grassland was more efficient at the peak of the growing season. Our data suggest that encroachment by V. karroo in semi-arid grasslands may result in significant declines in C sequestration despite higher rates of productivity, but is unlikely to lead to substantial increases in water use where trees do not have access to water in deeper soil horizons on the other. These results support growing arguments to preserve semi-arid grassland ecosystems even when C sequestration is the primary objective of land management interventions.
