An ecohydrological framework for grass displacement by woody plants in savannas

Abstract

During the past several decades, woody plants have been encroaching into grasslands around the world. This transition in plant dominance is often explained as a state shift in bistable ecosystem dynamics induced by fire-vegetation feedbacks. These feedbacks occur when woody plants are able to displace grasses because of their better access to soil water and light. On the other hand, grasses can displace woody plants because of their ability to increase fire frequency and of the higher susceptibility of woody plants to fire-induced mortality. In this study, we present an ecohydrological framework to investigate the displacement of grasses by woody plants. Considering the effect of lateral root spread and of soil water and light limitations, we found that woody plant encroachment can substantially suppress grass production even without the presence of grazers. Bistable dynamics emerge as a result of the grass-fire feedback for a wide range of rainfall conditions, fire susceptibility, and woody plant growth rates.

Global Environmental Change in Drylands: Ecohydrological Controls on Plant Communities

“…To this end, I consider the case with no woody biomass losses from fires (kη = 0) and investigate how the coexistence of woody plants and grasses may occur in dry savannas in the absence of fires (Sankaran et al, 2004(Sankaran et al, , 2005Scheiter and Higgins, 2007;Higgins et al, 2010). In dry savannas I also do not account for light limitation in grass growth because the woody plant cover is relatively low and thus light limitation in grasses is insignificant, in agreement with other studies (Scheiter and Higgins 2007;Higgins et al, 2010;Yu and D'Odorico 2014a). I then examine the disturbance-based mechanism of coexistence between woody plants and grasses in wet savannas (MAP>650 mm), and account for light limitation in grasses and woody biomass loss from fires.…”

“…Following other studies (e.g., Andeires et al, 2002;Van Langevelde et al, 2003;Beckage et al, 2009;Yu and D'Odorico, 2014a), the rate of change of shrub biomass is proportional to the existing shrub biomass, S, and to the resources available for new shrub growth, Smax-S, while fireinduced disturbance kills shrubs at a rate that is proportional to the existing shrub biomass and to fire frequency, , Consistent with other studies (e.g., Andeires et al, 2002;Van Langevelde et al, 2003;Yu and D'Odorico, 2014a), this model does not account for resprouting of shrubs after fires, a trait that is species-specific and is expected to favor State II (e.g., Chidumayo, 2004;Vest et al, 2004;Moreira et al, 2012). Thus, grasses may limit shrub growth only through fire dynamics but not through a preferential access to the available resources.…”

“…The model does not explicitly represent the spatial interactions among trees and grasses (Jeltsch et al, 1996;van Wijk and Rodriguez-Iturbe, 2002) and thus uses a lumped approach to simulate dynamics of trees and grasses. Since model 2 is adapted from model 1, in this chapter details of model 1 including soil water balance and vegetation dynamics are provided while the reader is referred to Yu and D'Odorico (in press) for details of the model 2. where the subscripts 1 and 2 refer to the shallow and deep soil layer, respectively; n is the soil porosity, Z 1 and Z 2 the soil layer thickness (mm), S 1 and S 2 the relative soil moisture (0< S 1 , (Yu and D'Odorico, 2014a). In general, the transpiration model should account also for seasonal changes in plant hydraulic conductivity; this effect, however, is here not accounted for because I prefer to limit the complexity of the model.…”

“…To this end, I use Beer's law to calculate solar irradiance under woody plant canopies as a function of the leaf area index (LAI ̅̅̅̅̅ w , m 2 m −2 ) of woody plants (Scheiter and Higgins, 2007;Yu and D'Odorico, 2014a). I then account for the impact of reduced light availability (under woody plant canopies) on grass growth through a light limitation coefficient (l) calculated as the ratio of actual to light saturation net photosynthesis rate, based on leaf-level physiological measurements (O'Halloran, 2007).…”

Woody plant encroachment into grasslands leads to accelerated erosion of previously stable organic carbon from dryland soils

“…Observations from the grass-woody vegetation transitions occurring in drylands across the southwestern U.S. suggest that the competitive ability of grass species is being reduced, allowing woody species to encroach and that ecosystems are adjusting to a change in current or historic controls on vegetation patterns [Archer, 1995]. Cited drivers of woody encroachment include climate [Seager et al, 2007], atmospheric CO 2 [Higgins and Scheiter, 2012], fire [Yu and D'Odorico, 2014], and animal activity, particularly grazing [Van Auken, 2009], and it is generally recognized that encroachment results from a combination of both climatic and land use drivers [Peters et al, 2013].…”