Hydraulic lift as a determinant of tree–grass coexistence on savannas

Abstract

SummaryThe coexistence of woody plants and grasses in savannas is determined by a complex set of interacting factors that determine access to resources and demographic dynamics, under the control of external drivers and vegetation feedbacks with the physical environment. Existing theories explain coexistence mainly as an effect of competitive relations and/or disturbances. However, theoretical studies on the way facilitative interactions resulting from hydraulic lift affect tree-grass coexistence and the range of environmental conditions in which savannas are stable are still lacking.We investigated the role of hydraulic lift in the stability of tree-grass coexistence in savannas. To that end, we developed a new mechanistic model that accounts for both competition for soil water in the shallow soil and fire-induced disturbance.We found that hydraulic lift favors grasses, which scavenge the water lifted by woody plants. Thus, hydraulic lift expands (at the expenses of woodlands) the range of environmental conditions in which savannas are stable.These results indicate that hydraulic lift can be an important mechanism responsible for the coexistence of woody plants and grasses in savannas. Grass facilitation by trees through the process of hydraulic lift could allow savannas to persist stably in mesic regions that would otherwise exhibit a forest cover.

Long‐term impacts of shrub plantations in a desert–oasis ecotone: Accumulation of soil nutrients, salinity, and development of herbaceour layer

“…The litter of H. ammodendron was annually decomposed and thus may also increase the ions in the shallow soil. Moreover, when the ions that accumulated in the roots exceed a certain amount, ions may be ‘left behind’ at shallow soil depths, then rise to the top soil via capillary transport (exfiltration) of soil moisture, and eventually accumulate at the soil surface as water evaporates; these processes are facilitated in arid regions by the small size and frequency of rainfall events, high rates of evaporation, hydraulic lift, and large soil temperature gradients between day and night (Armas, Padilla, Pugnaire, & Jackson, ; Y. Li, Wang, Wang, Zhang, & Lai, ; Yu & D'Odorico, ; Yu & Foster, ). Indeed, the increase in fraction of silt and clay reduced hydraulic conductivity and thus may significantly reduce leaching of salts through the soil column (Jackson et al, 2005).…”

Contrasting effects of two Acacia species on understorey growth in a drylands environment: interplay of canopy shading and litter interference

“…Finally, several mechanisms have been proposed to explain the facilitative influence of trees on understorey grasses (Sankaran et al. ), including hydraulic lift by woody plants (Yu & D'Odorico ). Although we did not have the means to test hydraulic lift in this study, the specific pattern of growth of the herbaceous vegetation beneath the canopy suggests a simpler mechanism related to canopy shading.…”

“…; Muhamed et al. ; Yu & D'Odorico ). In savannas, these interactions may result in different tree–grass relations, which are important determinants of the savanna, with implications for the provision of essential ecosystem services (Sala & Maestre ).…”

Influences of Root Hydraulic Redistribution on N2O Emissions at AmeriFlux Sites

“…There are two primary approaches to HR modeling: the scheme proposed by Ryel et al (2002) (e.g., Fu et al, 2016;Li et al, 2012;Wang, 2011) or its variations (e.g., Lee et al, 2005;Yu & D'Odorico, 2015), and the approach proposed by Amenu and Kumar (2008) (e.g., Luo et al, 2013;Quijano & Kumar, 2015;Tang et al, 2015). Ryel et al's scheme is an empirical method that describes HR flux based on the soil water potential (Wang, 2011;Zheng & Wang, 2007) and CLM4.5 (Fu et al, 2016).…”