Evapotranspiration from a Mediterranean evergreen oak savannah: The role of trees and pasture

Abstract

Mediterranean evergreen oak woodlands of southern Portugal (montados) are savannah-type ecosystems with a widely sparse tree cover, over extensive grassland. Therefore, ecosystem water fluxes derive from two quite differentiated sources: the trees and the pasture. Partitioning of fluxes according to these different sources is necessary to quantify overall ecosystem water losses as well as to improve knowledge on its functional behaviour. In southern Iberia, these woodlands are subjected to recurrent droughts. Therefore, reaction/resilience to water stress becomes an essential feature of vegetation on these ecosystems. Long-term tree transpiration was recorded for 6 years from a sample of holm oak (Quercus ilex ssp. rotundifolia) trees, using the Granier sap flow method. Ecosystem transpiration was measured by the eddy covariance technique for an 11-month period (February to December 2005), partly coincident with a drought year. Pasture transpiration was estimated as the difference between ecosystem (eddy covariance) and tree (sap flow) transpiration. Pasture transpiration stopped during the summer, when the surface soil dried up. In the other seasons, pasture transpiration showed a strong dependence on rainfall occurrence and on top soil water. Conversely, trees were able to maintain transpiration throughout the summer due to the deep root access to groundwater. Q. ilex trees showed a high resilience to both seasonal and annual drought. Tree transpiration represented more than half of ecosystem transpiration, in spite of the low tree density (30 trees ha À1) and crown cover fraction (21%). Tree evapotranspiration was dominated by transpiration (76%), and interception loss represented only 24% of overall tree evaporation.

Do ecological and silvicultural factors influence acorn mass in Holm Oak (southwestern Spain)?

“…In the CA location, the presence of graywackes (a hard bed rock) and slates probably limits the development and number of deep tap roots that can access the local groundwater reservoir. Paço et al (2009), also working in a dehesa, found a high resilience of Holm Oaks to drought. The cited authors found that the decline in transpiration during very dry years (2004 and 2005 in their study area) was only 16%, much smaller than the decline in annual rainfall.…”

Simple physics-based models of compensatory plant water uptake: concepts and eco-hydrological consequences

“…Indeed, water uptake from deep subsoil has been shown to be critical in meeting atmospheric transpiration demand in water-limited environments (e.g. Jipp et al, 1998;Oliveira et al, 2005), especially where roots can reach shallow groundwater (Zhang et al, 1999;Zencich et al, 2002;Lamontagne et al, 2005;Cleverly et al, 2006;Gazal et al, 2006;Paço et al, 2009).…”

Integration of hydrogeophysics and remote sensing with coupled hydrological models

“…That increase took place despite lowering of water table, which confirmed that water table depth had no or negligible influence on T g and that the trees are adapted to tap water from any accessible reservoir in order to satisfy the transpiration requirements. These results are in agreement with several other studies such as David et al (2004);Paço et al (2009);Miller et al (2010) that independently found that oak trees in savanna environment could compensate low soil moisture in dry season by taping groundwater at depth of 8 and 13 m. These authors also found that groundwater uptake was thermodynamically favorable over soil water uptake during dry seasons and that such behavior was compatible with the tree physiology. However, as the transpiration rates were below the atmospheric water demand, groundwater uptake constitutes a tree strategy to subsist during dry season and not a mechanism for continued plant growth.…”