How plant functional-type, weather, seasonal drought, and soil physical properties alter water and energy fluxes of an oak–grass savanna and an annual grassland

Abstract

Savannas and open grasslands often co-exist in semi-arid regions. Questions that remain unanswered and are of interest to biometeorologists include: how do these contrasting landscapes affect the exchanges of energy on seasonal and annual time scales; and, do biophysical constraints imposed by water supply and water demand affect whether the land is occupied by open grasslands or savanna? To address these questions, and others, we examine how a number of abiotic, biotic and edaphic factors modulate water and energy flux densities over an oak-grass savanna and an annual grassland that coexist in the same climate but on soils with different hydraulic properties.The net radiation balance was greater over the oak woodland than the grassland, despite the fact that both canopies received similar sums of incoming short and long wave radiation. The lower albedo and lower radiative surface temperature of the transpiring woodland caused it to intercept and retain more long and shortwave energy over the course of the year, and particularly during the summer dry period.The partitioning of available energy into sensible and latent heat exchanged over the two canopies differed markedly. The annual sum of sensible heat exchange over the woodland was 40% greater than that over the grassland (2.05 GJ m −2 per year versus 1.46 GJ m −2 per year). With regards to evaporation, the oak woodland evaporated about 380 mm of water per year and the grassland evaporated about 300 mm per year. Differences in available energy, canopy roughness, the timing of physiological functioning, water holding capacity of the soil and rooting depth of the vegetation explained the observed differences in sensible and latent heat exchange of the contrasting vegetation surfaces.The response of canopy evaporation to diminishing soil moisture was quantified by comparing normalized evaporation rates (in terms of equilibrium evaporation) with soil water potential and volumetric water content measurements. When soil moisture was ample normalized values of latent heat flux density were greater for the grassland (1.1-1.2) than for the oak savanna (0.7-0.8) and independent of moisture content. Normalized rates of evaporation over the grassland declined as volumetric water content dropped below 0.15 m 3 m −3 , which corresponded with a soil water potential of −1.5 MPa. The grassland senesced and quit transpiring when the volumetric water content of the soil dropped below −2.0 MPa. The oak trees, on the other hand, were able to transpire, albeit at low rates, under very dry soil conditions (soil water potentials below −4.0 MPa). The trees were able to endure such low water potentials and maintain basal levels of metabolism because * Corresponding author. Baldocchi et al. / Agricultural and Forest Meteorology 123 (2004) ecological forcings kept the tree density and leaf area index of the woodland low, physiological factors forced the stomata to close progressively and the trees were able to tap deeper water sources (below 0.6 m) than the grasses.

Comparison of four EVI-based models for estimating gross primary production of maize and soybean croplands and tallgrass prairie under severe drought

“…The use of LSWI is convenient as the data are available from MODIS and Landsat. All the comparisons conducted in this study are based on the limited flux towers of crops and grasslands, and both plant types are herbaceous which have lower tolerance capability to drought, compared to woody forests (Baldocchi, Xu, & Kiang, 2004). Therefore, further evaluation of LSWI performance in drought conditions and its improvement of the waterrelated downscaling regulation parameter (W scalar ) are still needed for other ecosystem types (e.g., forest) in the future study.…”

Mountain ecohydrology: quantifying the role of vegetation in the water balance of montane catchments

“…Although an explicit focus on ecohydrological interactions is fairly new, the recent advances in this interdisciplinary field has been built on the longer-term research trends in ecology, hydrology, biogeochemistry and geomorphology, which have focused on the interactions between vegetation and the water cycle as primary controls on ecosystem structure. The result of interactions between biologically mediated cycling of water, energy and biogeochemical species is readily apparent as emergent vegetation patterns in the landscape (van Wijk and Rodriguez-Iturbe, 2002;Rietkerk et al, 2004) and biogeochemical fluxes (McClain et al, 2003;Baldocchi et al, 2004). Unlike physical features of the landscape that affect water 188 P. D. BROOKS AND E. R. VIVONI…”

Soil moisture and vegetation controls on evapotranspiration in a heterogeneous Mediterranean ecosystem on Sardinia, Italy

“…Despite the attention these ecosystems are receiving, a general lack of knowledge persists about the relationship between ET and the plant survival strategies for the different PFTs under water stress [Baldocchi et al, 2004;Kurc and Small, 2004]. With this as motivation, we explore the issue of estimating ET and its relationship with soil moisture, q, for the typical PFTs of an heterogeneous water-limited Mediterranean ecosystem.…”

“…[4] These Mediterranean ecosystems are commonly heterogeneous savanna-like ecosystems, with contrasting plant functional types (PFTs, e.g., grass, shrubs and trees) competing for the water use [Scholes and Archer, 1997;Ramirez-Sanz et al, 2000;Jackson et al, 2002;Baldocchi et al, 2004;Fernandez et al, 2004;Williams and Albertson, 2004]. Despite the attention these ecosystems are receiving, a general lack of knowledge persists about the relationship between ET and the plant survival strategies for the different PFTs under water stress [Baldocchi et al, 2004;Kurc and Small, 2004].…”

“…In semiarid regions evapotranspiration (ET) is the leading loss term of the root zone water budget with a yearly magnitude that may be roughly equal to the precipitation [Reynolds et al, 2000;Rodriguez-Iturbe, 2000;Baldocchi et al, 2004;Kurc and Small, 2004;Maselli et al, 2004;Williams and Albertson, 2004].…”