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Aim Movement of water from the land surface to the atmosphere (evapotranspiration, ET) is the dominant output flux in the global terrestrial surface water budget. The partitioning of ET between soil evaporation ( E) and plant transpiration ( T) couples important ecological, hydrological and atmospheric processes. ET partitioning has been hypothesized to vary as a function of woody plant cover, yet a relationship between ET partitioning and woody cover has not been quantified empirically. Land surface models assume unit increase in T per unit increase in vegetation cover (woody cover), following a proportional linear relationship. Recent assessments have questioned the validity of this assumption for heterogeneous canopies, but we lack experimental data across an explicitly defined gradient of woody cover to characterize this relationship. Location North American monsoon region. Methods In a controlled dryland environment experimental facility, we manipulated woody cover and documented the response of ET and its component fluxes. We incorporated the resulting functions into a widely used coupled land-atmosphere model ( WRF- Noah) to document the implications of modifying specific model parameters that assume (1:1) proportionality. Results As total ET increased with woody cover, T/ET deviated below 1:1 proportionality. Using our experimentally determined relationship for ET partitioning and woody cover in the model, we observed reductions in ET of as much as 40% during the monsoon season and annual increases of almost 200% in regional E. Main conclusions Our results highlight a limitation of modelled ET that affects regional to global patterns of water flux, with implications for a number of earth surface processes. A better understanding of how changing woody cover influences patch-scale ecohydrological processes is needed, particularly under current changes in woody cover associated with deforestation, afforestation and drought-induced mortality. More specifically, improved representation of E and T fluxes will improve understanding and modelling of large-scale ecological, hydrological and atmospheric processes. [ABSTRACT FROM AUTHOR]

Abstract: Soil evaporation, a critical ecohydrological process in drylands, can exhibit substantial spatio-temporal variation. Spatially, ecohydrological controls of soil evaporation may generally depend on a hierarchical structure spanning from the presence or absence of litter, through canopy patches of woody plants and intercanopy patches separating them, up to the overall vegetation mosaic characterized by density of woody plant cover in the landscape, although assessment of these factors in concert is generally lacking. Temporally, ecohydrological controls can be further complicated by not only seasonal climate, but also phenology, particularly in seasonally deciduous drylands. We experimentally assessed the interactive controls on soil evaporation along a gradient of mesquite cover (Prosopis velutina) within the North American monsoon region, with respect to such hierarchical structure and seasonality/phenology. Our results indicate that presence of litter exerts a dominant control on soil evaporation, independent of seasonality; in absence of litter, both patch and mosaic attributes influence soil evaporation variably with season/phenology. Correlations from related measures of incoming energy suggest energy limits evaporation in many cases, although other factors such as wind may potentially influence hierarchical and seasonal/phenological combinations. Our results highlight the need to account for both hierarchical vegetation structure and seasonal/phenological variability to improve ecohydrological predictions of soil evaporation. [Copyright &y& Elsevier]

Abstract: Soil carbon often varies significantly among vegetation patch types, but less known is how the size and species of plants in the tree canopy patches and the cover types of the intercanopy patches affect the carbon storage, and whether vegetation characteristics affect storage in adjacent patches. To assess this, we measured fine-fraction soil carbon in a semiarid woodland in New Mexico USA for canopy patches of two co-dominant woody species, Pinus edulis and Juniperus monosperma that were paired with intercanopy patch locations covered by herbaceous grass (Bouteloua gracilis) or bare ground. Soil carbon at shallow depths was greater in canopy than intercanopy patches by a factor of 2 or more, whereas within intercanopy patches soil carbon in grass locations exceeded that in bare locations only after accounting for coarse-fraction carbon. Hypothesized differences among canopy patches associated with species or size were not detected (although some size-depth interactions consistent with expectations were detected), nor, importantly, were effects of species or size of woody plant on intercanopy soil carbon. The results are notable because where applicable they justify estimates of soil carbon inventories based on readily observable heterogeneity in above-ground plant cover without considering the size and species of the woody plants. [Copyright &y& Elsevier]

A large proportion of rainfall in dryland ecosystems is intercepted by plant foliage and is generally assumed to evaporate to the atmosphere or drip onto the soil surface without being absorbed. We demonstrate foliar absorption of intercepted rainfall in a widely distributed, continental dryland, woody-plant genus: Juniperus. We observed substantial improvement in plant water status, exceeding 1.0 MPa water potential for drought-stressed plants, following precipitation on an experimental plot that excluded soil water infiltration. Experiments that wetted shoots with unlabeled and with isotopically labeled water confirmed that water potential responded substantially to foliar wetting, that these responses were not attributable to re-equilibration with other portions of the xylem, and that magnitude of response increased with water stress. Foliar absorption is not included in most ecological, hydrological, and atmospheric models; has implications for interpreting plant isotopic signatures; and not only supplements water acquisition associated with increases in soil moisture that follow large or repeated precipitation events, but also enables plants to bypass soil water uptake and benefit from the majority of precipitation events, which wet foliage but do not increase soil moisture substantially. Foliar absorption of intercepted water could be more important than previously appreciated, especially during drought when water stress is greatest. [ABSTRACT FROM AUTHOR]

Much of the terrestrial biosphere can be viewed as part of a gradient, with varying amounts of woody plant cover ranging from grassland to forest--the grassland-forest continuum. Woody plant cover directly impacts the soil microclimate through modifications of near-ground solar radiation and soil temperature, and these interactive effects are relevant for key ecohydrological processes such as soil evaporation. Trends in how increasing woody plant cover affect soil surface microclimate have recently been evaluated for gradients of evergreen woody plants, but analogous trends for deciduous plants, where phenology should be influential, are lacking. We evaluated season-dependent changes in soil microclimate along a deciduous grassland-forest continuum of velvet mesquite (Prosopis velutina Wooton) using repeated hemispherical photography and continuous soil temperature measurements at the 5-cm depth. Both near-ground solar radiation and soil temperature decreased with increasing canopy cover, even during the leafless season. The trends varied substantially among seasons, however, with differences between canopy and intercanopy patches readily evident only during the period of full leaf-out, during which the correlation between near-ground solar radiation and soil temperature was strongest. Our results provide a more comprehensive understanding about the interactions of canopy cover, canopy structure attributes, and plant phenology that produce seasonally pulsed heterogeneity in the soil surface microclimate. Notably, our results add a new dimension to the moisture "pulse dynamics" perspective commonly applied to dryland ecohydrology, highlighting seasonally pulsed heterogeneity in soil microclimate that could influence soil moisture dynamics in drylands. [ABSTRACT FROM AUTHOR]