Ecohydrology and woody plant encroachment: A conceptual framework for evaluating landscape consequences

TitleEcohydrology and woody plant encroachment: A conceptual framework for evaluating landscape consequences
Publication TypeConference Paper
Year of Publication2003
AuthorsHuxman T.E., Snyder K.A., Wilcox B.P., Scott R.L., Breshears D.D., Small E.E., Jackson R.B., Pockman W.T., Hultine K.R.
Conference Name2003 Fall Meeting, American Geophysical Union
Date PublishedDecember 8, 2003
Conference LocationSan Francisco, CA
ARIS Log Number159658
AbstractIncreases in the abundance or density of woody plants in historically semiarid and arid grassland ecosystems have important implications for hydrology, ecology, and society. Using a simplified water-balance model, we propose a framework for conceptualizing how woody plant encroachment is likely to affect components of the water cycle within these ecosystems. In these systems, shrub removal has been proposed as a mechanism for increasing watershed yield, but such increases are likely only in systems with shallow bedrock, riparian systems, or climatic systems where precipitation exceeds potential evapotranspiration for extended periods. In semiarid and arid systems abiotic controls on evaporation overwhelm biotic control of streamflow. In these systems, the predominant effects of woody plant encroachment that promote ecohydrological feedbacks are changes in: infiltration, depth of plant water removal, generation of transient overland flow and near-ground energy budgets that affect the ratio of evaporation (E) to transpiration (T). In these more xeric areas, the ratio of E:T is critical because it indicates the biological use of water which influences landscape structure. Increasing woody plant cover is associated with larger open interspaces; at the landscape scale the differential contribution of canopy versus interspaces may change landscape E:T. Alterations of spatial structure and ecohydrology likely influence other biogeochemical processes, such as carbon cycling. At the landscape scale, total respiration depends on the magnitude of canopy/interspace respiration flux scaled by the spatial extent of interspaces and canopy space. Canopy respiration is strongly correlated to T, while interspace respiration is largely driven by soil water availability and, therefore, is driven by factors that control E. Resolving the partitioning of E:T should help resolve the net effect of woody plant encroachment on other biogeochemical cycles at the landscape scale. This framework for considering the effects of woody plant encroachment highlights important ecological and hydrological interactions that serve as a basis for predicting other ecological aspects of vegetation change.