An ecohydrological framework for determining the landscape effects of woody plant encroachment

Title	An ecohydrological framework for determining the landscape effects of woody plant encroachment
Publication Type	Conference Paper
Year of Publication	2003
Authors	Snyder K.A, Huxman T., Breshears DD, Wilcox B.P., Small E.E., Scott R.L., Jackson R.B, Hultine K.R.
Conference Name	88th Annual Meeting, Ecological Society of America
Date Published	August 3-8, 2003
Conference Location	Savannah, GA
ARIS Log Number	153300
Abstract	Woody plant encroachment into semiarid and arid systems is a global phenomenon with large potential hydrological and biogeochemical consequences. We considered systems in the southwestern United States (pinyon-juniper, mesquite and creosote) where dramatic changes in physiognomy create potential feedbacks between woody plants and hydrology at the landscape scale. The outcome is a new conceptual framework useful in directing future research and determining broad-scale potential ecological and biogeochemical consequences across a range of ecosystems. Shrub removal has been proposed as a mechanism for increasing water yields, but increased water yields are likely only in systems with shallow bedrock, riparian systems, or mesic systems where precipitation exceeds potential evapotranspiration. 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 (through feed-forward or feed-back effects) 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. The alterations of spatial structure and ecohydrology likely influence 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 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.