Vegetation-hydrology response to environmental change: Computational tools for scaling forest water use

Integrated models of the linkages between vegetation, soil, terrain features, and runoff demand a general theory for making them scalable from small field studies to global extent. Attempts to formulate a general theory of vegetation-hydrology relations have produced two apparently opposing concepts of optimality and equilibrium. Optimality suggests that vegetation maximizes water consumption; equilibrium theory suggests a more conservative water use. These theories were explored using a distributed vegetation-hydrology model in conjunction with thermal remote sensing in a semi-arid, conifer-covered watershed. High atmospheric vapor pressure deficit (VPD) inhibited leaf stomatal opening and prevented excessive transpiration rates. This conservative water use supports equilibrium theory. It was hypothesized that VPD-limitation would over time limit canopy density, which would support optimality. The climatic conditions were modified to reduce stomatal regulation and promote a more optimal water use as vegetation growth was simulated. The results showed that present-day vegetation would not be supported on drier sites, but greater canopy density could be supported in wetter sites. Such evidence of the existence of a general theory of vegetation-hydrology equilibrium suggests that simple and robust computational tools for scaling up investigations of forested water use.