Soil and xylem water potential and soil water content in contrasting Pinus contorta ecosystems, Southeastern Wyoming, USA.

The relationships between volumetric soil water content (ϕ), in situ soil water potential (Ίsoil) and predawn xylem pressure potential (Ίpredawn) were quantified in four contrasting lodgepole pine ecosystems in Wyoming, USA. On three of the sites, changes in Ίsoil correlated closely with Ίpredawn, but on a porous soil derived from coarse granitic parent material, Ίpredawn declines occurred much sooner than corresponding declines in Ίsoil, possibly because of local depletion of rhizosphere moisture and low molecular diffusivity of water in that soil. Exptrapolation of laboratory-derived characteristic curves for soil moisture to field conditions yielded different relationships between ϕ and Ίsoil than curves derived from in situ measurements, probably because of disruption of soil structure and porosity during sample collection and handling in laboratory studies. Although a close correlation between ϕ and Ίpredawn was observed, future efforts at modelling the soil-plant-atmosphere continuum should be directed towards a more detailed understanding of the complex relationships between Ίsoil at varying depths and plant water stress.