Soil salinity levels are an important determinant of plant evapotranspiration and carbon uptake. In this dissertation I develop, evaluate, and test a model of plant evapotranspiration and carbon uptake in the context of a saline soil environment, and drive the model using leaf physiological parameters determined from field measurements. This modeling work is performed in the context of three research questions: (1) How are leaf gas exchange parameters characterizing photosynthesis in perennial pepperweed best determined for seasonal scale landscape flux analysis? (2) How can the effects of soil salinity on root water uptake be represented in order to account for changes in the diurnal cycle and in the uptake of carbon dioxide by plants? (3) How sensitive are modeled results to changes in model input parameters, and how may these sensitivities limit the predictive abilities of the model? These questions are assessed using data from a relatively wet pasture-peatlands in the San Francisco Bay - Sacrament River Delta region of California, with the dominant land-cover species perennial pepperweed (Lepidium latifolium), a mildly salt-tolerant and invasive herbaceous weed.

Presented in this research is a characterization of pepperweed as a highly capable invasive species, able to take advantage of local resources such as light, carbon, water, and nitrogen. Modeling results from each section also demonstrate its ability to photosynthesize under higher temperatures and vapor pressure deficits than standard plant models suggest. Incorporating soil salinity into a whole-plant model increases the ability to describe how different soil and atmospheric parameters influence evapotranspiration and photosynthesis in such an environment. The model's sensitivity analysis reveals two pairs of parameters that may constrain each other, and demonstrates how improved measurements of plant conductance and leaf water potential can constrain other portions of the parameter space.