Stomata are the common pathways through which diffusion of CO2 and water vapor take place in a plant. According to the stomatal optimality hypothesis, plants regulate stomatal behavior to maximize carbon assimilation for a given water availability. The carbon cost for unit water loss, marginal water use efficiency (λ), is often described as a function of soil water availability and atmospheric CO2 concentration. Because parameters of the functions in previous studies were not consistent and varied by plant functional type, parameter estimation for individual species and assessment of the sensitivity of tree water use to the parameters are needed.
We measured sap flux density of loblolly pine (Pinus taeda) and sweetgum (Liquidambar styraciflua) with Granier-type heat dissipation sensor at the Duke Forest free-air CO2 enrichment (FACE) site. Water use of P. taeda and L. styraciflua in ambient and elevated (+200 µmol mol-1) CO2 concentration was modeled using Ecosystem Demography model 2 (ED2), a demographic terrestrial biosphere model that scales up individual-level competition for light, water and nutrients to the ecosystem-level.
Results/Conclusions
While the diurnal patterns of the modeled plot-average sap flux density generally agreed with observed values, the magnitudes varied widely depending on a parameter for λ with no water stress (λ0). When λ0 increased from 5 to 11 μmol mol-1 kPa-1 and the sensitivity of λ to soil water (β0) was fixed to -1.15 MPa-1, estimated daytime sapflow during growing season decreased by 33.9% in ambient CO2 and 31.4% in elevated CO2 for P. taeda and by 51.1 and 43.8% for L. styraciflua. Model results with fixed λ0 and β0 did not indicate interannual variations of sap flux density; the model underestimated sap flux density in dry years. Modeled sap flux density was not able to explain individual tree-level variations from observation. Further analyses regarding the effects of vertical hydraulic conductivity and leaf and stem capacitance are needed for better prediction.