The interaction between canopy resistance, water use, microclimate, and water table depth in a Cottonwood forest

In order to learn more about the magnitude and the controlling variables of ecosystem level fluxes of energy and mass exchange, eddy covariance sensors were installed above a cottonwood forest in a northern Californian watershed and the net ecosystem exchange of Carbon (NEE) and evapotranspiration (ET) were monitored continuously for 1.5 years. The focus of this presentation is on seasonal trends in canopy conductance and evapotranspiration and the effects of forcing variables such as flood duration, water table depth, air temperature, solar radiation and vapor pressure deficit (VPD). Our objectives are: 1) quantify the magnitude of the water fluxes in a riparian cottonwood forest in the Cosumnes River Preserve, 2) describe the daily and seasonal variability of ET and canopy resistance in this ecosystem, and 3) examine the effect of the interactions between local hydrological conditions and other controlling factors of the environment [temperature, VPD and photosynthetic active radiation (PAR)] on the energy exchange in the cottonwood forest. When controlled for changes in VPD, we found that canopy resistance decreases with increasing radiation. Supporting leaf level and stem flow estimates of other workers, our data show clear evidence of increasing canopy resistance with rising VPD. Canopy resistance also increases on seasonal time scales when the water table drops in the late summer. Our results support ET estimates found by others, and our measurements also suggest that growing-season flooding suppresses transpiration in this relatively flood-tolerant species.