MEASURING INSTANTANEOUS EVAPOTRANSPIRATION RATES OF A FOREST UNDERSTORY POST-DISTURBANCE USING A PORTABLE CHAMBER

A significant proportion of precipitation falling on natural watersheds is returned to the atmosphere via plant transpiration and evaporation from the surface of leaves and soil. In forested watersheds where the canopy has been disturbed, evapotranspiration (ET) by the understory is the main flux of water back to the atmosphere. The ability to take field measurements of water use by understory plants, therefore, is vital for a complete ecosystem water budget. However, little research has been apportioned to measuring understory water use, and the technology to do so is thus limited. Many field methods involve large, continuous ET chambers which are hard to transport or are installed as permanent structures. Thus, a portable rapid chamber which can be easily deployed and collect quick ET measurements of single plants was developed for measuring understory plant water use. Previous studies suggest that chambers of this design may perform better than a continuous chamber and similar in accuracy to a weighing lysimeter for estimating ET. The system was used for measuring ET rates of understory plants in three disturbed watersheds (75%, 55%, and 35% basal area reduction) and an unharvested watershed in a California coast redwood forest. A one-way ANOVA displayed significant differences in log transformed ET rates by watershed (p = 0.00776) at  = 0.05. The watershed with the highest harvest intensity had the highest average ET rate and was significantly different from the moderately harvested, lightly harvested, and control watersheds. We present the case study and results from the methodology of this study, which may be adopted to quantify ET for shrubs, crops, or grasses at the plant or landscape scale.