Estimation of leaf area index and simulation of evapotranspiration for intensively managed Douglas-fir forests

Graduation date: 2014Understanding the tradeoff between water use and productivity is critical for modeling growth of intensively managed Douglas-fir forests in the Pacific Northwest. Evapotranspiration is closely linked to carbon dioxide intake during the process of photosynthesis. However, summer drought characterizing the growing season in this region imposes a limit on carbon dioxide intake due to plant responses that limit water loss to reduce potential for cavitation. Therefore, understanding or predicting the rate of water use and the effect of soil water potential and vapor pressure deficits on foliar exchange of both H₂O and CO₂ is important for simulating the net primary production of a given forest site. This project explores methods for estimating daily and seasonal evapotranspiration, compares estimates of evapotranspiration to soil water drawn down, and tests the relationship between water use and productivity. A frequently used equation for simulation of forest evapotranspiration is the Penman-Monteith equation. Many forms of this equation can be found throughout the literature, covering a wide range of complexity. The performance of this equation depends on the accuracy of estimating its individual components, for example, Leaf Area Index (LAI). LAI is a key parameter of evapotranspiration equations because this index accounts for the surface area over which evapotranspiration occurs. Three common methods of LAI estimation were compared to determine the most accurate value for simulating evapotranspiration. Methods explored included, LAI estimation through measure of light attenuation, sapwood area allometrics, and estimation from foliage mass measurements. Methods were employed across Douglas-fir (Pseudotsuga menzsii) stands in the Oregon Coast Ranges representing a range in structural characteristics, due to management and age. To best predict LAI in stands with structural and management variability, estimates of LAI from foliage mass were determined to be most appropriate. LAI from light attenuation consistently under predicted LAI, and estimates of LAI from sapwood allometrics were unable to capture appropriate estimates from stands with an LAI greater than eight. Utilizing estimated LAI, seasonal and daily evapotranspiration was determined for the summer growing season of 2012. Evapotranspiration values were validated through comparison to soil water loss (m³/m³) measured throughout the growing season. Variability in stand and soil structural properties were thought to contribute to the range in measured soil water loss at both a daily and seasonal scale. Cumulative water loss over the growing season ranged from 0.0635 m³/m³ to 0.2706 m³/m³. Variability in evapotranspiration calculated from a simple Penman-Montieth equation was also seen at each plot at both the daily and seasonal resolution. Cumulative evapotranspiration calculated at each study plot ranged from 0.2 to 1.0 m³/m³. A plot level comparison of calculated evapotranspiration and soil moisture at both daily and seasonal scales showed that simple measures of soil water loss cannot currently be used to validate evapotranspiration