Influence of environmental variables on leaf area index in loblolly pine plantations

Abstract

The productivity of even-aged forest stands varies from one year to the next as a function of canopy size and its interaction with the effective radiation used for photosynthesis. To characterize this relation, ecologists use leaf area index (LAI), a metric that serves as an indicator of the photosynthetic capacity on a given site. In this research, we proposed a model describing leaf area index dynamics in loblolly pine plantations growing in the southeastern United States. The model implements a delayed differential equation using periodic coefficients that enforce the seasonality in resource availability. The equation was further expanded to accommodate climatic variables to evaluate their contribution in reducing the observed variability. The proposed model uses environmental modifiers to account for the changes in resource availability and to adjust the carrying capacity in forest stands. From a range of tested variables, we found monthly maximum temperature and monthly excess water to be the most influential on leaf area index dynamics. With the addition of environmental modifiers and a local carrying capacity parameter, root mean square error was reduced to 0.3802 units LAI (m2/m2) from a base model RMSE of 0.4427 units LAI (m2/m2). The results indicate that the delay component has a small 41-day effect in the model, which is contrary to our initial hypothesis that stored within-tree carbohydrates can be used for further seasons to build foliage.