Water Use of an Intensively Managed Loblolly Pine Plantation: Implications of Rapid Tree Growth on Stand Evapotranspiration and its Components

Abstract

Increasing demand for plant-derived bioenergy is projected to expand tree plantations with intensive silviculture and improved tree varieties. A criticism regarding these plantations is their large water use to support fast growth and high productivity. However, use of improved varieties and high fertilizer and herbicide inputs will also lead to faster stand development, faster canopy closure, and changes in stand structure that can significantly influence water dynamics. Here, we studied the evapotranspiration (ET) of a young intensively managed loblolly pine stand and investigated the components of ET to determine the contribution of each to overall water use. We also compared ET with similar plantations receiving less intensive management to determine if our study stand used more water. We used the eddy covariance method to estimate ecosystem-level total ET (ETEC), while plot-level estimates of ET (ETP) were obtained via soil lysimeters, sap flow sensors and throughfall collectors, enabling measurement of the components of ET (soil evaporation, transpiration, and canopy interception, respectively). Results showed that ET increased over the fourth year since planting but decreased during the fifth year. Soil evaporation was the largest component of ET (36%), while transpiration and canopy interception accounted for 27% and 22%, respectively. Soil evaporation decreased through stand development while transpiration and canopy interception increased. Leaf area index (LAI) and precipitation were the most significant factors controlling ET and its components. Comparing the ET in this study with similar-aged plantations with lower LAI showed a higher water use. This high water use in the early stages of stand development wasn’t necessarily due to tree transpiration, but from high soil evaporation when the canopy is not fully developed. However, the long-term implications of a shorter rotation age but more frequent harvest cycles can offset short-term advantages. While there are potential sources of uncertainty between the two methods, this study had the advantage of using multiple methods to understand the interconnected processes that contribute to ET. Therefore, it is recommended that long-term observation of ET using multiple measurement techniques to evaluate the impact of widespread loblolly bioenergy crops in the Southeastern US.