Evapotranspiration of Pine Forests: A Global Synthesis

Pine (Pinus spp.) forests are found in many areas with a wide range of bioclimates, providing a myriad of ecosystem services. Pine trees have been widely planted for various purposes such as commercial timber production, soil erosion control and water quality improvement of source waters, urban forestry, wildlife habitat, and bioenergy for carbon sequestration. Quantifying these benefits requires a clear understanding of the hydrological cycle in pine forests, especially the evapotranspiration processes. This synthesis study aims at synthesizing published literature documenting the water balance components including evapotranspiration (ET), the sum of canopy/litter interception, tree transpiration, soil/understory evaporation, water yield as surface runoff and subsurface drainage, and soil water storage of pine forests, natural or managed, at stand or watershed levels. We conducted the analyses using the Budyko’s water and energy balance framework to show how the aridity index, defined as potential evapotranspiration/precipitation ratio (PET/P), influences water use strategies and controls on evaporative efficiency (ET/P) of pine forests under different climatic regimes and management. We examine influences of tree and stand age, xylem anatomy, forest management such as thinning, species conversion (e.g., from loblolly pine to longleaf pine or deciduous to pine), and artificial drainage on pine forest ET and hydrology. We explain the large variability of forest ET and its components (i.e., canopy interception) and across a large global climatic gradient. We identify research gaps for fully accounting pine forest evapotranspiration such as water loss by understories and stress the importance of canopy interception and soil evaporation. Our review suggests that total ET rates of evergreen pine species generally are higher than native broadleaf deciduous forests under most climatic conditions. We demonstrate the success and challenges to use pine forests to meet multiple ecosystem services (soil erosion control, water supply, and timber yield) in a changing environment.