The role of tree age in triggering convective clouds: implications for plantation forestry expansion in the Southeastern US

Abstract:

The Southeastern United States includes some of the most intensively managed forests worldwide and conversion from natural forests to pine plantations is projected to increase. Currently, natural and unmanaged forests are being replaced with young, fast growing loblolly pine plantations to cope with an increasing demand for timber. These large scale land cover/management changes can impact key features of the hydrological cycle such as the generation of convective rainfall (i.e. thermodynamic precipitation). The role of stand age in regulating land-atmosphere feedback mechanisms is here investigated by the combined use of field observations and modeling. We use simple analytical solutions to describe the diurnal evolution of the atmospheric boundary layer (ABL) and the lifting condensation level so as to derive relations for cloudy/cloudless conditions (a proxy for the predisposition of the system to trigger convective rainfall) as a function of soil water content (SWC) and free atmosphere (FA) conditions. Results from four study sites suggest that young pine plantations maximize soil water depletion while minimizing rainfall suppression, thereby reducing the risk of water stress. In contrast, mature pine stands are more tolerant to drought, and cloud formation becomes nearly independent of the soil conditions. Our findings suggest that the impact of land cover, and therefore management practices, on ABL processes is encoded in the non-linear response of the Bowen ratio to changes in the SWC. Given the observed FA conditions in the Southeastern US, age-related changes in the surface energy fluxes can modify cloud cover and rainfall recycling mechanisms.