Loblolly pine trees (Pinus taeda L.) occupy more than 20% of the forested area in the Southern United States, represent more than 50%of the standing pine volume in this region, and remove from the atmosphere about 500 g C m−2 per year through net ecosystem CO2 exchange. Hence, their significance as a major regional carbon sink can hardly be disputed. What is disputed is whether the proliferation of young plantations replacing old forest in the Southern United States will alter key aspects of the hydrological cycle, including convective rainfall, which is the focus of the present work. Ecosystem fluxes of sensible (Hs) and latent heat (LE) and large-scale, slowly-evolving free atmospheric temperature and water vapor content are known to be first order controls on the formation of convective clouds in the atmospheric boundary layer. These controlling processes are here described by a zero-order analytical model aimed at assessing how plantations of different ages may regulate the persistence and transition of the atmospheric system between cloudy and cloudless conditions. Using the analytical model, the roles of ecosystem Hs and LE on convective cloud formation are explored relative to the entrainment of heat and moisture from the free atmosphere. It is demonstrated that the leading eco-hydrological mechanism governing the cloudy-cloudless regimes at the land surface is encoded in the non-linearity between the Bowen ratio Bo = Hs/LE and root-zone soil water content. This non-linearity is triggered by a soil type-specific threshold between soil water content, water potential and stomatal conductance and suggest that young/mature pines ecosystems have the ability to recirculate available water (through rainfall predisposition mechanisms). Such non-linearity in the
Bo-soil moisture relation was not detected in a much older pines stand. These results enable
the generation of hypotheses about the impacts on the thermodynamics of boundary layer cloud
formation driven by afforestation/deforestation and groundwater depletion projected to increase
following increased human population in the Southeastern United States.
