Removing adult overstory trees stimulates growth and transpiration of conspecific juvenile trees

During the last century, the density of Ashe juniper (Juniperus ashei Buchholz) has greatly increased in oak savannahs of central Texas. Recently, juniper removal has been advocated as a regional water conservation tool. In this study, we investigated whether juvenile trees released from an overstory canopy after clearing exhibited accelerated growth and water consumption. We compared leaf-level transpiration (E₁) and carbon assimilation (A_net) rates among juvenile juniper under three different treatment scenarios: 1) in the open, 2) under an adult juniper canopy or 3) recently released by the removal of an adult juniper canopy. Released plants apparently grew faster and used more water than other juvenile trees; average A _net of released plants was 94%-162% greater (P < 0.05) than those beneath an adult canopy and 22%-44% greater than open-grown plants. Furthermore, average E₁ of released plants was 22%-72% greater than those beneath an adult canopy and 13%-22% greater than open-grown plants. These differences persisted for at least two years after treatment. Rates of A_net were particularly elevated in released plants compared to other plants during periods of low water stress; whereas E₁ tended to be higher in released plants compared to other plants at all levels of water availability. Our evidence suggests released plants have better access to water, because at two out of three study sites, predawn leaf water potential (Ψ_p) was significantly more favorable for released plants than open-grown or under-canopy plants (P < 0.05). Although adult canopy removal temporarily reduced leaf area of juniper on a community level, and likely total water use, we demonstrated that released juveniles, at a minimum, partially compensated for the reduced overstory by increasing rates of water use and growth.