Crown retreat of open-grown Southern live oaks (Quercus virginiana) due to canopy encroachment in Florida, USA

Broad-crowned, open-grown southern live oaks (Quercus virginiana) standing in open, park-like landscapes figure prominently in the history of the southeastern USA where they represent important natural, cultural, and literary icons. Similar to other savannas in North America, Europe, and Australia that are maintained by frequent, low-intensity fires and grazing pressure, live oaks and the open landscapes they dominate have been extensively replaced by forests during the past century. To explore the process of savanna loss, we studied the response of open-grown live oak crowns to increasing crown encroachment by other tree species as stand density increased after fires were suppressed and cattle grazing ceased in north-central Florida. To document the rate and to describe the process of crown encroachment and resulting die-back, we mapped and measured open-grown live oak crowns in two formerly open stands with different degrees of canopy encroachment and three stands that remained open and savanna-like. To understand why open-grown live oaks are threatened by forest succession, we compared crown densities, light compensation points, and maximum photosynthetic rates of open-grown live oaks and the principal encroaching tree species but found no significant differences. Instead, the crowns of mature open-grown live oaks apparently retreat in response to canopy encroachment because they do not grow substantially in height in response to crowding from taller-growing neighbors. If open-grown live oaks are to be maintained in the landscapes of the South, active management interventions that mimic historical disturbance regimes are needed.