Timing of Nitrogen Resorption-Related Processes during Fall Senescence in Southern Oak Species

Oak (Quercus)species are prominent in southern US forests. The ability to recycle nitrogen (N) during dormancy is an important adaptation to conserve a limited resource, but N resorption in southern oaks is not well understood. leaf protein and chlorophyll are both degraded during senescence to release N that can be stored in stems and roots. We hypothesized that leaf N would decrease soon after degradation of leaf protein and/or chlorophyll. Chlorophyll, protein, and N content were measured in leaves of Q. texana, Q. phellos, and Q. nigra during fall 2016 and 2017, in Arkansas. Degradation of protein, which holds the majority of leaf N, started early, in September, whereas chlorophyll degradation and N export from leaves occurred in late November. The delay between protein degradation and N export indicates that N resorption is drawn out over months in southern oaks, because of an unknown mechanism. Protracted leaf senescence could be due to a physiological or biochemical constraint, or it could be an adaptive trait where fall is typically warm and water-limited, but occasionally wet. Our results lay a foundation for future studies to examine how environmental stress may affect nutrient resorption during leaf senescence in southern oak species.