Changes in leaf lifespan, nitrogen resorption, and mean residence time of leaf nitrogen along a soil fertility gradient in an evergreen oak tree

The ability of plants to retain nitrogen (N) for a long period of time is critical to their N use efficiency, growth, and fitness, particularly in infertile environments. The mean residence time of leaf N (MRT_L) and its two determinants, leaf lifespan and N resorption efficiency (r_N, the fraction of the total leaf N pool that is resorbed during leaf senescence), have been hypothesized to increase plastically with decreasing soil N fertility but this remains to be fully tested. To avoid confusion by random changes in these characteristics in a relatively narrow N fertility range, MRT_L, leaf lifespan, and N resorption efficiency were measured in Quercus glauca over a broad N fertility range. In the high to moderate N fertility range, leaf lifespan and r_N increased with decreasing N addition rate, and thus the MRT_L increased. However, in the moderate to low N fertility range, leaf lifespan increased but r_N decreased significantly, so MRT_L decreased. The decrease in r_N occurred because the senesced leaf N concentration was almost constant at the lower limit while the green leaf N concentration decreased in this range. The hump-shaped quadratic responses of MRT_L and r_N along the N fertility gradient suggest that incorrect conclusions about the response of these traits to N fertility variation may be drawn from experiments that include only a few fertility levels, and N recycling within leaf canopy alone cannot achieve efficient N use in infertile environments.