Seasonal dynamics of biomass and nitrogen in canopies of Solidago altissima and effects of a yearly mowing treatment

Seasonal dynamics of biomass and nitrogen allocation in ramets of the clonal perennial Solidago altissima in response to yearly mowing were assessed in a field experiment. Final biomass of all component organs of the ramets was lower in mown than in unmown plots. Except for a negative effect on rhizome nitrogen concentration, mowing did not influence tissue nitrogen concentrations but, as a consequence of the reduced biomass accumulation, pools of standing-crop nitrogen were reduced in all organs. At all times during the annual stand development, the nitrogen concentration in the leaf canopies declined exponentially from the top downwards. The gradient of this decline was most marked at the beginning of the season and then became less apparent. In the mature stands, the area-based nitrogen contents of 'upper-canopy' leaves (receiving > 50 % of incoming light), for a given Light availability, on average were greater in mown than in unmown plots. About half of the nitrogen still remaining in pre-senescent leaves sampled in July was re-absorbed before leaf death. Nevertheless, the decreases in the absolute leaf- and stem-nitrogen pools per ramet during reproductive growth were much lower than the concomitant nitrogen gains of the developing inflorescences, implying that more than half of the nitrogen allocated to reproductive structures was taken up from the soil. On the other hand, there was evidence for nitrogen re-allocation from old to new leaves during vegetative growth which apparently enabled individual ramets to maintain an exponential nitrogen profile even during the phase of rapid leaf production in spring, thus using nitrogen efficiently. (C) 2000 Editions scientifiques et medicales Elsevier SAS.