Relationships between leaf nitrogen and limitations of photosynthesis in canopies of Solidago altissima

Vertical distribution patterns of light, leaf nitrogen, and leaf gas exchan ge through canopies of the clonal perennial Solidago altissima were studied in response to mowing and fertilizer application in a field experiment. Co nsistent with the distribution of light, average leaf nitrogen content foll owed a 'smooth' exponential decline along the fertilized stands both in con trol and mown plots. The nitrogen profile along the unfertilzed stands in m own plots, however, was 'disrupted' by high-nitrogen leaves at the top of s horter ramets that only reached intermediate strata of the canopies. Hence, in these stands leaf nitrogen was significantly increased in shea ramets c ompared with tall ramets for a given light environment, suggesting suboptim al stand structure but not necessarily suboptimal single-ramet architecture . However, at least under the climatic conditions observed during measureme nts, such disrupture had no substantial effect on stand productivity: model calculations showed that vertical distribution patterns of leaf nitrogen a long ramets only marginally influenced the photosynthetic performance of ra mets and stands. This is explained by the observed photosynthesis-nitrogen relationship: the rate of photosynthesis per unit amount of leaf nitrogen d id not increase with leaf nitrogen content even under saturating light leve ls indicating that leaf photosynthesis was not nitrogen limited during the measurement periods. Nevertheless, our study indicates that consideration o f how architecture(s) of adjacent individual plants interact could be essen tial for a better understanding of the trade-offs between individual and ca nopy characteristics for maximizing carbon gain. Such trade-offs may end up in a suboptimal canopy structure, which could not be predicted and underst ood by classical canopy optimization models. (C) 1999 Editions scientifique s et medicales Elsevier SAS.