Do competition and herbivory alter the internal nitrogen dynamics of birch saplings?

Deciduous trees recycle nitrogen within their tissues. The aim of this study was to test the hypothesis that reductions in plant growth, caused by competition and herbivory, reduce the sink strength for N during autumn nutrient withdrawal, and reduce the storage capacity and hence the amount of N remobilized in the following spring. We used (15)N-labelled fertilizer to quantify N uptake, leaf N withdrawal and remobilization. Betula pubescens saplings were grown with either Molinia caerulea or Calluna vulgaris, and subjected to simulated browsing damage. Competition reduced B. pubescens leaf N withdrawal and remobilization, with C. vulgaris having a greater effect than M. caerulea. However, simulated browsing had no significant effect on sapling N dynamics. The patterns of leaf N withdrawal and remobilization closely followed sapling dry mass. We conclude that the effect of competition on sapling mass reduces their N-storage capacity. This reduces sink strength for leaf N withdrawal and the source strength for remobilized N. The ability of saplings to compensate for browsing damage removed any potential effect of browsing on N dynamics.