Additional carbon storage under elevated CO2 is not diminished – and may even be amplified – by nitrogen limitation

Background/Question/Methods

Enhanced carbon storage by intact forests may mitigate rising atmospheric CO2, but there is concern that nitrogen limitation – together with tissue stoichiometry and Liebig’s Law of the Minimum – may curtail that enhanced carbon storage. Nevertheless, most forest FACE experiments show sustained enhanced NPP despite nitrogen limitation, and there is evidence that the world’s nitrogen-limited forests are storing additional carbon as ambient CO2 rises. By what mechanisms can trees store additional carbon despite nitrogen limitation? And what does the answer say about the future of carbon storage? To answer these questions, we focus on the evolution and community ecology of wood, which is the primary living reservoir of stored carbon and a weapon that trees employ to kill – or be killed – by overtopping neighbors. A game theoretic analysis of the physiologically-based Perfect Plasticity Approximation of forest stand structure provides a framework for the talk.

Results/Conclusions

Driven by shifts in competitive allocation, additional carbon storage under elevated CO2 is not diminished – and may even be amplified – by nitrogen limitation. When tissue stoichiometry is allowed to vary (consistent with established empirical findings), competitive trees are predicted to allocate additional photosynthate from elevated CO2 to both wood and fine roots. Increased allocation to fine roots maintains competitive dominance for nitrogen, and increased allocation to wood maintains competitive dominance for light. The fractional increase tips slightly in favor of wood as nitrogen becomes more limiting, suggesting that relative (but not absolute) enhanced carbon storage will be greatest in the most nitrogen-limited forests. When tissue stoichiometry is held fixed (at different values for foliage, fine roots, and wood, as may occur if trees “run out” of stoichiometric plasticity), competitive trees are predicted to allocate additional photosynthate from elevated CO2 primarily to wood, the most nitrogen-poor tissue, using small amounts of nitrogen that would have otherwise gone to fine roots. Surprisingly then, fixed stoichiometry forces nitrogen-limited trees to amplify (!) carbon storage by shifting allocation to wood in order to remain competitive.