Why do forests respond differently to nitrogen deposition? A modelling approach

Elevated reactive nitrogen deposition can result in declines in belowground C allocation to mycorrhiza, potentially threatening the sustainability of forest ecosystems, especially with regard to ongoing global environmental changes. Previous studies showed that the vulnerability of mycorrhiza differs in different forests. In this paper, we introduce a model that explains such differences as resulting from the optimization of belowground tree C investment in forests with inorganic or organic N economy. According to the model, the optimization of belowground tree C investment in forests with organic N economy is governed by indirect competition between trees for available forms of N. We predict decreasing C investment in mycorrhiza with N deposition in systems dominated by an organic N economy. In contrast, in forests with inorganic N economy the optimization of belowground tree C investment is governed by direct competition between trees for available forms of N. We show that C investment in mycorrhiza does not depend on N deposition in the case of inorganic N economy. Parameterizing the model with data from spruce and beech temperate forests, we show that spruce forests are more sensitive to N deposition with a decrease of tree investment in mycorrhiza, which seems to be consistent with empirical findings. As mycorrhizae are essential for nutrients and water acquisition in trees, we suggest our approach could contribute to predicting the responses of forests to global environmental changes.