Evaluation of Plant Responses to Atmospheric Nitrogen Deposition in France Using Integrated Soil-Vegetation Models

The aim of this chapter is to give an overview of plant responses to nitrogen (N) deposition by using two dynamic biogeochemical models coupled with a vegetation module: VSD+-VEG and ForSAFE-VEG. The biogeochemical models were first validated on some French forest sites from the ICP-Forests network. A French vegetation table (which is now part of a European database) containing 230 species with their appropriate ecological environmental parameters, was set up. The outputs of each model in terms of plant response to atmospheric nitrogen deposition were compared to measured values for one forest stand. The two models underestimated the occurrence of certain herbs and grasses and overestimated (ForSAFE-VEG) or underestimated (VSD+-VEG) the presence of certain mosses. This allowed us to improve the validation and thus the calibration of some parameters. For the simulated period ForSAFE-VEG indicated some variations in the occurrence of the plant groups, the mosses group showing the highest increase and indicating a high sensitivity to atmospheric N deposition. No significant changes in the occurrence percentage of plant groups were observed by running the VSD+-VEG model, this model being less sensitive than ForSAFE-VEG to simulate tenuous vegetation changes. The observed changes over time in the dominant ground plant groups using ForSAFE-VEG could be related to changes in site environmental conditions, but only the influence of the maximum N deposition was obvious. Further investigations are needed to compare the performance of the two models on other sites, but these tests of the ForSAFE-VEG and VSD+-VEG models showed promise for simulating the link between N deposition and vegetation diversity.