Vegetation succession as affected by decreasing nitrogen deposition, soil characteristics and site management: A modelling approach

After many years of increasing nitrogen deposition, the deposition rates are now decreasing. A major question is whether this will result in the expected positive effects on plant species diversity. Long-term experiments that investigate the effects of decreasing deposition are not available. Model simulations may yield insight into the possible effects of decreasing nitrogen deposition on the vegetation. Therefore we developed the vegetation succession model SUMO which is closely linked to the soil model SMART2. In SUMO, the biomass development of five functional plant types is simulated as a function of nitrogen availability, light interception and management. The model simulates the change in biomass distribution over functional types during the succession from almost bare soil via grassland or heathland to various forest types.The model was validated on three sites in The Netherlands and one site in the UK. The aboveground biomass of two grassland vegetation types was well simulated, as well as the aboveground biomass of heathlands during succession of sod removal. Some of the stages of forest succession were simulated less well, but the calculated biomass in the older stages agreed with measured values.To explore the long-term effect of a decrease in nitrogen deposition, we applied the model to a heathland and a pine stand. In the heathland a major change was predicted as a result of decreasing nitrogen deposition in combination with turf stripping. The dominance of grasses changed into a dominance of dwarf shrubs, whereas at continuing high levels of nitrogen deposition grasses remained dominant. In contrast, the simulations indicated only very small effects of a decreasing N deposition in pine forests. This difference is due to the removal of excess nitrogen by management (turf stripping) in the heathland, whereas the more extensive management in the forest hardly removes any nitrogen from the system. The main conclusion from these examples is that a decrease of nitrogen deposition may retard succession, and consequently increase biodiversity in heathland but probably not in forest. The effects of declining N deposition depend on the amount of N that is removed from the system as a consequence of the various management regimes.