Modelling the impacts of atmospheric nitrogen deposition on Calluna-dominated ecosystems in the UK

1. The increased deposition of nitrogen (N) from the atmosphere over the last century has been associated in Europe with changes in species composition, including replacement of characteristic ericaceous shrubs such as Calluna vulgaris by grasses in heathlands and moorlands. However, these changes may also be associated with changes in management practices and environmental stresses, which may interact with changes in N deposition. Policies have now been implemented to reduce N deposition, but whether, and over what time scale, changes in vegetation composition will be reversed is uncertain. 2. A model was developed to simulate competitive growth between Calluna vulgaris and the grass species Deschampsia flexuosa and Molinia caerulea, driven by light and N availability. The model was parameterized for application to UK heath and moorland systems, and tested using a synthesis of data from 10 years of three field manipulation experiments in the UK. New routines to simulate management (burning, mowing, sheep grazing) were incorporated, and the model included a stochastic treatment of heather beetle Lochmaea suturalis responses. The effects of increases and decreases in N deposition over a period of 250 years were simulated under different management regimes. 3. Model runs demonstrated that changes in species composition in response to step changes in N deposition may occur over several decades and management cycles. The simulations showed a strong effect of management intensity, and in particular litter removal, in modifying the long-term impact of N deposition: recovery of Calluna dominance in lowland heaths was predicted within two to three decades of a decrease in N deposition under high-intensity management, but over five decades under low-intensity management. The timing of outbreaks of heather beetles, which were modelled stochastically, also had a strong effect on the balance between Calluna and grass species. Sensitivity analysis demonstrated the importance of mineralization rates, and Calluna growth rates and mortality rates, in influencing model outcomes, and also demonstrated significant interactions between these three factors and the probability of heather beetle outbreaks. 4. Synthesis and applications. Using a simulation model of competition between heather and grasses for light and N, scenarios were run for 250 years. Nitrogen deposition above 30 kg ha year(-1) initially increased Calluna biomass, consistent with field experiments, but after several decades led to grass dominance rather than heather dominance. The effect of N deposition depended on grazing pressure and the degree of litter removal through mowing, burning or sod-cutting. The benefits of policies to reduce N deposition, in terms of restoring heather dominance, may only be realized after several decades, and active site management may be needed to capture the full benefits of such policies.