Calibrating and Testing the Forest Vegetation Simulator to Simulate Tree Encroachment and Control Measures for Heathland Restoration in Southern Europe

We used the Forest Vegetation Simulator (FVS) to simulate tree encroachment and tree control measures in a continental heathland in Southern Europe, where the goal is the conservation of Calluna vulgaris (L.) Hull. We selected as the management target a maximum tree basal area of 4.6 m2 ha1, corresponding to a minimum Calluna cover of 25%, and modeled stand dynamics along a 50-year period under three management scenarios: no management; low-frequency prescribed burning; and low-frequency prescribed burning followed by annual grazing. We initialized and calibrated the Northeastern variant of FVS with data measured in the field and the Fire and Fuel Extension with additional data from a prescribed burning experiment. We carried out most calibrations by multipliers, including a novel way to simulate the impact of grazing on shoot height and survival. To simulate fire behavior, we designed a custom fire behavior fuel model for heath fuels. Successful validation of simulated stand-scale attributes and postfire mortality was carried out using equivalence testing. Model projections showed that stand basal area would exceed the target threshold under both the no-management and the prescribed burning scenario, 5 and 10 years into the simulation, respectively. Conversely, prescribed burning followed by yearly grazing successfully controlled encroaching trees throughout the simulation, by keeping their basal area below the threshold associated with the maintenance of the heathland cultural landscape.