The resistance of a chalk grassland to disturbance.
Concepts of ecosystem stability, resilience and resistance have been discussed theoretically for nearly three decades. Understanding the effects of habitat disturbance and mechanisms of recovery in practice are vital for successful conservation management and restoration, particularly of subseral communities with high conservation interest and sites subject to unavoidable disturbances. Chalk grasslands are one such habitat, with high European conservation importance, and the greatest remaining extent in north-west Europe lies within the Salisbury Plain Training Area (SPTA), the largest UK military training area. In order to understand the resistance and recovery of this habitat type, we undertook an experimental approach using imposed disturbance treatments. An experiment was designed to compare the disturbance effects of a Land Rover, a truck and a Challenger II tank on a tall Bromopsis erecta-dominated chalk grassland community on the SPTA. Permanent quadrats were established on a site experimentally disturbed by single and multiple passes of the three vehicles and a tank turn (slew). Post-disturbance changes were recorded in the vegetation and soils in permanent quadrats. One year after the disturbance, all the treatments still had significant soil compaction effects and all treatments except the single Land Rover pass resulted in a significant reduction in sward height. The grassland community sampled was significantly less resistant to disturbance by tracked vehicles than wheeled vehicle disturbance, with tracked vehicles creating the greatest recorded soil compaction and exposure of bare soil and longer-term changes in sward composition. Ordination techniques were used to characterize post-disturbance successional trajectories. These suggest that chalk grassland is significantly less resistant to disturbance caused by multiple passes of tracked vehicles and tracked vehicle turns. Chalk grassland recovery from these types of disturbance is less predictable. Identifying vegetation community resistance assists understanding of the ecosystem response to long-term and cumulative stress and facilitates strategic management of habitats where disturbance events are commonplace, especially in areas of high nature conservation interest. These data demonstrate that small-scale but acute disturbance events can have significant effects on plant community composition, and can have wider reaching impacts on other aspects of site management. There are important implications for the management of off-road vehicles in recreational and agricultural contexts, and for the formulation of a strategic sustainable management plan for the SPTA that incorporates both military and conservation objectives. Synthesis and applications. Resistance to disturbance is not necessarily an additive function. Managers of chalk grasslands should limit activities that create high intensity disturbance events because the succession trajectory following such events may be less direct and with less predictable outcomes than that following lower intensity disturbances. Increased predictability of succession trajectories following medium to low disturbance events means these types of disturbance might be used deliberately to create short-term and small-scale heterogeneity in both species composition and sward structure. Site managers should be aware that certain activities not previously considered to be potentially damaging might be creating significant habitat disturbance effects, as changes to soil structure, functioning and fauna can occur in the absence of changes in plant community composition.