The resistance of a chalk grassland to disturbance

Summary

Understanding the effects of habitat disturbance and mechanisms of recovery is needed for successful conservation management and restoration, particularly areas of high conservation interest and sites subject to unavoidable disturbances. Chalk grasslands are one such habitat, with high European conservation importance. The greatest remaining extent in north-west Europe is situated within the Salisbury Plain Training Area, a military training area in southern England. In order to assess the resistance and recovery of this habitat type, an experiment using vehicle disturbance treatments was undertaken.

Study site: The study was undertaken on the Salisbury Plain Training Area in Wiltshire, southern England. The site is underlain by chalk bedrock and between 150 and 155 m altitude. The grassland is classified under the UK National Vegetation Classification (NVC; Rodwell 1992) as CG3d (Bromus erectus grassland, Festuca rubra–Festuca arundinacea subcommunity) a diverse vegetation community. 33% of the training area contains a component of this vegetation type and it is one of the communities for which it has been designated as an Site of Special Scientific Interest. There is therefore particular interest in the ability of this vegetation community to withstand vehicular disturbance.

Experimental design: Three commonly used military vehicles, differing in mass and traction, were selected for the experiment: a Landrover (1-t light utility wheeled vehicle), a Bedford truck (4-t medium utility wheeled vehicle) and a Challenger II tank (64-t heavy tracked vehicle). There were six replicate blocks each containing eight lanes 25 m in length to which disturbance treatments were randomly allocated. Treatments included a control (no vehicle pass), single and multiple passes, and a tracked vehicle slew (a sharp turn). Lanes allocated to the wheeled vehicles were 5 m wide and for the tracked vehicle 10 m. Additional lanes 15 m in width were used for the tracked vehicle slew.

Monitoring the effects of disturbance on soil: Soil compaction was assessed in quadrats in each lane using a manual penetrometer.

Soil samples were taken during the 2 days following the disturbances. A 10 × 10 × 10 cm block of soil was removed from the centre of each of the three disturbed quadrats using a trowel, and separated into 0–5 cm and 5–10 cm depth fractions; 500 cm3 of each soil sample was air dried and passed through a 2-mm sieve. The following soil properties were measured:available phosphorus (P); total organic nitrogen (total N) estimated as a surrogate measure of organic matter; exchangeable cations (calcium, magnesium, sodium and potassium); and soil pH.

Monitoring the effects of disturbance on vegetation: Immediately before the treatment application in October 1998, the site was surveyed to identify any significant variation in species cover. Three 50 × 50 cm vegetation quadrats were randomly placed in each lane and the vegetation was recorded using the Domin scale.

Following disturbance, three permanent quadrats (0.5 × 2 m) were marked with stakes at 6, 12 and 18 m along the length of each disturbed lane centred on the vehicle track, and paired with parallel, undisturbed quadrats nearby. In July 1999 and 2000, vegetation was surveyed in the quadrats. Species cover was recorded using the Domin scale, including a score for bare ground. Three random vegetation height measurements were taken in each quadrat using a drop disc (30 cm diameter, mass 80 g) and ruler.

A year after disturbance, all the treatments still had significant soil compaction effects and all except the single Land Rover pass resulted in a significant reduction in sward height. The grassland community was significantly less resistant to disturbance by the heavy tracked vehicle (Challenger II tank) than the lighter wheeled vehicles, with tracked vehicles creating the greatest recorded soil compaction and exposure of bare soil and longer-term changes in sward composition.

The chalk grassland is significantly less resistant to disturbance caused by multiple passes of tracked vehicles and tracked vehicle turns. Grassland recovery from these types of disturbance is less predictable.

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 training area that incorporates both military and conservation objectives.

Conclusions: Managers of chalk grasslands should where possible limit activities that create high intensity disturbance events as these will be damaging to plant communities and soils. Less damaging medium to low 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 vehicular activities not previously considered to be potentially damaging might be creating significant habitat disturbance effects.


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