The use of soil disturbance in the management of Breckland grass heaths

Summary

Contemporary management of heath and grassland for nature conservation often reinstates or mimics traditional land use practice, particularly grazing and cutting. However, soil disturbance has often been overlooked as a management technique in Britain, despite successful use by other European heathland managers. An experiment was undertaken to examine the effects of different types of disturbance treatments upon the flora of a calacareous, lichen-rich Breckland grass heath in Norfolk, eastern England.

Study site: The study was undertaken on Weeting Heath National Nature Reserve over three years (1989-91). The grass heath had a history of sheep and European rabbit Oryctolagus cuniculus grazing.

Experimental design: The experiment was a randomized block design (to control for variation in soil conditions, grazing intensity and grazing history across the site). Four blocks, each 60 x 80 m, were sub-divided into four (15 x 80 m) plots. In 1989, each of four treatments (forage harvest, rotovation, plough, undisturbed control) was applied once within each block (randomly allocated to plots). Plots which had been rotovated or ploughed in 1989 were further sub-divided into two (7.5 x 80 m) plots, one rotovated in both 1990 and 1991 and the other left. The six treatments (each occurring once in each of the four blocks) were thus:

C - control;
F - forage harvested 1989;
RL - rotovated 1989 and then left;
RR - rotovated in 1989, 1990 and 199;
PL - ploughed 1989 and then left;
PR - ploughed 1989 and then rotovated in 1990 and 1991.

Forage harvesting was undertaken in late November. Rotovation and ploughing were carried out mid-February to mid-March. To avoid disturbance to ground-nesting birds, monitoring and soil sampling was undertaken in autumn.

Rabbit activity: The density of active burrows as a measure of rabbit activity was measured in August/September 1989-1991. Faecal pellet production was measured in 1989 and 1990 by counts in 1 m² quadrats within each plot.

Vegetation:  At the end of July/August 1989-1991, vegetation height was measured at 36-45 points per plot in 1989-1991, using a sward stick. In 1989, five, and in 1990-91, 10, 0.25 m² quadrats were placed in each plot and the percentage cover of each plant species was estimated.

Initial effects: Different treatments resulted in differing degrees of disturbance with implications for the composition of surviving vegetation and sources of innoculi in situ.

Immediate effects of forage harvesting included wheel compaction by the tractor, harvester and trailer, removal of vegetation and disturbance of the surface soil on hummocks, creating open patches of around 15-50 cm in diameter. Rotovation produced extensive areas of disturbed soil, but with fragments of vegetation including pieces of grass tussocks, moss and lichen, remaining within or on the soil surface. Ploughing buried vegetation to about 20 cm and produced a surface consisting almost entirely of subsoil. In places partially buried grass was exposed. Compared with rotavation, single ploughing produced fewer beneficial results for target species and in some cases caused a marked decline e.g. of winter annuals and lichens. Repeat disturbance after ploughing further impoverished the vegetation.

Soil disturbance, resulted in a considerable increase and high levels of bare ground, maintained in repeatedly disturbed treatments. Bare ground declined in both single disturbance treatments but was still significantly more predominant than in the control 2.5 years post-treatment. Forage harvesting gave an ephemeral increase in bare ground, with no significant difference to the control by 1990 or 1991. The effect of treatments on vegetation cover  in 1989-1991 are summarised in Tables 1a, 1b and 1c (attached).

Vegetation height: All treatments resulted in shorter vegetation compared to the control throughout 1989-1991 (Table 2, attached). In the first year, rotovation produced significantly shorter vegetation than ploughing, with forage harvesting intermediate between the two. In the second year, all four disturbance treatments reulted in shorter vegetation than forage harvesting, while repeated rotovation gave significantly shorter vegetation than both plough treatments. In the third year all disturbance treatments had a similar vegetation height, and were significantly shorter than forage harvesting.

Rabbit activity: Faecal pellet production rate (Table 2) show a high level of rabbit activity in rotovation treatments. In 1989 no significant treatment effect on rabbit burrow abundance was found, but ploughed plots had a density of burrows approaching that of controls and more than twice that of the other treatments (F and R). In both 1990 and 1991, single ploughing tended to have a higher abundance of burrows than the control and other treatments, while forage harvesting tended to have a lower abundance of burrows than other plots.

Conclusions: Suitable soil disturbance treatments may help conserve and reinstate Breckland grass heath vegetation. Rotovation in the short term increased the abundance of some critical vegetation components. Rotavation within grass heath areas on a rotational basis (once every 3-5 years) is recommended to provide a mosaic of plant sub-communities; some undisturbed areas should be retained. Single ploughing produced few beneficial results and in some cases caused a marked detrimental impact on target plant species.


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