Managing heterogeneity: the importance of grazing and environmental variation on post-fire succession in heathlands

Summary

The coastal heathlands of north-western Europe developed as a result of forest clearance around 4,000 years ago, and have since been maintained by disturbance regimes, including grazing by domestic and wild animals, burning, turf-cutting and harvesting heather Calluna and Erica and bracken Pteridium aquilinum. Over the past 50 or so years, these traditional management regimes have gradually been discontinued, resulting in reforestation through natural succession. Heathland ecosystems have recently been classified as greatly endangered (EC Habitats Directive 92/43/EEC). Different modern management regimes include grazing at different stocking rates, burning, mechanical cutting and turf stripping.

In this Norwegian study, the interactive affects of fire and grazing on plant species composition and diversity along a local moisture gradient in coastal heathlands was investigated. A replicated series of post-fire successions was initiated in three heathland habitats and the areas subjected to two grazing regimes. Floristic and environmental data were recorded over a 5-year period.

Study area: Lurekalven is an island situated in a fjord approximately 20 km inland from the Norwegian west coast. The highest point is 54 m above sea level. The climate is oceanic, with July and January mean temperatures of 12 °C and 2 °C, respectively, a long growing season (c. 220 days above 5 °C), high precipitation (1600 mm) and strong winds.

Deciduous forest covered most of the island until cleared in c. AD 700, with heather Calluna vulgaris heath developing over the larger part of the island, with smaller areas of meadows and cultivated fields. Around 1350, the island was managed for grazing but from 1950 onwards the island gradually fell out of use, resulting in rapid reforestation, mainly by downy birch Betula pubescens. Trees and shrubs were cleared and burning and grazing re-introduced in 1995. The overall grazing pressure has been c. 1 sheep/ha/year. Today, the island is largely covered by Calluna heath, with mires and willow Salix shrubs in wetter areas. Moisture is a major source of local environmental variability, with dry heath on shallow soils often on ridges or south-facing areas, and moist heath on deeper soils often on north-facing slopes or in poorly drained areas.

Experimental design and sampling: Three heath habitat types were chosen: M, moist, north-facing; I, intermediate, on relatively flat ground; D, dry, south-facing on shallow soils. These were chosen to span as wide a range of heathland environments, be relatively representative of the island's heaths, and occurring throughout so that replicate sites could be established. A fence was erected across the island, dividing it into two, excluding grazers from one part (i.e. two treatments, grazed vs. ungrazed). 12 experimental blocks (c. 100 m²) were established, two replicates for each combination of heath type × grazing treatment. Within each block, five 1 m² permanent plots were established in random positions, marked with metal poles, and frequencies recorded of all vascular plants, bryophytes and lichens using a 1 × 1 m metal frame divided into 16 subplots. After the baseline survey in July–August 1993, all 12 sites were burnt in April 1994 and re-analysed in July–August of the following 4 years. Control plots could not be established prior to treatment because of difficulties controlling fires, so two control plots were placed in unburnt heath, as close as possible to each site and at similar inclinations, aspects and soil depths, in 1995. Species diversity data were compiled at three spatial scales: site richness (number of species recorded in at least one plot/site/year for burnt and control areas), plot richness (number of species/plot/year) and average species density (average richness per 25 × 25 cm subplot/plot /year).

Environmental variability within and between heath types was not the focus of the investigation, but topography, slope, aspect and soil depth were measured. Soil samples were analysed for loss-on-ignition, pH, Kjeldahl total nitrogen and exchangeable cations.

Fire had strong effects on the heathland vegetation structure and species composition. Out of the 137 taxa encountered over the 5 years (69 vascular plant taxa, 58 bryophytes, and 10 lichens), 16 taxa, including the lichens Cladonia gracilis (18 occurrences), C.furcata (11) and C.uncialis (eight), were only encountered in the 1993 baseline census or in the controls. 24 species, including heath speedwell Veronica officinalis (26), the moss Leptodontium flexifolium (23), cat's-ear Hypochoeris radicata (nine) and foxglove Digitalis purpurea (seven), were encountered only after fire.

The primary floristic gradients in the data (as shown by principal components analysis) reflected this strong overall effect of burning.

Effects on species composition: Analyses implied that the strong compositional trends during post-fire succession were habitat-specific, modified by grazing, and had complex interactions. The effect of fire on compositional trends through time accounted for 7.8% of the variance, and was highly statistically significant, both overall and at all sampling dates except from the 1993 baseline census. Revegetation trajectories differed significantly among habitats and with grazing regime, but these effects were relatively weak, adding only 3.0% and 1.5% to the explained variability in the data, respectively. If different grazing effects among habitats were allowed for, a further 3.9% was added to the explained variance. Overall, post-fire succession in the 12 sites through the 5-year experiment explained 23.2% of the total inertia, and if the overall differences among sites and sampling times were also taken into account, then 62% of the total variance in the data could be accounted for. The 38% not accounted for was variation that occurred among the replicate plots within sites at each census.

In order to visualize and interpret the interactive effects of grazing and habitat on species composition, the data were split into a grazed and an ungrazed subset, and separate PRC allowing for differential responses among sites was run for each subset. The overall results were broadly comparable. However, the sequential tests and the PRC diagrams revealed some interesting differences. The immediate response was strong in the ungrazed subset, the habitats had distinct responses (dry south-facing > intermediate flat > moist north-facing), and succession towards the unburnt control appeared quickly. The grazed subset, in contrast, had less clear patterns, both among habitats and through time. While the overall species composition in the grazed and ungrazed data was very similar, there was an indication that individual species responded differently to post-fire successions in grazed and ungrazed heaths. There was considerable variation in these species-specific responses; in some species strong negative effects of fire under grazing would change to positive when grazing was removed (e.g. hard fern Blechnum spicant and heath bedstraw Galium saxatile), whereas others responded more positively to fire under grazing (e.g. Agrostis spp., viviparous fescue Festuca vivipara, heath-spotted orchid Dactylorhiza maculata and juniper hair cap Polytrichum juniperinum).

Effects on diversity: Univariate analyses showed that the strong compositional response to fire was paralleled by changes in species diversity. At all scales, species diversity decreased the first autumn after fire, reached pre-fire levels in the second year, and continued to increase for the following 2 years. Grazing or environment did not influence this strong trend in species diversity during the post-fire succession. There were overall differences among habitats, however, as richness was highest in moist heath and lowest in dry heath across scales. Grazing also influenced diversity, but in more complex ways, as effects differed among habitats and scales of study. Average species density and site richness decreased in the grazed dry and intermediate sites, but increased in the grazed moist sites. These effects, however, were not discernible at the plot scale.

Conclusions: These results demonstrate that grazing and prescribed burns do not have simple additive effects within this heathland system, as grazing created ecological opportunities for additional sets of species, increased variability among habitats, and added complexity to the post-fire successional dynamics. In order to preserve diversity, conservation management should thus aim to preserve the complexity of the traditional management regimes, both in terms of the actual disturbances (e.g. fire and grazing) as well as the spatial scales at which they are applied. The considerable variability in these effects among the heathland types suggests that local environmental variability should be taken into account in heathland conservation.


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