Individual study: Encouraging heathland establishment through the removal the humic layer on recently deforested heath at Tudeley Woods RSPB Reserve, Kent, England
Allison M. & Ausden M. (2006) Effects of removing the litter and humic layers on heathland establishment following plantation removal. Biological Conservation, 127, 177-182
The extent of heathland has declined markedly in the UK during the 19th and 20th centuries and heathland recreation is thus a desirable conservation objective. Whilst early attempts to recreate lowland heathland have been primarily on former arable land, it is now widely recognised that restoration of afforested heathland is likely to be more successful. Heathland restoration efforts are usually hampered by the presence of high nutrient levels in soils. This case describes the effects of removing the humic layer on the establishment of heather on a formally afforested heathland.
Study site: The study was conducted on Tunbridge Wells Sand at Tudeley Woods RSPB Reserve in Kent, UK between 1994 and 2004 following pine plantation removal during the winter of 1991/92. Prior to coniferisation in the 1890s, the site was thought to have comprised heathy ground dominated by heather Calluna vulgaris used for grazing, and gorse Ulex europaeus collection.
Experimental design: A randomised block experiment was set up containing the two treatments replicated in four blocks:
1) Surface 15 cm of litter and humus removed down to expose the mineral horizon.
2) A control in which just the surface 5 cm of leaf litter had been removed down to the humic layer.
Each treatment plot was 5 m square with 2 m strips between plots. None of the plots received any management during the course of the experiment, but some were lightly grazed by rabbits Oryctolagus cuniculus.
Measurement of soil properties: Soil properties were measured from six approximately 300 cm³ soil samples taken from the top 10 cm of each plot in July 2004. Each group of samples was homogenised, sealed in a polythene bag and analysed by an accredited laboratory (ADAS, Wolverhampton, UK) using standard soil analysis techniques.
Vegetation recording: The frequency of Calluna was assed in each of 250 10 × 10 cm squares within each plot in 1994, 1995, 1996, 1998 and 2004. More detailed vegetation recording was carried out on 26-27 July 2004. The frequency of vascular plant species, bryophytes and lichens was recorded within each of 10 0.5 × 0.5 m quadrats within each plot. The number of pine Pinus spp. and birch Betula spp. saplings more than 10 cm high were recorded and the number of bracken Pteridium aquilinum fronds were also counted. Using a sward stick comprising a 20 cm diameter plastic disk weighing 145 g, able to slide freely down a 1.5 m stick, vegetation height was measured.
Vegetation recovery: Changes in the frequency of heather over the course of the experiment are shown in Figure 1 (attached). For the first three years after the humic layer had been removed, a significantly greater Calluna frequency was actually found in the control plot than in the treated plots. After twelve years, both sets of plots become almost entirely heather-dominated. The mean height of heather, bryophyte species richness, lichen species richness, bracken frond density, pine density and birch density did not differ between litter-removed and control plots.
Soil properties: Twelve years after litter removal, the depth of the humic layer and the nitrate content of the soil were significantly lower in plots in which the humic layer had been removed. The organic content of the soil, as measured by percentage loss on ignition, was also significantly lower in plots in which litter had been removed. The soil content of ammonium, phosphorus, potassium and magnesium did not differ significantly between plot types. The effects of removing the humic layer on soil properties are shown in Table 1 (attached).
Conclusions: The fact that desirable heathland vegetation eventually became established in the control plots, suggests that there may be little benefit in removing the humic layer in addition to leaf litter (see also case 324). Furthermore, for three years after the start of the experiment Calluna occurred in greater frequency in control plots than in treatment plots, perhaps due to the presence of heather seeds in the humic layer. However, litter removal may ensure better long-term maintenance of desirable heathland vegetation by reducing soil nutrients, particularly nitrogen.
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