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No. 13: Restoration of dune habitats in Østerild Klitplantage - baseline monitoring 2011

Nygaard, B., Wind, P. & Ejrnæs, R. 2011. Restoration of dune habitats in Østerild Klitplantage - baseline monitoring 2011. Aarhus University, DCE – Danish Centre for Environment and Energy, 36 pp. - Scientific Report from DCE – Danish Centre for Environment and Energy No. 13
http://www.dmu.dk/Pub/SR13.pdf

Summary

The overall objective of the monitoring programme is to document the outcome of the restoration project which focuses upon creating open dune habitats following clear-cutting of dune plantations in the National Test Centre facility for wind turbines in Østerild Klitplantage. The monitoring programme includes a recording of soil conditions and plant species composition prior to clearing of dune plantations (baseline monitoring) and a systematic recording of the changes during the first 10 years of the succession towards open dune habitats (post-construction monitoring). This report presents the monitoring sites and plots as well as the main results of the baseline monitoring.

Monitoring sites and plots were laid out in a stratified random arrangement in order to accommodate the different starting points and restoration measures. Stratification was applied according to baseline condition (forest type), planned post-cutting treatments of litter layer and hydrology, expected management regimes, distance to appropriate seed sources and topography.

The dune plantations planned for clear-felling in the test area were predominantly coniferous forests, with introduced spruces, pines and firs (mainlyPicea sitchensis,Sitka Spruce andP. mugo,Mountain Pine) as well as the native pinePinus sylvestris(Scots Pine). The starting conditions are assumed to have a major impact on succession following deforestation and the monitoring sites thus include Mountain Pine, Sitka Spruce and Scots Pinestands.

In order to facilitate the restoration of grey dunes (habitat type 2130), dune heaths (habitat type 2140) and humid dune slacks (habitat type 2190) various post-cutting treatments of hydrology and accumulated soil organic matter will be implemented. The monitoring programme includes an assessment of the effect of these treatments on the rate and direction of vegetation development towards the target communities. Hopefully the results will lead to recommendations for improving the management of future conversions of plantations to dune habitats.

Prior to afforestation in late 1800, the dune areas in Østerild and Hjardemål Plantage were characterized by a high and presumably highly fluctuating water table and consequently moist and wet habitats were widespread in the area. A successful restoration of moist dune heaths (2140) and humid dune slacks (2190) will therefore require a restoration of the original hydrological regime. One of the planned actions in Østerild is thus to close drainage dykes and allow temporary pools and shallow lakes to develop or expand. The monitoring programme aims to follow the succession in dry and rewetted dune habitats, including areas that will be seasonally flooded.

Coniferous litter is acidic and the decomposition rate is very low, which leads to an accumulation of needles, cones and twigs on the forest floor. A thick litter layer in the coniferous forest may constitute a major constraint on the restoration of natural dune habitats. In cooperation with the Danish Nature Agency in Thy, we have included four different post-cutting experimental treatments of the accumulated organic matter: 1) removal of litter layer and exposure of bare soil in larger patches, 2) removal of litter layer and exposure of bare soil in smaller patches, 3) burning of litter layer and 4) intact litter layer (control treatment).

Baseline monitoring was carried out in July 2011, prior to clear-cutting of the plantations and the methods are based on the variables in the NOVANA programme for terrestrial habitats (Fredshavn et al. 2011). We recorded the plant species composition and vegetation structure in a pin point frame (0.5 * 0.5 m) and a documentation circle with a radius of 5 m for each of the 100 monitoring plots. Furthermore, we collected soil samples from all plots for pH measurements (100 samples) and C/N ratio analyses (24 samples) and we measured the thickness of the accumulated organic matter in the forest floor (the litter layer).

We found that species richness was highest in Scots Pine stands and lowest in Sitka Spruce sites. The most widespread species wereDeschampsia flexuosa,Picea sitchensis,Pleurozium schreberiandHypnum jutlandicum, all occurring in more than 50% of the plots. FurthermoreMolinia caerulea,Dryopteris carthusiana, Vaccinium uliginosum,Carex arenariaandDicranum scopariumwere found in more than one third of the plots.

We compared the vegetation composition in the afforested dune plots in Østerild with reference dune plots from the NOVANA monitoring and found that the vegetation in Mountain Pine stands have the largest floristic similarity with acidic grey dune and relatively dry dune heath, while plots in Scots Pine stands mainly share species with moist dune heath and relatively dry dune slack. The forest floor vegetation in the Sitka Spruce stands differs markedly from all open dune target vegetation types.

We found the highest average vegetation height in Scots Pine stands, while a distinct herb layer was absent or highly scattered in one of the Sitka Spruce stands. Furthermore, we found that two out of three Scots Pine stands had a relatively high cover of dwarf shrubs of whichVaccinium uliginosumwas the most abundant species followed byEmpetrum nigrum,Calluna vulgaris,Erica tetralixandMyrica gale. Dwarf shrubs were absent or highly scattered in the two Sitka Spruce stands.

We examined the cover and species composition of bryophytes and terricolous lichens (growing on soil surface). We found 18 bryophytes species and a relatively high cover of bryophytes in two of the three Scots Pine stands and the Mountain Pine stands. The most abundant bryophytes werePleurozium schreberiandHypnum jutlandicum, both very common species in coniferous forests.

Furthermore we found 7 terricolous lichen species, but a very low lichen cover in the forest floor. Lichen cover exceeded 5% in only 3 plots in the Mountain Pine stand.

We found a very high canopy cover in the Sitka Spruce stands with very few gaps for light penetration. Light penetration through the forest canopy was relatively high in one out of four plots in the Mountain Pine stand and plots from one of the Scots Pine stands.

We found highly acidic soils in the Sitka Spruce stands (pH around 2.75) and one of the Scots Pine stands. In the Mountain Pine stand and one of the Scots Pine stand soil pH was slightly higher, while in one Scots Pine stand 50% of the samples had a pH value above 3. Furthermore, we found that in grey dune (2130), dune heath (2140) and dune slack (2190) plots from the NOVANA programme, soil acidity was on average one pH unit higher than the afforested dunes in Østerild (3.8 on average versus 2.8). The low pH-values may impose significant constraints on the restoration of dune communities as acidification as it is known to lead to species loss in heathland and acidic grassland.

We found the thickest litter layer in one of the two Sitka Spruce plantations with a median litter depth of 12 cm. In the Mountain Pine stand the thickness of the litter layer was rather low, with a median value of 4 cm.  

Generally the measured carbon-nitrogen ratios in the top-soil were rather low. The highest values were recorded in the 4 samples from the Mountain Pine stand (median = 29), while the C:N ratio was considerably lower in one of the 3 Scots Pine stands (median =19). We found a significant negative correlation between soil pH and the C:N ratio, which indicates that the decomposition of organic material in the forest soil is closely connected with pH.