Brunbjerg, A.K., Ejrnæs, R. & Nygaard, B. 2022. Restoration of dune habitats in Østerild. Results from the monitoring programme 2011-2021. Aarhus University, DCE – Danish Centre for Environment and Energy, 42 pp. Scientific Report No. 496 http://dce2.au.dk/pub/SR496.pdf

Summary

The overall objective of the monitoring programme was to document the long-term effect of the restoration initiatives taken in Østerild Klitplantage to create open dune habitats following clear-cutting of the dune plantations in the National Test Centre facility for wind turbines.  

In order to facilitate the restoration of grey dunes (Annex I habitat type code 2130), dune heaths (2140) and humid dune slacks (2190), we implemented various treatments of hydrology and accumulated soil organic matter. The monitoring programme included an assessment of the effect of these treatments on the rate and direction of vegetation development towards the target communities. The monitoring programme included a recording of soil conditions and plant species composition prior to clearing of dune plantations (baseline monitoring in 2011), a systematic recording of the changes during the first 10 years of the succession towards open dune habitats (post-cutting monitoring) as well as a final monitoring in 2021. This report presents the final analyses and results from the final monitoring.

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

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 1) baseline condition (forest type), 2) planned post-cutting treatments of litter layer, 3) hydrology and 4) expected management regimes (grazing).

The monitoring programme was designed to identify the most important site conditions as well as the best post-construction treatments for a successful development towards target communities. We considered treatment of the litter layer as a key part of the restoration process, as a thick litter layer in the coniferous forest may constitute a major constraint on the restoration of natural dune habitats. Coniferous litter is acidic, and the decomposition rate is very low. This leads to an accumulation of needles, cones and twigs on the forest floor. In the implementation plan, four different post-cutting experimental treatments of the accumulated organic matter were suggested: 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). The monitoring plots were thus distributed in the clear-cut areas in order to detect effects of the different treatments of the litter layer. However, the implemented post-cutting treatments turned out to be more uniform than expected when the monitoring programme was designed. The tree stumps and litter layer were thus treated by the same methods with stump crushing (158 ha) as the most widespread treatment followed by stump removal (56 ha) and depth milling (12 ha). Only minor areas were left untreated (control) and no areas were treated with burning. This has reduced the possibilities of using the monitoring data to evaluate restoration potentials of different methods.

Successful restoration of moist dune heaths (2140) and humid dune slacks (2190) also required focus on restoring the original hydrological regime. Prior to afforestation in late 18th century, the dune areas in Østerild and Hjardemål Plantage were characterised by a high, and presumably highly fluctuating, water table and, consequently, moist and wet habitats were widespread in the area. One of the planned actions in Østerild was thus to close drainage dykes and allow temporary pools and shallow lakes to develop or expand. The monitoring programme aimed to follow the succession in dry and rewetted dune habitats, including areas that were seasonally flooded. The restoration initiatives were expected to create suitable habitats for light-preferring and low growing vascular plants, bryophyte and lichen species, species demanding nutrient-poor conditions and species depending on a fluctuating water table and changing moistness. The implemented actions lead to vegetation resembling wet dunes in the national monitoring programme, but our results did not indicate full restoration of the natural hydrological conditions with high and fluctuating water table.

Baseline monitoring was carried out in July 2011 prior to clear-cutting of the plantations, and the methods were based on the variables in the National Monitoring and Assessment Programme for the Aquatic and Terrestrial Environments (NOVANA) (Fredshavn et al. 2011). We recorded the plant species composition and vegetation structure in a pinpoint frame (½ x ½ 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).

Post-cutting monitoring of the vegetation was conducted in 2013 (20 plots in the dune area formerly dominated by Pinus mugo), 2015 (20 plots in the dune area formerly dominated by Pinus mugo), 2017 (all plots, n=100) and 2019 (n=76). The final monitoring was carried out in August 2021 (n=91) using the same methods as for baseline monitoring. Due to various construction work and deviations from the planned treatments, only 59 plots were monitored in 2011, 2017 and 2021. These were used for analyses comparing conditions in 2011 and 2021.

During the 10-year period after clear-cutting, we found that:

• Soil pH increased from very acidic conditions prior to clear-cutting to levels almost comparable to the target habitat types.
• Although the altered water regime did create new moist habitats in the project area holding species depending on fluctuating water coverage, a pronounced overall effect of changes in hydrology on species richness and composition was lacking.
• The number of vascular plant species increased significantly from 2011 to 2021, and grazing seemed to have a positive effect on species numbers. Forest stand age had a negative impact on plant species richness, indicating a need for detailed planning of restoration initiatives accounting for differences in forest stand age.
• Vegetation composition obviously changed after clear-cutting, with relatively quick colonization and establishment of species.
• The cover of sedges increased significantly overall, but the degree of dwarf shrub cover was dependent on forest type. The dominance of grasses compared to dwarf shrubs indicated high nutrient contents.
• The vegetation composition in previous Pinus sylvestris stands resembled the vegetation composition in previous Picea sitchensis stands more than Pinus montanus stands.
• The invasive bryophyte Campylopus introflexus showed a considerable increase in occurrence, but a decrease in coverage.
• The nutrient content (Ellenberg N) was higher in wet dunes and dwarf shrub heaths in Østerild as compared to a NOVANA reference data set.
• A number of plant species adapted to relatively nutrient poor conditions had colonized the former plantations, and c. 57 % of the plots (n=48) were predicted to belong to dry dunes and heaths. However, more than 11 % of the plots were predicted to belong to non-habitat nature, indicating that restoration initiatives were not fully successful.

We recommend introduction of grazers in natural densities to cause and maintain disturbance after clear-cutting and to investigate the effect of burning of litter (not tested in this study) as means to increase pH levels. The implemented actions did not allow for concluding on the effects of stumps and litter treatments, nor for recommendations on the overall optimal restoration procedure. 

Restoration of natural hydrology was not complete, most likely because closing of part of the ditches was not sufficient to rewet the full Østerild area. Full restoration of hydrology was restricted due to consideration of the technical facilities and privately owned agricultural areas.