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No. 175: Monitoring of recovering dune heath at Østerild 2019. Part 4

Wind, P. 2020. Monitoring of recovering dune heath at Østerild 2019. Part 4. Aarhus University, DCE – Danish Centre for Environment and Energy, 186 pp. Technical Report No. 175. http://dce2.au.dk/pub/TR175.pdf

Summary

The state-owned Østerild area, including the afforested plantations Østerild Klitplantage and the neighbouring Hjardemål Plantage, is located in Thy region in the northern part of Jutland in Denmark. The dominant tree species in the Østerild area are conifers that mainly are the introduced alien species Picea sitchensis (Sitka Spruce) and Pinus mugo (Mountain Pine) together with the original native species Pinus sylvestris (Scots Pine) which has been planted here.

The three species were the main conifers present in the afforested areas where the National Test Centre facility for wind turbines was established in 2011 and inaugurated in 2012. The assumption was that the starting conditions before the clear-cutting in summer 2011would have a major impact on the succession following deforestation.

Prior to afforestation in the late 18th and in the beginning of the 20th century, the dune areas in the Østerild area were characterised by a high-level, presumably fluctuating, water table. Consequently, moist and wet habitats were widespread as most of the area was and still is low-lying.

Therefore, successful regeneration of moist dune heaths and humid dune slacks (habitat type 2190) required recovery of the original hydrological regime. Thus, one of the implemented initiatives was to close drainage ditches and allow temporary pools and shallow waterbodies to develop or expand. Besides, various treatments of accumulated soil organic matter were planned in order to facilitate the recovery of the vegetation cover of grey dunes (habitat type 2130), dry dune heath (habitat type 2140), and the above mentioned moist habitat types. One of the aims in planning the monitoring program was to follow the succession in dry and moist dune habitats, including areas with seasonal flooding.

Aarhus University, Danish Centre for Environment and Energy (DCE), has developed the botanical monitoring program in 2011 prior to the clear-cutting of the conifer trees. The overall objective of the monitoring program running from 2011 to 2021 is to document the direction of the recovery of light-open dune habitats after the clear-cutting of the dune plantations in the Østerild area in 2011.

In accordance with the objective of the monitoring program, twelve monitoring sites were established in stands of the three main coniferous species. Hundred plots were appointed in the monitoring sites following a stratified random design in order to span the different times of planting of the different conifer stands and the applied regeneration measures. The baseline condition (forest type), the planned post-cutting treatments of the remaining stumps and the litter layer, the hydrology, the expected management regimes, the distance to appropriate seed sources, and the topography of the Østerild area were important parameters for the stratification. Thus, 20 plots were placed in the P. mugo, 30 in the P. sylvestris and 50 in the P. sitchensis stands.

Within the framework of the monitoring program, the first phase was to record the species diversity, the vegetation composition, and the soil condition prior to the clear-cutting of the dune plantations (the baseline monitoring). The next phase involved a systematic recording of the development of the vegetation composition and soil conditions (the post-construction monitoring) during the first 10 years after the clear-cutting in order to follow the changes from the baseline conditions towards recovery of light-open dry and moist dune habitats.

One of the objectives of the monitoring program was to assess the effect of the treatments on the rate and direction of vegetation development towards the target communities. Unfortunately, not all the planned treatments were implemented while the need for the clear-cutting of the afforested areas have had a lesser extent that originally planned causing that the trees in one monitoring site with plots investigated in 2011 have not been cut. Therefore, in 2017 thirty-five new plots were laid out in other parts of the clear-cut plantation areas of which five were placed close to the main unpaved field road of the Test Centre, partly covering the verge. None of the plots had been investigated previously neither in the baseline monitoring in 2011 nor in the preceding years.

Between 2017 and 2019, the establishment of a new telecommunication mast with its associated anchorage and the construction of an unpaved access road to the mast area from the existing main field road of the Test Centre have destroyed the three plots no. 32, 34 and 62. Besides, no. 31 and 35 have been heavily affected by the construction of the anchorage bases. A sixth plot, no. 17, was accidentally buried under a pile of wooden flakes. Thus, there are 96 plots left for the final investigation in 2021 of the development of the vegetation composition caused by the various treatments after the clear-cutting of the plantation in 2011.

In August 2019, DCE investigated the remaining 41 plots established in 2011 and the 35 new plots established in 2017. In accordance with the monitoring scheme, the 20 plots laid out in the former Pinus mugo stand in Hjardemål Plantage were not investigated in 2019. A pinpoint frame (0.5 * 0.5 m2) and a documentation circle with a radius of 5 m where the pinpoint frame function as the centre were used to investigate the composition of the plant species and vegetation structure in each of the 76 plots. The plots were recovered by the use of a GPS and the digital photos taken in 2017.

Within the pinpoint frame, all vascular plant species, bryophytes, and lichens were recorded and then supplemented with additional species in the rest of the 5 m circle. Besides, within the 5 m circle the total coverage area of bryophytes, lichens, bare soil and sand, the amount of dead wood, and the free water surface were estimated. The height of the vegetation cover within the pinpoint frame and general inclination of the plot were measured. An overall digital photo was taken of all of the 76 plots. All vascular plant species and some characteristic bryophytes and lichens were determined on the species level in the field. All collected data were immediately recorded on the field scheme. The names of the species, their presence in the pinpoint frame or in the 5 m circle, all the additional data, and the digital photos are compiled in the annexes of the report.

During the fieldwork in 2019, 122 vascular plant taxa were recorded in the 76 plots – 67 dicots and 34 monocots, including 16 grasses, and 9 sedges and rushes, respectively. Gymnosperms contributed with two species, and ferns and fern allies with three species. The cryptogam flora included nine determinable taxa of bryophytes and two lichen taxa.

Species richness varied widely between the 76 plots. Nine taxa were the absolute minimum number recorded in two plots in the former Pinus sylvestris and Picea sitchensis stands, respectively. The highest number of 39 taxa was recorded in a plot adjacent to the main unpaved field road where Picea sitchensis previously formed the canopy. In the former Pinus sylvestris stands the highest number of recorded in a plot was 30 taxa while in the former Picea sitchensis stands 23 taxa in a plot were the highest number. In the area where Pinus mugo previously dominated the lowest and the highest number in the plots were 14 and 21 taxa.

The dwarf shrub Calluna vulgaris was the most widespread species recorded in 97 % of the plots followed by the grasses Molinia caerulea and Avenella flexuosa, which were present in 89 % and 82 % of the plots, respectively. Other common species were the dwarf shrub Erica tetralix, the broadleaved herbs Hypochaeris radicata and Rumex acetosella, and the grasses Agrostis capillaris and Holcus lanata, although the two latter were not recorded in any of the 10 plots in the former Pinus mugo stand. Juncus effuses was the most widespread rush recorded in more than two-thirds of the plots while the most widespread fern species was Dryopteris carthusiana. Among the bryophytes, Dicranum scoparium and the invasive Campylopus introflexus were the most widespread species. The mean number of taxa per plot was 20.

The species quoted above were also the most widespread as the majority were found at all monitoring sites. Carex arenaria was an exception by not being found in any of the five plots along the main unpaved field road. The influence of the main unpaved field road on the species composition was obvious. In the five verge plots, nineteen species were more frequently or exclusively recorded in the vegetation cover.

The clear-cutting of trees in the project area has led to exposure of the bottom layer. The improved light penetration to the bottom layer has facilitated the spread of vascular plants, bryophytes, and lichens to the former afforested areas. Especially dwarf shrubs like Calluna vulgaris, Empetrum nigrum, Erica tetralix, and Vaccinium uliginosum and a number of prominent grass species like Agrostis capillaris, Avenella flexuosa, Holcus lanatus, and Molinia caerulea have benefitted from the improved light conditions. Besides, the 2019 investigation revealed the presence of two invasive alien species, Amelanchier spicata and Prunus serotina, not previously recorded in the plots.

In the plots, eighteen new common vascular plant species have been recorded presumable occurring in the surroundings of the National Test Centre. For instance, the establishment of the new telecommunication mast with its additional infrastructure in one monitoring site may have led to their presence because of the disturbance the construction activities have caused e.g. the construction of the access road paved with gravel and transport of building material.

  The prime objective of the clear-cutting of the former afforested areas in the project area was besides the establishment of the National Test Centre to try to direct the vegetation succession on areas not directly affected by the erection of the wind turbine facilities towards the target communities – dry and wet heathland and dune slacks. Secondly, the botanical project aims to improve the diversity of native species compared to the situation before the deforestation of the project area by creating suitable habitats for light-preferring, low-growing species with preference for nutrient-poor conditions, fluctuating water table, and shifting moisture regime. Whether the overall objective of the project has been successful must await the performance of a more in-depth analysis of all the gathered vegetation and additional data after the completion of the vegetation monitoring in 2021.