Strandberg, M., Nielsen, K. E., Damgaard C. & Degn, H. J. 2011: Status og plejemuligheder for klokkelyngdomineret våd hede. Danmarks Miljøundersøgelser, Aarhus Universitet. 52 sider – Faglig rapport fra DMU nr. 820.

Summary

Two wet heathland areas with the ”cross-leaved heath” Erica tetralix (habitat type 4010) were investigated to determine the ecological status of the habitat type, causes for the decline of Erica tetralix and to develop a basis for management methods for this nature type. The Danish habitat monitoring programme (NOVANA) shows that the cover of Erica tetralix has declined by 10 % since 2004, when the programme was introduced. A summary of yet unpublished Danish observations from wet heathlands was presented which demonstrated that the decline has appeared and accelerated since the mid 1990s, where larger coherent areas with 100% cover of Erica tetralix still existed. At the same time, review of management literature made it clear that no proven methods for successful management of wet heathland have been published.

Because of this lack of knowledge concerning the causes of the decline and effective management methods, two approaches were selected to investigate wet heathlands with Erica tetralix.

1. Statistiscal analyses of inland wet heathlands with Erica tetralix obtained through the nationwide Danish monitoring programme – NOVANA (Svendsen et al. 2005).
2. Transect analyses, including species cover and structure (compactness), Nitrogen concentration in vegetation, soil morlayer (0-5 cm) C, N and pH, were carried out on the shooting range Borris Hede in western Jutland DK with the objective of quantifying the current status and diagnosing possible causes for the decline.

The analyses showed that both cover and vegetation structure of Erica tetralix in the Erica tetralix-stands were reduced, and that the condition of the individual plants appeared poor relative to expectations from samples collected in the 1980s and 1990s. The fact that notably the Purple moor grass Molinia coerulea and the Common heather Calluna vulgaris were present in the former Erica tetralix dominated areas indicated ecosystem change.

The analysis of data from the NOVANA programme demonstrated a significant relation between the decline of Erica tetralix and nitrogen deposition, indicating a threshold at a deposition between 12 and 14 kg N ha^-1 year^-1 (fig. A).

The measurement of the C/N ratio in the morlayer showed an unfavourable ratio of 21 – 26 under the Erica tetralix-stands, whereas the ratio in a healthy ecosystem would be above 30 (fig. B). This demonstrates that the frequent fire management on Borris Hede has been insufficient to remove nitrogen from the areas with wet heathland, probably because these areas are too humid to burn the morlayer in the season when management by fire is allowed.

The overall conclusion is that the N-deposition exceeds the critical load for nitrogen for wet heathland and that the management by fire in the season from September 1 to March 31 is not sufficient to avoid accumulation of nitrogen in the morlayer. This again leads to a slow change in the ecosystem towards dominance of purple moor grass Molinia caerulea at the cost of typical species such as Erica tetralix. Because the present deposition endangers the stability of the nature type wet heathland (type 4010), it appears that the high end of the presently accepted critical load interval of 10 – 25 kg N ha^-1year^-1 (UNECE 2004) is too high for this ecosystem. Figure A indicates that the upper limit for the Critical Load for Danish wet heathlands should be below 12 – 14 kg N ha^-1år^-1. Consequently, efficient management methods for this nature type need to be developed. Such management should take into account measures for restoration of degraded wet heathhlands, as well as management methods that can remove excess nitrogen and increase pH from the present level of just above pH 3 to values around pH 4.

References see the end of the report.

Full report in Danish in PDF-format (1,77 MB)