Invasion of Calluna heath by native and non-native conifers: the role of succession, disturbance and allelopathy.
The coastal heathlands of North-western Europe are treeless anthropogenic landscape of high conservation value, now threatened by encroachment by both native (Pinus sylvestris L.) and non-native (Picea sitchensis (Bong) Carr.) conifers. This study wants to gain insight into the different external drivers which interact with the natural successional processes to determine the future development of the heathland landscape. We tested how seedling emergence and establishment is related to competitive effects from the standing vegetation. Heathlands are nutrient-stressed habitats and we tested the seedling emergence and establishment along a nutrient gradient using mean Ellenberg values. In addition we tested if phytotoxic substances are affecting the seedling emergence and establishment related to different successional stages. Study species differed in seed size and we expected different colonization rates and competitive effects from the heathland vegetation. A factorial experiment was carried out in three different successional stages to isolate the effects of competition versus phytotoxins on seedling emergence and establishment by combining vegetation cutting and charcoal addition treatments. Both conifers were able to invade all successional stages, but invasibility decreased during succession. This was largely attributable to competitive effects from the standing vegetation, although phytotoxic effects were also detected, especially in late-successional stages. Seedling emergence and establishment was higher in the larger-seeded P. sylvestris than in P. sitchensis. These results present a management challenge as fire is an important component of the traditional management of the coastal heathlands, while at the same time increasing invasibility. Managers at sites susceptible to invasion should ensure that resources for tree seedling control are available before fire or other management-related disturbances are applied.