Ecological differences betweenCalluna-andEmpetrum-dominated dry heath communities in Drenthe, The Netherlands

Abstract

In dry heaths of the north Netherlands, both Calluna vulgaris and Empetrum nigrum may be the dominant dwarf shrub. The differences in growth form, vegetative and generative reproduction, leaf size and leaf inclination are discussed here, as well as differences in behaviour of the two species in and around shrubs of Juniperus communis. It is suggested that the horizontal leaves enable E. nigrum to endure more shade than C. vulgaris. E. nigrum also differs ecologically by not being attacked by the heather beetle or other insects, not being eaten by sheep, but being affected adversely by mowing, burning and sod cutting. It is more tolerant to burial by drift sand, but less tolerant to trampling. It prefers somewhat richer soils. The occurrence and even dominance of E. nigrum on very poor, heavily podzolized sand is probably due to either the former presence of a pine wood or to a combination of a heather beetle plague and overgrazing. The present threat to C. vulgaris by Deschampsiaflexuosa does not seem to affect E. nigrum. E. nigrum heath differs in structure from C. vulgaris by being both lower and denser, without a hollow structure, with a better developed moss layer and an even, smooth canopy. The microclimate is therefore moister and milder, with much lower temperature maxima and higher minima, particularly when there is a snow cover. Dry E. nigrum heath in Drenthe, The Netherlands differs from C. vulgaris heath by 17 differential species; C. vulgaris heath differs from E. nigrum heath by 26 differential species. In E. nigrum heath, hepatics and tree seedlings are more numerous, in C. vulgaris heath herbs and lichens are more numerous. The latter has been described as Genisto–Callunetum. The former is described here as a new association, the Orthocaulio–Empetretum. It is slightly more allied to coniferous scrub and forest associations than the Genisto–Callunetum. The same applies to its mycoflora, which is slightly richer than that of the Genisto–Callunetum and resembles in particular juniper scrub and conifer forests.