The Establishment of Rhododendron Ponticum in the Killarney Oakwoods, S. W. Ireland

Abstract

(1) Rhododendron ponticum was introduced into the Killarney area of S.W. Ireland during the 19th century, and has subsequently spread by means of large numbers of very small, easily dispersed seeds, throughout the semi-natural oakwoods (Blechno-Quercetum association). It is now a serious threat to these woods by shading the ground flora and preventing regeneration of native woody species. (2) The woods have been subject to almost continuous human disturbance and to grazing for at least four centuries, and they are currently severely overgrazed by the introduced sika deer (Cervus nippon). This disturbance and grazing is a major factor in aiding the spread of Rhododendron ponticum. (3) Seedlings of R. ponticum are closely associated with bryophyte communities on slopes. Successful germination appears to occur almost exclusively in bryophyte communities of less than 1-cm depth. These `safe sites' seem to provide the necessary humidity and light for germination, as well as offering some protection from adverse factors. (4) The successful establishment of R. ponticum is due to a combination of (a) the abundance of `safe sites', which have arisen principally, if indirectly, as a result of overgrazing and other disturbance; (b) its tolerance of shade; and (c) its unpalatability. (5) Seedlings which died did so mainly as a result of competition with other R. ponticum seedlings, or, in some years, because of drought. (6) Management of the woods in relation to R. ponticum control is briefly discussed.