Comparison of native and invasive Rhododendron ponticum populations: Growth, reproduction and morphology under field conditions

The objective of this paper is to identify the factors that promote the invasiveness of Rhododendron ponticum L. by the means of comparative field observations. Rhododendron ponticum (Ericaceae) is an evergreen shrub with a natural distribution in the Mediterranean and Black Sea area and was introduced to the British Isles in 1763, Thenceforward, the species has been considered as a major threat to natural ecosystems there. We compared native and invasive populations of Rhododendron ponticum with respect to trait patterns that are associated with invasiveness. Six populations each in the natural part of its range, in Georgia (Caucasus) and Spain as well as six invasive ones in Ireland were examined with regard to biometrical, morphological and ecological characteristics. Invasive Irish populations differed from non-invasive ones mainly in growth patterns and showed much higher rates of annual shoot growth in the field and higher rates of seedling recruitment. In contrast, native Spanish populations were discriminated by their shape and a-e; whereas native Georgian rhododendron, above all, showed distinctive leaf characteristics. In general, the relationship between Irish and Spanish populations was closer than to the Georgian ones. Our results suggest that both genotype and environment account for the trait pattern found in Irish populations. Differences in genetically fixed traits had a greater effect in morphological differences to Georgian provenances. In contrast, the invasive Irish rhododendron were favoured by a more benign environment than the Spanish populations.