Evolution of germination strategy in the invasive species Ulex europaeus.
Aims: The study of the adaptive potential of the germination patterns of invading species enables us to identify some traits linked with their capacity to colonize new sites, and to gain a better understanding of their area of distribution. The aim of this study is to determine the germination pattern of Ulex europaeus, a cosmopolitan invasive species, in relation to temperature and to explore its potential evolution and invasiveness in a tropical region, following its introduction from Europe. Methods: We studied the germination pattern of U. europaeus (the common gorse) to test both physical dormancy and germination capacity within the range of temperatures found in the native and invasive regions. To understand its germination pattern and its evolution, the rate and the speed of germination, as well as the percentage of seeds that became mouldy during the experiment, have been compared between a native habitat, France and a habitat into which it has been introduced, the tropical island of La Reunion. Important Findings: The results show that gorse seeds germinate in large quantities, possess the ability to germinate under a wide range of temperatures and they confirm the physical dormancy of the seeds (caused by seed coat impermeability). The decrease in germination from 25°C upwards, coupled with an increase in the rate of moulding help to explain its restricted distribution at altitude in tropical environments. For scarified seeds, we have not detected any difference between the two regions, neither in the percentage of germinated seeds, nor in the percentage of mouldy seeds. However, seeds from Reunion germinate faster at 20°C than seeds from France and a greater number of seeds from Reunion are able to germinate without scarification (10-60% for Reunion versus 0-10% for France). These results suggest that while preserving the advantages of the native habitat, in Reunion gorse develops a strategy which favours the rapid occupation of new sites.