Temperature-related effects on the germination capacity of black locust (Robinia pseudoacacia L., Fabaceae) seeds

Germination is a key trait promoting species invasiveness and has become a major goal in invasion ecology. In this study, the effects of different scarification methods and temperature regimes on seed germination performance of the invasive exotic tree Robinia pseudoacacia L. were investigated. Ripe seeds were gathered from six collection sites in Tuscany (central Italy). Mechanical, chemical and thermal scarifications and no pretreatment were applied; seeds were incubated in a range (9–21°C) of constant temperature regimes. The scarification treatments proved highly important for the germination performance of seeds in term of both final response and germination rate in time, with mechanical scarification being the most effective treatment. Temperature significantly affected the germination rate and the mean germination time of the mechanically scarified seeds. The cumulative germination rate significantly decreased with the reduction of the temperature, with the highest value in the 18–21°C regimes (ca 98%), and the lowest in the 9–12°C regimes (ca 60%). The mean germination time slightly decreased with increasing temperature, but was substantially similar between 18°C and 21°C. Our data indicate that the temperature affected the onset of seed germination and that the lowest temperature tested delayed germination. These plastic germination responses, as well as the high germination capacity at lower temperature, may result in the spread of the propagule germination over a large time interval, contributing to the invasiveness of this species.