Thermal shock and germination in North‐West European Genisteae: implications for heathland management and invasive weed control using fire

Question: Is the stimulation of germination by thermal shock (resulting from the passage of fire) commonly observed for Mediterranean‐climate Fabaceae also apparent for NW European Genisteae (Fabaceae) species? Location: Southern England and NW France. Methods: The germination of Cytisus scopiarius, Genista anglica, Ulex europaeus, Ulex gallii and Ulex minor was examined following exposure to a range of temperatures (50°C, 65°C, 80°C, 95°C and 110°C) applied to seeds for 5 min. A sixth Mediterranean‐origin species ( Spartium junceum ) was also included since it is a common invasive in NW Europe and North America. Results: All five native NW European species displayed increased germination following thermal shock, even when seeds were heated to 110°C. However, there was some variation depending on provenance: in contrast to seeds collected from southern England, germination of French C. scopiarius seeds was unaffected by temperature. Spartium junceum germinated most at 95°C, but was the only species to show reduced germination when seeds were heated to 110°C. Conclusions: The NW European Genisteae appear to be pre‐adapted to the high temperatures associated with fire; a response attributable to their evolutionary origins in the fire‐prone ecosystems of the Mediterranean Basin. Consequently, projected increases in fire frequency linked to climate change may stimulate their regeneration in NW European heathlands, potentially altering the species composition of these ecosystems. Additionally, a clearer understanding of the interaction between thermal shock and germination may explain why fire has so frequently been ineffective in controlling invasive Genisteae populations worldwide.