Prescribed burning of heathland vegetation increases seed germinability and seedling emergence of introduced Scotch broom (Cytisus scoparius) in Northern Europe

Prescribed burning is a management technique to maintain coastal heathlands in Northwestern Europe. Fire rejuvenates characteristic plant species but may also stimulate regeneration of undesired plant species. In Denmark, Scotch broom (Cytisus scoparius) is an alien species spreading rapidly in vulnerable heathlands. The role of high temperatures in stimulating seed germination in Mediterranean-climate Fabaceae plants is well-known, however the germination response of Northern European C. scoparius to fire remains largely unknown. The effect of prescribed burning on regeneration of Scotch broom in Danish heathlands was assessed by (i) comparing germination responses of C. scoparius seeds collected in a burned and an unburned heathland area, (ii) evaluating regeneration from seeds and vegetative reproduction under natural conditions, and (iii) treating fresh seeds at different high temperatures to investigate the importance of fire intensity on seed survival and germination. Prescribed burning increased the germination percent and speed of germination of C. scoparius and caused a significantly higher Scotch broom seedling abundance in the field 1.5 years following the fire. Increased germination of seeds from the burned area was likely caused by high temperatures, as heat treatments at 65–95 °C stimulated seed germination. At 95 °C and 110 °C the proportion of dead seeds increased significantly, indicating the upper thermal tolerance limit of the seeds. In conclusion, application of prescribed burning with low fire severity aids generative reproduction of C. scoparius in North European heathlands, whereas fires with higher severities can provide invasion control if temperatures exceed the threshold for seed survival.