LAUSR

Scotch broom (Cytisus scoparius) is a global invasive species disrupting native species and habitats in several parts of the world. Its natural distribution spans across Europe, but seeds have also been transferred between countries, mixing plants of different genetic origins. In Denmark, Scotch broom consists of two gene pools: one is found in relatively small populations and is considered native; the other one is presumably introduced and is spreading as a noxious weed. To assess climate thresholds controlling the geographic ranges of the two gene pools, we evaluated seasonal changes in freezing tolerance (electrolyte leakage) and underlying variations in carbohydrate metabolism (HPLC) and water status of genetically different source populations representing the two gene pools. The data reveal that the introduced type is less freezing tolerant than the native type with differences being greatest in early autumn and when plants are most cold hardy, lending support to the hypothesis that insufficient freezing tolerance currently limits range expansion of the introduced Danish Scotch broom. In both gene pools, parallel increases in freezing tolerance and concentrations of sucrose and raffinose and decreasing levels of starch indicate the mobilization of soluble sugars from storage carbohydrates to achieve maximum freezing tolerance. However, genotypic differences in freezing tolerance are not explained by differences in carbohydrate metabolism, but may be related to different water contents. The observed low temperature limit for the introduced C. scoparius could provide valuable information for building process-based species distribution models predicting its geographic distribution and range shift following climate change.