Germination niche breadth and potential response to climate change differ among three North American perennials

Responses of species to environmental change are largely dependent on their niche breadth. To investigate the relationship between germination niche breadth and sensitivity to climate change, we selected three North American perennials: Physalis longifolia, Asclepias syriaca and Penstemon digitalis. Dormancy loss and germination requirements were determined for seeds from populations along a Midwest US latitudinal gradient. Fresh seeds were incubated at 1, 5, 9, 15/6, 20/10, 25/15 and 30/15°C, and seeds cold stratified at 1, 5 and 9°C for 4–12 weeks were incubated at 15/6, 20/10, 25/15 and 30/15°C. Germination niche breadth (Levins’ B_n) was calculated from final germination proportion. In addition, a sequence of temperatures evaluated the effect of future warming on germination phenology. Germination differed significantly among populations and collection latitudes (P < 0.001), but variation did not have a latitudinal pattern. Niche breadth was widest for Physalis and Asclepias and narrowest for Penstemon (P ≤ 0.05), with implications for germination phenology. Germination shifted to autumn for Physalis and Asclepias under future warming in northern collection regions, and shifted earlier in spring for Penstemon regardless of region. Due to limited stratification and germination requirements, resulting in the narrowest niche breadth, we initially predicted Penstemon to be most at-risk, as future warming would fall outside its stratification envelope. However, species with wide niche breadths (Physalis and Asclepias) may be more vulnerable to climate change due to maladaptive shifts in germination phenology.