Penstemon digitalis

Abstract: Seed production may be limited because flowers do not get enough suitable pollen or because plants lack the resources to make seeds. We used replicated plantings to test factors that influence effects of bumblebee behavior on pollen limitation, as measured by the difference in seed set between hand- and naturally pollinated flowers, of Penstemon digitalis in patches of four to 41 flowering individuals. Seed set per flower was 376% higher in the largest as compared with the smallest Penstemon patches. This positive density dependence reflects activity of long-tongued bees, which (1) have higher effective density as patch size increases, (2) visit greater proportions of plants as patch size increases, and (3) visit smaller proportions of flowers per visited plant as patch size increases. Our results suggest that economics of flight and maneuverability of large, long-tongued bumblebees lead them to transfer more pollen between than within Penstemon plants in large patches. Density of smaller, short-tongued bumblebees was not positively associated with Penstemon seed set, but these bees may be important pollinators at low plant densities. Our experimental system indicates a clear positive relationship between activity of effective pollinators and seed set in a species capable of pollinating itself.

Abstract: Natural selection is thought to have shaped the evolution of floral scent; however, unlike other floral characters, we have a rudimentary knowledge of how phenotypic selection acts on scent. We found that floral scent was under stronger selection than corolla traits such as flower size and flower color in weakly scented Penstemon digitalis. Our results suggest that to understand evolution in floral phenotypes, including scent in floral selection, studies are crucial. For P. digitalis, linalool was the direct target of selection in the scent bouquet. Therefore, we determined the enantiomeric configuration of linalool because interacting insects may perceive the enantiomers differentially. We found that P. digitalis produces only (S)-(+)-linalool and, more interestingly, it is also taken up into the nectar. Because the nectar is scented and flavored with (S)-(+)-linalool, it may be an important cue for pollinators visiting P. digitalis flowers.

Abstract: 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’ Bn) 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.