The aspen (Populus spp.) population at Smith’s Falls, located in the Niobrara River Valley of northern Nebraska, USA, has long been believed to have originated through hybridization. One of the putative parents, quaking aspen (P. tremuloides), is distributed in isolated pockets in the region. The range edge of the other, bigtooth aspen (P. grandidentata), has retracted since the Pleistocene, and is now located 650 km to the east of Smith’s Falls. The genetic identity and diversity of the Niobrara aspens had not been rigorously assessed and the stand has shown recent declines in tree health and abundance believed to result from climate change-induced post-budbreak springtime freezing, and drought stress. To ascertain the genetic identity and diversity of the population, we compared nuclear microsatellite markers and chloroplast sequences of Niobrara aspens to those of their putative parental species. Individuals of the parental species and the putative hybrids were also grown in a common garden for phenotypic comparison using traditional and spectral techniques. To assess the aspens’ climate change vulnerability, we harvested stems and leaves from common garden plants and measured morphological and physiological traits related to freezing (phenology, stem electrolyte leakage, chlorophyll fluorescence) and drought (embolism vulnerability, leaf osmotic potential) tolerance.
Results/Conclusions
Molecular analysis indicated that the Niobrara aspen population consists of only three genetically distinct individuals and that these individuals resulted from a Pleistocene-era hybridization event between quaking and bigtooth aspen. Comparison of microsatellite markers and chloroplast sequences of these Smith’s aspens (P. x. smithii) suggests maternal chloroplast inheritance from P. grandidentata. Leaf margin dentition, abaxial pubescence, and budbreak phenology differentiated taxa, with the hybrids showing intermediate values. Spectral profiles allowed statistical separation of taxa in short-wave infrared wavelengths, with hybrids showing intermediate values, indicating that traits associated with the internal structure of leaves and water absorption may vary among taxa. We also found that the Niobrara aspens, like their parental species, are resistant to post-budbreak freezing, including at temperatures that they are likely to experience in future climate scenarios. Drought stress is likely to be a more important threat to these genetically unique hybrids as the Niobrara aspens share some of the morphological and physiological vulnerabilities to drought typical of both of their parental species. As this population does not appear to be sexually reproductive, conservation efforts should therefore focus on mitigating drought stress to stems resulting from ongoing asexual suckering.