PREMISE OF THE STUDY: Aspen groves along the Niobrara River in Nebraska have long been a biogeographic curiosity due to morphological differences from nearby remnant Populus tremuloides populations. Pleistocene hybridization between P. tremuloides and P. grandidentata has been proposed, but the nearest P. grandidentata populations are currently several hundred kilometers east. We tested the hybrid-origin hypothesis using genetic data and characterized putative hybrids phenotypically. METHODS: We compared nuclear microsatellite loci and chloroplast sequences of Niobrara River aspens to their putative parental species. Parental species and putative hybrids were also grown in a common garden for phenotypic comparison. On the common garden plants, we measured leaf morphological traits and leaf-level spectral reflectance profiles, from which chemical traits were derived. KEY RESULTS: The genetic composition of the three unique Niobrara aspen genotypes is consistent with the hybridization hypothesis and with maternal chloroplast inheritance from P. grandidentata. Leaf margin dentition and abaxial pubescence 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 internal structure of leaves and water absorption may vary among taxa. However, reflectance values in the visible region did not differentiate taxa, indicating that traits related to pigments are not differentiated. CONCLUSIONS: Both genetic and phenotypic results support the hypothesis of a hybrid origin for these genetically unique aspens. However, low genetic diversity and ongoing ecological and climatic threats to the hybrid taxon present a challenge for conservation of these relictual boreal communities.
Bibliographical noteFunding Information:
We thank Jeffrey Carstens (USDA-ARS) for providing invaluable assistance in collecting root and leaf cuttings, sharing natural history knowledge, and suggesting collection locations. Bruce McIntosh (Western Nebraska Resources Council) and Joseph Zeleznik (NDSU) generously accompanied us in field collections and scouting of aspen stands. Mike Groenewold (Nebraska Game and Parks), Kevin Pape (Iowa DNR, Sioux City), Tim Hardy, Joe McNally (Iowa Arboretum), John Pearson (Iowa DNR), Todd Faller (Faller Landscape and Nursery), and the late Rick Hall (ISU) assisted with local collections and provided plant material. Pam Sprenkle and Gordon Warwick (NPS), Mike Groenewold (Nebraska Game and Parks), Rich Walters (The Nature Conservancy), Mel Nenneman (USFWS), Matthias Wallace (USFS), Dean Studnicka (Nebraska Game and Parks), and Mark Schneider (Iowa Arboretum) facilitated permissions and permits essential for our collections. Christopher Cole (UMN-Morris) provided early guidance with the aspen microsatellites. Laura Messman, Allen J. Butterfield, and ZhaaZhaawaanong Greensky assisted with propagation and outplanting of cuttings. Cathleen Nguyen conducted HPLC of plant pigments and CN analysis. Kali Hall and Shan Kothari analyzed fibers. Funding was provided by National Park Service grant #191779 to Jeannine Cavender-Bares, Mark Dixon, and Molly Nepokroeff. Finally, we thank two anonymous reviewers and the associate editor for helpful comments that improved the manuscript.
- Clonal growth
- Common garden
- Niobrara river
- Relictual communities
- Smith’s aspen