ECR feature: Emily Schumacher on temporal climatic responses of the butternut tree.

Emily Schumacher is a research assistant at the Morton Arboretum in the USA. She is a conservation biologist interested in using genetic tools to infer tree restoration measures. Here, Emily shares her recent work on temporal climatic effects on the butternuts.

Emily Schumacher with butternut tree at the Morton Arboretum, Lisle, IL.

Personal links. Twitter | GitHub | Google Scholar

Institute. The Morton Arboretum

Academic life stage. Research Assistant

Major research themes. Conservation genetics; improving conservation collections; modeling rare species’ distributions; species range shifts after the last glacial period.

Butternut trees from West Virginia.

Current study system. Butternut (Juglans cinerea) is a tree with a sweet-sounding name but a troubled present. Already a rare tree in eastern North American forests, butternut has significantly declined in abundance in the last 50 years due to disease, habitat loss, and habitat change. All of its current populations are infected with a fungus (Oc-j) that kills butternut trees, limiting natural regeneration. Additionally, suitable habitat for butternut is dwindling as it is a cold-tolerant mesic habitat species, which are areas predicted to be heavily threatened due to climate change.

Recent JBI paper. Schumacher, E., Brown, A., Williams, M., Romero‐Severson, J., Beardmore, T., and Hoban, S. (2022). Range shifts in butternut, a rare, endangered tree, in response to past climate and modern conditions. Journal of Biogeography 49(5), 866-878

Motivation behind this paper. We can work to prevent butternut from going extinct, but we need to know more about its modern diversity and past biogeography. There are plans to make ex-situ collections to create breeding programs for butternut, but for that, we need to understand its current genetic patterns and the past range movements that shaped these patterns. Our work built on Hoban et al. (2010) which found that Quaternary glaciations largely shaped butternut’s genetic diversity. However, that study may have under-sampled butternut’s northern range. With additional northern population sampling of modern trees and fossils, along with distribution modeling, we examined if glaciations were the main driver of genetic diversity in butternut or if diversity was shaped more by the central-marginal hypothesis. Glaciations are predicted to leave populations at a species’ northern latitudes lower in genetic diversity due to successive bottlenecks as organisms migrate northward. Alternatively, the central marginal hypothesis predicts that genetic diversity will decrease at modern range edges, where habitat quality is lower and fewer individuals can tolerate marginal conditions, leading to bottlenecks and reduced diversity in range margins. We also aimed to better document the speed of butternut’s range shifted following glacial periods and how this shaped the genetic structure of a disjunct set of populations.

Mature butternut flowers.

Key methodologies. Our methodologies focused on expanding the examinations performed in Hoban et al. (2010) to provide a detailed picture of how influential glaciations were on butternut’s genetic patterns and the extent of butternut’s range during and following the last glacial period. We added 757 butternut individuals sampled mostly from butternut’s northern range to the 1,004 individuals used in Hoban et al. (2010). We then tested for statistical relationships between genetic diversity metrics and geography to evaluate for support for the central marginal hypothesis or postglacial migration. Also, to predict how butternut shifted in response to glacial movements and hindcast butternut’s suitability, we mapped fossil pollen records in 1,000-year increments. We then used these datasets to assess how butternut shifted its range in response to the last glaciation, which could also help predict its response to modern climate change.

Unexpected challenges. Combining data collected at different times and by various researchers was the main challenge in generating this research. Samples were collected during many trips spanning a decade and under different spatial considerations. Similarly, genetic data was generated at different times by different people, but gladly used the same markers. We performed analyses to ensure consistency in the data. Although incorporating data collected with multiple aims in mind and by different people posed difficulties, it also allowed our study to be of a much larger scale – and, therefore, more interesting and exciting!

Butternut tree from Hoosier National Forest with large, visible butternut canker.

Major results. We found that while the last glaciation had a large impact on the past range shifts of butternut, it was not the only process affecting butternut’s modern genetic patterns. Unlike Hoban et al. (2010), we found that genetic diversity was more similar to patterns predicted by the central marginal hypothesis rather than postglacial migration, because we found that genetic diversity was highest closer to the center of butternut’s range and lowest at its range edges. However, glaciation was still shown to have had a large impact: fossil pollen was found near glacial margins and hindcast suitable habitat maps predicted suitable habitat near glaciers. We also observed two distinct areas of butternut’s suitable habitat throughout the past 20,000 years, suggesting butternut may have persisted in two distinct refugia (one in southern North America and one in Canada, near Nova Scotia and New Brunswick). Our findings add to a growing body of studies suggesting that eastern North American tree species have relatively complex genetic patterns.

Next steps for this research. Our insights will help improve ex-situ collections for generating seeds and restoring butternut. Our findings that the New Brunswick population is a distinct evolutionary lineage with specialized climate requirements due to glacial movements suggest these individuals may also require special protection as an independent taxonomic unit. A full determination will require more in-depth phylogenetic analysis to support this differentiation, but this area is currently under protection interest. We would also like to examine more North American tree species to determine if they are similarly shaped by multiple ecogeographic hypotheses, facilitating genetic diversity conservation.

Butternut tree fully leafed out, Shenandoah National Forest, VA.

If you could study any organism on Earth, what would it be? I love studying organisms that are foundational species for ecosystems, hence my obsession with trees and plants. In particular, I am fascinated by rare trees. But it would also be incredible if I could study coral reefs as they are foundations for ocean ecosystems. Plus, because I live in Chicago, I miss the ocean!

Anything else to add? This project was highly collaborative, and I feel really lucky to have such encouraging and supportive senior scientists that led me through this process. My coauthors, the journal editor, and reviewers were so helpful and kind as I worked through my first first-author manuscript. A huge thank to all of them because the study was such a team effort! It was also fun for my supervisor, Dr. Hoban, to revisit his PhD study system and test and refine his findings from that time, which shows that science is constantly generating new knowledge!

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