A recent paper in Molecular Ecology investigates what ecological processes cause tree species' geographic distributions to shift in response to historical climate change. Jordan Bemmels, a graduate student in ecology and evolutionary biology at the University of Michigan, is first author. The researchers studied canyon live oak (Quercus chrysolepis), a tree species from a California region with a Mediterranean climate characterized by seasonal drought during hot, dry summers.

“Shifts in geographic distribution and changes in demography over long time periods will leave patterns in genetic variation across a species' range, so we can look at these patterns to infer what happened to the species in the past,” Bemmels explained.

The researchers built models to investigate what ecological processes and factors may have driven past climate change response. Their models showed changes in the distribution of canyon live oak over time since the height of the last Ice Age (21,500 years ago) and simulated genetic predictions from these models, which they compared to real genetic data to determine which models were most likely. They collected DNA from 257 trees from 46 sites across California.

“We tested competing models where the response to climate change is driven by different ecological factors, such as climatic niche, availability of topographic microsites, drought tolerance, cold tolerance, and local adaptation, and our results suggested that drought tolerance likely played an important role,” Bemmels said. Topographic microsites include canyons, steep slopes and mountain ridges where canyon live oak could have a competitive advantage over other tree species.

“In addition to the finding that seasonal drought tolerance is particularly important in determining how the species responds to climate change (which could have implications for other trees from Mediterranean climates more generally), the most novel aspect of the project is in how we were able to test different species-specific competing ecological hypotheses about the response to past climate change.

“We constructed distinct models that showed how the species distribution would have changed over time if, for example, seasonal drought tolerance in particular is most important, and what the genetic predictions of this scenario would look like. Modelling and genetically testing these types of hypotheses had not been previously done this way, and it allowed us to test more detailed hypotheses better informed by ecological theory than is typically possible.”

According to the paper, spatially explicit models, despite some of their limitations, provide a window into a diversity of questions that would continue to go unexplored without their application.

Geographic distribution shifted by following changes in climate variables, such as summer precipitation, that are related to seasonal drought tolerance. When these variables shifted due to climate change, canyon live oak followed in response.

Coauthors from U-M EEB include graduate student Pascal Title and Professor L. Lacey Knowles. Joaquin Ortego, formerly in the Knowles Lab, is now a Ramón y Cajal Research Fellow at Estación Biológica Doñana. The paper involved close collaboration between all coauthors and evolved out of a group project in the Knowles’ Statistical Phylogeography class. The paper was published online Sept. 12, 2016.