“I can’t tell you how trees sing or cry,” Associate Professor Kai Zhu told the audience during the recent University of Michigan School for Environment and Sustainability (SEAS) Faculty Seminar Series. “But I can tell you whether they move or not.” The group of students, faculty and community members—all of whom had demonstrated their clear love for trees through their presence at this seminar—were hooked.

A climate scientist and global change biologist, Zhu has worked at the SEAS Institute for Global Change Biology since he arrived at U-M in 2022 while holding parallel positions with the Department of Ecology and Evolutionary Biology and the Michigan Institute for Data Science. In his talk, “Plant Biodiversity Responses to Climate Change: Contrasting Implications From Forests and Grasslands,” Zhu outlined recent research exploring how a changing climate will affect vital forest and grassland ecosystems throughout the country. While increasing temperatures and decreasing rainfall aren’t spurring trees to grow legs and walk away, it is causing the ranges of some key species to shift away from their geographic “homelands” over the course of a few generations.

Zhu divided his talk into two sections. The first discussed the findings of one of Zhu’s first major publications, a 2012 study on species range shifts in forests in the eastern United States titled “Failure to migrate: lack of tree range expansion in response to climate change.” The research, understanding that climate was changing over space and time, sought to understand how forests would respond. “How far, and how fast, do species have to move to perfectly track climate changing over space?” Zhu asked the audience.

Through predictive modeling and projections, his team was able to determine that the velocity of habitat change for many species was rapidly shifting in order to accommodate new temperature and rainfall thresholds, but there was one major problem. Very few of these predictions were backed by any observation studies.

A major reason for this lack of firsthand observational data is because trees simply live so much longer than people, and generational range shifts are therefore so much harder for scientists to track. However, thanks to an ongoing national forest inventory, or “census for trees,” being conducted by the U.S. Department of Agriculture, Zhu and his colleagues have been able to employ a new modeling technique known as longitudinal band analysis to more accurately record the distribution of seedlings compared to adult trees and infer migration patterns. “Individual trees don’t move—forests migrate through generations,” Zhu reiterated. Seeds are how these crucial species can adapt their ranges to new climates. 

Armed with these new datasets and modeling methods, scientists were able to compare observation results to the predictive models and arrived at a troubling conclusion: While trees should be shifting northward in response to warming climates, ranges are instead contracting. One explanation for this counterintuitive phenomenon (or, “failure to migrate”), Zhu suggests, is due to underground communities of fungi known as mycorrhizae.

Mycorrhizae are a symbiotic relationship between plant roots and underground fungi. The underground networks they form are mutually beneficial and often vital to the survival of the local plants and fungi, as the fungi pass on vital nutrients from the soil in return for extra carbohydrates from photosynthesizing plant cells. Alongside colleague Kabir Peay at Stanford University, Zhu has explored the macroecology of soil fungi by utilizing molecular data to interpret current distributions and project future suitable habitats for both tree and fungi species. The overlap of these projections points toward a disheartening "tree-mycorrhizal mismatch," posing significant hurdles for forest adaptation. If necessary mycorrhizal networks do not exist in the more northern geographies where seedlings are trying to take root, new forests won’t be able to flourish and effectively shift their habitat range.

In the second half of his talk, Zhu shifted his focus to ongoing research exploring ecological community structure in grassland habitats in California. Initial observations from experiments held throughout the California Floristic Province, an enormous biodiversity hotspot stretching from southern Oregon to California Baja Sur in Mexico, present bleak conclusions. Eighty percent of the area has experienced significant warming in the past 40 years, and twenty percent has experienced significant drying. This overarching trend of warming and drying is pushing localized grassland ecosystems, or “climate niche centroids,” to favor species that can better tolerate heat and drought while driving out or killing off those species that cannot.

To effectively measure these major factors, Zhu employed community temperature and precipitation indices to measure trends in overall temperature and precipitation changes of 12 selected climate niche centroids from across the region. These observational data were then corroborated by results from a related long-term experimental study, the Jasper Ridge Global Change Experiment, suggesting that community shifts are indeed driven by warming temperatures, and possibly accompanied by drying climates.

Interpreting these findings, Zhu concluded significant warming and drying is occurring, and that grassland ecological communities are changing as a result. Warming and drying means grassland climate niche centroids are gaining “warmer and drier” species, while losing “cooler and wetter” ones. By combining these observational and experimental datasets, Zhu has been able to find clear evidence that grassland communities are rapidly changing to match the pace of climate change. What this means for the future of grasslands species and communities, however, he couldn’t be sure.

Zhu concluded his talk by comparing the ways climate could impact forests and grasslands,  offering these new insights into the drivers of ecological change. Forests could change more slowly as a result of longer generations and microclimate buffering, while grasslands will likely change more rapidly due to their exposure to macroclimate trends and faster population turnovers.

While there are still some ways in which scientists can help mitigate the negative effects of such rapid change, such as through assisted migration of seedlings, the results from Zhu’s research provide critical information for scientists and nature enthusiasts in planning where and how future forests and grasslands will appear and look like. While trees and grasses can’t move in a typical sense, Zhu and his colleagues have shown that climate change can cause them to shift over time—and that the consequences can be serious.