What’s Driving the East-West Divide in Trees’ Response to Climate Change?

As temperatures rise due to climate change, many tree species across northern North America have begun migrating poleward in search of cooler, more suitable habitats, a new study led by Duke University researchers finds.

The study, which is the first to tease apart and quantify the effects of seed production and tree recruitment — two critical factors that drive tree migration — provides new insights into how trees are responding to climate change at a continental scale.

It also sheds light on what factors might affect their ability to keep pace with warming temperatures in coming years.

“We see evidence of movement at the northern frontiers of many populations,” said James S. Clark, Nicholas Distinguished Professor of Environmental Science at Duke. “They are responding, but it’s not very fast and there are some important differences between what’s happening with species in the West versus the East.”

Seed production determines trees’ capacity to regenerate after large-scale disturbances by dispersing seeds to habitats where the odds of future survival are more favorable. Recruitment is the number of young trees, grown from those seeds, that survive at least to an age when their trunks measure 5 inches in diameter.

In the arid West, seed production appears to be leveling off or declining, but it remains high enough for now that, coupled with healthy recruitment rates, tree species at higher latitudes are keeping pace with the northward spread of warmer temperatures. The question is: For how long? And can it compensate for forest losses due to decades-long droughts coupled with increasingly intense wildfires?

“If seed production continues to decline, as our models suggest, that could exacerbate the effects of large-scale diebacks,” said Clark.

In the East, on the other hand, tree migration currently lags behind climate change because while recruitment remain strong at higher latitudes, the center of seed production, the warm and moist Southeast, is located too far away from these northern frontiers to help facilitate species’ poleward movements.

“The centers of seed production and recruitment for most species don’t align as well in the East as they do in the West, so for now, Eastern migration is being limited, in part by seed production,” said Shubhi Sharma, a doctoral student in ecology and evolutionary biology at Yale University who led the study while she was a Master of Environmental Management (MEM) student at Duke.

The outlook for Eastern forests may, nonetheless, be brighter than for the West because “there’s still a lot of seed being produced, so even as centers shift we may be OK,” said Clark.

The researchers published their peer-reviewed study Jan. 18 in the Proceedings of the National Academy of Sciences. They used statistical software developed by Clark to analyze data on hundreds of thousands of trees, representing 81 species, in the MASTIF monitoring network, which includes more than 500 long-term field research sites nationwide.

“Jim’s model let us drill down into a massive amount of data, much of which didn’t exist before or wasn’t used for this purpose, and predict fecundity at specific locations,” Sharma said. “That helped us see patterns in seed production and recruitment we otherwise would have missed.”

Being able to identify and predict those patterns is critical for future forest conservation and management, she said.

Forty-five researchers from 30 institutions co-authored the study. Many, including Jonathan Myers, associate professor of biology at Washington University in St. Louis, directed the painstaking work of collecting and analyzing seeds at the different MASTIF network sites.

“Our team has collected, counted and identified seeds of every woody plant species in 200 seed traps distributed across (our research forest) each year since 2012,” Myers said. “As one of only two research sites located in the central forests grasslands ecoregion, our Tyson ForestGEO Plot helps to bridge an important biogeographic gap in the MASTIF network.”

The new study was funded by the National Science Foundation, Belmont Forum, NASA, and France’s Programme d’Investissement d’Avenir (“Make Our Planet Great Again”) initiative.

Sharma received her MEM in ecosystem science and conservation from Duke’s Nicholas School of the Environment in 2020. In addition to Clark’s primary faculty appointment at Duke’s Nicholas School, he holds an appointment at the Université Grenoble Alpes through the Institute National de Recherche pour l’Agriculture, l’Alimentation et l’Environnement.

CITATION: “North American Tree Migration Paced by Climate in the West, Lagging in the East,” S. Sharma, R. Andrus, Y. Bergeron, M. Bogdziewicz, D.C. Bragg, D. Brockway, N.L. Cleavitt, B. Courbaud, A.J. Das, M. Dietze, T.J. Fahry, J.F. Franklin, G.S. Gilbert, C.H. Greenberg, Q. Guo, J.H. Ris Lambers, I. Ibanez, J.F. Johnstone, C.L. Kilner, J.M.H. Knops, W.D. Koenig, G. Kunstler, J.M. LaMontagne, D. Macias, E. Moran, J.A. Myers, R. Parmenter, I.S. Pearse, R. Poulton-Kamakura, M.D. Redmond, C. Reid, K.C. Rodman, C.L. Scher, W.H. Schlesinger, M.A. Steele, N.L. Stephenson, J.J. Swenson, A.V. Whipple, T.G. Whitham, A.P. Wion, C.W. Woodall, R. Zlotin and J.S. Clark; Jan. 18, 2022, Proceedings of the National Academy of Sciences. DOI: https://doi.org/10.1073/pnas.2116691118

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