Climate models predict that certain tree species will likely migrate northward in the coming century. But two studies led by researchers from the National Park Service and the University of Maine found several barriers that may make it difficult for tree migration to keep pace with climate change in the Northeast.
Park Service plant ecologist Kathryn Miller and UMaine Professor Brian McGill simulated seed dispersal and tree regeneration from Tennessee to Maine over the next 100 years based on climate predictions, the distance seeds of different tree species can travel, and how long it takes each species to grow from seed to mature tree. They found very different migration rates for oaks compared to pines, but more importantly, they found a large area from the District of Columbia to southern New Jersey through which trees could not migrate in 100 years despite suitable habitats becoming available.
“Species whose range ended south of D.C. couldn’t disperse northward at all,” Miller said. “They just couldn’t get past that barrier, which we attributed mostly to non-forest land use – farmland and human development.”
Part of the problem is that forest regeneration in that area is severely lacking, and even where regeneration is occurring, species regenerating on the forest floor are different from those making up the forest canopy. Miller and McGill call the phenomenon “regeneration debt,” which they say is caused by invasive species, too many deer, and extensive land development.
The researchers found that widespread farmland was another – albeit less severe – barrier to northward tree migration in central New York.
The implications of these barriers for forests in the Northeast could be significant.
“It means that the tree species predicted to gain suitable habitat in the Northeast aren’t going to make it here on their own,” Miller said. “If we just let things happen and assume everything will be OK, then the end result may not be what we want. We have to start to think about [what we’ll do] if the current tree species in the Northeast start to decline – like spruce and fir, which are predicted to be vulnerable to the changing climate. Then we’re going to have to be ready to figure out which species should be here to replace them.”
Miller is especially worried about what she said is the worst-case scenario: forests transitioning to invasive shrub thickets, which is already beginning to happen in some parts of the mid-Atlantic states.
“It’s happening not just because of climate but because the invasives and the deer are interacting and creating conditions that trees can’t regenerate in,” she said. “It’s not as widespread here in the Northeast yet, but the same invasive species that are causing problems in the mid-Atlantic are also up here.”
Without intervention, Miller believes that many areas can expect long-term declines in forest cover that will exacerbate the existing barriers to climate-driven tree migration. “This may already be occurring, as migration studies have yet to detect a strong signal of northern expansion for some tree species,” she said.