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Are you interested in how to adapt our natural systems to climate change?
Our debate today works with the article titled Facilitating adaptive forest management under climate change: A spatially specific synthesis of 125 species for habitat changes and assisted migration over the eastern United States from 2019, by Louis R. Iverson, Anantha M. Prasad, Matthew P. Peters, and Stephen N. Matthews, published in the MDPI Forests journal.
This is a great preparation to our next interview with Jasen Johns in episode 448 talking about urban and community forestry.
Since we are investigating the future of cities, I thought it would be interesting to see how our natural systems, specifically forests in this case, can be prepared for climate change. This article synthesises two decades of research to provide a comprehensive framework for managing species under the changing climate.
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Welcome to today’s What is The Future For Cities podcast and its Research episode; my name is Fanni, and today we will introduce a research by summarising it. The episode really is just a short summary of the original investigation, and, in case it is interesting enough, I would encourage everyone to check out the whole documentation. This conversation was produced and generated with Notebook LM as two hosts dissecting the whole research.
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Speaker 1: The trees in your local forest are currently trying to run away from home.
Speaker 2: Yeah, just at a very, very slow pace.
Speaker 1: Over millennia, as climates slowly change, tree species naturally migrate. They basically drop seeds slightly further north to chase the temperatures and rainfall patterns that actually keep them alive.
Speaker 2: But the problem is that they move at a glacial pace.
Speaker 1: Exactly. Natural migration maxes out at roughly fifty kilometres per century. But right now, human-induced climate change is shifting the environment underneath their roots at warp speed.
Speaker 2: So the climate a tree needs to survive is sprinting north infinitely faster than the tree can actually follow?
Speaker 1: Which brings us to the central question facing forest managers across the Eastern United States: Do we defend our native tree species at all costs through traditional conservation?
Speaker 2: Or do we embrace assisted migration, meaning deliberately picking up non-local tree species and planting them further north to secure our future canopy? Even if it risks creating an invasive ecological crisis.
Speaker 1: And grounded in a comprehensive spatial synthesis of 125 tree species, I argue that assisted migration is an absolute urgent necessity to prevent massive canopy collapse.
Speaker 2: And I argue for traditional conservation. Artificially relocating species is frankly a profound ecological gamble that seriously underestimates our current ecosystems.
Speaker 1: I see why you think that, but let me give you a different perspective by looking at the sheer mathematical reality in the data. The researchers used two models, Distribute 2 and Shift.
Speaker 2: The habitat and colonization models.
Speaker 1: Exactly. Distribute calculates where a tree’s ideal climate will be in the future, which is its habitat quality, and Shift calculates how far a tree can actually spread its seeds over that same time period, its colonization likelihood.
Speaker 2: And those two metrics start to diverge pretty aggressively.
Speaker 1: Yes, especially under high emission scenarios. You know, what the data calls RCP8.5, which essentially models what happens if we just keep pumping carbon into the atmosphere. The gap between habitat quality and colonization likelihood becomes an insurmountable chasm.
Speaker 2: But I come at it from a different way because we have to look incredibly closely at how these predictive models are actually built.
Speaker 1: How do you mean?
Speaker 2: When we talk about these climate scenarios, the models operate on incredibly coarse grids. We are talking about 10 by 10 or 20 by 20 kilometre squares. In the context of ecology, a grid that large is a blunt instrument.
Speaker 1: It’s an average.
Speaker 2: Exactly. It takes a massive piece of land, like a towering mountain, a river valley, maybe a shaded ravine, and just averages them all out into one uniform surface with a single projected temperature.
Speaker 1: But let’s look at the Allegheny National Forest in Pennsylvania as a prime example of why those averages matter Under that RCP eight point five scenario, native species that form the backbone of that forest today, like red spruce, red pine, black ash, they are projected to completely lose their suitable climate by the year twenty one hundred.
Speaker 2: Their survival capability rating drops to lost.
Speaker 1: Yes, literally lost. Expecting tree species to naturally outrun climate change is like asking someone to outrun a bullet train on a bicycle. The physical mechanisms like wind blowing seeds or animals carrying nuts a few miles away are completely inadequate.
Speaker 2: I hear you, but jumping straight to assisted migration because of an average migration speed of fifty kilometres per century ignores the nuance of the local landscape.
Speaker 1: In what way?
Speaker 2: Because the models completely miss local on-the-ground complexities like microrefugia. These are specific little pockets of a landscape that physically buffer native species from macroclimate changes. If a species is already abundant locally, that topographic diversity will likely buffer them far better than coarse models predict.
Speaker 1: I mean, that’s an interesting point, but I’d frame it differently. The models aren’t just a blunt instrument predicting doom. They actively identify the exact species that are ripe for assisted migration.
Speaker 2: You’re talking about the migrate species classification?
Speaker 1: Exactly. In the Allegheny National Forest, there are fourteen species trees like post oak, sweetgum, and sycamore that do not currently exist there but will have highly suitable habitat by the end of the century. The data classifies them as migrate plus.
Speaker 2: They have high biological adaptability.
Speaker 1: Yes, to thrive in the hotter, drier future climate of Pennsylvania. But they just can’t disperse their seeds fast enough to cross state lines. By planting them now, we guarantee that when the red spruce dies out, there is already a canopy ready to take its place. We have no choice but to put them on the train ourselves.
Speaker 2: But hold on. Why take the massive risk of introducing a sweetgum tree when the data offers a much safer, far more ecologically sound alternative?
Speaker 1: You mean the infill strategy?
Speaker 2: Yes. Let’s unpack that Why introduce a sweetgum to Pennsylvania, a tree with zero historical presence in that specific forest, when the Allegheny already contains rare naturally occurring species like mockernut hickory or scarlet oak? The data classifies these as infill plus.
Speaker 1: Meaning they’re already there, just uncommon.
Speaker 2: Precisely. And here is why that distinction is so biologically critical. These infill species are already fully adapted to the local soil microbiology. They already have symbiotic relationships with the specific underground fungal networks that allow roots to absorb nutrients.
Speaker 1: Sure, they have a local foothold.
Speaker 2: Right, and they are already recognized by the local insects. Promoting these rare local species respects traditional conservation boundaries while adapting to warming. It avoids rolling the dice on importing a stranger.
Speaker 1: I’m not really convinced by that line of reasoning, because I think you’re treating the introduction of a near native species as if we’re importing some genetically engineered alien life form.
Speaker 2: Well, from an ecological standpoint, it is alien to that specific grid.
Speaker 1: But we are talking about assisting the migration of a North American sweetgum or oak just a couple of states to the north, and we have to be willing to do this because of foundation species. Let’s talk about oaks.
Speaker 2: They are certainly the bedrock of eastern biodiversity. I won’t argue there.
Speaker 1: They support massive amounts of life. In the Mid-Atlantic alone, over 500 lepidopteran species, so your moths and butterflies rely on oaks to survive. And if you lose the caterpillars, you lose the massive bird populations that eat them. The entire food web collapses.
Speaker 2: It’s a keystone genus.
Speaker 1: But across the eastern forests, oaks are currently suffering from a mechanism called mesophication. They are failing to regenerate while being replaced by shade-tolerant maples.
Speaker 2: Which is largely due to our own history of fire suppression.
Speaker 1: Exactly. The maples cast incredibly dense shade, and oak seedlings need sunlight to grow. So the mature oaks are in the canopy, but their seedlings are dying in the dark. Now, the models consistently show oaks will thrive in warmer, drought-prone futures, but the highest oak diversity is currently clustered down in the South Central states.
Speaker 2: So they’re trapped down south.
Speaker 1: So if we refuse to assist the northward migration of the rich oak diversity currently clustered in the south, aren’t we guaranteeing the ecological collapse of northern forests by starving them of their future foundation?
Speaker 2: That is a compelling argument. The Lepidopteran food web is a critical factor. But have you considered the explicit warnings in the very research we’re discussing?
Speaker 1: The warnings about evaluating species?
Speaker 2: Yes. The source warns that while species will be moving, quote, “Not all will be perceived as valuable. We have to evaluate them for persecuting, protecting, or ignoring.” Think of moving a southern oak or a sweetgum into a northern forest like an organ transplant.
Speaker 1: An organ transplant? That seems a bit extreme.
Speaker 2: It’s really not. It might look like a perfect physical match for the new climate, the temperature is right, but the local ecosystem, the forest’s immune system, might react completely unpredictably. When you move a species outside its historic range, you are leaving behind its natural checks and balances.
Speaker 1: So without its natural predators-
Speaker 2: Sweetgum seeds and keep its population stable don’t exist in Pennsylvania. Without those predators holding it back, its population could absolutely explode. It could choke out the saplings of the very local info oaks we should be protecting.
Speaker 1: You’re saying it becomes an invasive species.
Speaker 2: Yes. We are directly risking the creation of the next invasive crisis. Just with a domestic tree instead of an international one. We simply cannot fully predict how a near-native tree will interact with a novel fungal or insect network.
Speaker 1: But isn’t the alternative worse? I mean, the risk of doing nothing, of letting the canopy simply die off as the climate outpaces it, and watching that entire bird and insect food web collapse, surely that outweighs the risk of a sweetgum becoming overly abundant.
Speaker 2: I just don’t buy that the choice is a binary between doing nothing and performing assisted migration.
Speaker 1: Then what’s the realistic alternative for the oaks?
Speaker 2: The solution is intensive local silvicultural treatments. We can use repeated prescribed fire, intentionally burning the understory to clear out those shade-making maples, and partial harvesting to open up the canopy.
Speaker 1: To promote the local oaks?
Speaker 2: Yes. This specifically promotes the regeneration of the local infill species that are already perfectly adapted to the ecosystem, rather than trying to play God and engineer a brand-new forest from scratch using models that can’t account for biological warfare.
Speaker 1: Well, as we summarize this, my position remains rooted in the staggering velocity of the changes we’re facing. The sheer mathematical reality of climate velocity makes assisted migration an ecological imperative.
Speaker 2: And you feel the models justify that intervention?
Speaker 1: I do. When habitat suitability moves north at a rate that completely dwarfs a tree’s physical ability to disperse seeds, traditional conservation risks becoming a recipe for extinction. We must proactively manage migrate species to ensure we actually have a forest canopy tomorrow.
Speaker 2: And my summary is that the infinite complexity of local ecosystems makes assisted migration a dangerous hubris, prioritizing our infill species Relying on local landscape refugia and using active silviculture is the only ecologically sound path. It respects the delicate balance of our forests.
Speaker 1: I think where we definitely converge is on the profound reality of the threat itself. Human-induced climate change is fundamentally altering the habitat suitability of Eastern United States forests.
Speaker 2: Oh, absolutely. The old do-nothing approach is simply no longer a viable option for forest management.
Speaker 1: We definitely agree there. Our divergence just remains on whether the risk of initiating an invasive ecological crisis outweighs the risk of total canopy loss due to slow natural migration rates.
Speaker 2: Which is a massive debate that will define the next century of forestry.
Speaker 1: It really is, and honestly, this reflects the immense value of utilizing spatial modelling to understand multiple futures. Listeners should really dive into the one-by-one degree grid data themselves to draw their own conclusions about the forest in their own backyards.
Speaker 2: It’s an incredible tool for visualizing what’s to come.
Speaker 1: So which path will best preserve the ecosystems of tomorrow? That’s a decision we’ll all have to weigh.
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