IN THE FACE of shifting weather patterns influenced by a changing climate, the garden can be a really confusing place these days. What stressors are coming next, and which plants will have the resilience required to stand up to whatever those prove to be?
With no group of plants is that more essential environmentally to think about, or trickier to figure out, than with the trees. Because of their long lifespans, a tree planted today will be reaching maturity in what may be a whole different world.
At Mt. Cuba Center in Delaware, the esteemed native plant research site, a new effort called the Resilient Tree Canopy Project is under way to begin to think about tree choices for the future.
I learned about that work recently from Mt. Cuba’s Director of Horticulture, George Coombs, a role he assumed in December 2018. Before that he was the Manager of Horticultural Research, and oversaw studies in its famed Trial Garden area, where species and cultivars of native plants are tested for their performance side by side.
We discussed how the Mt. Cuba team is beginning to assess native tree species for their roles in the landscape of the future. Plus: Be sure to have a look at Mt. Cuba’s extensive educational offerings, including many virtual programs.
Read along as you listen to the Nov. 4, 2024 edition of my public-radio show and podcast using the player below. You can subscribe to all future editions on Apple Podcasts (iTunes) or Spotify (and browse my archive of podcasts here).
trees for the future, with george coombs
Margaret Roach: Hi, George. Any fall color there? I guess that’s what we look to the trees for at this time of year.
George Coombs: Oh my gosh. It is like perfect fall color here right now. We’ve had such a dry, warm fall. I think that some of the best fall color I can remember here at Mt. Cuba.
Margaret: It’s funny, same up here in the Hudson Valley, I always thought that we needed moisture to have good fall color, but it seems like stress [laughter], drought stress, has brought out the wildest colors in the trees.
George: You know what, we were talking the other day that it’s just the fact that maybe the leaves haven’t fallen off quite as quickly, because the rain hasn’t come around to knock them off. So there’s more hanging on.
Margaret: Maybe. Who knows? But in a world of madness, I enjoyed the colorful show we’ve been having recently as a respite from all the madness [laughter].
So yeah, so the matter of which trees will make it, which trees will make up our forest canopy and understory in 20 or 50 years and beyond. It’s one I’m hearing more about lately from various people I interview, like Jeff Lynch at Wethersfield, a historic estate in Connecticut, Daniel Weitoish at Cornell Botanic Gardens, experts at Arnold Arboretum. I mean, they’ve all talked to me recently about thinking about this. So how did this become the topic of a project at Mt. Cuba and when? Give us a little quick history of that.
George: Yeah. So at Mt. Cuba Center, our gardens are little unique in that much of what we call the naturalistic gardens, which is the bulk of our gardens here, is developed from an abandoned cornfield. And so, a lot of the trees that we have as our existing kind of mature canopy, kind of germinated in this abandoned field around the same time. And so, we have this predominantly tulip poplar canopy [above, and top of page]. These trees are about 100 years old now, which is kind of the expected lifespan of a tulip poplar, when that’s maybe not what would live forever on this type of site.
And so, we’re finding ourselves with a kind of rapidly aging canopy, and we historically have not done a great job of allowing new trees to kind of regenerate underneath of this canopy, kind of the next generation waiting in the wings. And so, we recognized several years ago that we really needed to do a lot more tree planting.
However, I think to what you’re talking about with other folks is just, the trees that we plant today are going to be alive, living through and alive kind at the end of the century, hopefully, which is going to be a very different climate. And so, we just want to make sure that we’re planting the right things so that this kind of canopy that shades this beautiful garden that we have remains healthy and intact, and we’re not kind of setting ourselves up for future failures as we go about trying to prevent one from happening right now.
Margaret: Right. And there’s no guidebook [laughter]. We can’t look at the reference book. And I mean, I think gardeners listening, a lot of us were, they remember about a year ago last November, a lot of us were reclassified into warmer zones or half warmer zones, USDA hardiness zones. And it kind of felt like that made sense. People were like, “Oh yeah, that’s right. The dahlias, I forgot to dig up, came back for the first time.” [Laughter.] They overwintered or the perennial plants that were sort of iffy ones or zone-stretching ones were being marketed now in local garden centers, and are doing well in my garden, that kind of thing.
But it’s more than that. In the matter of looking ahead to say the year 2100 or even 2050, and especially with trees is more complicated. And it’s not just temperature shifts and frost dates, and literal hardiness of a particular plant in all its parts, but it’s about what the shift in climate does to trigger more pests and diseases, too, to give an advantage to their opponents, yes?
George: Yeah, I mean, that’s the other aspect to all of this, Margaret, is really just, there are so many threats for our native trees and not just in natural forest communities or things, but in horticulture we use a lot of native tree species in horticulture. And you’ve seen things with emerald ash borer, beech-leaf disease, there’s all kinds of what I would call species catastrophic diseases that really are going to change the way that our landscape looks and exists, and the types of animal interactions that these trees provide for the wildlife.
So there’s just a significant amount of change in that realm. And then, when you add on to it, this like climate change, and there’s so much that’s unknown about that and to what extent we’ll experience. But it’s just really stacking the deck against our trees in a way that we really need to be careful about what we may have been, just playing things fast and loose with species selections and where we put them, and all of those things. I think it’s going to have to get a lot more dialed in to avoid these large-scale failures.
Margaret: Yeah. So you have, at Mt. Cuba, a long history of research, your trial garden programs, like I mentioned in the introduction, but you can evaluate trees in a three-year or five-year garden planting. Plant rows of them and keep your eye on them, and watch this one versus that one. You can’t do that like you might a herbaceous plant, a perennial or an annual, or even a biennial or whatever. You can’t do that with trees. So what’s the methodology here? Because it is a different beast. [Laughter.]
George: That’s just it, it’s one of those things we will never have the perfect answer, because by the time that we actually know it’ll be too late. We kind of have to make our best guesses now and hope that we’re kind of being as informed as we possibly can.
Margaret: So you’re using current data, and data from other places that are warmer than you, or where are you getting any insights that you might utilize?
George: Yeah. So when we started this project, I think the most well-known source and rightfully so, is the U.S. Forest Service. They have a really very cool tool called the Tree Atlas. And this website provides a lot of information about how the Forest Service is modeling, how different species will react to climate change. So you can go on there and kind of pick out any kind of typical native species that you might see in the woods around you, and kind of look at different models based on the projected warming of, this species might shift 100 miles to the north, or it might just expand in general.
And so, we started there and when we started really digging into that data, it really was throwing off some red flags for us, at least from a horticultural standpoint, because some of the things that they were recommending didn’t really follow logically. And so, the more that we educated ourselves about their system, it became clear that they were modeling all of these kind of recommendations that they were providing on how a species would perform in nature. So not necessarily in a horticultural setting, in a kind of person-made environment, but it’s really about how that species would shift. So it depends on-
Margaret: So a self-sown seedling of whatever, the real next generation in a forest setting or something is that they might be thinking about, yes?
George: Yeah, like how many seeds does it produce? How is that seed distributed? How many plants currently exists in that type of forest in the region? All of those things. And so, obviously we’re kind of a lot more controlled and can choose things to put into our gardens that maybe wouldn’t naturally seed themselves into our area.
So that was kind of the main shortcoming there. So we looked at another tool that was a little bit more of a horticultural perspective, and that’s something called the BGCI Climate Assessment Tool. And BGCI stands for Botanic Gardens Conservation International, which is really an organization that compiles a lot of records that public gardens maintain individually, and kind of puts it into one data set. And so, they’re looking at… You can go to this website, the climate tool website and plug in a public garden near you and it’ll tell you, depending on the warming model, what the future climate is expected to be. And then, it uses that information to say, O.K., well we know that public gardens at these temperatures are growing these species, so we have kind of a general temperature range that we can provide you for a species. And they also have that information for naturally existing examples.
So it’s a huge data set, and with all this information, they’re kind of able to give you guidelines or boundaries of, at these temperatures you’ll see this plant growing in nature. And then it’ll give you kind of horticultural temperature ranges, too. And so, it’s really interesting to look at that because they’re not always the same.
There’s plenty of plants that will grow in warmer or cooler places than where they naturally grow. And that’s an important thing for everybody to keep in mind, because one of the best things that I learned in college, is plants grow where they compete best. It’s not necessarily they grow where they thrive the most, or what they can tolerate. It’s really about where they carve out this ecological competitive niche. And so, there’s gaps in our understanding about just how much you can push plants outside of where they naturally would occur. So some of this horticultural set is really helpful in picking at that a little bit.
Margaret: And then, I think I read in a document that you gave me that’s about what you’ve thought about so far and done so far with this project, that you also looked at something called, I think it’s the Future Urban Climate Web application. Did you also, where it kind of tells you what is thought to be the future climate in certain areas, what’s going to happen, the sort of predictions?
George: Yeah, I think that website needs a disclaimer. It can be a little scary to check it out.
Margaret: [Laughter.] I’d say so; my goodness, I thought I needed to take a sedative after I read that.
George: But it’s a really cool tool, and I think it’s a great way for people to kind wrap their head around what level of change is possible.
Margaret: Yes.
George: And so, this tool basically says you can pick any city near you. I picked a couple of different ones, Philadelphia, Dover, Delaware, etc. And it’ll give you in a moderate-warming scenario, what kind of other part of the country you would be most like. So for example, if you look at Philadelphia in a moderate-warming scenario, Philadelphia will be more like a city in southern Maryland. But in a high-warming scenario, Philadelphia could expect it to be more like Memphis, Tennessee.
Margaret: That put me over the edge, George, that was a lot.
George: Yeah, it’s wild.
Margaret: That was a lot. But I think in a way it brings to front of mind exactly why we need to be thinking about this for our longest-lived creatures and how essential this is, because this is where it very well may be headed or possibly is headed. Yeah, yeah.
George: And the bad news is I built this report or this kind of analysis on the analogs this web tool was providing a couple of years ago, and I recently revisited it, and it’s not being revised for the better. So it’s like the further in time we get, the more we understand where we will or will not be able to kind of hit this climate target, and it’s going to keep shifting.
Margaret: Right, right. So you looked for data that’s out there of different types, and analyses that are out there of different types, and you’re sort of taking all that into consideration. And again, this is specific to your location and another entity, another organization might be doing this for their area. Are you also taking into… Because you’re talking about in a human-made setting, as you said, using these trees, are there also cultural practices or any other shifts in the way you grow the trees or plant the trees, I don’t know, that you’re thinking of to give them better chances? Is that part of the analysis? I don’t know.
George: It is and it isn’t. So we’re very interested in this idea. Well, let me back up. So all of this data is based on yearly average temperatures, which all climate modeling information is based on this kind of yearly average temperature, which is a helpful metric, but it’s not exactly like what plants experience. They live and die at those extreme moments, where either it’s really cold or it gets really hot.
And so, you could have a mild increase in yearly average temperature and assume everything’s going to be fine. But what could actually happen is both of the extremes expand, or you get very kind of a erratic from year to year, or month-to-month temperature swings or weather swings, and all that is adding an extra layer of stress. And so, in terms of thinking about the horticultural aspects of how we plant these trees and where we want to put them, it just becomes all that more important to make sure that we’re siting them in ways that they are going to kind of be able to weather those stressful moments as best as possible.
Margaret: And as gardeners, we’ve always really, well, we should have always really been thinking about, it’s almost like you’re talking about microclimate in a way or hinting at that kind of thoughtful process of like, O.K, where am I placing this in this property that I’m managing or whatever? It’s not the north side and the south side and out in the field versus downhill/uphill. Those are not all the same places, even though they’re on your “property” or whatever. Do you know what I mean?
George: Exactly. When you look at a natural forest community on a hillside, there will be very different types of tree species on the south side than there are on the west side or the east side, and it’s very much different than the north side. And all of that has to do with the microclimates of it gets more sun during the hottest parts of the day. So the plant is under more water stress and different types of trees are better at dealing with those stresses. And so, that’s a level of detail that we’re not used to really thinking about in horticulture, but we probably need to start getting that specific.
Margaret: Right. So in this kind of, again, it’s not a report, but the beginning of the notes on what you’re doing that you shared with me, you have some charts of species of trees. And some are assessed by level, so to speak, using some of this data that we’ve just been talking about, and some that are kind of “no concern;” you’re rating them as no concern. And some that are “avoid,” a level 4: avoid, avoid, there’s a red light going off and a warning beeper or something, I think.
Were you surprised by, were you surprised by who ends up on what list? And can you tell us a little bit about that, how that happened?
George: Yeah, no, I wasn’t surprised. It was actually kind of reassuring. And honestly, I think the moral of this story or this project was there’s a lot of native trees that are going to do well. A lot of the things that are already in our area, we expect to continue to be here. So that was honestly a bit of a relief. So that was really great to see. And I think for us, some really common species that we expect to do well would be things like red maple, sweetgum, tulip poplar, white oak, sycamores. These are large, dominant trees in our landscapes, in our natural areas. [Above, swamp white oak, Quercus bicolor.]
Margaret: I saw Eastern red cedar, the Juniperus virginiana was on there and-
George: Yep, flowering all the way, all these things.
Margaret: Yeah. So familiar mainstays of the native landscape. So it wasn’t that they were all banished, so to speak, for their lack of resilience in this early analysis.
George: And I think we kind of have different tiers of confidence. So those that first level, what we call level 1, we don’t really expect them to have any issues. Even if we get to this high-warming scenario, which when you read more about that, it’s a little scary. And then, we have ones that are a little bit on this kind of gray area. We call them level 2, they’re likely to be fine. And so, those are plants like sugar maple, cucumber magnolia, chestnut oak. These are plants that for the most part we think that they’ll be good. But if we get to this high warming, they may start to develop some stress and struggle a little bit.
Margaret: Well, and something like the sugar maple, it’s been thought for a long time that yes, it may survive, but no, it’s not going to be a maple-sugar resource. It is not going to perform the same way in terms of the way that we think of it, as this source of this resource that we utilize. Do you know what I mean?
George: I do. Yeah. People often talk about sugar maples disappearing, and our work and analysis didn’t really show that. I could see maybe the forest that is dominated by sugar maple will look different, but as a tree species, it seems like it would probably do well here for the most part.
Margaret: Right, right, yeah.
George: And then as we get further down the levels, things start to get a little bit more dicey, where we feel like, O.K, there’s a couple of things that are going to be really stressed if we do get to that high-warming environment. If it stays moderate, they’re probably not going to be as much in jeopardy. And so, those are plants that we would still want to use, but we’d want to pay attention to them and just see how they’re performing, and make sure that our assumptions are panning out. And so, some things there like swamp white oak, yellow buckeye, butternut, which is a type of Juglans. So there’s a handful of things that we’re like, yeah, we’ll use them, but we’re going to use them sparingly.
And then kind of below that, that’s when we get to this category of trees that we want to avoid. Again, it’s not that we would never plant them. I think some of these plants do provide different kind of ecological value to different insects and things that we want to have in our garden, but we just want to be very thoughtful about where we put them that they’re not going to, if they do start to struggle, they’re not going to pose a safety hazard to any of our guests, or make sure that we’re not using them in large concentrations where if they were to fail, it would really be a big detriment to the garden.
Margaret: Right, right. And so I was, of course, I’m looking through the list, even though I’m in a completely different zone, I’m looking through the list of familiar species to me, and I wasn’t really that surprised that I saw some of the things like for instance, what I call moosewood, Acer pensylvanicum [above], I think. Is that striped maple, do you call it? Or also, I don’t know what the common name-
George: We call it moosewood, too. Yeah.
Margaret: Moosewood. O.K. And so, for me, that’s right inside the forest edge here, it’s an understory tree. It’s very, very common, but it likes that cozy place, where it’s moister, where it’s shadier, etc. I can’t imagine it dealing with hotter, drier, do you know what I mean? Its niche could be rendered inhospitable.
George: And I think it’s interesting, because we actually just planted a garden that has this as a very dominant feature before we did this project. And we’re kind of excited about that, because we’re going to be able to kind of monitor these trees very closely and see: Are they starting to struggle? If they are, what are they struggling with? Are they getting pests and diseases more often? Are they dying in droughts? Are they just kind of unable to handle the heat, and kind of wither in the hotter months? It’ll just be very interesting to watch these trees and their journey, knowing that they’re already there. We’re not going to remove them by choice, but we’ll be able to kind of watch their potential decline.
Margaret: Yeah, I mean, I don’t know if in your forest, their beeches are a big element, but they are here where I am, the native beech I mean. And there’s so much pressure on beeches and it’s just unimaginable what it would be, the forest would be without the beeches. Do you know what I mean?
George: It’s frightening.
Margaret: Yeah. So there are some species like that. I was just shocked, horrified, whatever. And again, I’ve heard about this from the person at Wethersfield, the historic garden in Connecticut, where that’s a big part of both their ornamental with the European species of beeches, as well as their native surrounding forest lands, of which they have hundreds of acres. That’s really, really scary stuff. So yeah, I don’t know.
So I want to make sure we have time to just talk about you, something about Mt. Cuba. You have how many visitors come a year now and so forth? It’s growing, growing, growing, I think, yes?
George: It is growing. We’re getting close to almost 30,000 visitors a year.
Margaret: And you’re open like April to November, I think. Is that right?
George: We are open April 1st, and we’re open through the weekend preceding Thanksgiving.
Margaret: Yeah. And one thing that people may not know about, but people listening might want to explore, your educational offerings are just incredible. You have some incredible virtual ones, as well as in-person. And speaking of the topic of trees that we’ve been speaking of, I know in February you’re going to be doing with a colleague, I believe, a presentation about some of this. So maybe you just want to, and it’s part of a three-webinar series, I think.
George: Yeah, so we have a winter lecture series that we do every winter. Starting in January there’s a talk and then there’s one in February, and then another in March. And they all are kind of typically tied together with a theme. And so, the theme that we’re going to have this year is just climate change in general. So Nicole and I, Nicole is one of our arborists here, we’re going to be talking about this tree canopy project in greater detail on Feb. 19th. So we’re on the February time slot, but there’s other kind of talks in this lecture series that people can watch. It’s all virtual, it’s all online. And it’s a really easy way to learn about some of the work that we’re doing and some other things related to climate change as we think about what do our gardens need to be doing differently as we navigate that.
Margaret: Well, George, I’m always so glad to speak to you, and like I said, it’s been too long, but thank you. And I’ll give the link to all the other educational offerings. It really is quite the lineup of great speakers, great presenters, and great topics for all kinds of gardening. So thank you. Thanks for making time today.
George: Thanks, Margaret.
(Photos courtesy of Mt. Cuba Center.)
prefer the podcast version of the show?
MY WEEKLY public-radio show, rated a “top-5 garden podcast” by “The Guardian” newspaper in the UK, began its 15th year in March 2024. It’s produced at Robin Hood Radio, the smallest NPR station in the nation. Listen locally in the Hudson Valley (NY)-Berkshires (MA)-Litchfield Hills (CT) Mondays at 8:30 AM Eastern, rerun at 8:30 Saturdays. Or play the Nov. 4, 2024 show using the player near the top of this transcript. You can subscribe to all future editions on iTunes/Apple Podcasts or Spotify (and browse my archive of podcasts here).
