A recent article on the High Line blog is in recognition of Arbor Day and our shared appreciation for trees — there are 1,340 of them on the High Line (!) — we sat down with Bill Logan, renowned arborist and author, and Luka McLean, High Line horticulturist and member of our brand-new tree maintenance team, to talk about the unique joys, challenges, and impact of taking care of trees in the city.
High Line: Arboriculture, in its simplest definition, is taking care of trees in service of people. How do you consider the impact of trees on biodiversity—other plants, pollinators, wildlife, etc.—in addition to garden visitors?
Bill Logan: I like that question a lot. When we’re working in arboriculture, we’re not just working on specific trees, but we’re working to create—you could call it an ecosystem, but I prefer the word holobiont.
Lynn Margulis (née Sagan), an important evolutionary biologist, was studying the eukaryotic cell—the modern cell out of which all of the higher animals and plants are made—and she proved that that cell was itself a community. They were a community of cells that had previously existed as prokaryotes that decided to cooperate. Those cells developed a way to reproduce that would allow them to create that cooperative organism. So her idea is that even at the cellular level, we’re not talking about individuals, we’re talking about communities. And she called that cell a “holobiont.” We may study the larger living thing and consider it a whole, but it’s actually composed of all these other living things.
And Lynne Boddy, the great English mycologist, now does the same thing. She will look at an oak tree, and say the oak tree is the center of this holobiont that includes an extensive microbial world.
And other people are even going, “Let’s look at the holobiont as the entire earth.” But as people who care for, design, and love urban gardens, we might decide on the boundaries of a holobiont where we wish to study a whole, be it a garden, a city, a forest, a region.
So when you look at a place like a High Line, we need to look at the whole thing, rather than talking about the individual elements, because the individuals are subservient to the whole creature. The super-creature is really the garden itself, which benefits people for sure, and we can talk specifically about those benefits. You can see by the number of people on the High Line and how much delight they get from using the space. But you also see all the wildlife and insects that come there, too.
So the High Line really acts as a holobiont, and we have to treat it as a whole, which I think is wonderful and necessary. This type of biodiversity in one place makes people happier. At this time of year, we may go out into the city and see bulbs coming up, like daffodils—we expect those and forsythia—and you check them off, and you kind of walk on. But when you see a whole garden that’s all operating together, it has a different effect on human beings as well as on all the other creatures, because they’re experiencing a larger living being.
High Line: That’s wonderful. In thinking of your answer of looking at the whole, how can you do that while you’re focusing on the trees? How does that focus in service of the whole, or how are you able to look at that big picture through your work specifically with trees?
Bill Logan: Partly, we try to create a more diverse palette of tree species in the garden that will, hopefully, attract other organisms. One example I think about often is a tree that we never used to use: chestnut oak, which wasn’t used for many years. In reality, they’re valuable trees because they’re tap rooters, they do well in difficult situations, and they’re host to a whole set of organisms. Oaks, in general, are great because they also often have a very wide set of organisms—both macroscopic and microscopic—that live in communication with them.
That’s just to say that adding a chestnut oak, for example, into a planting would have an effect, not only because of the beauty of the tree itself, but also because of the impact of all the things the tree will attract. There’s also an impact on the other plants that surround it. We may be looking plant by plant, but what we’re trying to do is improve the way in which the overstory and understory plants work together. So very often what we’re doing is going in during dormancy and looking at ways that we can—without doing any damage to the trees—open them up and let more light come through. That way, the understory can be richer. And maybe that’s my prejudice but I think making the holobiont richer, the ecosystem richer, is far better.
Luka McLean: The holistic view Bill has so wonderfully described is also central to the High Line horticulture team’s work. From our team’s long-term goals for the health and beauty of the garden, which are part of the five-year strategic plan we’re in the midst of, we bring in partners like Bill to help provide expertise and guidance when we need it.
The High Line has a newly formed tree maintenance team—that I’m part of—which is putting that holistic view and more specific guidance into practice with our trees. That means, for example, pruning the trees to clear up the canopies so the understory can thrive as much as the other plants in a garden section. And while certain tasks may focus on individual trees or garden sections, we need to continue to consider the entire park and have that big picture in mind.
As a horticulturist on the High Line, one of my main jobs is to take care of the garden and improve upon it. The best way to do that is by applying knowledge and best practices—sometimes gained from professionals like Bill or others on our team—and keeping that holistic picture in mind.
High Line: What is the role of an arborist and how might arboriculture be distinct from urban forestry?
Bill Logan: I remember when I was getting started in this field 30-some-odd years ago, urban forestry was regarded as something brand-new. When I started an arborist company based in New York City, people looked at me like I’d lost my mind. They said, “There can’t be enough to do there!” I would reply, “What do you mean? We’ll never finish—there’s so much to do!” And that was because people didn’t know about urban forestry at the time.
As arborists, we have to be able to work back and forth quickly and seamlessly between the holobiont and the organisms that are part of it. It is such a pleasure to look at and to touch a tree.
I often tell people that I can be in the worst mood in the world. But when I knock on the client’s door, I say hello and walk through to look at the tree in their back garden. And suddenly I feel a lot better because I’m looking at this living thing and I’m saying, “What should we do? How can we improve this?” There’s a lot of problem-solving on a more focused scale, which has a visible, tangible impact.
Luka McLean: I agree with that. The role of the arborist is specific to the individual trees and woody material (shrubs), and looking at the big picture through the impact of trees, which are the largest—and often most impactful—organisms in the garden. All the players at work towards the success of the garden include a large group of individuals, not just arborists, but landscape architects, city planners, non-horticulture park staff, and even volunteers, in some cases. The High Line has such a rich culture of volunteering and learning. It’s great to involve volunteers—and even Teen staff—in all areas of our horticulture work.
High Line: Thank you, both. Could you tell us: how do trees, and by extension arborists, contribute to making New York City healthier, in terms of the environment and its people?
Bill Logan: We know a lot about trees in terms of ecosystem services (or how trees benefit the environment). So we know that trees and woody plants sequester carbon and absorb and clean air pollutants. They’ll regulate the city temperature, cooling it in summer. But they also open up to light in the winter, which is important and less frequently discussed. When a deciduous plant loses its leaves in the winter, it brings more light into buildings. People also often suffer from a lack of light in the winter, so the fact that the trees have no leaves then is a positive benefit to their psychic health. Trees also improve stormwater by keeping runoff from happening. They also often provide the kind of nucleus for a holobiont to form: they’re habitat.
I believe one of the most distressing experiences for people is losing their connection to the living world. So every time people come into contact with trees—walking along the High Line or any place they do it—they’re establishing a valuable relationship. We talk about forest bathing (editor’s note: defined as a conscious process of relaxation in natural or forested environments), but you can do forest bathing in an adventitious forest of all volunteer plants that have just come up because somebody stopped mowing the area.
Luka McLean: I would add that arborists play an important role because they’re maintaining the trees that provide all this quantifiable and non-quantifiable value to people. It’s especially important in cities like New York City, where there’s a lot of hardscape—or pavement and buildings—and fewer natural or planted areas. I’m proud of what the High Line provides and how I support that through my work. By gardening with a holistic view, we can ensure all these positive benefits, in addition to creating beautiful gardens.
High Line: Bill, this question is for you. In your latest book, Sprout Lands: Tending the Endless Gift of Trees, you took a closer look at the historic and global symbiotic relationships between people and trees. What can gardeners—or even just tree lovers today—learn from what you discovered?
Bill Logan: That’s a really interesting question. People have been taking care of trees since well before the Industrial Age. There’s a wonderful town called Murua in the north of Spain that has two square miles of pollarded trees around it. [Editor’s note: Pollarding is a pruning technique that involves removing upper branches, which creates a condensed head of foliage and branches.] You can tell, even today, that people wanted to keep the trees alive as long as possible. And they did a good job because—even though they were cutting them fairly dramatically—there are trees in Murua that are 500 to 700 years old.
So if you do it right, you don’t make trees live less long, you often make them live longer. And, at the same time, people got what they needed from the trees. If it was a chestnut, for example, they got the fruit. And they’d also get chestnut wood that they could make into charcoal and use for everything, including heating, cooking, pottery- and iron-making, and so much more.
I think there’s a place for learning and copying the kind of techniques that were used in pre-modern forestry. For example, at the Metropolitan Museum of Art, we pollarded all of the London planes and we’re hedging a whole group of linden trees. So, we can do exactly the kind of techniques that they would have done historically, with good benefit to the modern city.
But the other thing is just a basic attitude we can adopt: Trees are our partners. We can work to help them live the longest, healthiest possible life, and, at the same time, they’ll give us the things we want from them. One of the main lessons I had from writing Sprout Lands was the extent to which that is true. There is a 2,000-year-old linden tree in England that has been cut to the base every 20 years for 2,000 years that’s still growing strong.
We still need things from trees. There are all of the ecosystem services we discussed, but urban areas today are very different from the little town of Murua. At that time, if you wanted to go out into the woods, you would just walk out into your pollarded woodland, and from there you could be in a wild space in five minutes. Now, we’re in the middle of these large cities, and I think bringing more and more woodlands into cities makes people healthier and happier. Not only because of the ecosystem services but also because of that sense of relationship to the natural world that trees provide.
Luka McLean: Yes, agreed. A big part of the High Line’s mission is to build that relationship, creating a space where New Yorkers can be in nature. What effect does that have on them as they go through the park? And what can we as staff do to add to that relationship? It’s an ongoing process and so important to keep in mind.
High Line: What are the unique challenges of doing arboriculture in an urban environment or, more specifically, on the High Line?
Luka McLean: One of the major challenges on the High Line is risk management. It’s such a big part of tree care and tree maintenance. Making sure that the people walking through the space are safe is our main priority. So in terms of major events like a branch failure or tree failure, consistent maintenance is important to avoid those issues. The High Line’s tree maintenance team is also training the other High Line horticulturists to be more aware and proactive about possible issues related to trees.
The other challenging thing about the High Line is the extremes in weather—sunlight, wind, and sometimes even shade in some places. We have to find the right trees for the right space, and continue monitoring and curating them over time.
Bill Logan: In many ways, the High Line is a microcosm of a city because it has the same kinds of advantages and limitations of a city. You’re blessed to have an old railroad trestle that can hold huge weights, so you can use heavy, natural soil, rather than the synthetic materials used in other rooftop gardens. So you have good soil but a tremendous number of plants, so the plants are all challenged with respect to how much soil area they have. As Luka said, the wind and the sun working together can wreak havoc on plants, even more so since in cities it can be unpredictable. Not only because you’re not sure how the wind’s going to come down this particular street today and how it’s going to spin as it does it, but also because the buildings change and the trees change around you. Or a building might come down and all of a sudden your trees are exposed to something they were never exposed to before. Or the reverse—a new building goes up, which is what has happened around the High Line, and suddenly you have a lot more shade. So that’s a microcosm of what happens in the city with rapid change, young soils, and usually not enough soil. These are huge points of struggle, particularly in urban areas.
High Line: What roles does climate change play in how you approach your work, and what adjustments have you had to make or what new challenges have you faced?
Bill Logan: There are some obvious challenges as it gets warmer. Certain pests and diseases are encouraged. For example, hemlock, which was one of our most important shade conifers in the Northeast has become almost unusable because of the hemlock woolly adelgid. This is certainly a result of climate change, but also pests, diseases, and invasive species are moving in because they’re being brought by international trade. This will strongly impact what we can plant in the future.
We see changes happening and we try to keep ahead of them. For example, we’ve started to use southern oaks that wouldn’t used to have grown north of southern or mid-New Jersey like the chinkapin oak. There’s a lot of creative thinking needed to make tree selections that we will be sure will be well-adapted in the future as the climate gets warmer.
Luka McLean: As we consider new tree species to add to the High Line, we are looking for trees that exhibit more climate resilience. For instance, the trees at the 10th Avenue Square (on the High Line at 17th Street) will be replaced this year. Originally, three-flowered maples were planted, an Asian species noted for its graceful flowers and form and brilliant fall color. But these trees have to be happy planted in the large containers that exist under the decking in this area, and 15 years later, they have outgrown their soil space and are struggling. Confined root space and urban pollution are already huge stressors for all plants; when you add temperature and precipitation swings, it’s even more difficult for plants to thrive. So the resilience of new plants we introduce is more important than ever.
High Line: What role can native trees play in the garden?
Bill Logan: Native trees are great but some like the chestnut oak or even the pin oak may not work in New York City, even though they’re native. One of the reasons they struggle could be climate change, but it’s also because they like low pH soil. Because of all the rubble, all the junk, and run-off in New York City, the soil is highly alkaline. The city is a novel ecosystem that includes both elements of the native ecosystem around it and the things that happen because we’re in a city, where soils are affected in this way, the winds and sun, and everything. So we have to find things that are adaptable to both of those.
Luka McLean: About half of the High Line’s trees are native to the region. Native trees are such a cornerstone of the High Line’s design, bringing back some of that feeling of old growth New York forests to the city, creating opportunities for wildlife, native pollinators, birds, and bees. They help create a healthier ecosystem, which can prevent invasive species.
High Line: Thank you, both. Arbor Day is a moment when people are encouraged to plant trees. Would you have any recommendations for local gardeners?
Bill Logan: It’s fun to play with your selection of trees. Especially if you’re planting small trees, it’s not a disaster if it doesn’t work out and if you have something that you really want to try. I’m an oak guy. So I think anytime I can find an interesting new oak of any kind, I will try it as long as I have enough room. But if you’re a gardener, you have to consider your conditions, your light. Often people want a plant that might not be appropriate for the setting. There are a lot of variables that go into it that should be considered, but it’s wonderful to experiment and see if you can stretch the limits of what a plant can do.
Luka McLean: Absolutely. Within horticulture, there’s a great saying: “Right plant, right place.” I’ll always try to recommend native trees from the US and Canada wherever possible. But for trees at street level in New York City, it’s also important to take into account salt tolerance levels. We need hardy trees because it can be an unforgiving environment. There are higher temperatures, wind events, and [laughs] we just had an earthquake 20 minutes ago.
For trees and shrubs, I recommend looking at NYC Parks’ list of street trees for the city. It’s free and available online.
My favorite native trees on the High Line is the serviceberry, which is currently in bloom in the park. There’s also redbud, which is very popular in other New York City parks. Red maple too—it has fantastic spring color.
And personally, I took care of staghorn sumac and smooth sumacs on the High Line for many years. They’re kind of a finicky tree, but very nice to work with and uncommon in gardens. It’s a wild tree and shrub. It’s very generous in terms of anybody who’s just starting out learning how to work with trees. It was one of the best learning experiences for me during my time at the High Line so far.
Bill Logan: Agreed. In general, the High Line could be a really good place for inspiration if you want to be adventurous.
Luka McLean: And we’re actually experimenting more now that you mention it. We’ve recently introduced pitch pines, Pinus rigida, which are native to coastal New Jersey and Long Island, on 30th Street near the Rail Track Walks. That area has relentless winds due to the tall buildings on either side, so we’re trying to choose a tree that is more drought- and wind-tolerant.
High Line: Bill, this question is for you. You’ve done a lot of work around New York City. Aside from the High Line is there a project you’re most proud of or that’s most memorable to you and why?
Bill Logan: It’s hard to choose! I have a great affection for Madison Square Parkbecause I did the inventory on which the renovation was based. That was at least 30 years ago, and there was maybe a decade when I didn’t work there. But now, for the last 20 years, we’ve been working there helping Stephanie Lucas, with the trees there. It’s so delightful to go to a place and get to know the trees so intimately. I know those trees so well, I could see them in my sleep. I love that space and I love what Stephanie and her team have done with it.
I also have a great affection for the MET, as we were involved from the start. We selected the trees for them at plaza, we helped install them, and we have taken care of them ever since. And that site was the inspiration for my book Sprout Lands, because I had to learn how to properly pollard the London planes there. We love going there every year; we love fostering a long-term relationship with a site and its trees.
I also have a tremendous affection for Fresh Kills, even though I started by hating it. We had to do a tree inventory for a section they wanted to create a park in. The plants were fighting with one another, the trees were getting knocked down by vines and starting over again. After a while, we realized that the trees and vines were not only in fierce competition but were also trying to regenerate that landscape. That they would keep living and dying until they had done it, even if it took a thousand years. It was really cool.
High Line: How did you get into arboriculture? How did you get your start?
Luka McLean: I got interested in arboriculture through the work I’ve been doing at the High Line. I was managing the [Diller – von Furstenberg] Sundeck and the Hudson River Overlook. I had these 15+-year-old sumac trees that were aging. I wanted to learn how to maintain them and understand their life cycle. Partially, it was in regards to risk management and making sure the trees were safe for the public, since that’s a heavily trafficked area with families and lots of other people who would sit beneath the trees. I was also curious about best practices for the environment and how the trees fit into the health of the entire garden.
I had the opportunity to learn tree maintenance techniques from Bill at the New York Botanical Garden school of professional horticulture. I’m still on that educational journey, still getting new information, and doing more work in the field with the newly created tree maintenance team, which was formed this year at the High Line.
Bill Logan: I loved trees since I was a kid. I had the good fortune to grow up in the San Francisco Bay Area, in a small house nestled in trees of all kinds. My brother and I climbed them we would do all kinds of things, including things we shouldn’t have done. So I’ve loved trees since a young age but I didn’t know that you could make a living taking care of them.
At one point many years ago, I was a garden journalist and had to write an article on pruning. There was this guy at New York Botanical Garden I was supposed to interview, Wayne Cahilly. Wayne agreed to be interviewed but he said, “You have to climb a tree with me.” He didn’t know I was an old rock climber.
So we got 85 feet up this oak tree at the gardens, and I didn’t want to come down. We talked for three hours, and at the end of it I said, “Can I do this again?” He said, “Sure.” I said, “How about tomorrow?” [Laughs.]
He became a mentor to me. I was starting out in the field rather late at that time. I’ve been doing it for 35 years now, but I started it when I was in my 30s and never looked back. It was—and is—such a wonderful opportunity to work with these living things.
I never imagined the possibility of a career with trees; it was really thanks to Wayne that realized I could. And as Luka said, the education goes on and on. You can never know it all—we don’t know half the amount about trees that people know about the human body. So every day we’re learning something new, which is great. I never get bored.
High Line: Luka, you mentioned the tree maintenance team at the High Line. What is the role of that team? And how do its help the entire park’s trees stay healthy?
Luka McLean: The tree maintenance team is primarily responsible for the maintenance and observance of the trees and the woody vegetation (shrubs) on the High Line. The team serves as extra support for the entire horticulture team. Individual High Line horticulturists can call on our team for consultations about pruning needs or design questions; we’re there to keep the holistic nature of the park in scope. We’re also working on recording tree information and storing it in a capacity that can be used to better maintain the trees in the future.
In the future, the tree maintenance team would also like to do more educational outreach with the public, sharing the best practices of tree care and management that we do on the High Line.
High Line: You’ve touched on mentorship and lifelong learning, which are so great. What would you say to someone interested in getting into urban arboriculture but hasn’t started yet?
Luka McLean: If it were me, I would just say to take a class at NYBG. [Laughs.]
Bill Logan: Yes, that’s great. Really any place where you can get your foot in the door if you’re interested. There are some great local tree organizations that are related. One is New York State Arborists, which is our chapter of the International Society of Arboriculture. And the other is the Long Island Arboricultural Association. When I was first starting out, that was one of the things that Wayne sent me to do immediately was to join them, and it was wonderful. I still know some of the people I met there, and they still advise me. It’s wonderful to have that kind of long-term relationship with people who also care very deeply about trees.
Volunteering at a place like the High Line, as Luka was saying, is also a learning opportunity. What a great place to begin to learn how to do things! There’s also the group called Trees New York, which has been teaching people since the 1970s to be street tree-pruners and you get a card after you pass their course. It allows you to prune any tree from the ground in New York City, any public tree. So, there are lots of ways in, but I think it’s so important to just get in and begin meeting other people in the field.
As Luka mentioned, New York Botanical Garden has an arboriculture concentration in continuing ed, which is very good. We are also blessed with great Cornell Cooperative Extension people who are extraordinarily knowledgeable and generous with their knowledge.
Luka McLean: With regards to the International Society of Arboriculture, you can be certified by taking an examination, which is pretty rigorous. At the end of it you have gained all the basic foundational knowledge needed to practice arboriculture. You’re aware of the soil science, plant selection, risk management, tree care, etc.
If you would like to learn more about Bill Logan and his work, visit williambryantlogan.com.
Source: High Line Blog 25 April 24
