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news

A Note From the Editors

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© Arnold Arboretum, Harvard

interviews

Novel Ecosystems Represent The Future

by Peter Del Tredici
September 4, 2019

This interview with Peter Del Tredici was conducted and condensed by frank news. Peter Del Tredici holds a BA degree in Zoology from the University of California, Berkeley, a MA degree in Biology from the University of Oregon, and a Ph.D. in Biology from Boston University. He's worked at the Arnold Arboretum of for 35 years as Plant Propagator, Curator of the Larz Anderson Bonsai Collection, Editor of Arnoldia, Director of Living Collections and Senior Research Scientist. Dr. Del Tredici taught in the Landscape Architecture Department at the Harvard Graduate School of Design from 1992 through 2016.

My name is Peter Del Tredici, and I'm a senior research scientist emeritus at the Arnold Arboretum at Harvard University located in Boston, Massachusetts, where I have worked for some 35 years. I'm a specialist in urban ecology and  the author of this field guide to urban vegetation – Wild Urban Plants of the Northeast (Cornell University Press, 2010). I've spent 20+ years teaching landscape architecture students at Harvard Grad School of Design in urban ecology and the principles of urban ecology.

One of the issues I always like to talk about is the role trees play in the urban environment. These days a lot of people are talking about climate change and how important trees are in terms of fixing carbon to help mitigate climate change impacts. But, the fact of the matter is, urban environments emit huge amounts of CO2 as part of their normal metabolism and in no way is there ever enough vegetation to offset the carbon that cities produce. This isn't to say that trees don't have an important role to play in the urban environment, just that their primary role is not in mitigating climate change. To my mind the most important thing that trees can do is make cities more livable for both their human and non-human inhabitants.

They create shade and lower temperatures. That's one of the things that all trees do really well. Parks are often structured around the plantings of trees so that people can escape the intense heat-bed that builds up in intensively paved areas. Of course, trees also provide habitat for wildlife, especially birds and squirrels, but also insects and a whole host of other organisms that few people are aware of. When you think about the overall role that trees play in the urban environment, fixing CO2 is pretty low down on the list compared to making cities more livable for people and creating habitat for wildlife. 

Still, you're an advocate for more trees in urban environments, right?

Yes, I think that we do need more trees. It's a complicated issue because putting a tree on the street in a meter square tree pit is actually a death sentence for most trees. There's just not enough soil volume to allow a tree to live more than 10 or 15 years. If you want a tree to live a long life and fix lots of carbon, you actually have to give it some real estate. This is a big problem. Developers always want to max out the amount of the building footprint so they can maximize their profit. Giving trees the land they need to grow large is something that a lot of developers don't like to do. But, the fact is if you want trees to become part of the permanent infrastructure of the city, they need a reasonable amount of soil volume. They also need a fair amount of care and maintenance.

This idea that you just put the trees in the ground and they will grow is just a fantasy.

If you have a park situation where you've got a lot of open space and a lot of soil volume, the trees will usually do pretty well. But the street, that's a very different matter. It's actually very expensive to install street tree plantings. Most cities have a lot of infrastructure embedded in the street and putting trees in the same space can be complicated. You also have very complex drainage issues. Actually, getting trees to grow in the urban environment, particularly in the densely-paved urban environment, that takes real money. So, wealthy cities like New York and Boston, of course, will invest in tree plantings because they can afford it. Middle class or poorer cities, on the other hand, often have a hard time generating enough income to plant and take care of trees in the densely urbanized environment.

Why is the line of planting more trees to mitigate climate change something we’re attached to?

I think it's because it's planting trees means we don't have to change our behavior, basically. 

There was a study that came out of Europe a month or so ago saying that if we planted a trillion trees we could offset all of the CO2 produced by human activity on a global scale. A trillion trees, oh my god! It says nothing about the fact that it takes them 20 years or so before they actually can have an impact on the global carbon cycle and we need the impact right now.

I think that people like the idea of planting trees because it’s a simple answer to a complex problem. If we just plant more trees, they'll solve the problem by growing, and they take care of themselves to boot. For years, people have been trying to offset their flying miles by planting trees. I'm going to fly from New York to San Francisco, and I'm going to plant 25 trees in some third world country to offset that carbon contribution. I'm sorry, but this just doesn't make a lot of sense to me. I think people are attracted to the idea because it means you don't have to change your behavior.

Right, we plant, but then what?

Putting a tree in the ground is just the very first step in a long process. The goal is getting the tree to live for more than 20 years, and that requires the input of real financial resources. Most people think trees can take care of themselves, but in the urban environment, this is not typically the case. 

The other aspect of this issue that people don't think about much, is that all trees fix carbon, create shade and lower temperatures. It really doesn't matter whether it’s a weedy, non-native tree like the tree of heaven or the Norway maple or a wonderful native species like sugar maple or red oak. A lot of the trees growing spontaneously along roadsides, in vacant lots, and coming out of sidewalk cracks are actually contributing significantly to the ecology of the urban environment, but people dismiss them as weeds or invasive species. 

People don’t acknowledge the fact that they're growing in very inhospitable conditions, totally on their own, and they don’t require any maintenance so they’re totally sustainable. They're making a contribution to the urban environment without actually costing anybody anything. We need to give the trees that grow spontaneously in the urban environment a little bit more respect.

Just because they're not necessarily native to North America when the pilgrims landed, I don't think that's all that relevant when it comes to mitigating climate change.

Non-native plants adapting and thriving is a sign that they belong.

Absolutely. They're better adapted to the constantly changing urban conditions than a lot of our native species which evolved under dense forested conditions with well drained soil rich in organic matter and diverse microbial populations. We need to plant super-tough trees that can tolerate urban conditions regardless of where they may have come from.

How can we be helpful to environments that are already in existence and are contributing positively?

Trees in park situations with lots of soil and a good amount of space require minimal, but on-going maintenance – what you might call normal tree care. I think it’s really important for cities to have an inventory of their trees in order to track their condition and to monitor mortality over time. How many trees do we actually have? And is that number going up or down? A good inventory also allows people to identify which areas of the city need more trees. In many ways a lack of tree density is associated with poverty. 

Tree planting is not a numbers game where the more trees you plant, the better it is. It really should be about how well the trees are planted, especially making sure that the tree has enough soil volume to live a long life. Just putting a tree in a meter square tree pit is a death sentence. So making sure that the trees are planted in such a way that their chances of living for 20 years are increased is the single most important thing.

The difficulty is that in many parts of our older cities like Boston and New York were not designed to support the growth of trees, where the doorways and the steps come right down to the sidewalk. Literally, there’s no room for trees in these environments unless you redesign the street, which is an incredibly expensive endeavor. This is often a social justice issue because a lot of these neighborhoods are poor. Unfortunately, there is not a lot that we can do about it, so the best thing to do is to create parks nearby where people can get some respite from the intensity of the summer sun.

You are also an expert in the Ginkgo tree.

Yes, the Chinese Ginkgo tree.

One of the most adaptable trees we have. What do they tell us about adaptation for climate change? What have we learned from that specific tree?

Wow, that's a really interesting question. It's actually an evolutionary question. It's one of the trees that survived the atomic bombing of Hiroshima. Amazingly a number of Ginkgo’s within a few kilometers of ground zero were able to sprout back following the blast. The Ginkgo tree is amazing. It's a deciduous tree, and its leaves turn a beautiful yellow in the fall. Ginkgo appears to be the one species that invented the ability of losing your leaves when growing conditions are unfavorable. That can be the winter time, it can be with drought, or in the case of the Ginkgo I think it was that because it grew in the Arctic Circle there was no sunlight. Of course that was 50 million years ago when the Arctic was much warmer than it is now.

This ability to lose your leaves when conditions aren't favorable is what I would call resilience.

The other thing that Ginkgo has is it has the ability to sprout from the base following injury. That's another life history trait that ginkgo seems to have invented, so if something happens to the main trunk, it has extra buds stored below ground that will sprout back and continue the life of the tree. This is what the trees in Hiroshima did after the atomic bomb. The thing about a tree like Ginkgo, as well as a few other trees, like the California redwood and the European olive, is that they're very resilient in their capacity to deal with disturbance. Part of this means they know how to deal with climatic uncertainty. Any tree that's been around for 40 or 50 million years, that's a tree that has adapted to a broad range of environmental conditions. People talk about resilience as some abstract attribute of an ecosystem, but I have always had trouble visualizing what a resilient ecosystem look like. But I know what resilient trees are. They are the ones that sprout back following catastrophic disturbance.

I think we need to be planting resilient trees like Ginkgo that can tolerate the unpredictability that lies in the future in terms of climate.

If there is some catastrophic disturbance like a surge from a hurricane or something, these trees may lose their original trunks, but their roots will hold the ground and sprout back. We need to start thinking about resilience as an attribute of plants, and we need to plant resilient plants as part of our defensive infrastructure, particularly against sea level rise and climate uncertainty.

There's so much to unpack there.

I could go on and on about Ginkgo.

I do think the key is that resilience is not an abstraction.

There are certain traits plants have that make them resilient. That's what we really need to focus our attention on when we try to design landscapes that are going to survive 50 years into the future. This goes to the heart of why the idea of ecological restoration is very problematic, because the environmental conditions that existed when Columbus landed no longer exist, and the native vegetation adapted to those conditions are not necessarily the species that are best adapted to the conditions that exist today. There's very little evidence to show that the native species are going to do best under climate change conditions. In fact, the spread of invasive species across the planet suggests that they are better adapted to changing environmental conditions than native species.

That's a really easy way to put that. The idea of restoration sounds nice. It sounds progressive even. But, it's ignoring, literally and figuratively, the climate we're living in now.

Oh, absolutely. I think that's the real critical issue. We have to get real about what's actually happening. Restoration is a romantic idea, that somehow we can go back to the way things used to be because there's a certain comfort in that. That's just a fantasy. There is no going back in time.

We're past the point of return. 

Yes, we are, long past.

So, I guess what’s next? What’s productive for our future?

A lot of people talk about trees as a renewable resource. You cut them, but the forests will come back. New England is a textbook example of that. The forests of New England have been cut three or four times since the original settlement in the 1600s, and the forests keep coming back because we have a very good climate for trees. But, the fact of the matter is that when you think about trees as a renewable resource – as an alternative to burning fossil fuels – that’s when you run into problems. When you cut a tree and burn it for fuel, whether it's a power plant or whether you turn it into pellets for home heating systems, not only are you putting carbon dioxide into the environment at high levels, but you’ve cut down a tree that was fixing carbon.

Burning a tree for fuel is very much a lose-lose proposition.

There are some states that actually give subsidies for wood-burning power plants as an alternative energy source. But burning wood to offset fossil fuel consumption is not a particularly good strategy. This is a long way of saying that I think a really important way to think about strategy for dealing with climate change is to really embrace conservation of our existing forests. There you've got 70, 80, and 90-year-old trees that are fixing carbon at a tremendous rate that planted forests can only match after many years.

In terms of a short-term solution, conservation of existing forests is a much better plan for dealing with climate change than planting more trees.

If you want to fix the most amount of carbon in the shortest period of time, this can only be done through the preservation of intact forests.

That's part of the tragedy of all these fires burning in the Amazon right now. Not only are we putting all that CO2 into the atmosphere, but the trees that were fixing all that CO2 are gone. It's a double whammy that happens when you burn trees for fuel. 

Or burn for agriculture, right? Is that the same category?

In the Amazon most of the fires were set to help clear the land for agriculture. I'm sure they're harvesting the trees for lumber as well, but when you harvest trees for lumber, there's a lot of material left over, the slash it's called. That has to be disposed of, and the easiest way to do this is to burn it.

Is this thought process the same in California too [with all the recent fires]?

California is the poster child for this, but the California situation is different because it's not associated with agriculture the way it is in the Amazon. In California, there are three things happening that are problematic. One is the weather is changing. It's less favorable to trees and more favorable to grass and shrubs. That's the long-term prognosis. The other thing is that because California is so densely populated, people are moving into forests and the areas where people interface with forests is increasing. When people move into these forest situations, they're perforating the fire so the forest is no longer an intact forest, and once you begin perforating it, they come together and cause fragment the forest into disconnected pieces. 

The interface between wild lands and human habitat is where a lot of these fires are starting and because you have valuable houses that need to be protected, they need to be controlled. So unlike 50 years ago when people might have just let the fires burn, they have to go in there and try to put those fires out which is getting harder and harder to do because the amount of fuel in the forest has greatly increased over that time period. Fire suppression has become a big part of California's history since the end of World War II, and has led to the build up of fuels in these forests which eventually reach a critical mass from a fire perspective.

So when a PG&E power line falls down and creates a spark, like it seems to have done in the Paradise fire, the whole thing goes up. The fire suppression, the changing climate, and increased development have created a very problematic situation in many parts of the Western United States.

It sounds like everything humans try to do, well intentioned or not, ends poorly.

You could say that.

We have the best of intentions, but we should let nature be.

Our track record with the environment when we try to manipulate ecology is really bad.

It's the law of unintended consequences. The fact is that even though the given law or regulation may be well intended, the system is always more complex than we can ever understand or predict. There are all these things we don't understand about the system, but that never stops us from trying to manipulate it to some human advantage. If we really want to do something for the environment, people argue with me all the time about this, but leaving it alone is the best thing. If you really want to help the environment, just leave it alone.

The other thing that’s important to consider is that the spread of invasive species is really driven by disturbance. The more the landscape is disturbed for agriculture, for housing, for whatever, the more invasive species like it. If you have some of these highly disturbed areas and you leave them alone, over time they will go to an ecological succession process and become more stable and productive. These are called novel ecosystems. They're not native, but they're functional from a biological point of view. But, they don’t develop unless you leave them alone. When we try to intervene, go in there and rip out the invasive species, we create more disturbance, and it becomes a vicious cycle – disturbance begets disturbance. The only way to break that cycle is to stop going in there and trying to manipulate the environment for whatever purpose we’re trying to impose on the landscape.

Whether a species is native or non-native, is almost an irrelevant issue, because these non-native species, particularly in the urban environment, are contributing tremendously to the ecology of the city and the city is not a native ecosystem. It's a novel system with a novel ecology. It's not a question of whether it’s good or bad. It just is what it is.

I like that.

I think that's the single-most important concept that I try to introduce my students to. What are these novel ecosystems that represent the future? How do they function? And how can we manipulate them to increase their aesthetic and ecological potential? We need to engage these novel ecological processes as opposed to saying, "Oh, my god, look at all these non-native species. We have to get rid of them.”