Peter Del Tredici was invited to speak at a conference sponsored by the Presidio Trust in San Francisco, “Bridging the Nature-Culture Divide Conference by the Cultural Landscape Foundation,” January 23, 2015. Professor Del Tredici recently retired as senior research scientist at Harvard’s Arnold Arboretum after 35 years of service. He is an Associate Professor in Landscape Architecture at the Harvard Graduate School of Design where he teaches courses on soils, plants and urban ecology. He advocates for a pragmatic approach to urban landscapes, which values novel ecosystems for the functions they perform and their sustainability in stressful environments.
Professor Del Tredici’s presentation at the Presidio Conference was entitled, “Saving Nature in a Humanized World.” The presentation is available on YouTube.
We attended his presentation, which was warmly received by an audience of about 180 people. We paraphrase some of his key points.
Update: Professor Del Tredici has requested that the following statement be added to this post: “Professor Del Tredici has graciously allowed us to paraphrase–for educational purposes only–some of his key points in his lecture. In no way should his permission to reprint the lecture on this website be considered an endorsement of the political or ecological agenda of “Death of a Million Trees”
Professor Del Tredici began his presentation by complimenting the Presidio for what it has accomplished in the past 20 years and congratulating the Presidio for “…what it has become. But I’m going to do something very different today. I hope you’re ready for this.”
Professor Del Tredici spoke about spontaneous urban nature, some of which we control but a lot we do not control. What does spontaneous nature look like? The reason why this is important is because it’s about the future. What is the world going to look like 20 years from now? The answer to that question is in urbanized nature.
Detroit is a depressing place from a sociological standpoint. It is so economically depressed that the land has lost its value. Forty percent of the land is no longer occupied or managed by public or private entities. From the standpoint of a botanist, it is a fascinating place because we can see how nature develops without human interaction. Detroit is a case study for urban ecology.
Globalized Ecology
The vegetation of most cities is as cosmopolitan as its human population. Asa Gray’s “Manual of Botany” reports that 10.7% of plant species in Northeastern United States were non-native in 1856. By the 1990s, 25-35% of plant species were non-native. This number is not going down. It’s a strongly upward trend over the past 150 years. We can create little islands of native plants by eradicating non-native species, but the reality is that our ecology is becoming as globalized as our economy. These changes mirrored the changes in the ethnic and racial composition of American cities. The same forces that produce socio-economic changes in cities are also changing the biological environment.
Urbanized Environment
A significant portion of land area in the Northeast is fully urbanized. Urbanization in the West is just as rampant as the Northeast. Looking at an aerial view of Los Angeles, you can see that it is completely developed. You can talk about what used to grow there, but the concept that there is a vegetation that is native to these current conditions, Professor Del Tredici said, “Personally I find that an absurdity. I hate to be so harsh, but nothing is native to LA as it now exists.”
- Cities have distinctive environmental characteristics, such as the urban heat island effect. Cities are significantly warmer than rural adjacent areas, which means they are important predictors of the impact of climate change because they have already warmed as much as other places are projected to in the future.
- Urbanized areas can also be defined by the amount of impervious surface they contain. When 25% or more of the land is covered with an impervious surface such as roads, parking lots, houses etc., the environment is urbanized from the standpoint of the vegetation because impervious surface fragments the environment, compacts the soil, and interrupts the hydrology. Using the definition of 30% impervious surface, urbanization describes not only our cities, but also many of our suburbs.
- Glaciation is analogous to the urbanized environment because the heavy equipment that is used to clear the land leaves in its wake compacted glacial till. What you find after the glaciers recede is barren land; the vegetation has to come back from nothing—a condition known as primary succession.
- One-sixth of the city of Boston is built on land fill. What is the native vegetation of filled soil? There is no going back when you’re talking about filled urban landscapes. Not quite as much of San Francisco is built on landfill, but most of the eastern and northern edges of the city are on landfill.
- There is a huge difference between native soils and fill soils. Fill soils support the development of novel ecosystems. Native ecosystems cannot be created without native soils. There are some native species that are adapted to urban conditions, such as roadside areas. Urbanized vegetation is a cosmopolitan mix of native and non-native. Urbanization favors species that grow well in soils that are relatively fertile, dry, sunny, and alkaline.
- Where it snows, the roads are repeatedly salted to prevent dangerous, icy conditions. This creates alkaline conditions along roadsides to which many plant species are not adapted.
Urban Ecology
Professor Del Tredici studies modern urban ecology which was born in post-war Germany, where urban environments were reduced to rubble and ecologists began to study what was growing in that rubble. That was the birth of modern urban ecology. It’s important to study, not for what it used to be, but for what it is now and what it can become in the future. Nature reclaiming the urban environment on its own terms is an interesting process, an evolutionary process that we should pay attention to. Post industrial succession—the process of rebuilding ecology in an intensively urban environment– should be studied with the same level of academic intensity as we studied the post-agricultural succession in the Northeast.
Novel Ecosystems
When native forests are converted to urban ecosystems and then abandoned—as seen in Detroit– they don’t go back to their original state, rather they become novel ecosystems. There is no going back. Once we achieve the level of compaction and impervious surface of an urbanized environment we have limited what the landscape can become in the future. Some of these changes are permanent. There are long term disturbances caused by chronic stress factors that permanently alter ecological conditions. Professor Del Tredici said, “These conditions are not reversible. Invasive species aren’t going anywhere. If you remove invasive species you are gardening. When you garden you are deciding who lives and who dies. You are just playing god. This gives you the illusion of control, but it is a never-ending effort to control a process that can’t be controlled.”
In 1996 the Arnold Arboretum was given a 24 acre parcel of derelict land, called Bussey Brook Meadow. In 2011, Del Tredici succeeded in preserving it as a site for research on urban ecology by leaving it alone. The land had a 300 year history of use and abuse, all left more or less alone. Plant species—both native and non-native–have sorted themselves out and restored a functional wetland in the middle of the site. It doesn’t matter that it isn’t a native landscape if it is providing the necessary ecological functions.
The Bottom Line
Ecology is not about stasis, it’s about flux. Stasis is achieved by maintenance, but the natural state is flux. Evolution is based on competition, which species is the best adapted to current conditions. Sustainability is about reducing maintenance in order to promote ecology. Landscape architects look at the Bussey Meadow site and ask, “When are you going to fix it?” Professor Del Tredici’s answer is, “I’m not sure this site needs to be fixed. It has value just the way it is.”
We have quoted Professor Del Tredici’s work in previous articles and we consider it important everywhere, but we bring this presentation to your attention today primarily because of where it was delivered. The Presidio Trust has engaged in some of the most aggressive “restorations” in the San Francisco Bay Area and some of the most successful: Inspiration Point, El Polin Spring, Thompson Reach, etc. All fish in Mountain Lake were recently poisoned in order to “restore” the lake to exclusively native species. Pacific chorus frogs were recently reintroduced. The intention is to reintroduce the Western Pond Turtle to Mountain Lake, a species that is notoriously easily disturbed and being considered for endangered status. It is also a species that requires hundreds of meters of unshaded nesting habitat in proximity to its water source.
These projects have required the destruction of thousands of trees because the native vegetation is grassland and scrub. However, the Presidio has also made a commitment to the preservation of its historic, non-native forest which was planted by the military over 100 years ago. Major investments have been made in reforestation of the aging forest with similar tree species.
In other words, the Presidio Trust seems to have assigned itself a schizophrenic mission to simultaneously destroy an existing landscape in order to re-create it and preserve that same landscape: the re-creation of an idealized landscape vs. preservation of the novel ecosystem within the historic forest. We suppose that is one definition of “balance.” However, we would like to believe that the invitation to Professor Del Tredici to speak of the sustainability of urbanized novel ecosystems is an indication that the Presidio Trust will assign more value to what exists and less effort to attempts to re-create an historic landscape that may no longer be adapted to the real world.
Conservation Sense and Nonsense 
Thank you for sharing all of this Millie. The slide ‘design vs ecology’ is gold.
Thanks, NNLC. It IS gold and the fact that it was delivered in the heartland of nativism was a bonus. If you watch the video, you will see some interesting reactions in the audience, ranging from anxious stares to appreciative laughter. We are not alone, but changing public policy is still a big hurdle.
Interesting, but it has absolutely nothing to do with the East Bay hills, where removal of non-natives is mostly pretty easy. It also totally ignores the animals and the Endangered Species Act..
The two endangered animals in the East Bay will not benefit from spraying thousands of gallons of herbicides known to be harmful to reptiles and amphibians. The endangered plants will not benefit from spraying herbicides known to kill beneficial microbes and fungi in the soil.
Furthermore, the ESA does NOT obligate us to reintroduce endangered species to places where they have not existed for over 100 years.
I oppose the use of pesticides. The removal of exotics doesn’t require them. The ESA may not obligate us, but ethics do.
Your personal opinion regarding pesticide use does not prevent land managers from using them. The California Invasive Plant Council conducted a survey of land managers to determine the methods used by land managers to control plants they consider “invasive.” Ninety-four percent of land managers report using pesticides: 62% use them “frequently,” 10% use them “always.” “Restoration” projects will be judged by the reality of what is done and how it is done. Your opinion of what “ought” to be done is irrelevant in that judgment.
Why RoundUp shouldn’t be used:
I wrote this about French broom, but it applies to all plants:
1. Plants get their energy only through their leaves.
2. Their store of energy is finite.
3. Therefore, if the leaves are removed every year (e.g. by cutting the stem at its base), the plant will eventually die, probably within a few years.
4. Every time you cut the broom, any sprouts that grow will be much smaller than the parent plant, and far
easier to cut the next time.
5. You obviously don’t like to return every year, but that is exactly why you never succeed in eradicating the broom.
6. No matter how you try to control the broom, you can’t avoid returning, since there are seeds in the ground.
7. When Robert Stebbins was around, he pointed out that Round-Up is harmful to the wildlife.
8. Goats aren’t the solution, because they are always confined to an electric-fenced area, and miss all the plants outside the fence. I’m also not sure that they eat all the exotics, such as Italian thistle and poison hemlock.
9. I asked a previous park supervisor to set up a volunteer weed-removal day, but I never heard from him. When I asked the current supervisor about this, all she could say was “goats”. She also insisted that the only acceptable methodology is pulling the broom out of the ground, and wouldn’t listen to my ideas (see #6). Have you ever seen a goat pull a French broom stem out of the ground? Not everyone is capable of pulling it out of the ground, but anyone can cut off the leaves – even a child.
Can’t imagine why you are sending this to us. We aren’t the ones who are spraying our public lands with herbicide. We are strongly opposed to the use of herbicides on our public lands. Why don’t you send it to the land managers who are doing this?
As a conservation contractor who works in rural settings and urban fringe, I love the idea of design vs ecology. What a brilliant article!
And to share some good news that there is an alternative to glysophate which has been used in urban environments for ten years in Sydney… saturated steam… read more at http://www.weedtechnics.com
Excellent post.
Yes this post was more than excellent and appropriate. I am particularly interested in the concept of schizophrenia in environmental and conservation organizations. It happens too often. Spartina alterniflora eradication is a good example. There has been alot of good information on that in this blog. In Washington state in the name of salmon habitat restoration they have sprayed pesticides to kill japanese knotweed on the banks of the rivers. And there is absolutely no science to support it. In fact the science that exists implies that because the plant is so loaded with antioxidants, the leaves that it drops in the creeks which deteriorate just like native brush could actually be giving a boost to the salmon. The environmental organizations that are xenophobic and science illiterate about so called non-native species (frequently described as invasive) have been silent about habitat restorations that involve spraying pesticides to get rid of unwanted vegetation. Do they not see the connection between this destruction and global warming et al.
I am particularly saddened by the Sierra Club’s obsession with eucalyptus destruction, and going further and proposing pesticides to deter regrowth. Perhaps this is just an aberration of the Northern California Sierra Club, I know that the local organizations have a great deal of autonomy. But for my part I throw all mail from the Sierra Club immediately into the trash.
We didn’t quit the Sierra Club without first trying for several years to convince them that they are on the wrong side of this issue. As you know, the local chapters are run democratically. That means if you can find someone who is willing to run for local office and then campaign for them, it is theoretically possible to turn this around. Here in San Francisco, it only takes about 100 votes to be elected to the local leadership. It’s do-able if you can find someone with the fortitude to run. If you lived in the San Francisco Bay Area, I don’t doubt for a minute that you could turn this around with all your contacts here.
Who in the Sierra Club supports pesticide use? I’ve never heard of that. I certainly never have, and I was the chair of the Wildlife Committee for about a decade..
http://theyodeler.org/?p=7539
That’s unfortunate. If I were there, I would have explained why herbicides aren’t necessary. Most people seem to thing that re-sprouting is FOREVER! Even a brief exposure to physics would convince them otherwise.
They say Spartina is invasive to the west coast yet it thrives in the pacific near japan! They are using round-up to “controll” not eradicate this migrating NOT invading plant! PLEASE let our world evolve naturally without biosides!
Urban environments are often ignored by the more traditional ecologists. Which is a shame because they can be just as fascinating. I am going to study ecology myself, and am myself turned off by the natural environments in my region. As I mentioned earlier, those are usually cultural landscapes that relied on human intervention. They cannot exist on their own, indeed they are more or less gardens. I think nature should be able to develop, instead of intensively managing to remain more or less the same. To me, species and environments are not the most important, more important is that life or evolution keeps on going. I forgot to say this in my previous post, but I think that wanting nature to remain the same is much more anthropocentric, because nature is not a moral agent, we are, and so we decide what an ecosystem should be.
I prefer urban ecology, since the field is less judgmental. It lacks the “this is what it should look like, these are the species that should life here” attitude.
Some more examples of my region why the non-native species phenomenon is often absurd. Stinzenplanten were introduced plants that grew around old human estates, these plants are on the red list. Meaning that effort is spend to protect certain non-native species, while effort is spend on others to remove them. Of course, it surely depends on how common the plant is, there’s a certain fetish for rare and endangered species.
Another phenomenon was the ice age, I’ve read that China and North-America lost less species as Europe because of it. We are talking mainly plants here. In Europe there weren’t enough so called refugia during the ice ages, and so our floral diversity is lower.
The biological globalization must also not be exaggerated; some conservationists worry that we end up with a “McDonaldisation”, in which environments across the globe end up the same. I claim we should not fear this, because there will always be regional variety. Take the urban areas in my region. The most common birds in urban areas are the native blackbird, the native jackdaw and the more or less native house sparrow. Yes, there’s the pigeon McFeral, which has populations in urban areas everywhere. But it seems, and I forgot about this one, that feral pigeons in some areas are outcompeted by local crow species and the wood pigeon. This wood pigeon is another highly successful urban species, and also very common, and it is native. Like said on this blog, novel ecosystems end up being a mixture of native and non-native elements and can end up being unique even if they have features from one another.
Even without human introductions, there are red foxes, beavers, wolves and brown bears in both North-America and Eurasia. Yet there’s also species that are unique to one of the continents. Let us also not forgot that there are subspecies, and that fast evolution can result in new subspecies or species. The Eurasian badger might be distributed alongside Eurasia, but it differs in appearance and behaviour in across this distribution. There’s an emphasis on species diversity today, but there’s also within species much diversity.
Since species composition is unlikely to be entirely the same, there will always be some differences. The dingo and wolf are not that distantly related, but still differ substantially. Different species composition, different abiotic factors, will select for different traits.
Indeed, with novel ecosystems we are seeing new co-evolutionary relations establishing, thus new, ongoing, and sometimes fast evolution, and new species composition. It is fascinating. These non-native species can sometimes be the future of future ecosystems. We shouldn’t be so judgmental, life finds a way.
It is very exciting to read your comments. You have reached the same conclusions as we have, but we have taken a different route to those conclusions. Your conclusions seem to be rooted in observations. Whereas ours are the result of reading hundreds of scientific studies. We were pointed to those studies by the claims of nativists that didn’t make sense to us. They made arguments to destroy our landscape supported by their claims about how nature works. When those claims seemed mistaken, we dug around in the scientific literature to find out how nature works.
Thanks again for visiting and commenting. It is very rewarding to hear from someone who is watching closely what is happening in nature and reaching the same conclusions, particularly the desire to leave it alone to do what it knows best to do.
Where can I obtain Oxalis seed?
I have no idea.