“Restoration is just horticulture dressed up to look like ecology”

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.

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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.

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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.”

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  • 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.

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  • 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.

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  • 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.

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  • 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.”

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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.”

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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.

Tennessee Hollow
Tennessee Hollow “Restoration” is 270 acres, 20% of the Presidio. Presidio Trust photo.

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.

The San Francisco Presidio, painting by Richard Beechey, 1826
The San Francisco Presidio, painting by Richard Beechey, 1826

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.

The Living Roof: A failed experiment in native plant gardening

Living Roof, California Academy of Sciences, March 2011

When the California Academy of Sciences reopened in San Francisco in August 2008, its “living roof” was considered its most unique feature.  Thirty species of native plants were candidates for planting on the roof.  They were planted in test plots with conditions similar to the planned roof and monitored closely.  Only nine species of native plants were selected for planting on the roof because they were the only plants that were capable of self-sowing from one season to another, implying that they were “sustainable.”  A living demonstration of “sustainability” was said to be the purpose of the living roof. 

So, 2-1/2 years later, what have we learned from the living roof about the sustainability of native plants in San Francisco?  The results of monitoring the roof since June 2009, are reported on the “fromthethicket” blog about Golden Gate Park.

Two of three of the predominant species on the roof after 2-1/2 years are native.  The third–moss–is not.  It is described by “fromthethicket” as “varieties of early succession mosses, the types that commonly show up in disturbed soil.”

The monitoring project has divided the roof into four quadrants.  Non-natives now outnumber natives in two of the quadrants, those which are not being weeded.  Although natives outnumber non-natives significantly in the other two quadrants, non-natives are also growing in these quadrants.

California Academy of Sciences, April 2011

We had the privilege of meeting the ecology consultant who designed the plant palette for the  living roof for the academy and many other institutions around the world.  He would not be surprised by this monitoring report.  He advised the Academy to walk the streets of San Francisco and identify the plants growing from the cracks in the sidewalks.  These are the plants he advised the academy to plant because these are the plants that are adapted to current conditions in the city.  The Academy rejected this advice because they were committed to planting exclusively natives on the roof.

The designer also advised the academy not to irrigate the roof, because the point of the roof is that it is a demonstration of sustainability.  Again, the Academy refused because they knew that without irrigation most of the native plants would be brown during the dry season, roughly half the year.  They wanted the public to believe that the plants that are native to San Francisco are beautiful year around.

There is a lesson to learn here for anyone who is willing to learn from it.  The living roof is not natural because it is irrigated and intensively gardened (e.g., weeded, fertilized, replanted, reseeded), yet non-natives not only found their way there on their own, but are dominating it within only 2-1/2 years.  Native plants are not sustainable in San Francisco without intensive gardening effort.

Peter Del Tredici has been telling us this for several years.  He is a Senior Research Scientist at the Arnold Arboretum at Harvard University and a Lecturer in the Department of Landscape Architecture at the Harvard Graduate School of Design.

In a recent publication*, he advises the managers of public lands in urban areas to abandon their fantasy that native plants are sustainable in urban settings:

“The notion that self-sustaining, historically accurate plant associations can be restored to urban areas is an idea with little credibility in light of the facts that 1) the density of the human populations and the infrastructure necessary to support it have led to the removal of the original vegetation, 2) the abiotic growing conditions [e.g., temperature, salinity, moisture, etc.] of urban areas are completely different from what they were originally; and 3) the large number of non-native species that have naturalized in cities provide intense competition for the native species that grew there prior to urbanization.”

Sure, he says, we can grow native plants, but they require at least the same amount of effort as growing any other plant and are therefore just another form of gardening:  “Certainly people can plant native species in the city, but few of them will thrive unless they are provided with the appropriate soil and are maintained to the same level as other intentionally cultivated plants.”

He concludes that native plant advocates are making a “cultural value judgment:”

“…people are looking at the plant through the subjective lens of a cultural value judgment which places a higher value on the nativity of a given plant than on its ecological function.  While this privileging of nativity may be appropriate and necessary for preserving large wilderness areas or rare native species it seems at odds with the realities of urban systems, where social and ecological functionality typically take priority over the restoration of historic ecosystems.”

We hope that the managers of our public lands in the San Francisco Bay Area will soon catch up with the scientific literature as well as acknowledge the actual experience of years of failed “restorations.”  Aside from the waste of scarce resources, these efforts are poisoning our parks with toxic herbicides and destroying beautiful and healthy plants and trees to no useful purpose. 


* “Spontaneous Urban Vegetation:  Reflections of Change in a Globalized World,” Nature and Culture. Winter 2010, 209-315.