“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.

Hybridization: “Genetic pollution” or a natural process?

 
Presidio variety of California poppy. NPS photo

The San Francisco Chronicle’s gossip columnist, Leah Garchik, recently published a story about California poppies in the Presidio.  Apparently, someone planted the “wrong” poppy, or it migrated there.  The poppy that is native to the Presidio is small and yellow.  This “alien” poppy is the large orange poppy that most of us consider the classic California poppy.  The historical record indicates that this classic poppy grew elsewhere in San Francisco, but since it didn’t grow in the Presidio it must be removed because the Presidio’s Vegetation Management Plan “contains the requirement to remove any plants that could jeopardize the integrity of the genetics of native plants in the Presidio.” 

The “wrong” poppy

This incident reminded us of an article published in the newsletter of the local chapter of the California Native Plant Society several years ago, entitled “Contaminating the Gene Pool.”  In this article Jake Sigg instructs gardeners to beware of planting the wrong variety of a native plant because it will cause “genetic pollution.”  The California poppy is one of the examples he gives of a variety of California native being planted in San Francisco that doesn’t belong here.  It isn’t sufficient in his opinion, to plant a California native if that specific variety of the species didn’t historically occur in San Francisco.  He asks gardeners to “think in terms of preserving the genetic integrity of the local landscape.”  And he speculates many negative consequences of selecting the wrong variety, such as “genetic swamping, you’ve got all these foreign genes that are going to overwhelm the native population.”  We were reminded of Mr. Sigg’s vocal opposition to human immigration. 

For the benefit of our readers who aren’t gardeners, we should explain what Jake Sigg and the Presidio are worried about.  In a word, they are worried about hybridization, defined as “to breed or cause the production of a hybrid,” which is defined as “the offspring of two animals or plants of different breeds, varieties, or species.”  Hybridization is as likely to occur between two native plants as two non-native plants, but native plant advocates are concerned about the possibility of a native and a non-native plant producing a hybrid variety that is distinct from the native plant.  Hybridization is not inevitable, but it does occur naturally as well as through human manipulation.  The “From the Thicket” blog recently told the fascinating story of the development of a valuable garden cultivar variety of a favorite California native, ceanothus or California Lilac. 

Aside from the unpleasant association with eugenics, Mr. Sigg’s advice raises several practical questions.  How is the gardener supposed to know exactly which variety of a native plant “belongs” in San Francisco or even in a specific neighborhood within San Francisco, such as the Presidio?  And, in the unlikely event that gardeners might have such esoteric knowledge, where would they get the seeds of this specific variety?  Jake Sigg acknowledges this practical obstacle, but advises gardeners to get their seeds and plants only from the annual plant sale of the local chapter of the California Native Plant Society.  One wonders how many gardeners will follow this rather restrictive advice.

However, the more important question is the scientific question.  Is hybridization an inherently harmful process that always reduces species diversity?    We turn to Mark Davis for a less gloomy view of hybridization.  Like most scientific questions, there is evidence of both positive and negative effects of hybridization on species diversity.  Since you’ve heard the negative view from Mr. Sigg, we’ll let Mark Davis speak for the positive view.  Professor Davis reports in his book Invasion Biology* that “the fossil record generally shows that following the invasion of new species, the number of species resulting from adaptive radiations and evolutionary diversification exceeds the number of extinctions.”   And he concludes his discussion of hybridization and evolution by saying, “…a fair appraisal must also acknowledge that species introductions can enhance diversity as well, through hybridization, and the creation of new genotypes.”

The native plant movement has a narrow view of nature, which we do not share.  Their ideology is based on dire predictions of ecological disaster if we don’t follow their restrictive advice.  And when the managers of public lands choose to follow their advice, the consequences are usually the destruction of plants and animals that we value, in this case a field of California poppies. 


* Mark Davis, Invasion Biology, Oxford University Press, 2009, page 78-82.