Today, I’m publishing an excerpt of “The Post-Native World,” which was originally published by Ground Up, the Landscape Architecture and Environmental Planning Journal of the University of California, Berkeley.
The author of the article, Mark Wessels, received his Master in Landscape Architecture at UC Berkeley. He is a licensed landscape architect and certified arborist. He is a Senior Associate with PGAdesign in Oakland, California.
Mark sent the following excerpt of his article in Ground Up to the members of the Oakland Urban Forest Forum, of which I am a member. Mark’s article reflects my own belief that resilience of urban landscapes in a changing climate requires diversity, redundancy, and flexibility that can only be achieved with both native and introduced plants.
Conservation Sense and Nonsense
CITIES AS HARBINGERS OF A POST-NATIVE WORLD
Native plant enthusiasts argue that native plants have evolved for thousands of years to be optimized for their environments. This is based on the assumption that environmental factors like soil type, climate, and ecological communities change very slowly, at the rate of geologic time. The fitness advantage of native plants depends on a relatively static, unchanging environment.
Yet cities are anything but static. Urban soils are altered by construction, compaction, and contamination. Impermeable surfaces and water infrastructure change urban hydrology. Urban heat island effect and microclimates affect soil and air temperatures. Cities are defined more by how urban they are than by where on the planet they are located. A tree adapted to urban environments, for example, is much more likely to flourish in San Francisco than one adapted to coastal dunes. In short, cities are post-native; they no longer reflect the environmental conditions for which native plants evolved. They are something new.
Cities are not the only places irreversibly altered by human activity. Human influence ripples out through resource extraction, food and energy production, and global climate change. Cities are already several degrees warmer than their historical temperatures, and many native plants cannot survive in this altered environment. Climate models predict several degrees of warming globally in the next 50 years. Native plants face challenges in urban settings today, and 50 years from now they will face challenges everywhere. As the effects of climate change spread beyond cities, landscape architects will need to move beyond geographic provenance to find plants adapted to a post-native world.
DIVERSITY OF APPROACHES VS. SINGLE STRATEGY
Globalization has irreversibly altered the planet, but it may also hold the key to surviving climate change. Designers today have unprecedented access to plants from around the world. For millennia, plants have been continuously evolving new, more efficient ways to survive in an astounding array of environmental conditions. In a post-native world, we will have to reconsider the idea that each plant is custom-evolved for a particular place on the earth, and instead think of global biodiversity as a library of adaptation. This library holds the key to successful planting in urban areas today, and hope for an uncertain future.
What I’m suggesting is that we embrace global biodiversity while we still have it; that our cities become hotbeds of plant species richness, hybridization, and cross-pollination; that we start a thousand divergent experiments, in small and controlled ways; and that we embrace this moment of globalization to produce an unprecedented explosion of diversity with which we can begin to replant and repopulate this irreversibly altered planet.
The resilience of natural systems lies in diversity, redundancy, and flexibility. Individual plants, and even individual species, die off frequently, but there is always another individual or another species to fill the void. Relying on a small set of native trees without embracing the redundancy and diversity of natural systems is a recipe for disaster.
Mark Wessels
Conservation Sense and Nonsense 
San Francisco is an example of a city that has often embraced the nativist dogma. The ongoing slaughter of non native trees by Rec and Parks department as well as other agencies in the city in control of public land has left the public land open, windy, sterile and dull. Where once there was woodlands thick with verdent stands of eucalyptus, Monterey pines and Monterey cypresses as well as willows, buckeyes, mock oranges, Himalayan blackberries etc we now have open grasslands. And the remaining eucalyptus and other trees that have been left have often had their limbs cut so they look like some kind of park setting with no trace of woodland that can be attributed to it. What was once a thick diverse woodland is now scattered trees.
One only has to go to the Presidio to see the land maintained by the National Parks rather than Rec and Parks of San Francisco to see thick lush trees growing unmolested. But what about the rest of the city? And who gives Reck and Park the right to have unbridled control of our trees. They always say that the arborist that they hire say that these trees are too old and have outlived their lifespan or are diseased or are not native. This is one-sided thinking at best and is not proven by science. Some of the facts are completely unscientific. For instance the nativist point of view is that you need native plants for pollination. One can only look at the monarch butterfly in the eucalyptus coexisting to see that that is false science.
The use of the chainsaw and herbicides by the people in charge of our urban forest is detrimental to the health of the forest and the humans that live amongst them. San Francisco was once a progressive City. If it had any sense of still being a progressive city it would maintain its forest by the organic method rather than using herbicides.
The mindset of many articles on gardening is that it is important to plant native plants. While native plants should be part of the ecology of California as the article above points out what was once native is no longer necessarily native with different microclimates and the evolution of climate change. DIversity is the key to successful ecology. Humans need to adapt to the ever-changing evolution as flora and fauna has adapted. Diversity is the answer not isolation and reverting back to some nativist ideology however popular it might sound to some.
Totally agree with all of this. I’m a gardener in Maine. I have a client who is native obsessed and I have taken many gardening/ecology classes here, and all of them have the same dogma around “invasives.” I try to ask questions like, “Isn’t it better to leave a huge tree that is getting old because we know that ancient trees provide more life and habitat and biodiversity due to their size and age, rather than cut it down simply because it’s an invasive?” Or I try to ask, “If there are over 700 non-native species in Maine, but not all of them become invasive, does that mean there is a reason only a few of them are successful, and that the environment is creating the conditions for them and perhaps the land benefits?” Or I try to ask “Native plants weren’t adapted to high levels of pollutants, the chemical poison in the soil, human development and habitat fragmentation, and hundreds of years of degraded land use, so why do we say they are adapted here?”
Inevitably, I am interrupted and they repeat “invasion biology” basics and don’t actually listen to my question or answer it. I’ve found in the native garden/ecology world an absolute inability to question the “science.” If I bring up long term studies that show that over time “invasives” help the land and lead to greater biodiversity, I am literally treated like a crazy, misinformed conspiracy theorist. It’s very disappointing, considering all these people consider themselves ecologists. The basic idea of ecology is that individuals in an environment are creating and created by their environment and vice versa. Why are invasive plants treated as though they are separate from the environment?
I’m so sorry to hear about the destruction of healthy and thriving forests that you’ve witnessed. I’ve tried to find out how many pesticides are used in Maine in the name of invasive plant removal but it’s a weirdly difficult thing to find.
My client who insists on “native plants only” has a garden that is struggling and the insects and birds and soil biology that is supposed to have immediately responded is hard to find. Meanwhile, I have another client who has let her property go free for 6 years and the place is covered in insects and birds. There is a very strong and balanced mix of native and non-native plants – they are so much healthier and there is a huge variety of plants, and shrubs and trees and it has all happened “naturally.” You can feel the life in that property, whereas the native garden feels like it’s just a fight against what the land wants. It really does feel like a cult to me sometimes. I think actually working with the land and doing the labor gives me a different perspective than people who are “doing research” and reading the “science” of the native garden world. They are so convinced that invasive plants are the greatest problem, but if you actually do the gardening and work with the land, it’s obvious they are extremely useful for the degradation we’re facing.
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Thank you so much for your insightful comment and your efforts to gently guide your clients to a more diverse garden that is adapted to current environmental conditions.
I have been debating with native plant advocates for 25 years, so I am sympathetic to your frustrating experiences with the cult-like devotion to the botanical past. I admire your willingness to engage in these difficult, sometimes unpleasant, conversations.
I agree that horticultural professionals are in the best position to judge the value of a diverse garden because their top priority is results. Many academic scientists seem to be wedded to an outdated paradigm on which their careers were based. Another generation of scientists is needed.
I so agree. Beautifully said. Every time I have a chance to try to get people to think or re-think their cultish dogma, I try, whether in person or online. With so many beloved Bald Eagles choosing Eucalyptus to nest in now in the Bay Area (like most other raptors from Hawks to Owls are also doing), it’s a great opportunity to explain to people that these bird are deliberately choosing Eucs, which I believe is because the open canopy is safer for the fledglings to learn to fly in and perhaps the strong medicinal scent of Eucalyptus helps keep parasites from the nests. And then if they are responding, I bring up the Monarchs who are in such danger and yet also clearly choose Eucalyptus. The animals know. But what a sad waste in the meantime, with destroyed and poisoned habitats.
I recently found out that they are spraying poison in the Las Gallinas wildlife ponds over fear of West Nile Virus. They must know that small fish eat the larvae, and so do many predatory water insects, like Striders, Backswimmers, etc. I’m horrified and don’t know what to do, especially since it’s no surprise that poison is heavily sprayed in the East Bay, but Marin has laws protecting the land and water. There was no warning for the people who went there looking for birds.
Thank you so much. I so agree and have been fighting the destructive nativism for years now.