The need for diverse urban forest and the obstacles to achieve that goal

Matt Ritter is a professor of biology at Cal Poly San Luis Obispo and Director of Cal Poly Plant Conservatory.  He is the author of several books about California’s unique flora, including A Californian’s Guide to the Trees Among Us.  He is considered an expert on the horticulture, ecology and taxonomy of the Eucalyptus genus.

Click on picture to view Professor Ritter’s presentation

In October 2021, Professor Ritter gave a presentation to the California Urban Forests Council, entitled “Underutilized Species for the Future of Urban Wood and Urban Forestry.”   He began by explaining why it is important to identify new tree species for our urban forest.

  • “Baja is moving to Oregon,” said Ritter to set the stage.  Within 50-80 years trees living in California now will no longer be adapted to the anticipated warmer, drier climate.  Trees killed by wildfire in California are not returning.  Forests are quickly converting to grassland and shrub.  As of 2018, California had lost 180 million trees to drought, disease, bark beetles, heat, and wildfire, which is nearly 5% of the total tree population in California.  Adding subsequent years to date, we have probably lost 7% of all of our trees.
  • Trees in urban areas will help Californians cope with warmer conditions because they cool our cities and reduce energy consumption.  Fewer trees will mean a lower quality of life, for us and for birds.  The loss of our trees reduces carbon storage, which contributes to more climate change. 

Ritter then explained why we must diversify tree species in our urban forests.

  • There are over 60,000 tree species in the world and only 7% of tree species are found in urban areas around the world.  In California our urban forests are even less diverse.  There are only 234 tree species on average in California’s urban forests.  The average number of approved tree species for planting in California’s municipalities is only 49 and few species on those approved lists are native to California.
  • Diversity of tree species ensures greater resiliency that enables our urban forests to survive changing conditions.
  • Only 9% of tree species in California’s urban forests are native. 
The native ranges of tree species in California’s urban forest.

An inventory of Oakland’s urban forest (street trees, medians, and landscaped parks only) was recently completed.  With 535 tree species, the diversity of Oakland’s urban forest is greater than average for California.  With 14% native trees, Oakland’s urban forest is more native than average. There are 59 species on Oakland’s list of approved trees, of which only 4 are native to Oakland.  The most significant finding of Oakland’s tree inventory is that our urban forest is only 64% “stocked,” meaning that of existing tree wells, only 64% are currently planted with trees.  When trees die in Oakland, they aren’t being replaced.  I don’t doubt there is a will to plant trees in Oakland.  I assume it is a question of means in a city with more pressing needs than resources.

Ritter and his colleagues at Cal Poly have created a website called SelecTree to help Californians choose the right tree for the right site and conditions.  There are 1,500 tree species described on SelecTree, using 60 characteristics, such as drought tolerance.  SelecTree rates blue gum eucalyptus “medium” for drought tolerance, the same rating as native coast live oak and bay laurel.  Ritter clarified that drought tolerance on SelecTree is a measure of how much water the tree species uses.  Claims that eucalyptus uses more water than native trees is bogus, like most bad raps about eucalyptus.  

Ritter recommended specific tree species, based on their drought and heat tolerance.  He said that when diversifying our urban forests “we have to think about Australia” because it is the hottest, driest, flattest, and oldest place on the planet, which is another way of saying that tree species in Australia have survived terrible conditions that are comparable to the challenging conditions in urban environments.

Ritter recommended oak species that are native to Texas; eucalyptus and closely related tree species; and several tree species in the legume family, especially acacia.  In each case he mentioned the suitability of tree species based partly on the quality of its wood.  Apparently, I’m not the only person in California who is disturbed by huge piles of wood chips wherever trees have been destroyed.  Ritter also thinks we should be thinking about how we can use wood when trees are destroyed, rather than building potential bonfires.  

Obstacles to diverse urban forests in California

When Professor Ritter took questions from the audience, we learned that the main obstacle to a diverse urban forest in California, adapted to our climate conditions, is the myopic focus of native plant advocates:

Question:  “Are we introducing new pathogens to our natives by importing new species?”

Answer:  There are many laws and rules that restrict the importation of plants to prevent that from happening.  We also import only the seeds of plants, not grown plants.  The seeds are sterilized and they don’t carry the pathogens that may exist on grown plants in their native ranges.

Question:  “Do we know how quickly birds and insects adapt to new species?”

Answer:  “No we don’t, but who cares?  We are facing a climate emergency.   We have 50 years before life in our cities becomes hell.  We have a responsibility to protect the quality of life in our cities.  We should stop developing the wild, but cities are different.” 

Ritter anticipated a question that is often a concern of native plant advocates by saying we should not be concerned about “weediness,” AKA “invasiveness.”  He said, “That should be far down on our list of priorities of what to worry about.  We need to be primarily concerned about what tree species will grow in our changed climate.”

Rhetorical Question:  “But insects need native plants!

Answer:  Ritter instantly recognized the mantra of Doug Tallamy.  He replied that it is not well established that there are more insects living on native plants than on introduced plants.  He mentioned a single study that inventoried plant and animal species in eucalyptus compared to oak forests, presumably Dov Sax’s study which concluded:  “Species richness was nearly identical for understory plants, leaf-litter invertebrates, amphibians and birds; only rodents had significantly fewer species in eucalypt sites.  Species diversity patterns…were qualitatively identical to those for species richness, except for leaf-litter invertebrates, which were significantly more diverse in eucalypt sites during the spring.” 

Rhetorical Question:  “We are still dealing with a legacy of blue gum eucalyptus in the Bay Area.  Why should we repeat that mistake?”

Answer:  Ritter agreed that blue gum eucalyptus is “inappropriate” in many places where it was planted in the Bay Area, but we’re not planting blue gums.  There are 800 eucalyptus species and many are ideal for our conditions.  He said, “Why not plant eucalyptus?  It would be dumb not to plant suitable eucalyptus species just because it shares a name.”

Ritter added that, “Planting only natives just doesn’t work in San Francisco.  There would be no trees in Southern California because we don’t have very many native trees in California.”  The pre-settlement coast of California was virtually treeless in most places and that’s a fact. For example, a study of historic vegetation in Oakland, California reported that only 2% of pre-settlement Oakland was forested with trees. “Vegetation before urbanization in Oakland was dominated by grass, shrub, and marshlands that occupied approximately 98% of the area.” (1)

San Francisco in 1806 as depicted by artist with von Langsdoff expedition. Bancroft Library

Oakland as a case in point

The San Francisco Chronicle recently published an article about a guerilla tree-planter in Oakland who is planting native oak trees on public land, wherever he wants.  Oakland’s Director of Tree Services, David Moore, gently suggests that many of these tree plantings are ill-advised:  “‘There is a part of all of us that loves with our hearts the coast live oak tree because of its heritage, the symbolism of our city, and just the legacy that they have,’ Moore said. ‘But we have to diversify, and we are diversifying to other ones that are recommended to be more adaptable to climate change…The reality is that we have created a world that is not the native conditions of these plants,’ Moore said. ‘If we want trees to survive in these non-native conditions, we have to pick trees from around the world that can survive these conditions.’…Moore said oaks, while beautiful, are not the ideal tree for today’s hot, dry and cramped urban landscape. Without careful and costly maintenance, he said, oaks could destroy sidewalks, block light from street lamps and grow their branches into streets and walkways, creating hazards for motorists and pedestrians. The city still plants oaks, but mainly in parks rather than streets because that’s where they do better, Moore said…”

Stalemate

So, here we are.  We have a pressing need for a more diverse urban forest that is adapted to present and anticipated conditions, but we are paralyzed by the ideological commitment of native plant advocates who are demanding that we destroy our urban forest because it is predominantly non-native.  In a recent edition of Nature News, Jake Sigg said, “Hysterical tree planting is worse than a waste of time and resources…”

I am grateful to Professor Ritter for being bluntly frank with members of the arborist community who should know better.  Dare we hope they learned something from that presentation? 

I wish you Happy New Year.  Please join me in my hope for a more peaceful year.  Thank you for your readership.


(1) Nowak, David, “Historical vegetation change in Oakland and its implications for urban forest management,” Journal of Arboriculture, 19(5): September 1993

8 thoughts on “The need for diverse urban forest and the obstacles to achieve that goal”

  1. Thanks for your most recent article about Matthew Ritter’s presentation. I was especially impressed by the comments and questions in this article. Your writings always give me hope that all is not lost in our struggle. Best wishes for a 2022 that will be better in every way then the past 2 years.

  2. Thank you so much for this post. It’s so important and makes so much sense. It’s ironical that most people who have yards and money to landscape usually choose the gorgeous variety of trees and other plants from around the world, while the pressure is to limit yards to natives only. And that is dangerous for many reasons.

    From what I’m seeing, the native animals, including migrating birds, just love and need the beautiful varieties of exotic plants we are lucky to have here. (Yet, the same mistakes continue to be made, such as saying to cut everything down in the Burrowing Owl habitat at Cesar Chavez park in Berkeley, though we knew over ten years ago that the Owls need Fennel and other plants to help camouflage them when they are in danger from off leash dogs. Fennel, which is edible from root to seeds, seems to be a recent target to kill by nativists, yet those who love birds see how much birds need and use fennel, from food to light perches.)

    Thank you for continuing to protect nature! And may your next year be wonderful.

  3. He made some great points, but he rubbed me slightly the wrong was in his first paragraph. To be sure, Baja is NOT moving to Oregon. The globe may be warming slightly but the extreme climate change that’s being described in California is getting exaggerated.

    I believe that a great deal of the warming is actually coming from the heat island effects from massive urban sprawl. Can I prove this? No. But neither can anyone prove that the recent droughts are being caused by climate change. That’s speculation. It’s a most logical theory but we don’t actually have proof of that, not yet anyway.

    For the climatic zone of Baja to ship northward all the way to Oregon is even more speculative.

    If I’m right about the heat island effects of urban sprawl, then the rest his article makes great sense. That’d be a great way to cool our cities. However, the only problem is finding the water needed for the trees to thrive.

    1. California’s drought may not be caused by climate change, but climate change exacerbates California’s natural tendency to drought cycles because higher temperatures increase evaporation rates, as explained in this article by National Public Radio: https://www.npr.org/2021/06/09/1003424717/the-drought-in-the-western-u-s-is-getting-bad-climate-change-is-making-it-worse

      Ritter’s statement that “Baja is moving to California” refers to a trend, but not to the current state of affairs. Yes, by the end of this century, that statement could be literally true: “We are currently on track for a rise of between 6.3° and 13.3°F, with a high probability of an increase of 9.4°F by 2100, according to the Massachusetts Institute of Technology.” The forecasted high in Ensenada, Baja today is 70⁰, compared to a forecasted high of 59⁰ here in Oakland today.

      Ritter is asking us to look to the end of this century when planting trees, so his opening statement is consistent with his goal, which is to suggest that we plant trees now that will be capable of surviving in the anticipated climate. Given the long life of trees (if well-adapted species are planted), it’s appropriate that we look that far forward.

      Although I didn’t cover the issue in this summary, Ritter also mentioned the heat island effect in urban areas as one of the reasons we need to plant trees that are capable of surviving the challenging conditions of urban areas.

      1. Dear “Million Trees”,

        Right! And some trees that would be ideal to plant in urban areas are the so-called “non-native” eucalyptus trees. They cool the air and have a generally low demand for water. I have seen where eucalyptus trees have thrived in Tucson with little water.

        Regards,
        Fred M. Cain

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