Should cities plant native or introduced tree species?
This is a topic of some controversy in urban forestry right now. While the concept of a native species is not necessarily a scientifically valid category, it has been shown that native species can support more biodiversity on average than introduced species. But, contrary to public perception, introduced species can also support considerable biodiversity and, in some cases, that biodiversity can be more distinct. So while an individual introduced plant may have lower insect richness (on average), they can potentially increase regional biodiversity as long as they do not displace too many native plants by hosting species that would otherwise be rare or absent. Additionally, richness of species associates varies widely among these groups, such that it may be that some introduced species have more benefit than some natives.
Of course, preservation of biodiversity is not usually a high-priority objective in urban forestry, and native species don't always perform well on other metrics. Particularly in California, many native plants have very particular climatic or soil needs, and many don't perform well in urban areas as a result. Additionally, because trees don't migrate very quickly, many species have been stranded south of their ideal climate conditions. When combined with the urban heat island, climate change, and poor urban soil conditions, this has led to poor performance and survival among many natives in urban areas. And because most native trees are not suitable for urban conditions, the few species that are (like coast live oak) are not sufficient to meet tree diversity goals on their own. Diversity in the urban forest is an essential strategy in the face of an uncertain climate, unpredictable introduced pests and diseases, and for biodiversity in its own right.
My personal view is that we need to take a broader view than just native vs. non-native and look at things from a more bio-regional perspective. Some southern California or desert Southwest natives that have proven well-adapted to urban areas may be key to Northern California's future. And because most wildlife species can migrate much more effectively than plants, it's reasonable to believe these species will have similar biodiversity benefit to our local natives. However, I would like to see further research on this question.
Here are some general principles that I think can inform our decision-making on this topic:
Plant species that have proven adaptation to the climate and soils they will be occupying, including our future climate. Avoid species that have been proven to perform poorly or have marginal heat and drought tolerance today. I list this first because a healthy introduced species will more effectively support biodiversity than a native that never grows.
Avoid species that are invasive or that we have evidence to suggest they could become invasive. But keep in mind that most non-native plant species found growing in the wild are not truly invasive. Invasiveness should be assessed using quantifiable metrics and research. I like Cal-IPC's approach, but I would like to see more species assessed, not just the ones that are already abundant in the wild. They may have done further assessments that aren't posted publicly. This assessment would be useful for decision-makers who may be introducing new species.
Make biodiversity an explicit goal of urban forestry, instead of a barely considered co-benefit. Make decisions on the basis of that goal, rather than using it as a justification for what you wanted to do anyway. Most urban foresters I've spoken to seem to believe that maximizing canopy coverage is necessary and sufficient for biodiversity and don't see a reason to change their decision-making on species selection for this reason. But if the evidence shows that some native species are important for biodiversity conservation, this should be appropriately weighed in decisions.
Follow the science, and not ideology or conventional wisdom. Support and make use of research on finer categories of plants and how they affect biodiversity rather than broad, loose categories like native and non-native. Ideally this would be done on a species-level basis in each region but there is very little research in this vein. Broader concepts like geographic distance to native range, genetic distance between native and introduced species, or even prehistoric species assemblages should be researched and considered instead of treating this issue as black and white.
Plant larger species whenever possible. In the space-limited context of cities, trees with large canopies are the best way to maximize the volume of living plant matter while occupying little space on the ground-level. For most vertebrate species, tree size and structure is actually more important than which species you plant.
Consider novel management approaches that can reduce risk while leaving some deadwood and cavities for wildlife use. These resources tend to be scarce in highly managed urban trees.
This represents my thinking at the current time, though I expect it may evolve as further research reveals the full picture. I welcome any comments or discussion on this topic.
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here are some introduced tree species I've been seeing growing wild around Sacramento county a lot. Red gum eucalyptus, Atlantic pistache, Chinese tallow, coast live oak, callery pear, cherry plum, a few different elms, olive, common fig, pecan, catalpa, sweetgums, and privet counts I think. If humans were to suddenly go extinct for some reason I wonder how the plant community would evolve with all these new species competing. I get excited when I see hummingbirds and carpenter bees visiting non native plants. It's nice to see native and non-native things co-existing. Maybe things like Oleander should be avoided, because I found it out the flowers have no benefit to insects (no nectar or anything) and the foliage is rather toxic too I think. Things are forever altered by human activity and I see so much disturbance and non native introductions that I'm just happy when things at least provide a food source for local animals. There are also lots of cool introduced insects, birds, rodents, etc. that make things interesting
I understand the open-mindedness of alexbinck's assessment, but given the choice, I favor native species. Mu own community, a western Chicago suburb, is currently replacing trees in the downtown area and taking advice from The Morton Arboretum. The suggested species include natives, nonnatives, and what I would consider cultivars. I am disappointed that so many nonnatives (by number of individual trees) are being planted. On a parallel note, several years ago I reviewed the Arbor Day Foundation's criteria for designating a community a "Tree City USA." As I recall, the criteria were largely budgetary: Nothing was said about the species of tree being planted.
What @johncebula just said sounds very disappointing to me too. I'm always excited to see any natives in landscaping here in Sacramento such as toyon, coffeeberry, and ponderosa pine. Amazon is pretty cool for planting that stuff around their new buildings in my neighborhood. I fantasize about what California looked like before humans were around, so planting things that evolved here gives me a glimpse of that. Plus there are so many unknown/underappreciated natives that I feel ashamed planting the same ecologically out of place stuff as everyone else
@alexbinck do you assume climate change will make California hotter and dryer? I think that may be the general trend in a lot of places, but I'm not convinced certain areas such as California will be effected very negatively in that way. Some parts of the world will probably benefit from climate change
@scottmcneir I sometimes wonder if non-native species in wild areas will add or subtract from the overall ecological health of an area. My instinct is uncommon species might just add extra biodiversity with little downside, but a few of the species you mentioned are reaching densities that might be displacing enough native species so as to be a net negative. I don't know of any research on this specific topic, but I haven't really looked either. Invasive species are generally known to be harmful, but the exact line of when a species becomes invasive is not clear cut.
Regarding future climate trends, there is still some uncertainty over our future climate, particularly in the far future. We do know that things will be substantially hotter, and likely with more intense droughts and floods. Minimum winter temperatures and average precipitation are more uncertain, but the research I've shown suggests steady or slightly increasing average precipitation, and warmer winter temperatures. Overall, I see these as being pretty negative in aggregate, especially because the heat, floods, and droughts (and associated wildfires), may be extremely destructive. Less winter cold may have some benefits but also negative impacts by reducing winter mortality of insect pests and disease vectors.
@johncebula Keep in mind that this post was written from a California perspective. We have relatively few native tree species that are suitable for urban areas--mostly oaks, which are widely utilized, but not appropriate for every situation. Urban foresters also prefer to plant a diversity of trees because historically urban forests were dominated by one or a few species that were devastated when introduced pests and diseases wiped out those trees. But I expect that the Chicago area probably has a wider palette of native trees, being located in more of a humid, forested biome. I do support the use of those native species that are well-suited to urban conditions and as long as the overall diversity of the urban forest can be maintained. In your area I would think there should be a variety of native species for almost all circumstances, particularly if you include species from slightly more southerly climes as a hedge against climate change.
In California, our climate gets substantially drier as you move south, so we can add a few more species from Southern California and Baja, but we may also need to expand further east into the Sonoran Desert to reach the needed diversity. Thankfully, the consensus in the research I have reviewed suggests that species from nearby regions may be as good or nearly as good as local natives in terms of insect/wildlife habitat. And, as I alluded to in my initial post, the added diversity of a small number of exotics can also support unique and rare species that otherwise would be absent, though this finding is somewhat debated in the science.
These are all good points and comments. Thanks for putting this together Alex.
I'll add this paper we recenlty published about this issue: https://doi.org/10.1016/j.ufug.2023.128125
There just aren't very many native species available for California cities, and we currently grow most of them.
Thank you, Matt Ritter.
@alexbinck I saw a lot of desert willows (Chilopsis linearis) at costco in Lodi I think. Southwestern US and Mexican natives seem like a decent option too
@slowplants Thanks for sharing the work of your lab! Some other very interesting papers referenced in there as well. I've heard your lab is working on some region-specific climate-ready tree lists and I would be interested to see them or even discuss and collaborate if that's of interest to you.
@scottmcneir Desert willows are a great example of a near native we can expect to do well here--wide geographic range, from a climate slightly hotter and more arid than our own. I've also noticed that species from the drier edge of riparian zones tend to make good urban trees for whatever reason, and this species matches that pattern as well. However, there is one problem with many of these desert species that desert willow highlights as well--there are very few large stature trees from extremely hot, arid climates. Desert willow is great for smaller spaces but it can't and shouldn't replace elms, sycamores, oaks or other large species that shade our cities currently. With these historic European and Eastern North American species threatened by climate change, we can and should promote those larger SW natives that do exist but we may need more exotic trees as well to reach the needed diversity in this class of trees. Dave Muffly (@oaktopia though he's no longer active on this platform) has done great work in researching and promoting a variety of interesting oaks from the American Southwest, but we'll need more than one genus to solve this problem.
@alexbinck Before Europeans came, weren't the trees in the central valley mostly Quercus lobata? They are so good for shade, and I'm having a hard time coming up with a reason not to mostly plant them. I'm thinking there was a good amount of black walnut and riparian trees like poplars, willows, box elder, and ashes too. Otherwise I can't think of a lot of other big trees. I like Ponderosa pines and redwoods, but maybe trees that spread more at the top (like oaks) are nice for urban areas for shade.
@scottmcneir As you moved away from water bodies there was generally a rapid transition from water-loving riparian species like willows, cottonwoods, alders, etc. to more mesic species like valley oak, sycamore, walnut, etc. and then finally smaller shrubs and prairie in more dry locations. But dry soils with subsurface moisture that flank the rivers were dominated by Q. lobata, and this was a fairly large area compared to other treed landscapes. The only other nearby treed ecosystems of notable size were in the foothills where Q. douglasii dominates. In terms of area, the prairie ecosystem was probably the largest but maybe second was woodland dominated by Q. lobata surrounding rivers where there were better soils and the water table was shallow. Unfortunately, the vast majority of this valley oak woodland ecosystem was destroyed and replaced by agriculture, since it is extremely productive farmland.
So, Q. lobata is definitely the best adapted locally native tree for most urban areas, but again, planting too much of one species opens you up to risk in terms of pest invasions (one of which we are experiencing right now for Q. lobata) and uncertainty around future climate tolerance. At most, it's typically recommended to have one species be 10% of your urban canopy, so we still need the other 90%, and some have recommended a more conservative 5%.
Conifers certainly have their place in urban forestry, but generally, most of our native conifers are adapted to cool, moist conditions in the mountains with very different soils and slope compared to most urban areas. Redwoods need consistent irrigation to stay healthy, and are stressed by our current level of heat. Most people are not planting them anymore, and I expect many of the existing trees will die off in the coming decades as heat and drought intensify. Pines are a better choice, and some CA natives may play a role like gray pine, ponderosa pine, torrey pine, etc. But not all pines are equally heat tolerant, and many come with other drawbacks (messy, soft wood, flammable).
If you look at the southwest as a whole, we get lots more oak species but also some new genera like montezuma or bald cypress, pecan, and probably a few others. There are also some species from NW Mexico (Ficus, Cedrela, Conzattia, Ceiba) that might do well here but for the cold, so maybe they will be good options in the future once we've warmed a bit more.
On the whole, we have a solid foundation of larger trees native to Western North America, but the future climate and pest/disease resilience of some of these species may be challenged in the future. For this reason, I also advocate for more exotic species that are well adapted to our climate, even though they probably will provide a smaller benefit to local biodiveristy (though keep in mind that there is large and poorly researched variance between taxa). But I do think there is room to expand regional natives in many cities without compromising other important urban forestry goals, and this may be beneficial to local biodiversity if it can be achieved.
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