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.
