Trees soften the hardscape and add eye pleasing greenery to an otherwise bleak concrete environment. More importantly, trees provide tangible environmental and health benefits in an urban community. They generate oxygen, filter pollution, reduce the heat island effect, and provide an urban habitat for a region’s native birds. Generally speaking, larger trees provide more environmental benefits in urban areas than smaller trees. Urban forests, as they are sometimes called (although this article refers to landscaped trees rather than remnant forests), are important both in cities’ urban cores and in their suburbs.
However, some cities are less successful than others in fostering the growth of an urban canopy. Moreover, some of these cities are urban canopy-deprived despite years of pro-tree planting policies and efforts. My Southern California home-base, San Diego, is one such city. The paucity of an urban canopy can have many causes. For example, the absence or inadequacy of policies promoting road verges (sometimes called parkways) results in less urban canopy, especially near roads (a principal cause of the heat island effect). Lack of water and inhospitable soil also suppress both the number and size of trees. Nevertheless, Southern California’s shade-canopy deprived areas don’t lack in lush landscapes. In contrast, many native shade trees are relatively low water users and drought tolerant. Even where natural hydrology is inadequate to support shade trees, urbanization brings far more than the minimal water necessary to support native shade trees.
Southern California’s native shade trees include several types of oak tree, the western sycamore, the fremont cottonwood, the white alder, and several types of pine tree. Several non-native shade trees thrive here as well. Instead, Southern California landscapes are dominated by an assortment of exotic non-shade trees that provide far fewer environmental benefits. This choice of tree type is a contributor to the paucity of tree canopy.
Based on casual observation, the following are some of the popular Southern California tree selection pitfalls:
California Dreamin’ Trees: Postcards, T.V. shows, and nearly every bit of media portraying the California lifestyle exhibit non-native lifestyle flora. The most ubiquitous of these non-native image trees is the palm tree; and the most ubiquitous of the palm trees is the mexican fan palm (washingtonia robusta aka sand palm). These tall slender palms line so many boulevards and beaches that they have come to symbolize Southern California in many people’s minds. Of course, there are innumerable other palms that overwhelm California landscaping, from diminutive pygmy palms to gigantic canary island date palms. Palms are so associated with California that the visitor might be shocked to learn that the California coastal landscape was completely devoid of palms upon the first europeans’ arrival. There is only one native palm tree specie, the California fan palm (washingtonia filifera), and it’s natural range is limited to the desert canyons on the eastern side of the coastal mountain ranges.
Yet palm trees in Southern California are now so prevalent, they largely supplant all other tree species in urban landscapes. Their single non-branched trunks topped by a plume of fronds provide little foliage in comparison to shade trees.
Easy Street Trees: Nearly every discussion of urban tree selection involves a discussion of maintenance issues. This is especially true for municipal landscapes and streets. Concerns include fallen leaves and berries and roots uplifting sidewalks or invading water & sewage utilities. As a result, diminutive perennials predominate in many urban landscapes, largely blunting the environmental and aesthetic benefit of street tree programs. While cities such as San Diego are providing increasing recognition to the benefits of native trees and shade trees, it remains to be seen whether enough is being done.
Exotic Trees: Another hindrance to a truly eco-friendly urban canopy is our fascination with exotic trees. It seems to be human nature to pick the most noticeable and colorful plants regardless of location. We also seem to favor plants that remind us of the places we favor for vacation. Southern California’s temperate climate is hospitable to a wider range of non-native flora than other areas of the country. As a result, the region hosts an abundance of tropical and exotic trees in landscapes. These trees share little symbiotic relationship with native flora or fauna. Moreover, these trees are often water intensive. In general, they contribute to the displacement of urban canopy trees.
Insta-Tree: Some landscape plants, especially trees, are selected because they grow quickly into mature specimens. In California, eucalyptus trees and mexican fan palms are examples of such trees. This same growth factor tends to make them prolifically invasive, again displacing the native flora and fauna eco-web. Moreover, invasives like the eucalyptus tree create dead zones underneath. The chemicals released by fallen eucalyptus leaves are toxic for native flora (aka “negative allelopathy”). Falling eucalyptus tree branches, which are heavier and weaker than other trees, have caused several deaths in San Diego alone. While some of these trees provide shade and other benefits of larger trees, these benefits are often offset by their detriments or short-lived existence (due to efforts to remove them resulting from their harmful effects).
NIMBY Proof Tree: Another factor that sometimes undermines the development of an urban canopy is the “Not In My Back Yard” resistance to large trees. This resistance can come from stores concerned with obstruction of the visibility of their frontage or signage, or residents concerned with view obstructions. Concerns about views and visibility sometimes leads to a preference for benign diminutive tree types.
Conclusion: Education and incentives are necessary to guide the selection of trees that have an environmental benefit. In our climate-changing world, it’s more important than ever to promote the use of shade trees and native flora for urban environments.
All photos by author. All photos taken in the San Diego region, except the first photo, which was taken in Portland.
A wonderful tree selection resource is the Tree Selection Guide hosted by Cal Poly San Luis Obispo