Growing a Desert Urban Forest
If you think about it, urban forests grow in two separate and distinct ways. One is geometric and the other is linear. Each does not exist alone, and we cannot materially expand the forest, in numbers or physical size, unless we understand and appreciate these two processes.
The Geometric Urban Forest:
Trees, like all plants, tend to grow geometrically. Leaf canopies increase in height, width, and depth simultaneously throughout the entire growing season. Similarly, branches and trunks grow in both length and caliper (radial growth). During most of the juvenile (pre-mature size) growth stage of a tree, they can often double in size within a single growing season. Out of our view, the root system is following a similar pattern of growth and expansion. This geometric growth of established trees is the wellspring of our aspirations for urban forests to be a tool for heat island mitigation, promoting of pedestrian-friendly environments, creating a regional sense of place (like the Sonoran desert) and improving environmental sustainability.
The essential, professional care and maintenance of this established portion of the urban forest has been the focus and inspiration for nearly half the articles we have published in Arid Zone Times and Arid Zone Tree-mail since 1994. Proper, professional care and maintenance, from pruning, fertilization, pest control, irrigation design and scheduling, proper tree selection, installation, and proper tree placement are critical to a vigorous, durable, aesthetically pleasing urban forest. Without attention and proper care, the initial investment in urban trees is squandered. The expenses of landscape design and purchasing and planting trees must be reinforced by consistent, rigorous maintenance if the urban forest is to perform at its highest potential. It is the mission of landscape architects, growers, landscape contractors, arborists, and maintenance professionals to foster the longevity of these trees.
The Linear Urban Forest:
New planting of individual trees in streetscapes, commercial/residential landscapes, community open space and parks or as parts of landscape reclamation/conservation programs are the “seed corn” of the geometric forest. These one-by-one plantings are, at this stage, more promise than certainty as too many perish in the early years. Even the most casual observer can recognize the differing contributions made by a freshly planted 48” box tree (a luxury item for most new landscape budgets) relative to an established, maturing landscape tree; their respective visual and heat island mitigation values are obvious. The promise of these new plantings is only realized when the trees survive a gauntlet of environmental, physical, biological, horticultural and bureaucratic hazards to ultimately become part of the geometric forest.
Great fanfare is afforded these new tree plantings by some municipalities, utilities and city departments (“…City of X is on pace to plant 100,000 trees by year ‘Y’”). Virtually no reporting is offered regarding the NET growth of the geometric forest that results from these new plantings; in part because planting data is easier and more flattering than survival data. Landscape and horticultural professionals are trained to anticipate and manage the physical and environmental challenges of the urban forest to the extent possible. Landscape architects, arborists, landscape construction and maintenance companies have, as the core of their business models, the goal of designing, building and nurturing long lived, robust planted environments.
These efforts are too easily vanquished by development regulations enacted by some municipal, county, state and utility company intrusions that are, in many instances, detrimental to the survival of the linear urban forest and contrary to sound, accepted horticultural/arboricultural practices. Here are a few examples:
New streetscape tree plantings shoehorned into narrow landscape easements separating vehicular traffic (especially large trucks and buses) from sidewalks and pedestrians are rarely visually or horticulturally successful. If modern streetscapes are to take their place within the geometric urban forest these plantings need to be seriously re-imagined with more input from landscape architects, wholesale tree growers, arborists and maintenance professionals.
Some city requirements that trees be specified in landscape designs exclusively on the basis of trunk caliper, without designating the appropriate container size to support a proportional root system, continues to be a major contributor to early tree death and is a prescription for long term tree failure.
Conflicting requirements between municipalities and utility companies can create locations that restrict tree height, planting depth and landscape easement width. While these competing conditions are uncommon, they present settings where no tree can sustain long term-growth. The resulting landscape features trees that have been harshly pruned to accommodate the regulatory constraints of the sites or have died and been removed. Such pruning compromises the natural symmetry of trees reducing their visual appeal and makes them more prone to branch breakage, wind throw, sunburn and the subsequent infestation by tree bores and other wood eating insects.
Building a sustainable, dynamic urban forest requires careful attention to promoting and nurturing vigorous linear and geometric tree populations. To fully realize the promise of these trees, their potential to mitigate the urban heat islands, to foster more pedestrian friendly communities and to establish a sense of place in the Sonoran Desert, we must stop seeing them as an accounting task or a public relations gimmick. The challenge to cultivate greater collaboration among all desert horticultural professionals and policy makers, with the goal of re-imagining and redesigning our urban forests, could not be more urgent or timely. Acknowledging that these trees are a complex, biotic, vibrant, and fragile component of the modern built environment will be an excellent first step to building more attractive and livable cities.
Recommended Additional Articles:
http://www.aridzonetrees.com/common-pests-dt.html
http://www.aridzonetrees.com/azt-root-management-dt.html
http://www.aridzonetrees.com/variety-azt.html
http://www.aridzonetrees.com/pruning-trees-dt.html
http://www.aridzonetrees.com/watering-trees.html
http://www.aridzonetrees.com/nitrogen-fixation-and-desert-trees.html