   Urban Forests - September 22, 2004 Jeff Schalau, County Director, Agent, Agriculture & Natural Resources Arizona Cooperative Extension, Yavapai County What is it about trees that cause people to become so emotional? When a tree dies or must be severely pruned for safety reasons, people feel a sense of loss and often grieve over that loss. We can avoid most tree “problems” by putting the right tree in the right place and providing proper care for the tree over its lifetime. Insects and disease rarely overcome trees that are appropriately selected (locally adapted species and properly grown), correctly planted, and properly irrigated. Conversely, if any one of these factors is neglected, it can be an uphill battle to keep a tree alive. Trees have many tangible benefits-especially large trees. Some new information from the Center for Urban Forest Research (CUFR is part of the U.S. Forest Service Pacific Southwest Research Station) indicates that large-stature trees deliver greater benefits to communities than small-stature trees. Large trees benefit societies by conserving energy by shading buildings and cooling paved surfaces, cooling the air by transpiring water, providing habitat for wildlife, filtering airborne pollutants, reducing stormwater runoff, and hiding unsightly views. According to CUFR, a large tree is one that is 40 feet tall, 40 feet wide, and a trunk diameter of 30 inches by the time it is 40 years of age. There are some problems with this size/growth rate concept in the arid regions of Arizona, but I think we could justify waiting a few years longer for a tree to reach this size. Many residential lots would not be suitable for trees this large, but parks are ideal locations for urban forests having large trees. Here in the Verde Valley, we also have riparian corridors that border residential areas and act as suburban, if not urban, forests. In addition to parks and riparian areas, urban forests can be created by planting large street trees. Mulberry or cottonwood trees would qualify as large trees in our climate, but these trees can be more disease-prone and shorter-lived than other slower growing species. Better choices for large trees are Arizona sycamore (Platanus wrightii), black walnut (Juglans major), Chinese pistache (Pistachia chinensis), cork oak (Quercus suber), Texas red oak (Quercus texana), and live oak (Quercus virginiana). Arizona cypress (Cupressus arizonica) and deodar cedar (Cedrus deodara) are large evergreen trees, but evergreens may not be as suitable or provide the desired effects in public areas. While urban forests enhance communities, they also require commitment and investment by the communities they benefit. This includes considering long-term maintenance by qualified tree professionals, adequate irrigation, utility line clearance, awareness of damage that can be caused by human traffic and/or construction, and plans for replacement when mortality occurs. The benefits of large trees must be balanced with needs for utilities, road, and sidewalks. However there are ways to accommodate both trees and infrastructure. Tree roots are generally in the upper 18 inches of soil and utilities can be laid underground below this depth. Trading concrete and asphalt for more pervious paving materials (pavers or gravel) can also create a more hospitable root environment where water and oxygen are more available to plant roots. Landscape architects are also employing new engineering techniques that enhance root zone aeration. Large trees must also have adequate irrigation water budgeted for the future and irrigation delivery systems that will ensure their long-term survival. Urban forests add value to our communities and create outdoor havens where people can gather and recreate. We should nurture our existing urban forests and support the strategic placement and care of large trees within our communities. The University of Arizona Cooperative Extension has publications and information on gardening and pest management. If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113 ext. 14 or E-mail us at mgardener@verdeonline.com and be sure to include your address and phone number. Find past Backyard Gardener columns or submit column ideas at the Backyard Gardener web site: http://cals.arizona.edu/yavapai/anr/hort/byg/. |