16 Urban Tree Decline

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Figure 16.1 Maple declining from paving

mportance. – Among the problems leading to urban tree decline are: air pollution, soil compaction, mower- and machinecaused injuries, poor pruning, heat reflected from streets and buildings, direct root damage from excavations and turf cultivation, paving (figure 16.1), improperly applied herbicides, potting above and below ground level, overplanting, and lack of understanding about tree growth and development. For these and other reasons, urban trees generally suffer a diseased existence and must be frequently replaced. Those that survive are often aesthetically unpleasing.

Identifying the Cause. – In addition to the above, a variety of fungi can attack trees that have been weakened. Most are heart, butt, and root rotting fungi that can affect trees structurally, making them unsafe. Others attack the roots, causing the tops to die back. Only rarely will all the causal agents in urban decline be identified.

Identifying the Injury. – Identifying the injury is usually easy. Affected trees show a dieback of the crown, beginning with the uppermost and outermost branches first. In the final stages, the trees may have only a few green sprouts and leaves attached to the main stem.

Biology. – Tree crowns most frequently begin to die back when the roots have been damaged or are diseased. This is due to the fact that plants grow with a carefully balanced root/ shoot ratio. When a portion of the roots ceases to function, a portion of the crown dies as well. Often, disease fungi enter the weakened portion of the tree and further damage it.

Control. – Protect, fertilize, and irrigate trees that are declining. Plant trees that are resistant to air pollution injury and drought, and provide trees with adequate root space and aeration. Remove dying trees to avoid danger to people and property.

(1817 signs)

17 Forest fires a Positive Effects of Forest Fires

In North America, Native Americans used fire to herd deer and bison and to keep forests from spreading onto the grasslands. These man-made fires and fires ignited by lightning usually burned only underbrush and a few small trees and promoted new plant growth. This young, healthy growth attracted animals. Forest fires also thin forests, causing less competition among plant and animal species. This reduced competition makes the surviving species healthier because they have an abundant supply of nutrients. Burnt out stumps are important for cavity-nesting birds. Tree trunks that fall into steams provide a habitat for fish and aquatic insects, and debris and ash release nutrients into the water. Burned trees that fall to the ground release nutrients into the soil. Forest fires can also kill soil pathogens and change soil properties to allow the absorption of more water. Some species, like the Giant Sequoia tree, need forest fires to reproduce. The Giant Sequoia tree opens its cones and release seeds only in the extreme heat produced by fire. Also, the seeds need the bare soil and lack of competition that results from a fire to grow. Author George Wuethner writes, “Fires can regarded in the same manner as predators. Just as wolves maintain the fitness of a deer herd, fire helps to maintain the fitness of the forest ecosystem”.