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Wind
 A
tree is most likely to blow over under high wind conditions in an area where
its roots are confined. |
Wind increases the amount of water lost from a tree to the atmosphere. This may not be a big problem for drought tolerant trees if roots can expand into surrounding soil uninhibited by urban structures such as compacted soil, curbs, buildings and streets.
However, trees growing in restricted soil spaces are especially susceptible to wind desiccation in summer because the root system is too confined.
A good example of this type of site are street trees planted in a sidewalk cutout (planting wells in a sidewalk) with tall buildings nearby. Unless the site is properly designed, or the soil is sandy, roots frequently concentrate in the small area of the cutout because they are unable to extend beneath the pavement due to soil compaction or water logged conditions. Because roots are concentrated in a small area, the soil in the root zone dries quickly. They could also become unstable and blow over.
Well managed irrigation can overcome some of this water deficit, but irrigation is difficult to deliver to trees in highly urbanized sites, unless it was properly designed and installed during construction and has been well maintained.
The best method for managing water loss in a windy site is with proper site design and proper species selection. Species tolerant of drought usually grow best in windy areas unless soil is poorly drained. Species that tolerate both wet soil and drought are best suited for poorly drained windy sites. In wet soil, ill suited trees often appear drought stressed because oxygen is unavailable for root growth. The underdeveloped root system can not absorb enough water to keep pace with transpiration demands.
Wind also should be considered in areas subjected to hurricanes. Species have been reported to show differences in resistance to hurricane force winds.But wind resistant species only buys you so much protection. I think the more important factors are 1) soil and site conditions, and 2) tree defects in the crown and root system; I think these outweigh the importance of species, perhaps by a lot. Lists of wind tolerant trees vary because factors 1 and 2 above over-ride species so often. So lets dispel the myth that we can make wind resistant landscapes by selecting the right species; we make landscapes resistant by understanding the information we present on the hurricanes and trees web site. There is much to this and simply recommending species does a great disservice to our community.
