Mike Davison, Revised September 30, 2020
Street trees are what we sometimes call parkway trees. They are planted in small areas along streets and sidewalks. Planting street trees evenly spaced, is common practice, and a street lined with mature trees is always attractive. Street trees add value to the community, both as property value and as esthetic value. Maintaining that value requires planning. All long term urban forest management plans must include a way to deal with the life expectancy of these trees.
In Serrano Park, all of the streets were originally planted with trees evenly spaced in the parkways. Of these, the Brazilian pepper trees along Paseo Sombra and Paseo Tranquilo have been maintained by the HOA, and are now about 40 years old. Unfortunately, they are now at their end of useful life without ever having a long-term management plan in place.
Street trees are grown similar to crops. In other areas of the community, young trees can be planted to grow near mature trees so that as the mature tree reaches a point in its lifespan where it no longer is viable, the younger tree is ready to take its place. This is not practical with street trees. These we plant in neat rows, grow to maturity, and then replace and repeat, much like a crop.
Street trees are first planted all at the same time, evenly spaced and they grow quickly. The tree management is focused on fast growth. All of us want the look of a street lined with mature trees and the sooner the better. Unfortunately, the young trees usually have a high mortality rate due to various reasons. (See Urban Tree Mortality) It is common to experience a 25% mortality rate during the first 5 years after planting as the trees get established. The higher mortality rate is managed through replacement of the failed trees. The young trees have not yet established large root systems so removing the stump and roots, fixing the soil, and replanting is not too difficult. Since the young trees grow quickly, by 10 years into the planting cycle, all of the trees are of similar size. The survival rates improve greatly, as the trees get established.
The trees continue to grow quickly to approach their mature size and then growth slows. The tree’s life time is more dependent on size and growth rate than years. A fast growing urban tree will not grow large and will not live as long as the same species tree in a rural environment. As the street trees approach mature size, they behave like old trees. “Live fast, die young” is the life cycle of an urban tree. As the tree matures, the risks of limb and trunk failures start to increase and mortality rates again rise. Replacing lost trees at this stage is difficult and the results are unsatisfactory. The soil is a mass of living and dying roots, making planting difficult and planting young trees at this time has poor yields. The early replaced trees catch up in size but gaps exist where old trees were lost too late in forest cycle to be replaced. As the risks increase and failures accelerate, the trees reach their end of useful life. At this point, the only thing to do, is to remove all the trees, clean out the soil of the dense mass of roots, replant the next crop of trees, and start the cycle again.
Reference: Urban Tree Mortality