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Concerning street trees, as with many things in life, it is not good to put “all of your eggs into one basket”. This was demonstrated by the widespread devastation of the urban forests that occurred in the middle of the 20th century in many of the cities of the eastern and central United States. Because of their over-reliance with one species, Ulmus americana, American Elm, many cities were ill-prepared when Dutch Elm Disease came into town wiping out many of the elms. Along with the loss of the trees was also the loss of the cooling shade of their canopies for a whole generation. Since insects and diseases tend to affect certain species or their close relatives, having a variety of trees will limit the effect that one pest can have on a city.
Through this experience with the American Elm, urban foresters have developed a recommended “10-20-30 Rule”, where no more than:
10% of the street tree population is composed of one SPECIES,
such as Ulmus americana, American Elm;
20% of the street tree population is composed of one GENUS,
such as all of the various other Ulmus or Elms collectively;
30% of the street tree population is composed of one FAMILY,
such as those genera also in the Ulmaceae family, including Ulmus, Celtis (Hackberry) and Zelkova (Zelkova).
Let’s see how San José is doing on having a diverse urban forest. Here is a table showing of the most common street trees in San José and their percentage of the total street tree population:
The spreading canopies of Zelkova serrata arch over a street on a west side neighborhood of San José.
Analysis:
San José passes the test! We have no one species that exceeds 10%, no genera that exceed 20% or no family that exceeds 30%. It is remarkable that are no duplications on the list among any genera or families, so we are well below the level of concern on those counts. However, we need to be mindful as we start to reach our recommended capacities for two commonly planted street tree species, the Chinese Pistache and London Plane Tree (also called Sycamore) as they get close to 10%. They have been used for decades because of their abilities to withstand the urban planting conditions, but we need to keep in mind that there are other species that should also be considered.
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