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Lack of diversity in urban forests may lead to problems

The health of metro Atlanta's urban forest is becoming more and more important as the state of Georgia rapidly loses forestland to development. The trees we plant in new subdivisions, in parks, along streets and in cities are the future forests of much of our state.

Fortunately, as a result of urban forestry education, citizens and municipalities are becoming savvier where trees are concerned. Citizens participate in civic tree plantings and plant trees at home while municipalities establish and enforce buffer, landscape and tree ordinances to make sure developers replace what they remove.

But unfortunately, there are several trends many of us in the tree industry recognize as future problems. Evenly aged stands of trees, low species diversity and poor tree siting and planting will lead to issues that will be very costly to taxpayers and municipalities in the future. In 50 to 80 years, urban forests in places like Gwinnett County will begin to mature and decline.

Counties like Cobb and Gwinnett have grown rapidly in the past 20 years. Many trees have been plated in the past two decades in subdivisions and cities, making the present urban forest evenly aged.

In urban settings, the lifespan of a tree is much shorter than in forest situations.

Fifty to 80 years is about all you can expect to get out of an urban tree such as an oak. Trees like maples and elms may have an even shorter lifespan. If all of the trees are the same age, then there will be a crushing amount of tree work that will need to take place all at once.

Also, there is very little seedling regeneration in urban areas because of landscape management practices such as mowing and weed control. Generally, the only regeneration that occurs in urban areas is in abandoned or unmanaged areas, and those tend to fill with undesirable, non-native, invasive weeds and trees.

Urban forests are unlike natural forests in that there is very little species diversity.

One can scan through any number of tree nursery catalogs and see that everyone is selling the same mix of trees. Red maple, willow oak, Chinese elm, sugar maple and Leyland cypress are standard fare for nurseries and landscapers.

"I think Elm and Bradford pear trees are very good illustrations of the dangers in planting monoculture species," said Larry Morris, associate chief of the Sustainable Forestry Community Program with GFC. "Urban forests need the diversity of tree species for the same reason that rural forests thrive with a mixture of different trees; they are less susceptible to insect and disease attacks that would affect the entire forest.

"Many pathogens attack one genus/species of tree rather than a multitude." One example, Morris said, is Dutch elm disease.

"The American elm was widely planted as a street tree in the early to mid 1900's," he said. "Then the elm bark beetle and Dutch elm disease became prevalent and changed the face of many communities by killing over the course of a couple of decades a majority of the elms planted as street trees."

There are other examples of tree problems that hit closer to home.

The Southern Pine Bark Beetle attacks Southern yellow and white pines but not other coniferous or deciduous trees in the landscape. Another example is redbud, crape myrtle and Japanese maple being widely killed by the Asian ambrosia beetle.

Some species are more affected by abiotic factors than are other species.

The Southern red oak and the Northern red oak in the landscape are very susceptible to prolonged drought. Dogwoods are very susceptible to drought and heat stress that often leads to attack by dogwood borers.

Many varieties are touted as "ideal" landscape trees early after their introduction from the horticultural industry, only for it to be discovered later that they have long-term problems that no one expected.

Bradford pears, for instance, with their tight branching structure, tend to split apart during storms as they grow older. Leyland cypress is affected by four different diseases today now that they have been planted by the millions across Georgia.

The solution to metro Atlanta's problem of low species diversity in the urban forest should be addressed now.

Educators should be informing the green industry of the dangers of low species diversity, while landscapers and tree farms should broaden their tree selection.

The more diverse our tree population is on the genus level, then the less likely the chance that a single pathogen or insect will decimate the landscape in the future.

Stephen D. Pettis is an agriculture and natural resources agent with the Gwinnett County Extension Service. He can be reached at 678-377-4010 or Steve.Pettis@gwinnettcounty.com.