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This is the second part of a three-part story. The first part is about urban forestry and the urban heat island effect. The third part will appear on Monday, November 10.

Grove of urban trees

Excellent urban forestry allows trees to have adequate soil volume to grow to maturity

As we saw in the last article, the urban tree canopy in most cities in the US is declining, even though a healthy urban forest is a key to reducing the impacts of climate change and the urban heat island effect. What can be done to “fix” urban forestry?

In many cities, the answer is a simple “plant more trees.” Many urban areas have made a commitment to substantially increasing the number of trees planted each year, mostly with the use of citizen volunteers. Yet this is not necessarily the right approach. Too many trees planted in the wrong place are doomed to a short life. Trees planted in the right place but with no maintenance plan or budget are unlikely to live long.

Row of trees

Trees with adequate space will survive. Straight rows allow for efficient mowing.

In the right place and with the right care, trees in urban areas should be able to live for their natural life – from decades to hundreds of years, depending on species. All too often, trees die after only a few years. There have been few studies of urban tree mortality – most cities do not keep track of their trees – so it is often difficult to know how long trees really last beyond anecdotes. The urban ecologist Lara Roman has shown that measuring the half-life of trees, rather than their average longevity, provides us with a better measure of the long-term benefits of urban trees. This demographic approach makes it easier to project tree mortality under different survival rates.

If we take two mortality rates, 3.5% and 5.1%  per year, we can calculate a population half life of 13-20 years – meaning that only 50 of every 100 trees will live longer than 13-20 years, and only a tiny fraction will live out their natural lives.  The consequence of this is that communities are constantly trying to catch up – planting more and more trees to increase the city’s tree population even as most of the trees soon die.

sidewalk

Trees are planted behind the sidewalk. The narrow strip between the street and sidewalk is a good place for shrubs or very small trees

The high rate of mortality of planted urban trees is unsustainable. Cities will continue to lose canopy cover unless the mortality rate can be reduced.  This is where excellent urban forestry comes into play. Too often, we plant the wrong tree in the wrong place and it dies.  This is especially true of street trees – trees planted in a narrow strip of soil between the sidewalk and street. This is often an inhospitable habitat for trees, and they don’t last long.

The pictures in this article are examples of an urban forest plan that holds a high promise of success.  When Tates Creek Road in Lexington, KY, was redesigned, an aggressive planting program was put in place.  Initially, a lot of the planting met with failure because the contractors had little knowledge of proper planting and management practices.  After a few false starts, they appear to have gotten it right.  The photographs in this article, and the slide show linked to the photos, show that the right trees were selected, planted properly and with adequate space to thrive.  There is no reason why most of the trees in these photos should not live as long in the urban environment as they to in the forest.  Most of the oaks, yellow-poplars, maples and walnuts planted have the potential to live for hundreds of years.

Here is what was done right:

  • The right species were selected. Long-lived, large trees were planted where there was adequate soil volume, and smaller trees in smaller sites.  No trees were planted in the narrow grass strips between street and sidewalk, but were moved back into the larger soil volume of yards.
  • Most of the trees were seedlings, not horticultural clones.  Although there is limited research, my own experience and that of others shows that horticultural clones often do not survive as long or grow as well as seedlings.  Seedlings also have much higher diversity than do clones.  We will say more about this in a future article.
  • Most of the trees in this project are Kentucky natives. Trees do not need to be native, but need to be well suited to the site. Trees that score well on Doug Tallamy’s Bringing Nature Home list should be favored.  So, for example, it is better to plant a non-native oak than a ginkgo, because any oak is likely to harbor more insect and bird biodiversity than a ginkgo.
  • Trees were mulched and wrapped in tree guards to minimize mower damage.
  • Residual native trees – trees that were already on the site – were kept whenever possible.

The difference between excellent urban forestry and bad urban forestry comes down to this: only with excellent urban forestry will trees survive long enough to provide the environmental benefits that we need.  Citizen tree planting programs are important, but they are a waste of money if the tree mortality rate is too high to allow a substantial number of trees to survive more than a few years.

Stand of urban trees

Use of seedlings, rather than clones, of native trees is likely to lead to longer lived urban forests.

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