Tuesday, February 19, 2013

Calculating Durham’s Goal of 8,800 Trees Annually

I am responding to a challenge I took this morning to what were kiddingly referenced as “my statistics” regarding a benchmark I computed last October for the number of urban trees that should be planted each year in Durham, North Carolina, where I live.

Appropriately, the comment surfaced indirectly during the meeting of a group with which I have the pleasure of interacting every other month named the Durham Appearance Advocacy Group, aka DAAG.

Members include a cross-section of business leaders and representatives of community groups concerned about appearance, paired with an equal number of public agency representatives with missions that include issues such as appearance and recycling.

The collaboration charges itself with identifying performance gaps and collaboratively advocating solutions to close them but it is wholly reliant on the willingness of participants to ask tough questions.

To respond to the comment in the form of an answer to a question, below is how I came up with my calculation that Durham should be planting as many trees each day as it currently plants in an entire year.

  • Currently, according to the City of Durham, 350 trees are reforested each year while about 200 are removed for disease etc.

The net is 150 additional trees per year.

  • According to estimates by Joint City-County Planning, county-wide, Durham has 51% of its tree canopy remaining including 40% remaining in the city.


  • Durham has lost 1 million trees over the past 30 years, and according to analysis by the UNC-Charlotte, it is losing an average of 4 acres a day to development and impervious surface.


  • Using the UNC-Charlotte data shown over time, the ratio of trees remaining to impervious surface in Durham County has eroded to a ratio of 3 to 1 and 1.4 to 1 in the City, threatening appearance and sense-of-place and surrendering hundreds of millions if not billions of dollars in ecosystem services.


  • Nationwide, according to the National Forest Service, urban foresters replant one tree for every three that regenerate naturally and the survival rate for natural regeneration is 1 in 20.


  • My calculation then that Durham needs to plant 243 trees a day for at least the next thirty years is based on the need:

To gradually replace those lost over the last thirty years during the next thirty years by plant each day –    91 trees

To replace those removed each day by development, net those that may naturally regenerate by each day planting –                                                                   152 trees

Equals a daily urban forestry goal of–                  243 trees

Adjusted for the .41 trees currently planted –      242.6

Even if Durham’s urban forestry was to surrender any responsibility for the community and continue to restrict its focus to just city property, the number of trees planted each year should be at least 1,400.

But my hunch, based on surveys showing that things such as trees are a priority by 6 to 1 and based on those who strongly agree 19 to 1, is that Durham residents would favor the more holistic approach, or more than 8,800 tree plantings per year.

There are many ways to reach the goal but setting the goal is primary.  Hatching tactics that aren’t illuminated by a goal that is based on a strategic analysis and sources such as those cited above is a well-proven recipe for activity traps.

A goal such as the one computed above can be re-calibrated by future in-depth analysis if performed but there is no reason to wait and get even further behind.

Currently, there are some isolated efforts, other than urban forestry, that can and should be counted against the goal including adopt-a-tree, soil conservation tree sales, minimum zoning requirements for tree retention or replacement.

There should be metrics developed and maintained for each of these and the results accumulated to help inform progress toward the goal and closing the gap.

And whoever said I am a bit defensive?

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