A 5,000-Tree Pledge Built on Two Incompatible Numbers
Fairview has a 2019 inventory of 12,400 public street trees recording species, DBH, and condition. It has no inventory of park, institutional, or private trees. A council member cites a 2011 satellite study reporting 32% citywide canopy cover and proposes a "5,000 trees in five years" initiative. Public works has $85,000 per year for planting, an installed cost of about $325 per tree including two years of watering, and roughly 0.3 FTE of staff time available for planting oversight and follow-up. The city has also had two summers of drought and a 22% first-year mortality rate on last year's 240-tree planting. The council wants a recommendation within 30 days and has asked staff to "confirm the 32% number."
1. Identify the data gaps between the two figures and explain how each one changes the recommendation.
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The two numbers measure different things on different dates, so they cannot be compared directly.
- Scope: 12,400 is public street trees only. In most cities the majority of canopy sits on private, institutional, and park land, so a street-tree count says little about citywide cover.
- Age: the 32% figure is from 2011. Fourteen years of development, drought, and removal means current cover is unknown and may be lower.
- Metric mismatch: the proposal targets a tree count; canopy is a function of survival, growth, and mature size. Planting 5,000 small trees does not produce a measurable canopy change for 15–30 years, and 5,000 trees that die in year one produce none.
- Missing operational data: plantable space (stocking level along streets), species and genus mix, size-class distribution, and maintenance capacity. A 22% first-year loss tells you establishment, not planting, is the binding constraint.
The honest answer to council is that neither number is wrong - they answer different questions, and the current canopy figure does not yet exist.
2. Describe a defensible method to estimate current canopy cover and species diversity, including sample design and reported error.
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Use stratified random point sampling, which is standard, cheap, and defensible.
- Strata: right-of-way, park, residential private, commercial/institutional. Allocate points proportional to area, with a minimum per stratum so small strata are not swamped.
- Sample size: roughly 800–1,200 points citywide gives a 95% confidence interval of about ±2–3 percentage points at typical canopy levels; compute the exact interval from the pilot proportion.
- Interpretation: high-resolution imagery at 1 m or finer, with a documented rule set (what counts as canopy, minimum patch size). Use a tool such as iTree Canopy or manual photo interpretation by two analysts, and ground-truth 5–10% of points on site.
- Diversity: for street trees use the existing inventory; for parks and private land, sample plots. Report genus-level and species-level proportions and an importance value, and flag any genus above about 10–20% of the population.
- Reporting: give a point estimate with a confidence interval and a methods appendix, and date every figure. Do not present a single number as exact.
3. How do you turn this into a realistic recommendation to council?
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Separate the long-term goal from annual capacity, and give council options with price tags.
Capacity math: $85,000 ÷ $325 ≈ 260 trees per year if every dollar goes to planting, which leaves nothing for replacement of the 22% losses. Staff oversight of about 0.3 FTE realistically caps the program near 300 trees per year with volunteer or contractor support.
Options:
- A - status quo+: 260 trees/year, redirect part of the budget to watering and replanting, target under 10% first-year loss. Roughly $85,000/year.
- B - scale with partners: 400–500 trees/year using a partner or contractor for planting and a paid watering crew; approximately $150,000–$170,000/year.
- C - pledge as stated: 1,000 trees/year, about $325,000/year plus staff, and a high risk of failure without an establishment budget.
Recommend B, framed as "5,000 trees planted and 4,000 alive at year five," and add a canopy target measured on a five-year cycle (for example, a stated percentage-point gain by 2050). Commit to reporting survival at one, three, and five years. That converts a press release into a program council can fund and be held to.