Benefits of the 10-20-30 Rule

Dr. Frank Santamour, a Research Geneticist at the US National Arboretum, is credited with the first published reference of the rule. His 1990 paper Trees For Urban Planting: Diversity, Uniformity, And Common Sense notes, “I am not sure who first propounded the “10% rule”, nor am I sure that anyone would want to take credit for it, but it is not a bad idea.”

The urban environment is a harsh place for trees to thrive. When city foresters identify a tree species tolerant of city conditions, that grows quickly, AND offers great aesthetic value, there is a temptation to plant it everywhere. However, going all in on a few species is a high-risk strategy. For the potential consequences, see the cautionary tales of Dutch Elm Disease or Emerald Ash Borer.

The 10-20-30 rule is a way to safeguard against catastrophic losses from future pest and disease threats. If a deadly new tree threat arises, losses are limited and the majority of urban tree canopy is preserved. This protects against a drastic loss of ecosystem services and a municipal budget roller coaster.

A wide urban forestry gene pool also creates a stronger pest defense. A diversity of plant species supports more natural enemy communities and pests have a harder time locating suitable hosts when they are less concentrated. Additionally, diverse urban forests are more resilient to shifting climate conditions, as different species have varied tolerance to heat, drought, excessive rainfall, strong winds, and pollution.

There are compelling benefits to a highly diverse urban forest, but planting that diversity into reality is challenging. Many cities around the world still struggle with lopsided species composition. For some examples, check out the DiversiTREE project, which visualized urban forest diversity for eight cities around the world. No city successfully hits the 10-20-30 mark, but some are much closer than others. There are also stark differences between the tree diversity in city centers versus outer areas, which leads us to one of the rule’s limitations: scale.

On the flip side, Professor Norman Richards argued a rule like 10-20-30 would do more harm than good if it means proven-adapted species are replaced by poor-adapted or unproven species for the sake of species equity. “Simple percentage limits on species do not safeguard a population from poor species choices. Ill-adapted species can be a problem at only a few percent of a large street tree population,” writes Richards. Could the benefits gained by heavily planting a long-lived, well-adapted species outweigh the risk of catastrophic loss? Elm trees served cities with benefit-rich, arching canopies for decades before the Dutch Elm Disease decline.

A Target For Moving In The Right Direction

If you google just “10-20-30 rule”, the first 100 results are about presentations. It’s a popular rule for creating a pithy PowerPoint: no more than 10 slides, no longer than 20 minutes, and no less than 30-point font.

This more popular rule nicely demonstrates why, even with its limitations, the 10-20-30 rule for tree diversity remains valuable. It communicates an important concept concisely and memorably. A high visibility guideline like the 10-20-30 rule helps create awareness and motivation to keep tree diversity top of mind for urban forestry professionals. As long as communities still struggle with lopsided species composition, there is value in shifting the status quo with the 10-20-30 rule.

Tree species are only one component of urban forest diversity. Variety in sizes, ages, shapes, bloom times, and leaf colors, also contribute to the benefits, resilience, and aesthetics of urban forests.

If you are interested in tracking and improving the diversity of your community’s forest we have software solutions and expert services to assist at any step in the process. Get in touch to have a conversation about your urban forest goals.