Announcing a tree planting goal is a go-to move for politicians trying to win some popularity points. Everybody loves trees—they improve air quality, reduce sound levels, decrease crime, and infiltrate stormwater. And yet, even though we all agree it is a public good, we’re approaching tree planting all wrong, says Dark Matter Laboratories.
The first problem with the typical public pronouncement is that getting a certain number of trees in the ground doesn’t mean squat. In the UK, up to 350,000 saplings were planted along a high-speed rail route, 80% of which later died because the transit agency said replacing the trees was more cost effective than watering them. In Turkey, 90% of 11 million saplings may not have survived even though the project broke records and received lots of publicity.
The trees that do survive we often replace before they deliver the maximum ecological benefits. In urban settings, 50% are felled and replaced with saplings before they reach 10 years of age. Cities fear that, if allowed to grow larger, the trees may be more costly to maintain, drop branches in storms, and be too difficult to remove.
Those practices optimize for cost, but not for benefits. One analysis has found that a tree that is replaced every 10 years provides no meaningful ecological benefits as the tree never develops adequate leaf area. However, if left to grow for 50 years, the ecological benefits “start to rise exponentially” (See Biggest Trees Sequester the Most Carbon, Study Reveals.)
One suggestion, already taken up by some U.S. cities, is to set a target around canopy cover. Washington DC hopes to cover 40% of the city with trees by 2032. Seattle’s goal, which it has already almost achieved, is 30% coverage by 2037. Maintaining older trees will help those cities reach their canopy goals faster than planting a bunch of smaller trees.
Dark Matter Laboratories has developed further recommendations for funding and maintaining urban forestry projects here.
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