In response to cutting down trees for development and sidewalk repairs the City of Los Angeles has a policy of planting two trees for every one street tree cut down and four trees for every protected tree cut down.
Planting trees is both good and essential in order to keep the urban forest healthy. But it is insufficient, in and of itself. This perception that planting two or more trees for every mature tree cut down replaces the mature tree, or perhaps even doubles it, is wrong. In fact, the City’s tree replacement policy may ironically create a smaller and less effective urban forest in the future.
Planting a sapling only replaces another sapling, not a mature tree. Sixty years of growth are needed to realize the environmental cost-benefits of a mature tree, called Ecosystem Services. Never realized, are the ecosystem services of a large-stature tree that has been downsized or replaced by a small-stature tree. Small trees like crape myrtle deliver far fewer benefits. In fact, studies at The Center for Urban Forest Research shows that the crape myrtle benefits are up to eight times less. These are important distinctions as the city of LA uses small trees to replace large trees.
Despite ambitious tree planting programs in cities, tree canopies have been in decline. The decline may have to do with the fact that new trees are particularly vulnerable to premature mortality. One research study showed that a quarter of the trees planted through volunteer tree projects will die in the first six years (Lu, Svendsen, Campbell, Greenfeld, Braden, King, and Falxa-Raymond, 2010). In the study about Los Angeles’ million tree program (E. Gregory McPherson, et al., Los Angeles 1-Million tree canopy cover assessment. USDA Forest Service, Pacific Southwest Research Station, GTR-207 2008), a low mortality scenario projected that 17% of newly planted trees would be dead after 35 years, and a high mortality scenario projected 56% mortality.
The steady removal of mature trees paired with the loss of land available to plant them on is another reason cities see a decline in tree canopy. Despite LA’s “Million Trees” tree planting campaign, Los Angeles tree cover declined along with the potential to increase tree canopy cover as the amount of land available to be planted was covered over with larger houses, paved with driveways and hardscaped, according to a study published in Urban Forestry & Urban Greening by a team led by USC researchers (Lee et al. 2017).
As more land becomes dedicated to driveways and buildings, fewer permeable surfaces are available to support trees. It’s common for a developer in Los Angeles to buy a mature-treed property, cut the trees down, halve the amount of permeable land by putting up a larger building or buildings, and then plant twice as many sapling trees on the remaining open land. But one cannot keep halving the planting space and doubling the number of trees. There is a tree carrying capacity built into every piece of land. Two-for-one and four-for-one tree planting policies consistently fail to take this into consideration. To avoid overcrowding, replacement tree plans need to be tied to the amount of land available – not the number of trees put in the ground. Recommendations in the study about Los Angeles’ million trees program are 16 ft2 of pervious surface for small, trees, 36 ft2 for medium trees, and 100 ft2 for large trees .
Hence, tree planting programs can be a form of green-washing. This is because these programs allow governments, individuals, and companies to avoid taking more meaningful steps to preserve the urban forest as land continues to be sold, subdivided, built and overbuilt, and mature trees cut down without challenge.
The seductive thing about tree planting initiatives is that they are politically uncontroversial. Most people love to get a free tree, and those that don’t, just decline the offer. Tree preservation, on the other hand, is much more complicated and unpopular with many groups and individuals. Tree preservation policies can decrease the profit margins of developers, they can appear to be in opposition to other city goals such as increasing housing density and transportation improvements, and they can restrict people’s property rights. But the need to protect mature trees, and to preserve the required amount of permeable surface to support them and their replacements, is becoming increasingly urgent.
The summer of 2018 gave Angelenos record heat waves and 87 consecutive days without a day of clean air, the longest stretch of bad air in at least 20 years. This is part of a worrisome trend.
The combination of heat and extreme drought spell trouble for air quality in Los Angeles through a complex chain of events. Prolonged dry spells bring more temperature inversions, with a layer of warmer air trapping cooler air below, concentrating pollution near the ground. Higher temperatures accelerate the chemical reactions that form ozone, a key ingredient in smog, while also boosting demand for electricity, which further increases the smog-forming emissions from power plants. Hot, dry weather also creates ideal conditions for wildfires, which release still more smoke and soot.
Emissions from cars, trucks, ships, power plants, and industrial facilities are falling. Yet, the City is heating up twice as fast as the surrounding countryside, and experts predict Los Angeles air quality is in further danger. Given the threat increasing temperatures due to climate change poses to air quality, we need to look to other strategies to reduce smog and greenhouse gas emissions at the local level.
Trees, of course, supply both these benefits (air purification, carbon sequestration) at once, as well as other co-benefits: helping mitigate stormwater run-off, reduce the heat island effect, reduce noise, provide habitat for birds and other wildlife.
Trees, therefore, deserve to be a major tool for City planners as they struggle to clean and cool the air. Tree planting should be a complementary strategy, not a replacement for tree preservation.