The Replacement Tree Myth

Posted on Sep 3, 2018

thelasttreeIn response to cutting down trees for development and sidewalk repairs the City of Los Angeles has a policy of planting 2 trees for every one street tree cut down and 4 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-stature trees like crape myrtle deliver far fewer benefits. In fact, research 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 like crape myrtle to replace large trees.

Despite aggressive tree planting programs, tree canopies in cities 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, ambitious 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 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.

Los Angeles needs a new tree ordinance to replace existing laws that are out of date and don’t adequately address current challenges including climate, development, disease and replacement. Pasadena has strict protections for all trees with a 19” diameter and rules against overcrowding of new and existing trees. Other cities have tree spacing rules. Portland has tree canopy coverage goals. Sacramento requires that trees be planted and maintained in order to provide a minimum of 50% shade over a parking lot. Neighbors in San Francisco must be notified of a pending tree removal and given an opportunity to appeal.