Reuters

Most city trees aren't the result of tree-planting programs.

Trees seem to know how to make forests better than we do. According to new research from the U.S. Forest Service, an average of two-thirds of all trees in cities are the result of natural regeneration. Only one-third of trees are deliberately planted.

By looking at randomly located plots in 12 North American cities, the researchers found that the vast majority of the urban tree population is naturally occurring growth. Trees are reproducing and filling in empty spaces far more efficiently than any tree planting program ever could.

The amount of planted trees does vary from city to city, according to the research, published in the journal Urban Forestry and Urban Greening. Cities located in grassland-type areas have much higher rates of planted trees than cities in forested areas. And the proportion of planted trees increases in cities with higher population densities and amounts of impervious ground cover. Planted trees ranged from about 11 percent in Hartford, Connecticut, to nearly 90 percent in Los Angeles.

Trees, like any species, are in the business of reproduction. But that doesn't mean high-profile round-numbered tree planting programs and urban forestry managers can just take the rest of the week off. In looking at specific tree populations in Syracuse, the researchers found that exotic invasive species made up the bulk of new natural tree growth. Unless urban forests are managed, they could begin to shift more heavily toward these non-native species. And while non-native trees are better than no trees, the researchers point out that non-natives tend to have shorter life spans, and their invasiveness can often crowd out longer-lived native trees.

So while it's unsurprisingly impressive that nature can efficiently regenerate tree populations, the researchers argue this sort of data should become a more important part of the job of managing urban forests. But given the much higher rate of naturally regenerated trees, maybe we should start to think of it as assistant managing.

Photo credit: Mariana Bazo / Reuters

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