Nature Dominates in City Tree Regeneration
Although humans may think they dominate the city environment, nature plays a significant role in affecting tree species composition and tree cover in cities. Forest Service scientists used field data from 12 cities in the United States and Canada to estimate the proportion of the existing tree population that was planted or regenerated naturally on a given site.
On average, humans plant about one in three city trees. Natural regeneration in cities can be dominated by exotic invasive species, which will change the forest composition over time.
The study found the percentage of trees planted by humans varied with land use (residential land has with the highest proportion of trees planted), species (the most commonly planted was honeylocust), and the natural environment (cities developed in grassland areas have a greater proportion of trees planted than cities developed in forests). Percent of tree population planted by humans also increases with population density and impervious cover in cities.
New tree influx rates ranged from 1.6 trees per acre per year in Baltimore, MD, to 3.5 trees in Syracuse, NY. In Syracuse, the recent tree influx has been dominated by European buckthorn, an exotic invasive species. Without tree planting and management, the urban forest composition in some cities will likely shift to more pioneer or invasive tree species. Because these species are typically smaller and have shorter life spans, the ability of city systems to sustain larger, long-lived tree species may require human intervention. These data on tree regeneration and planting proportions and rates can be used to determine tree planting rates necessary to attain desired tree cover and species composition goals.
Forest Service Partners