Contrasting natural regeneration and tree planting in fourteen North American cities

Field data from randomly located plots in 12 cities in the United States and Canada were used to estimate the proportion of the existing tree population that was planted or occurred via natural regeneration. In addition, two cities (Baltimore and Syracuse) were recently re-sampled to estimate the proportion of newly established trees that were planted. Results for the existing tree populations reveal that, on average, about 1 in 3 trees are planted in cities. Land uses and tree species with the highest proportion of trees planted were residential (74.8 percent of trees planted) and commercial/industrial (61.2 percent) lands, and Gleditsia triacanthos (95.1 percent) and Pinus nigra (91.8 percent). The percentage of the tree population planted is greater in cities developed in grassland areas as compared to cities developed in forests and tends to increase with increased population density and percent impervious cover in cities. New tree influx rates ranged from 4.0 trees/ha/yr in Baltimore to 8.6 trees/ha/yr in Syracuse. About 1 in 20 trees (Baltimore) and 1 in 12 trees (Syracuse) were planted in newly established tree populations. In Syracuse, the recent tree influx has been dominated by Rhamnus cathartica, 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 in the near term. As these species typically are smaller and have shorter life-spans, the ability of city systems to sustain more large, long-lived tree species may require human intervention through tree planting and maintenance. Data on tree regeneration and planting proportions and rates can be used to help determine tree planting rates necessary to attain desired tree cover and species composition goals.