Ecological change in the urban forest of six Midwest, USA cities over twenty-five years.

摘要

This is one of the first long term studies that takes into consideration the entire urban forest, both public and private trees, over time. In 1980, an inventory of the urban forest in 10 cities was established to monitor the spread of Dutch elm disease. Shortly after the original study was concluded, the entire study ended. In 2003/2005, Bowling Green, Bucyrus, Delaware and Wooster, OH, Hutchinson, MN and Lincoln, NE were resurveyed to document the changes that have occurred over the years in the urban forest structure.;Between 1980 and 2003/2005, the urban tree density, species richness, and diversity did not change significantly. The public to private tree ratio also did not change significantly over the years, 8.5 and 7.9 to 1, respectively. The species richness of the urban forest is significantly greater than the natural forest. In 1980, 27%, and in 2003/2005, 42% of the tree taxa were considered overplanted. There were even more public trees that were overplanted; in 1980 and 2003/2005 there were 74% and 73%, respectively, deemed to be overplanted. The percentage of private trees that were regarded as overplanted is very similar to the total percentages. In 1980, 27% and in 2003/2005, 41% were considered overplanted. Of these overplanted taxa, many of them are native to North America, which creates an awkward situation for any emphasis to be further placed on the planting of "native species".;The size and condition of the trees were also measured. The average dbh of the urban forest trees change significantly over the years. One peculiarity that was noted was the significant loss of small trees between the years. This is due in part to in-growth between size categories, and fewer small trees are being planted. Also, over the years, the percent of trees in the different size classes of the urban forest begin to resemble what is found in a natural forest. The condition of the trees has gotten significantly worse between 1980 and 2003/2005. However, there are significantly more trees in the best condition category in both 1980 and 2003/2005.;As a result of the damage by an unusually large snowstorm that hit Lincoln, NE, in October 1997, when the trees still had leaves, there was a loss of 48.2% of the trees in the city. More than 90% of the smallest trees were lost, which produced a significant change in the average tree size, and the cleanup of the storm produced a significant change in the condition of the trees, for the better. The diversity of the trees did not change. But, there was a significant change in the number of the trees present in the urban forest. Finally, no correlation was found in the relationship between any of the wood properties (wood density, specific gravity, modulus of rupture or modulus of elasticity) and the percent lost of any of the species of trees.