How urban forest tree species composition compares with that of the natural forest

Background/Question/Methods

It has been suggested that urban forestry tree planting practices are creating a homogenous urban forest that bears little resemblance to the surrounding natural forest, with the same limited number of species dominating urban landscapes across North America.  To begin to address this question, tree species data were obtained from i-Tree (UFORE/STRATUM) analyses of 100 U.S. and Canadian cities.  Relative basal area of each tree species was calculated by city and land-use class and (where possible) for each city as a whole.  Those data were combined with data from nearby natural forests from the U.S. Forest Service Forest Inventory and Analysis Program.  The data were then analyzed using PC-ORD to determine which cities’ urban forests were most similar to one another, how they compared with nearby natural forests,  and how species composition related to large-scale environmental variables (using cluster analysis and Nonmetric Multidimensional Scaling ordination). 

Results/Conclusions

Preliminary results indicate that urban forests as a whole resemble nearby natural forests, especially in eastern North America.  Data for urban forests as a whole clustered along rough geographic and climatic lines.  Urban forests were most similar to natural forests in the northeastern and north central USA and eastern Canada.  They resembled natural forests least in the western USA.  More intensively managed portions of the urban forest (e.g. street trees) tended to cluster in ways less related to geography and climate than the urban forest as a whole.  These more intensively managed segments of the urban forest were also less similar to adjacent natural forests.  These results suggest that while the intensively managed portions of urban forests are being planted with largely the same suite of species throughout North America, this represents a relatively small proportion of the total urban forest in most cities.