Analysis of North American urban forest tree species composition

Background/Question/Methods

It has been suggested that urban forestry tree planting practices are creating a homogenous urban forest, with the same limited number of species dominating urban landscapes across North America. There is also concern about urban forests as pathways for invasive pests and pathogens to adjacent natural forests. However, to date urban forest inventory data have been limited, and data collected from different cities were often incompatible.

To begin to address the issue of urban forest composition, tree species data were obtained from i-Tree (UFORE/STRATUM) analyses of 45 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 (using cluster analysis) and how species composition related to large-scale environmental variables [using Detrended Correspondence Analysis (DCA)]. 

Results/Conclusions

Preliminary results indicate that urban forests as a whole resemble nearby natural forests. Data for urban forests as a whole clustered along rough geographic and climatic lines.  However, more intensively managed portions of the urban forest (e.g. street trees) tended to cluster in ways less related to geography and climate. 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, they represent a relatively small proportion of the total urban forest in most cities. The implications of this for providing pathways for forest pests are unclear. Urban forests whose compositions strongly resemble adjacent natural forests provide ready pathways for forest pests from urban areas to natural forests. However, the similarity of the intensively managed portion of urban forests suggest that these highly valuable forests might all be at risk from the same set of introduced pests.