Increasing tree diversity is a high priority for many urban forest managers in order to improve resilience against stressors such as pest outbreaks and climate change. However, in the urban environment, even if planting plans do include a wide array of tree species such plans may be undermined by stressful environmental conditions such as poor soil, invasive pests, or construction activities. To understand how the diversity of urban tree planting projects can change over time in challenging urban environments, this study compares current tree diversity in a highway right-of-way with diversity at the time of planting 10 to 30 years ago. The study focuses on three different tree planting campaigns which took place in 1988, 1997, and 2008 along 93 linear kilometers of highway in northern Illinois. In 2018, researchers used as-built planting records provided by highway managers to visit the planting locations of 3,163 trees to determine the number of trees which were still alive. Shannon Diversity, species richness, and Pielou evenness were calculated for these tree plantings both for the time of planting (based on planting records) and in 2018.
Results/Conclusions
At the time of planting, Shannon Diversity (H’), richness (S) and Pielou evenness (J) were highest in the 1988 cohort (H’ = 3.08, S = 30, J = 0.91) and lowest in the 1997 cohort (H’ = 2.02, S = 12, J = 0.82). The achieved values of H’, S, and J for the 1988 and 1997 cohorts in 2018 were all lower compared to the values at time of planting. The 1997 cohort had the largest decrease in H’ and J, reflecting large losses of Fraxinus to Emerald Ash Borer - Fraxinus represented 21% of trees planted in 1997. Planted and achieved diversity metrics for the 2008 cohort showed little change. Survival rates within each cohort varied widely by species which accounts for the difference between planted and achieved diversity. For example, of the clusters sampled in the 1988 cohort, Picea abies had relatively high survival rates (80%) whereas Populus deltoides survival was only 3.1%. While an urban tree planting project may begin with a diverse collection of trees, low and uneven survival rates can diminish its diversity over the course of multiple decades.