Soil quality and urban forestry: Using GIS to maximize tree longevity

Background/Question/Methods

Urban trees are increasingly being valued for their economical contributions based on the environmental functions they provide to urban ecosystems. Based on 2002 data, total value for urban forests in the U.S. was estimated at $2.4 trillion. Forest value increases when a greater proportion of forest trees are healthy with large diameters yet, maintaining urban tree longevity is met with a vast array of challenges (e.g. diverse management, limited root space, soil compaction, etc.). Although maximizing tree growth is recognized as a critical urban forestry objective, few forest health preservation programs exist. Including urban soil function and integrating urban soils into forest management can help managers understand the effects of tree management decisions. This study tested for the presence of feedbacks between soil quality and tree health, as well as between soil quality and tree species in Chicago, IL. Samples were taken from an area of approximately 4,000 trees, using 36 random individuals in a blocked sampling design, accounting for health categories (good, fair, poor, dead) across the four most common tree species (Gleditsia triancanthos, Acer saccharinum, Acer platanoides, and Tilia americana). Nine soil properties were measured and statistical analysis was performed using univariate, multivariate, and geospatial statistical tests.

Results/Conclusions

No patterns were found linking any of nine soil properties to tree conditions. The same disconnection appears in krig interpolation and tree diameter maps of the study area, reinforcing the lack of condition-based soil patterns. In contrast, tree species did show differences in the values for the soil macronutrients nitrate and phosphorus, as well as for soil pH. These results indicate that soil function in this urban forest may vary due to geographical associations with individual tree species. The distinct disconnection between health condition and soil runs counter to associations typically found in agro-ecology concepts where soil quality drives plant production. However, our findings that link soil patterns with tree species supports previously identified mechanisms within temperate savannah and tropical rainforest tree-soil relationships, where aboveground processes appear to alter associated soil properties. GIS technology can help manage the large amounts of heterogeneous data required to assess these types of ecosystem and landscape level patterns through easily accessible databases that can be used to generate practical tools, such as basic maps of above- and belowground characteristics and relationships.