Variation in tree growth and response to drought across an urban land-use gradient in the Chicago metropolitan region

Background/Question/Methods

The heterogeneous urban forest landscape, with a wide range of land-use classes and edaphic conditions, plays an important role in the development of community climate change mitigation and adaptation plans. The capacity of urban forests to mitigate climate change will depend on maintaining healthy canopy cover and protecting urban carbon storage, particularly as urban lands increase and land-uses change. In order to understand how urban forests will respond to future climatic changes and land-use conversions, we need a comprehensive knowledge of carbon storage and tree sensitivity and resilience across the urban forest continuum. This study begins to address these issues through dendrochronological analysis of urban trees across various land-uses and species in the Chicago metropolitan region. A preliminary data set sampled trees across land uses within the property of The Morton Arboretum in Lisle, IL. A full regional data set includes plots randomly chosen from a 2010 regional tree census. In each plot, increment cores were extracted from trees >10cm in diameter. For this study, productivity, variation in productivity, and drought resistance and resilience were calculated for sampled trees and compared among land-use categories and species.

Results/Conclusions

Preliminary results focused on four common urban tree species (tulip-tree - Liriodendron tulipifera, eastern white pine - Pinus strobus, Kentucky coffeetree - Gymnocladus dioicus, and sugar maple - Acer saccharum) indicate that urban tree growth and resistance to drought vary both among species and land-use categories (residential, built, forest, park). Productivity differed strongly among land-uses (F_{3, 112} = 12.97, p < 0.001), with park demonstrating the highest tree growth. Species showed highly individualistic responses in growth in relation to land-use. Most trees (86%) showed some reduction in growth in a particularly intense drought year (2005). Resistance to drought (relative to pre-drought growth rates) differed among species (F_{3, 113} = 5.30, p = 0.002) but not land-uses (F_{3, 113} = 1.64, p = 0.184), though there was also a significant interaction between species and land-uses. Species in all categories were highly resilient to drought effects in the five years post-drought (2006-10). These variations in response are likely associated with differences in site conditions and species physiology. Further assessing the interactive relationships in climate response between species and land-uses at the regional scale is critical in understanding the potential impacts of climate change on urban forest health and carbon sequestration.