Assessing variation in the resistance and resilience of tree growth to drought across an urban landscape
Carbon sequestration by urban landscapes could play an important role in climate change mitigation. Sequestration associated with urban forests will be dependent on sustained tree growth under future climatic regimes, which are predicted to be characterized by rapid change and extreme fluctuations. A better understanding of urban forest response to climatic fluctuation is needed to predict future conditions and preserve canopy cover. The Chicago Urban Forest Study addresses this need through analysis of tree growth resilience to climatic fluctuations based on a data set of increment cores obtained in 2013 from all trees (diameter at breast height ≥ 10 cm; 648 trees total) within a network of 190 0.1-ha plots that span an urban-rural gradient and a variety of land uses across the Chicago metropolitan region. Cores were prepared for measurement using standard dendrochronological techniques and tree-ring widths were converted to basal area increment (BAI) for computation of annual growth and variability in growth. Relationships between growth and climate variables, particularly drought and precipitation, were evaluated and drought resistance (growth during drought year relative to 5 year pre-drought mean) and resilience (mean growth in 5 years post-drought relative to 5 year pre-drought mean) were compared among species and land-use classes.
Initial observations demonstrate some variability in productivity among species and land-uses, and a strong correlation between precipitation and basal area increment (r=0.65). For the two primary genera, ash (Fraxinus) and oak (Quercus), growth during the 2005 drought was 91% of prior growth, 61% of trees had >10% reduction in growth, and resilience was 1.12 (increased growth following drought). The 2012 drought, the most intense in 50 years, generated a greater response, with growth 82% of prior and 68% of trees exhibiting >10% growth reduction. Two-way analysis of variance of the effect of genus and land use on drought resistance indicated a difference among land uses (but not genus) in resistance in 2012 (F=3.50, p=0.03), but not 2005 (F=1.03, p=0.36). Forest trees exhibited greater resistance than residential in 2012 (t=2.53, p=0.04) but were less productive overall, suggesting limitation by other factors. Results indicate that drought resistance and resilience were high in both oaks and ashes and only varied slightly across land-uses. Variation in fine-scale edaphic conditions within land use categories likely contribute heavily to variability in response and sensitivity of species to climatic fluctuations, further work will address these relationships and their effect on carbon sequestration in urban forest landscapes.