Arboriculture & Urban Forestry 39(6): November 2013 283 Figure 6. Indices of a) resistance and b) resilience to 2005 drought by species and land-use, with analysis of variance results for interactions among species and land-use categories. Means with same letter did not differ based on least-squares means com- parisons with Tukey-Kramer adjustment; bars indicate standard deviation. species and land-use types. Growth was reduced in the year of the drought, but the trees rapidly returned to pre-drought growth de- spite the very low moisture and unusually high temperatures as- sociated with the 2005 drought, conditions that have been termed “global-change-type” drought and have been shown to result in tree mortality in the southwestern United States (Breshears et al. 2005; Breshears et al. 2008). The relatively short duration of the 2005 drought, which was restricted to a single growing season, could have had some bearing on this response, as longer-term droughts may be more likely to result in high levels of tree mortality (Muel- ler et al. 2005). Long-term drought can lead to protracted water stress, which may result in carbon starvation (prolonged negative carbon balance) or hydraulic failure in roots and stems (McDow- ell et al. 2008). The period following the 2005 drought was char- acterized by high precipitation levels (Figure 3), which also likely contributed to the apparent resilience of the trees in this system. Resistance to the drought was also relatively high across all of the land-use categories, and two of the species (white pine and tu- lip-tree) showed somewhat higher resistance in the built areas of the landscape. Previous research has shown that excess soil water is often more of a problem in urban tree plantings than water defi- cits (Berrang et al. 1985), and built locations may not be especial- ly susceptible to drought if well maintained (Whitlow and Bassuk 1987). The high water-holding capacity and low belowground competition often found in urban plantings could explain the find- ing that trees in the built landscape were slightly more resistant to drought impacts. However, another potential explanation for the apparent drought resistance of trees in this study system, espe- cially in built areas, is that they may have received residual water from irrigation of nearby landscape plantings during the drought period (although direct watering is not known to have taken place). The variance in growing conditions across the Morton land- use gradient assessed in this study is much narrower than what is likely to be seen across an entire metropolitan region. Grow- ing conditions in built areas at Morton likely offer much less stress than those in the urban core or in developments with highly compacted soils (Whitlow and Bassuk 1987). For example, the urban heat island effect could exacerbate the effects of drought, but the heavy dominance of tree canopy cover at Morton prob- ably mitigates this effect to some degree, increasing the resilience of trees in this landscape to drought (McPherson et al. 1997; Cregg and Dix 2001). Therefore, the finding that there were some differences in growth and response to drought within the relatively narrow gradient represented by the Morton site, sug- gests that investigation of a full continuum of land-uses across an urbanized landscape would likely show even more variation. Previous studies of urban tree growth have focused on char- acteristics of specific planting sites (Pan and Bassuk 1985; Whit- low and Bassuk 1987; Kjelgren and Clark 1992) and urban-rural gradients (Iakovoglou et al. 2001; Iakovoglou et al. 2002). The first approach is superior in some respects to the land-use cat- egory analysis presented here, because it offers a more mecha- nistic understanding of the factors that impact growth variability. However, with such a reductive approach it would be difficult to assess how variation in urban tree growth will respond at a landscape- or regional-scale to stressors such as climate change. Studies that assess urban to rural gradients have this capac- ity but have often overlooked much of the variation that oc- curs within the urban portion of the gradient (Iakovoglou et al. 2001; Iakovoglou et al. 2002). The approach outlined here, ap- plied at a regional-scale and expanded to the full continuum of urban land-uses, would be very useful in assessing the poten- tial impacts of climate change and urban land-use change on urban forest health and carbon sequestration. However, some degree of characterization of specific growing conditions and their variation within land-use classes would be useful in avoid- ing issues associated with land-use categorization (Cadenasso et al. 2007) and still allow for application at a regional-scale. Acknowledgments. Funding for this project was provided by The Morton Arboretum, and data collection assistance was provided by volunteers in The Morton Arboretum Forest Ecology Lab. For comments on the manu- script we thank two anonymous reviewers. LITERATURE CITED Angel, J.R. 2011. Illinois Climate Normals (updated to 1981–2010). Illinois State Water Survey, Champaign, Illinois, U.S. Benoit, L., J. Skelly, L. Moore, and L. Dochinger. 1982. Radial growth reductions of Pinus strobus L. correlated with foliar ozone sensitivity as an indicator of ozone-induced losses in eastern forests. Canadian Journal of Forest Research 12:673–678. ©2013 International Society of Arboriculture
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