Arboriculture & Urban Forestry 39(6): November 2013 Arboriculture & Urban Forestry 2013. 39(6): 279–285 279 Tree Growth and Resilience to Extreme Drought Across an Urban Land-use Gradient Robert T. Fahey, Margaret B. Bialecki, and David R. Carter growth to extreme drought was generally high and did not vary across species and land-uses. In this study, individual tree species responses to drought varied across land-uses, illustrating the difficulty of predicting the reaction of urban forests to projected increases in the frequency of extreme climatic events. Tree growth response to drought varied even across the relatively narrow range of growing conditions studied here. Investigation of a broader range of sites, encompassing the full urban forest continuum, would likely demonstrate even greater variation in tree response to extreme climatic events. Key Words. Climate Change; Drought; Growth; Gymnocladus dioicus; Land-use; Liriodendron tulipifera; Pinus strobus; Resilience; Urban Forest. Abstract. Understanding the response of urban forests to extreme climatic events, such as drought, will be essential to predicting impacts of climate change on the urban tree canopy and related ecosystem services. This study evaluated variation in tree growth and drought resistance (growth during drought) and resilience (growth in period following drought) across four land-use categories (built, transportation, park, and semi-natural forest) and four species (Acer saccharum, Gymnocladus dioicus, Liriodendron tulipifera, and Pinus strobus) at The Morton Arboretum in suburban Lisle, Illinois, U.S. Tree growth and resistance to drought both varied as an interaction between land-use and species (F15, 100 = 5.25, p < 0.001; F15, 100 = 2.42, p = 0.005). Resilience of tree Trees in metropolitan regions grow in sites that span a wide range of land-uses and specific environmental conditions. What is com- monly referred to as the urban forest (street trees, landscape plantings, urban parks), is only a small component of a larger continuum that also includes natural areas, agricultural remnants, and interstitial stands. The wide variation in growing conditions across this continuum likely has a strong impact on the growth, resiliency, and mortality of the trees that make up the urban forest (Iakovoglou et al. 2001; Nowak et al. 2004). For example, the volume, structure, and composition of soil in the rooting zone can have a considerable impact on tree growth (Whitlow and Bas- suk 1987; Lindsey and Bassuk 1992). Variables that are linked to specific land-uses, such as pollution associated with transporta- tion corridors, can also affect tree growth (Benoit et al. 1982; Muir and McCune 1988). Environmental factors that occur at a larger scale can also be important; for instance, the urban heat island effect can influence both tree growth and resilience to envi- ronmental perturbations, such as drought (Cregg and Dix 2001). Urban forests will play a very important role in climate change mitigation and adaptation; however, the effects that global cli- mate change will have on the urban forest are difficult to predict and may be affected by changing urban land-uses (McPherson et al. 1997). Carbon sequestration associated with tree growth in urban areas could have an important mitigating effect on climate change (Nowak and Crane 2002). Average annual temperatures are projected to increase with global climate change, but specific changes at the regional scale, especially changes in precipitation amounts and timing, are difficult to predict accurately. However, the frequency of extreme climatic events, such as drought, floods, and heat waves, is projected to increase, and these events may be especially influential on urban ecosystems (Meehl et al. 2007). Urban forests may have the capacity to help mitigate some of these events, through such functions as their influence on water move- ment and the shading of buildings (McPherson et al. 1997). How- ever, the capacity of urban forests to mitigate deleterious climatic changes will depend on maintaining healthy canopy cover. In the long-term, the response of the species that make up the urban forest to general climate warming will be very important to main- taining a healthy forest (Woodall et al. 2010). In the near-term, though, the response of trees in the urban landscape to extreme climatic events will likely be more important to maintaining cano- py cover. In order to understand how the urban forest will respond to future climatic changes and land-use conversion, planners need to know how tree growth and resilience to extreme climatic conditions vary across the urban landscape and among species. This study addresses three research questions regarding the response of trees to extreme climatic events across a gradient in urban land-use: 1) How does tree growth differ across urban land-use categories and species? 2) How does sensitivity of tree growth to general climatic conditions vary with land-use and species? 3) Does resistance and resilience of urban trees to extreme drought vary among species and land-use categories? METHODS Study Area and Sampling Methods The study was conducted on the grounds of The Morton Arboretum (hereafter “Morton”) in Lisle, Illinois, U.S., which is in the western part of the Chicago metropolitan region. Mean temperatures in the area range between -5.3°C in January and 22.3°C in July, and mean annual precipitation averages 98.5 cm ©2013 International Society of Arboriculture
November 2013
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