Assessing the Impact of Climate Change on Urban Tree Species Selection: A Case Study in Philadelphia

United States and concluded that 54 species would lose at least 10% of their suitable habitat under the predicted climate change. A similar study by McKenney et al. (2007) predicted that an average northward migration of 700 km would happen for 130 tree species in North America under a full-dispersal scenario. However, the shift of the distribution ranges of trees is also influenced by many factors other than climate and the real change may not be so dramatic (Pearson 2006, Thuiller et al. 2008). The growth of trees can be accelerated in the future climate because of increased CO2 level and rising temperature under the condition that other environmental factors such as nutrients availability and soil moisture are optimal. This increase is more obvious in young trees and species with indeterminate growth habits (e.g., poplars; Ceulemans et al. 1999, Saxe et al. 2001). Besides faster growth, for cities in the temperate zone the most visible impact on trees may be on the on the phenology of trees such as leafing, flowering, and fall leaf color. It has been observed that the leafing date of horse chestnuts (Aesculus hippocastanum) in Geneva, Switzerland, has advanced about 40 days over 200 years (Defila and Clot, 2001). Studies also show that the flowering time of many plants worldwide has advanced in past decades (Fitter and Fitter 2002, Lu et al. 2006). In the United States, Primack et al. (2004) observed that trees in the Arnold Arboretum flowered 8 days earlier from 1980 to 2002 than they did from 1900 to 1920. Besides the timing of leaf-out and flowering, the onsets of the fall leaf color will also be impacted by climate change. As the climate gets warmer, people can expect the delay of fall color in general. Menzel (2002) found that, on average, the end-of-season phenological phases were delayed by 0.3 to 1.6 days/decade. Trees will face different pest and disease problems in future climates. Reviews by Coakley et al. (1999) and Garrett et al. (2006) pointed out that, in general, the rising temperature would lead to a geographic expansion of pathogen and vector distributions, thereby increasing the number of potential hosts. The pathogen evolution may also be more rapid and overcome the resistance of plants. Similar predictions were also made on tree pests. Most entomologists expect that the increase in temperature may speed up the growth and development, increase winter survival, and extend the distribution ranges of pests (Porter et al. 1991, Ayres and Lombardero 2000, Harrington et al. 2001). However, by far, studies on individual diseases and pests have only been conducted on a small number of species with important economic values (Hill et al. 1999, Logan et al. 2003, Gan 2004). Common pests and diseases of urban trees are rarely studied. The catastrophic damage on forests by Hurricane Katrina is an example of the impact that extreme weather events can have on urban trees at the local and regional scales. The impact of extreme weather events could increase in the future because they may become more frequent or intense as part of global climate change (IPCC 2007). The latest report from the US Climate Change Science Program (USCCSP) predicted that extreme weather events (e.g., heat waves and warm spells in northwestern America; intense hurricanes in the Atlantic and West Pacific; intensity of heavy snow events in Alaska and high mountain areas) would become more frequent in North America (USCCSP 2008). Even though there are still many uncertainties in those predictions, urban foresters can benefit from preparing urban forests for worst-case scenarios (Rodbell 2005). So far, there are few studies that focus on the effects of climate change on trees in urban environments. Sukopp and Wurzel (2003) observed that many of the nonnative invasive species that originated in warmer areas are benefiting from the climate change in European cities. Bisgrove and Hadley (2002) suggested that it is possible to find garden trees suited to the climate anticipated by the scenarios for the 2050s or even the 2080s. Nevertheless, they did not give details on how to implement their suggestions. In a study conducted by Broadmeadow et al. (2005) on broad-leaved tree species in urban woodlands in England, the authors recommended the use of provenances from a region with a current climate well matched to a planting site’s predicted future climate. This lack of study on the potential impact of climate change on urban trees, particularly in the United States, is not commensurate with the importance of urban trees. More than 80% of the US population is now living in cities. Urban trees provide valuable ecological, social, and economic benefits to urban residents (Appleyard 1980, Kuchelmeister and Braatz 1993, Nowak et al. 2006). Although findings from studies on the potential impact of climate change on trees in general can be applied to urban forest management, the uniqueness of the urban environment should be kept in mind when doing so. Trees in natural forests and plantations and trees in urban environments are managed for different purposes so climate change will likely impact the two groups differently. Furthermore, the effects of climate change on urban trees tend to be modified by strong anthropogenic influences. There is an urgent need to integrate climate change science with urban forestry research to understand those impacts and generate applicable adaptation measures. This study attempts to answer a question that is fundamental for adapting urban forests to climate change: which tree species should we plant when anticipating a fast changing climate? A case study was conducted in Philadelphia, Pennsylvania, as an attempt to address this question. The objectives of this study include (1) evaluate changes in suitable habitats for tree species currently planted in Philadelphia in predicted future climates, (2) assess the impact of climate change on major diseases and pests of urban trees in Philadelphia, and (3) make recommendations on choices of tree species. Although this study focuses on tree species in Philadelphia, the method developed in this study should be applicable in other cities. Study Site and Methods Study Site This study was conducted in Philadelphia, Pennsylvania. The center of Philadelphia is located at 40°00 N latitude and 75°09 W longitude (Figure 1). The city has a land area of 349.9 km. Philadelphia is the sixth largest city in the United States with a population of 1.4 million in 2006 (US Census Bureau 2008). Philadelphia is the northernmost city in the United States that falls in the humid subtropical climate zone, which features a hot and humid summer and a mild winter. The average annual precipitation is about 1,068 mm and the distribution is relatively even throughout the year. The original forest type in the Philadelphia region was coastal plain forest, which contains species such as willow oak (Quercus phellos) and sweetbay magnolia (Magnolia virginiana) (Rhoads and Block 2004). The original forests disappeared by 1900 as European settlers cleared forests for farming and timber production. Urban trees were first intentionally planted in Philadelphia by the late 1700s (Zube 1973). Today, the urban forest Journal of Forestry • October/November 2009 365 in Philadelphia is estimated to contain 2.7 million trees and cover 15.7% of the land surface of the city (Nowak et al. 2007). Change of Optimal Distribution Ranges of Trees in Philadelphia A list of 73 tree species currently planted in Philadelphia was extracted from Nowak et al. (2007). The list contains the most common tree species in Philadelphia encountered by Nowak et al. (2007) in their survey of urban forests in Philadelphia but not all species currently growing in the city. The change of the optimal distribution range for a tree species was analyzed by comparing the climate envelope of each species to the predicted future climate in Philadelphia. Climate envelopes for 60 tree species were obtained from Natural Resources Canada (2008). Six bioclimatic indicators were used to represent the climate envelope of each species: annual mean temperature, minimum temperature of the coldest month, maximum temperature in the warmest month, annual precipitation, precipitation in the warmest quarter, and precipitation in the coldest quarter. According to McKenney et al. (2007), this set of variables defines important climatic constraints on tree survival and growth in North America. In this study, the climate envelope is assumed to contain the bioclimatic indicators within the core distribution area of a species (from 5 to 95% of the cumulative occurrences of the species). For the remaining 13 species, the climate envelopes could not be found in the literature. Their general climate requirements were obtained from Gilman