294 influence tree size within other urban landscapes beyond street trees and residential areas are less studied though no less critical for identifying management interven- tions to improve growth. One such urban land-use type that could have the potential to support substantial tree growth is highway rights-of-way (ROW; also referred to as road verges or road reserves). Highways, high-speed and high-volume roadways which can be flanked by strips of undevel- oped land, are sources of air and noise pollution, stormwater runoff, and habitat and neighborhood fragmentation (Forman et al. 2002). Trees planted adjacent to highways can help mitigate these negative impacts (Rogers and Evans 2015), provided they can grow to maturity in a reasonable time frame. Unlike street trees which are often planted in limited soil vol- umes and in close proximity to the road, highway trees are generally planted in much larger soil volumes with open growing conditions and set farther back from the highway to provide clear zones for run-off-road accidents. The highway construction process can sig- nificantly alter the land adjacent to the road by topsoil removal and compaction, creating site conditions unfavorable for tree survival and growth (Somerville et al. 2018; McGrath et al. 2020). Highway trees are also exposed to pollution as well as de-icing salts in cold climates (Bryselbout et al. 2000; Fay and Shi 2012). While typically a narrow strip of land, ROW can have highly variable growing conditions, as many soil properties exhibit distance-dependent relation- ships with the road (Bryson and Barker 2002; Akbar et al. 2012; Werkenthin et al. 2014). Highway ROW can also contain multiple microtopographic features such as drainage ditches, foreslopes, and backslopes (Jimenez et al. 2011; Neher et al. 2013). Identifying the relationship between locations and features in the highway ROW and tree growth could inform decisions about where and what species to plant in a setting that would greatly benefit from trees. The goal of this study was to examine the impacts of species and cultivar choices and location within the highway ROW on tree size and consequently eco- system service provision in a highway setting. We used a set of trees planted between 1988 and 2008 along a highway in northern Illinois to investigate the following questions: 1) Do location and site factors in the ROW (e.g., the distance from the highway or location on sloped ground) influence tree growth? ©2022 International Society of Arboriculture Salisbury et al: Long-Term Highway Tree Growth 2) How does highway tree size compare to allom- etry estimates of similarly aged urban trees as documented in existing growth models? 3) What is the potential for highway trees to pro- vide carbon storage, carbon sequestration, stormwater reduction, and air pollutant removal according to ecosystem service estimation pro- grams such as i-Tree (USDA; Madison, WI, USA)? The results of this study can inform decisions about species and planting-site selection for highway planting projects in order to maximize the potential growth and benefits of these trees. MATERIALS AND METHODS The study trees were located next to Interstate 355 (I-355) in the greater Chicago region (United States; 41.932118, −88.037547; 41.538545, −87.960314) (Figure 1). This region has a temperate, continental climate with 10 °C average annual temperature; daily temperature extremes ranging from 40 °C to −32 °C; 94 cm average annual rainfall; and a median growing season of 177 days (Illinois State Climatologist 2007). The natural soils in the study area formed on glacial deposits, primarily moraines and till plains (Calsyn 1999; Hanson 2004). I-355 has 6 lanes, runs primar- ily north to south for 48 km, and has an average daily traffic volume of 271,980 cars per day (Illinois State Toll Highway Authority 2019). To locate the I-355 trees, we reviewed landscaping drawings that were annotated to indicate the final quantity and species of planted trees (i.e., as-built landscape record drawings) from 3 planting projects on I-355 from 1988, 1997, and 2008. These plans documented a total of 14,806 total trees planted along the highway during these 3 time periods. We refer to the study trees as part of the 1988, 1997, and 2008 planting cohorts, which are based on the year when the landscaping projects were begun. The 1988 and 1997 cohorts are located in the northern portion of I-355 while the 2008 cohort is located in the southern, newer section of the highway. Northern I-355 is an older stretch of highway in a more densely developed area and typically has a narrower ROW with more sloped planting areas compared to the southern sec- tion. In a survey of the I-355 trees in 2018, we docu- mented survival of trees from a randomly selected subset representing 20% of the total 14,806 trees doc- umented in the landscape drawings for the 3 cohorts
September 2022
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