Arboriculture & Urban Forestry 48(5): September 2022 patterns occurring during the planting and establish- ment periods for the 1988 and 2008 cohorts, respectively, could have also contributed to the less-than-expected growth for some species. Considering predictions of more frequent droughts and intense rain events for Illinois (Wuebbles et al. 2021), it is reasonable to expect such weather-related stressors will continue to be relevant to urban tree growth in the future. Addi- tionally, management practices can influence survival and growth (Vogt et al. 2015), potentially offsetting or exacerbating unfavorable weather. Unfortunately, we lack records documenting practices such as irriga- tion and the season when trees were planted and can- not draw conclusions about their possible effects. Highway environments can contain elevated lev- els of de-icing materials in cold-weather climates (Fay and Shi 2012), heavy metals in roadside soil (Werkenthin et al. 2014), and other air-borne (Kuttler and Strassburger 1999) and soil pollutants (Brysel- bout et al. 2000; Marusenko et al. 2011), which all have the potential to affect tree growth. Exposure to sodium-based de-icing salts has been associated with diminished stem growth in roadside trees, though these impacts vary among species and roadside con- ditions (Blomqvist 1998). Street trees (Tipuana tipu [Benth.] Kuntze) in São Paulo, Brazil, had reduced annual increment growth in association with elevated levels of particulate matter (PM10), zinc, barium, and aluminum (Locosselli et al. 2019). By contrast, in Finland, Populus tremula L. × P. tremuloides Michx. clones growing along highways had greater metal concentrations compared to rural sites, though con- centrations were not at levels considered toxic and likely did not severely impact tree growth (Nikula et al. 2011). Considering the many potential stressors trees can encounter in the highway roadside in addi- tion to other challenges associated with transplanting trees (Watson 2014), it is likely that multiple stressors could be impairing growth in the highway setting. Though notably, ameliorating compaction in high- way soil using deep ripping or subsoiling and the incorporation of compost improved tree growth (Somerville et al. 2018; McGrath et al. 2020), offer- ing a potential approach for alleviating at least some stressors in the roadside. Stressful conditions in the highway ROW not only impair growth, they also limit trees’ ability to provide size-dependent ecosystem services. Nevertheless, highway trees have the potential to contribute 301 substantial ecosystem services. For instance, the 303,000 trees in the United Kingdom’s Area 1, a 972-ha ROW (or soft estate), were estimated to annually remove 29 tons of air pollution, sequester 1,980 tons of carbon, and avoid over 75,000 m3 of runoff (Rog- ers and Evans 2015). And in Florida the 37,660 ha of vegetated ROW in the State Highway System (esti- mated to be about 40% woody vegetation) were esti- mated to provide approximately $5.95 million in air-pollution removal, $39.5 million in carbon seques- tration, and $465 million in runoff reduction (USD, 2014 prices)(Harrison 2014). In our study, we observed that small-stature 31-year-old trees (e.g., Malus spp.) and large-stature 10-year-old trees (e.g., Acer saccha- rinum) had similar estimated carbon sequestration rates and storage quantities. This observation high- lights how even after multiple decades, species with small-growth forms contribute little to carbon storage and sequestration, though small-statured species can provide other benefits such as aesthetics and can be appropriate for sites with space constraints. Addition- ally, size alone is not the only factor which influences ecosystem service provision. Q. macrocarpa on aver- age is estimated to store and sequester the most car- bon on a per-tree basis in the 1988 cohort, while other 1988 species have greater contributions to estimated avoided runoff. With the proliferation of large-scale tree planting initiatives across the globe (Eisenman et al. 2021), our results illustrate the role species selec- tion can play in a planting project’s ability to provide meaningful ecosystem services. Indeed, in order for highway plantings to effectively provide particular benefits such as air-pollution removal, thoughtful spe- cies selection and arrangement are necessary (Bal- dauf 2017; Barwise and Kumar 2020) in addition to mitigating site stressors such as poor soil conditions. Study Limitations Diameter is only 1 metric of tree performance, and on its own does not capture long-term growth patterns or a tree’s overall condition (North et al. 2018). How- ever, for the purposes of this research, diameter pro- vided an efficient way to compare large numbers of similarly aged trees. Planting density is 1 factor which influences a tree’s allocation of resources to growing taller or wider, consequently influencing caliper (Drew and Flewelling 1979). For the I-355 planting sites, tree spacing was consistent among planting sites (ranging from 4.6 m to 6 m) and tree planting areas ©2022 International Society of Arboriculture
September 2022
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