Restoring the iconic American elm to urban landscapes: early tree growth responds to aboveground conditions
Date
2021
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Publisher
University of Delaware
Abstract
Native, urban trees provide a range of benefits from supporting native species, to climate control and recreation, so there is a desire and need for native tree planting and restoration. Ulmus americana (American elm) was once a great tree across urban America but has been decimated by Dutch elm disease (DED). The goal of this research was to study elm establishment and growth across urban landscapes. We planted 90 trees along a gradient of urbanization intensity in Newark, DE and Philadelphia, PA. Our research entails multiple physiological and morphological measurements on three different American elm clones (RV16, RV474, and Sunfield) developed by the USDA Forest Service that are tolerant to DED. We studied how elm clone growth, chlorophyll fluorescence, and foliar chemistry relate to impervious surface area, ozone concentrations, and soil characteristics. We hypothesized that there would be no differences in tree growth or foliar chemistry between the clones, yet there would be differences in tree growth and foliar chemistry between the cities. We also hypothesized that tree growth would increase with increasing impervious surface due to warmer temperatures, greater ambient CO2 concentration, and greater N deposition in highly urbanized areas. Alternatively, elm clone growth would be depressed by greater ozone concentrations. After 9 months in the field, the mortality rate was extremely low (4%), demonstrating that these elm clones can withstand urban environmental conditions when provided ample water supply and protection from deer. Contrary to our expectations, the clones had variable growth responses to urban conditions. At the time of planting, Sunfield and RV16 clones were larger (height = 1.68m and 1.70m; DBH 30 = 8.42cm and 8.40cm; DBH 137 = 7.91cm and 7.40cm, respectively) than RV474 clones (height = 1.54; DBH 30 = 6.17; DBH 137 = 6.12). However, RV16 clones had the greatest growth rate in the first year (DBH 30 = +8.35cm; DBH 137 = +4.66cm) compared to Sunfield (DBH 30 = +4.96cm; DBH 137 = +1.30cm) and RV474 (DBH 30 = +7.93cm; DBH 137 = +3.72cm) clones. As expected, the elm clones differed in growth rate, chlorophyll fluorescence, and foliar chemistry between the cities. Clones planted in Philadelphia had greater photosynthetic capacity in July (Fv/Fm = 0.76) and September (Fv/Fm = 0.75), while Newark clones had greater photosynthetic capacity in August (Fv/Fm = 0.78). Depleted foliar δ13C signatures in Philadelphia clones suggest they are experiencing greater fossil-fuel derived atmospheric CO2 than Newark clones, possibly contributing to the greater growth rates observed in the Philadelphia clones (height = +0.49cm; DBH 30 = +7.42cm; DBH 137 = +2.57cm) compared to Newark (height = +0.29cm; DBH 30 = +4.71cm; DBH 137 = +1.66cm). Alternatively, enriched foliar δ15N and greater foliar %N in Philadelphia clones suggests they are experiencing greater N deposition from NOx-derived sources compared to Newark clones. Clones growing in Philadelphia had greater foliar nutrient concentrations yet were growing in soils with greater heavy metal concentrations. These foliar-soil chemistry patterns suggest that clones growing in Philadelphia respond positively (i.e., chlorophyll fluorescence, growth rate) to urban environmental conditions in a large city (e.g., warmer temperatures), whereas clones growing in Newark may be experiencing N limitation in the first year of growth after planting. This research is vital for helping return the once iconic American elm tree to urban landscapes.
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Keywords
American, Ecology, Elm, Landscape, Restoration, Urban