86 programs aimed at producing EAB-resistant cultivars will be essential to preserve ash trees in North Amer- ica. These programs have focused on Asian species as a source of resistant germplasm to hybridize and back cross with North American species (Koch et al. 2012). The degree to which Asian ash species and North American x Asian hybrids are physiologically suited to growing conditions in North America relative to common North American ash species has not been empirically tested. Manchurian ash grows in open woodlands and val- leys and is considered an adaptable and stress-tolerant species that is widespread in northeastern China, Japan, Korea, and eastern Russia (Aiello 2012). Green ash is the most widely-distributed ash species in North America; it is fast-growing and capable of sur- viving under a wide range of moisture conditions, although it is generally considered a wetland and ripar- ian species (MacFarlane and Meyer 2005). White ash is a common component in many forest types; it occupies bottomlands, open up-slope woodlands, and mixed hardwood forests (MacFarlane and Meyer 2005). White ash grows well in rich, well-drained, moist soils (Woodcock et al. 1993; MacFarlane and Meyer 2005). Black ash is a hydric species found in swamps and along streams that tolerates flooding, poorly drained areas, and grows well in shallow, organic peat, fine sands, and loam soils (Wright and Rauscher 1990; Iverson et al. 2016). Blue ash is toler- ant of high soil pH and drought and generally grows in upland forests on calcareous soils, but is also found in moist and riparian forests (Braun 1961). Blue ash appears to be more resistant to EAB than other North American ash species, as it has experienced lower mortality in forest stands (Tanis and McCullough 2012) and common garden studies (Herms 2015; Tanis and McCullough 2015). Comparisons of physiological traits among ash species and cultivars resistant and susceptible to EAB will inform planting decisions in urban forests. Hence, our objective was to characterize the physiological performance of North American ash cultivars highly susceptible to EAB including ‘Autumn Purple’ white ash, ‘Patmore’ green ash, and ‘Fall Gold’ black ash, with the more resistant blue ash, Manchurian ash, and Manchurian × black ash hybrid ‘Northern Treasure’ in a common garden plantation in northeast Ohio, U.S.A. White and green ash are both native to northeast Ohio and grow in the forest immediately adjacent to ©2019 International Society of Arboriculture Haavik and Herms: Ash growth and physiology in Ohio, U.S.A. the common garden. White and green ash served as reference species to compare physiological perfor- mance of the other ash species sampled. Over the growing season under differing precipitation levels, we measured growth and key leaf physiological traits, including leaf nitrogen (N) and specific leaf area (Reich et al. 1998), net photosynthetic rate and stomatal con- ductance (Lambers et al. 1998; Larcher 2003), photo- synthetic nitrogen use efficiency (PNUE) (Poorter and De Jong 1999), and chlorophyll fluorescence as an indicator of the efficiency of photosynthesis (Krause and Weis 1991; Mohammed et al. 1995; Sanchez and Quiles 2006). MATERIALS AND METHODS Study Site In 2015, we measured growth and physiological traits of six ash taxa growing in a common garden in north- east Ohio located at the Ohio Agricultural Research and Development Center campus in Wooster (UTM Zone 17T; 422253 E, 4514605 N). These included the North American blue ash (seedling origin), white ash (F. americana ‘Autumn Purple’), green ash (F. pennsylvanica ‘Patmore’), black ash (F. nigra ‘Fall Gold’), the Asian species Manchurian ash (seedling origin), and the Manchurian × black ash hybrid ‘Northern Treasure’ (Table 1). The garden was estab- lished in 2011 on a field previously planted to forage crops. Ash saplings were planted in a randomized complete block design. We sampled eight replicates of each taxa for a total of 48 trees in the study (Table 1). EAB was present in the common garden at low levels, as indicated by captures of beetles in intercept traps, but none of the trees we sampled exhibited signs or symptoms of infestation (crown dieback, cracked bark, visible larval galleries, or adult emergence holes). Growth and Physiology Measurements To quantify growth during the 2015 growing season, we measured stem diameter at breast height (dbh at 1.4 m [4.5 feet] from the ground) before and then again after the 2015 growing season. In July of 2015, we collected 10 to 12 leaflets from sun-exposed leaves on each of the 48 ash trees and stored them in a cooler during collection and transport to the labora- tory, where we measured area (LA, cm2 leaves with a leaf area meter (LI-3100, LI-COR® Lincoln, Nebraska). We then dried the leaves at 70°C ) of individual ,
May 2019
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