96 Although leaf N varied significantly among ash species, SLA did not, which is also consistent with low variation in growth rates. SLA is a function of leaf thickness and density and is a key character that determines capacity for growth; a high leaf area rela- tive to leaf mass allows plants to grow fast (Lambers et al. 1998; Poorter and De Jong 1999). Within spe- cies, SLA was not significantly correlated with growth but was highly correlated with leaf N (r ≥ 0.93) in all ash except Manchurian × black (r = 0.67). This suggests that leaves with a higher SLA would also have a greater photosynthetic capacity (Lambers et al. 1998), although this was not reflected in any of our measurements (i.e., A was not correlated with SLA on an intraspecific basis). Many measures of ash physiology were not sig- nificantly correlated with growth, perhaps because (1) environmental conditions in 2015 were not limit- ing for growth, and (2) instantaneous measurements were not representative of season-length physiologi- cal status. Chlorophyll fluorescence increases in response to excess light, drought, or other stressors that decrease intercellular CO2 supply and thus nega- tively affect phytochemical reactions (Krause and Weis 1991; Lambers et al. 1998). Lack of a relation- ship between variable fluorescence or the efficiency of photosystem II and growth may indicate that the degree of stress experienced by ash in our study was not appreciable enough to affect growth rate. Other studies also reported high, low, negative, or no cor- relations between CO2 uptake and growth rate (Lam- bers et al. 1998; Poorter and De Jong 1999; Larcher 2003). Short-term (instantaneous in our case) mea- surements do not reliably reflect seasonal patterns in photosynthesis (Pallardy 2008). Also, the relationship between photosynthesis and respiration, partitioning of photosynthate within the tree, and total leaf area were not measured and all contribute to variation in growth rate (Pallardy 2008). Although A on particular sampling dates did not correlate with growth among individual trees, high rates of A in green ash may have contributed to its greater diameter growth rela- tive to other ash taxa. Within a species, measures of physiology that are known to be tightly linked were often significantly correlated. For example, stomata regulate the rate at which CO2 Larcher 2003; Pallardy 2008), and A and gs diffuses into the leaf (Lambers et al. 1998; were often significantly positively correlated for most spe- cies on most sampling dates. ɸPSII was more ©2019 International Society of Arboriculture Haavik and Herms: Ash growth and physiology in Ohio, U.S.A. frequently correlated with A, gs, and PNUE on the last sampling date of the growing season, relative to other sampling dates. Low precipitation and the onset of dormancy on the final sampling date probably rep- resented the most stressful conditions experienced by trees over the course of the growing season. CONCLUSIONS The lack of significant variation in growth and physi- ology among ash species, both when environmental conditions were favorable and during dry, late sum- mer conditions, suggests that all taxa that we tested are generally adapted to environmental conditions in central Ohio. Notably, the Asian ash species Manchu- rian ash and the Manchurian × black ash hybrid ‘Northern Treasure’ are both EAB-resistant (Herms 2015) and performed physiologically just as well as North American species native to Ohio. These results indicate that Manchurian ash and the Manchurian × black ash hybrid ‘Northern Treasure’ are both suffi- ciently adapted to growing conditions in the Mid- western United States. We suggest that either could be planted in Midwestern urban forests in place of ash species that are susceptible to EAB. LITERATURE CITED Abrams, M.D., M.E. Kubiske, and K.C. Steiner. 1990. Drought adaptations and responses in five genotypes of Fraxinus penn- sylvanica Marsh.: photosynthesis, water relations and leaf morphology. Tree Physiology 6:305-315. Aiello, A.S. 2012. In search of Chinese ashes. The Plantsman 11:188-192. Braun, L.E. 1961. The Woody Plants of Ohio. Ohio State Univer- sity Press, Columbus, Ohio, U.S. 362 pp. Cappaert, D., D.G. McCullough, T.M. Poland, and N.W. Siegert. 2005. Emerald ash borer in North America: a research and regulatory challenge. American Entomologist 51:152-164. Carlier, G., J.P. Peltier, and L. Gielly. 1992. Water relations of ash (Fraxinus excelsior L.) in a mesoxerophilic mountain stand. Annals of Forest Science 49:207-223. Davidson, C.G. 1999. ‘Northern Treasure’ and ‘Northern Gem’ hybrid ash. HortScience 34:151-152. Davies, W.J. and T.T. Kozlowski. 1977. Variations among woody plants in stomatal conductance and photosynthesis during and after drought. Plant and Soil 46:435-444. Herms, D.A. 2015. Host range and host resistance. USDA Forest Service FHTET-2014-09:65-74. Herms, D.A., and D.G. McCullough. 2014. Emerald ash borer invasion of North America: history, biology, ecology, impacts, and management. Annual Review of Entomology 59:13-30. Iverson, L., K.S. Knight, A. Prasad, D.A. Herms, S. Matthews, M. Peters, A. Smith, D.M. Hartzler, R. Long, and J. Almendinger. 2016. Potential species replacements for black ash (Fraxinus nigra) at the confluence of two threats: emerald ash borer and a changing climate. Ecosystems 19:248-270.
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