Genotypic variation in water relations and gas exchange of urban trees in Detroit, Michigan, USA

Increasing tree species diversity has become a key underpinning for communities to improve resilience of urban and community forests. Increasingly, urban forestry researchers are examining physiological traits to aid in selecting trees for urban sites. Knowledge of physiological responses also has implications for understanding species' resilience to increased stresses associated with climate change. Here, we compare growth, leaf SPAD chlorophyll index, water relations, and gas exchange of seven genotypes of shade trees planted in two locations in downtown Detroit, MI, USA. Genotypes included Redpointe (R) maple (Acer rubrum 'Frank Jr.'), Flashfire (R) maple (Acer saccharum 'JFS-Caddo2 '), Pacific Sunset (R) maple (Acer truncatum x platanoides 'Warrenred'), Emerald City (R) tulip tree (Liriodendron tulipifera 'JFS-Oz'), Chanticleer (R) pear (Pyrus calleryana 'Glen's Form'), swamp white oak (Quercus bicolor), and Emerald Sunshine (R) elm (Ulmus propinqua 'JFS-Bieberich'). Trees were planted in either Lafayette Plaisance Park (Park), a large urban greenspace, or on the median of St. Aubin Avenue (Median), a nearby major thoroughfare. Tree height growth and leaf SPAD index were higher for trees planted in the Park location than on the Median. However, genotypic variation was larger than the effects of location or the interaction of Genotype x Location for most traits. Across measurement dates, midday leaf water potential was lowest for Pyrus trees and highest for Ulmus and Liriodendron trees. Pyrus and Quercus trees had relatively high rates of net photosynthesis (A) and stomatal conductance (gs) while Liriodendron, Acer saccharum, and Ulmus trees had low rates of A and gs. Liriodendron trees closed their stomata rapidly as leaf water potential (psi w) declined (isohydric response), while Pyrus and Quercus trees maintained gs across a range of leaf psi w (anisohydric response). Liriodendron trees also had the highest relative growth rates, suggesting that drought stress avoidance through isohydry is a viable drought tolerance mechanism in urban trees.