An assessment of the drought tolerance of Fraxinus genotypes for urban landscape plantings

The Fraxinus genus consists of ca. 65 deciduous, rarely evergreen trees found mainly throughout Europe, Asia and North America. Fraxinus species possess the aesthetic characteristics desired of urban trees (autumn colour, attractive bark and flowers); however, no scientifically based tolerance ranking exists for many genotypes within this genera. Aims of this study were to identify whether differences in drought tolerance exist within the Fraxinus genus and thereby provide information as to their usefulness for planting in urban landscapes where water stress poses a threat to transplant survival. At the cessation of a 2-week drought stress and subsequent recovery period, marked differences in drought tolerance among genotypes were distinguished based on net photosynthesis, leaf chlorophyll fluorescence and chlorophyll concentration. In addition, measurement of light absorbance, light trapping, electron transport and dissipation fluxes per leaf cross section of photosystem II at the cessation of the drought period provided an insight into mechanisms of drought damage and resilience among the genotypes tested. The results of this investigation indicate that F. excelsior ‘Aurea Pendula’, F. nigra Marsh., F. ornus L., F. angustifolia ‘Raywood’, F. excelsior L. and F. excelsior ‘Jaspidea’ should be considered for urban planting sites, where water supply is limited. Care should be taken when planting F. americana L., F. americana ‘Autumn Purple’ and F. velutina Torr., into urban environments as results indicated these species are sensitive to drought.