Selection of Shrubs for Urban Environments—An Evaluation of Drought Tolerance of 120 Species and Cultivars

Abstract

Shrubs comprise a very large proportion of the plant material used in public as well as private green spaces. Yet, there is currently a lack of quantitative assessments of stress tolerance in a large proportion of available species and cultivars of shrubs, thus complicating any design process involving shrubs. The aim of this study was to evaluate drought tolerance of many common and less traditional shrubs intended for public planting. Through this compilation, a first contribution (dataset) to species selection to obtain expected ecosystem services of shrubs is offered. As water stress is a major constraint for landscape plants in urban environments and is likely to increase in many regions under future climate scenarios, the quantitative drought tolerance of a species or genotype must be a fundamental consideration for plant selection for urban environments. In this study, we used water potential at the turgor loss point (ΨP0) as a key trait for evaluating drought tolerance of different species of shrubs. ΨP0 is a highly instructive trait because it represents a quantifiable measure of physiological drought tolerance. More negative ΨP0 values represent greater drought tolerance by allowing the leaf to maintain physiological function over a greater range of leaf water potentials. ΨP0 was estimated for a wide range of shrubs, representing a total of 44 genera and 120 species and cultivars. The mean ΨP0 value for all 120 shrub species and cultivars was −2.76 MPa, with the overall species ΨP0 value ranging from −1.48 MPa to −4.23 MPa. Intraspecific variation (variation between cultivars) was evaluated using five cultivars of Spiraea japonica and one wild collected genotype. Within this species, there was a range of ΨP0 values of 1.66 MPa, with S. japonica ‘Little Princess’ having the highest estimated drought tolerance (ΨP0 = −2.78) and the wild-type S. japonica having the lowest (ΨP0 = −4.44 MPa).