Assessing the relationship between trait-based and horticultural classifications of plant responses to drought

Water availability can be a major abiotic constraint for plant success in urban areas. To ensure resilient green spaces, understanding which species are able to cope with low water availability is paramount for plant selection. However, in the horticultural industry, descriptions of species’ responses to low water availability are oftentimes poorly defined, inconsistent across sources, and informed from the natural habitat of species, personal experience, or both. Therefore, a method to objectively assess tolerances of species to low water availability is needed. Using a multivariate approach, we characterised the drought strategies of 113 horticultural plant species spanning five different growth forms (graminoids, herbs, climbers, shrubs and trees) by using five leaf-level traits (leaf mass per area (LMA), leaf thickness, unit leaf area, leaf dry matter content (LDMC) and leaf water potential at turgor loss point (TLP)). Based on this approach, we found that species clustered into three distinct groups, which correspond to fundamental drought strategies based on previous ecological literature (i.e., ‘dehydration avoiders’ (species with high LMA and LDMC, thick leaves and low (more negative) TLP), ‘dehydration tolerators’ (species with low LMA and LDMC and high (less negative) TLP) and an ‘intermediate’ mixture of the two strategies). We found that the majority of species that were considered as ‘drought tolerant’ by the horticultural industry were classified as either ‘dehydration tolerators’ (31.9 %) or ‘dehydration avoiders’ (42.5 %) using our trait-based approach. We did not find any strong connections between the climate of origin of our species and our trait-based classifications. Distinguishing between dehydration avoidance and dehydration tolerance may help to enhance horticultural guidelines by better informing which species are suitable for particular biophysical and landscape contexts. This trait-based approach allows for evidence-based evaluation of drought strategies of new and emerging species and cultivars that may be suitable for urban landscapes.