As climate change progresses, urban plantings in many regions of the world are increasingly exposed to hot and dry climate extremes, posing new challenges for the health of urban vegetation. For example, extreme drought and heatwaves during the 2019-2020 austral summer caused the death of groundcover plants and widespread canopy dieback of street trees throughout Sydney, Australia. Discovering which horticultural plant species can tolerate the future climate of Australian cities is a major aim of our ‘Which Plant Where’ project.
We screened 113 horticultural plant species, both native and exotic to Australia, in glasshouse experiments by exposing both well-watered and droughted plants to a six-day heatwave with maximum air temperature of 41°C. We sought to identify which plant functional types can tolerate heat and drought stress. To achieve this, we measured leaf critical temperature, the temperature where dark-adapted chlorophyll fluorescence rises rapidly and photosystem II is disrupted, irreversibly impairing photosynthesis. To quantify and rank species for drought tolerance, we estimated leaf turgor loss point, or wilting point, through leaf osmotic potential measurements and a set of supporting morphological traits. We also measured stomatal conductance to determine if trees use transpirational cooling to tolerate the combination of heat and drought stress.
Results/Conclusions
Most horticultural species survived the experimental heatwave, even under moderate drought stress. Subtropical and tropical species had higher heat tolerance than temperate species. Leaf critical temperature was plastic, however, and increased significantly with both heat and drought stress. Despite increases in leaf critical temperature, drought exacerbated plant thermal damage by increasing leaf temperatures, decreasing leaf thermal safety margins, and increasing canopy dieback. Rapid osmotic adjustment (i.e. decrease in leaf osmotic potential) was observed in droughted plants after the fifth day of heat stress, suggesting increases in cell sugar/osmotica concentrations may provide a mechanism for rapid temperature acclimation in water-stressed leaves. Another observed mechanism for avoiding thermal damage during heatwaves was increased stomatal conductance and transpirational cooling, which surprisingly, was more common in droughted plants than well-watered controls.
We classified about 50 species as either ‘drought-tolerant’ or ‘drought-intolerant’, but found that horticultural classifications of drought-tolerance had poor correspondence with our trait-based approach – 65% of species classified as ‘drought-intolerant’ using leaf traits were described as ‘drought-tolerant’ by the horticulture industry. The results and overall experimental approach can be used to screen large urban floras and inform species selection for urban plantings to achieve greater resilience under future climate conditions.