Understanding urban tree heat and drought stress by tracking growth and recovery following an extreme year

Most cities are likely to experience hotter summers and less precipitation in dry months by 2050. Urban tree species selected based on historical climates may be vulnerable to future climate extremes, particularly heatwaves during drought. In Sydney, Australia, an extreme summer in 2019–2020 caused extensive canopy dieback in 20 % and the death of 8 % of surveyed trees (n = 150 trees), but it was unclear why certain trees were vulnerable to severe drought stress and hydraulic failure. Here, we measured environmental variables (i.e., volumetric water content of surface soil, percentage of impervious surfaces), tree physiology, and growth of 19 urban tree species from 2019 to 2023, spanning the extreme and subsequent three wet summers. We found that the most drought-stressed trees with low predawn (Ψ_pre) and midday (Ψ_mid) leaf water potentials had small diameters (<10 cm) and were surrounded by a high proportion of impervious surfaces (>75 %). Severe drought stress only affected ∼ 7 % of studied trees (n = 118 trees) but was correlated with low tolerance of heat stress and caused long-lasting declines in tree growth. On average, the extreme summer decreased growth by 64 %, with the greatest reductions and slowest recovery for the most heat-stressed trees, drought-stressed trees, and some exotic species. A better understanding of dynamic changes in urban tree water access and water use is needed. Future efforts should include physiological traits related to both drought and heat tolerance, considered at the individual tree and species-level, to understand urban tree vulnerability to future climates and improve the selection of climate-resilient urban species.