The role of leaf morphological plasticity in maintaining crown condition: A case study of London plane (Platanus x acerifolia) and valley oak (Quercus lobata)

In the coming decades, urban trees will face warmer conditions than ever. Street tree responses to heat are a key topic among urban forestry professionals, who commonly use crown condition to assess tree status and function. The extent to which crown condition responds to heat is understudied, as are the mechanisms that underlie tree resilience to climate. Across the Sacramento, CA, USA urban heat gradient, we assessed the extent to which London plane ( Platanus x acerifolia ) and valley oak ( Quercus lobata ) display plasticity in leaf morphological traits relevant to heat tolerance. We then assessed crown condition to determine whether leaf plasticity underlies the sensitivity of trees to urban heat. Valley oak leaves were smaller at hotter sites, and had higher leaf mass per area under hot irrigated conditions, which functions to improve leaf heat tolerance. At hotter sites, London plane developed larger leaves which were less suited for hot environments; although leaf dissection increased slightly, the increase in leaf perimeter per area was much smaller than the increase in overall leaf area. These patterns were reflected in crown condition. Leaf plasticity appears to allow valley oak to avoid crown damage if enough water is available. Our findings point to leaf developmental plasticity as a mechanism by which trees respond to urban heat and, to some extent, maintain crown condition in hot environments. Developmental plasticity of leaves under warming may be a key characteristic of trees that are able to maintain crown condition as the climate warms. However, supplemental water may be necessary even for species with high levels of plasticity in extreme heat. • Valley oak had smaller leaves in hotter locations • London plane had larger leaves in hotter locations • Canopy condition sensitivity to heat differed by species, dependent on irrigation • Valley oak maintained condition if irrigated; London plane only partly benefited • Plasticity may be key mechanism for acclimating to heat and maintaining condition