PS 48-246 - Urban tree physiological responses to summer heat: parallel responses of leaf gas exchange and thermal measures of canopy transpiration

Abstract: The degree to which urban trees maintain gas exchange through hot summers affects not only their own health but also influences regional water and carbon balances and provides a preview of vegetation responses to climate warming. Assessing these factors city-wide requires, simultaneously, a more nuanced understanding of leaf-level physiological responses to urban conditions and the development of techniques capable of measuring tree responses remotely. To help fill these knowledge gaps, we conducted a series of campaigns on the University of Maryland, Baltimore County campus. We measured leaf-level physiological parameters and leaf spectral patterns in about 50 trees from nine species that were planted across a gradient of surrounding impervious cover. Concurrently, we collected hourly thermal imagery using a drone to estimate canopy-level transpiration. Although trees experienced temperatures around 35° C during both July and August campaigns, antecedent conditions were hotter and drier leading up to the August campaign. As a result, most trees showed signs of physiological inhibition in August (e.g., lower carbon assimilation, electron transport, and photochemical quenching) with the magnitude dependent on local environmental conditions such as surrounding impervious cover and soil water availability. Moreover, the difference in heat emitted from tree canopies vs. nearby sidewalks (ΔT_cs) was found to be a reliable metric of transpirative cooling. ΔT_cs increased with vapor pressure deficit more strongly in July than in August, suggesting that most trees restricted transpiration in response to environmental conditions. The correspondence between our results at the leaf and canopy scales suggests that drone-mounted sensors can be used to assess tree gas exchange at broader scales to more accurately quantify spatial and temporal variation in water and carbon fluxes across cities.