Distance to canopy edge and tree trunk proximity affect understory temperature and humidity in urban tree stands

Climate change is increasing extreme temperatures in urban areas, degrading environmental quality and threatening the health of vulnerable human populations. Planting trees has become a key strategy for regulating urban temperatures, particularly for reducing heat extremes during hot weather. Trees provide shade and increase moisture, lowering below-canopy temperatures. This cooling effect could be enhanced by planting trees in groups. However, information on how the spatial arrangement of trees within tree stands influences below-canopy microclimatic conditions in urban areas is scarce. In this study, we evaluate how below-canopy temperature and relative humidity are affected by the spatial arrangement of trees within urban tree stands, specifically testing the impact of distances to the canopy edge and tree trunks on these microclimatic variables. We assessed these relationships by collecting empirical data from five even-aged tree stands, located in an urban park in Santiago, Chile. A 10 × 10 m grid of 25 sensors was used to measure below-canopy climatic variables, and the distance from each sensor to the edge of the canopy and to the closest tree trunks was recorded. Data was collected on three different days for each stand on warm, sunny days, and linear mixed models were applied to test the relationships. Our results show that all tree stands reduce below-canopy temperatures compared to surrounding areas. Additionally, we identified distance-dependent effects of tree canopy and trunks on below-canopy temperatures and relative humidity, with a relatively larger effect of distance to the canopy cover. However, these results varied significantly between stands, with some stands showing no effects. Increasing our understanding of how the spatial arrangement of trees influences their cooling capacity is crucial for planning urban vegetation to mitigate rising temperatures in urban areas.