From Ground to Crown: Analyzing shade provision and thermal comfort benefits of multilayered vegetation structures

Urban vegetation provides essential shade that reduces heat stress in cities. However, there are nuances in shade provisioning and how that affects thermal comfort at the pedestrian scale. While overstory vegetation like trees intercepts direct solar radiation, understory vegetation—shrubs and grass—could serve as a crucial secondary buffer, reducing surface temperatures and long-wave emissions. However, the complex interplay of radiative transfers between the different vegetation layers still is insufficiently understood and inadequately quantified. Despite extensive research establishing the importance of factors such as surface type, canopy coverage, and leaf area index, we lack a comprehensive understanding of how diverse vegetation structures collectively influence pedestrian thermal comfort. This knowledge gap has significant practical implications, as landscape architects must make evidence-based decisions between various configurations, from simple grass lawns to complex multi-layered vegetation systems. Through empirical studies conducted in several public squares in Munich, Germany, we investigate the structural diversity of urban vegetation and its impact on thermal comfort, quantified through Physiological Equivalent Temperature (PET). Our results show that sites with high vegetation structural complexity achieved significantly greater cooling on hot summer days (ΔPET = -5.2 ± 0.08°C) compared to sites with low structural complexity (ΔPET = -1.7 ± 0.13°C). This difference highlights how multiple vegetation layers work synergistically to enhance cooling effects. Additionally, low and medium-height trees (5m to 15m) demonstrated stronger cooling effects (Correlation coefficient r = -0.53 and -0.51, respectively) than tall trees (>15m) (r = -0.39). Among understory vegetation, shrubs exhibited moderate cooling potential (r = -0.28), while grass showed limited effects (r = -0.11), primarily influenced by soil moisture conditions. These findings provide urban decision-makers with evidence-based insights into the relative cooling effectiveness of different vegetation structures and demonstrate how both horizontal and vertical vegetation structural diversity can be leveraged for enhanced urban heat mitigation.