Speaker
Description
Research Problem:
Urbanization increases impervious surface cover, altering soil properties, disrupting hydrological cycles, and modifying microclimatic conditions under tree canopies. This change may reduce physiological performance, tree growth, and the provision of essential ecosystem services such as habitat provision, cooling, and carbon sequestration. Urban forests play a very important role in climate adaptation strategies; however, the vulnerability of urban trees to sealing surfaces is not well understood. In temperate European cities, where both tree species composition and infrastructure vary widely, evaluating tree responses to the surface is vital for informed urban planning.
This study addresses this gap by assessing the effects of urban sealing on growth, physiological traits, and ecosystem services in three common urban tree species. By integrating species-specific functional traits with environmental gradients, the study aims to identify resilient species and inform site-specific planting strategies to enhance urban forestry multifunctionality.
Methodology:
Field measurements were conducted in the southern region of Munich, Germany, across a gradient of surface sealing categorized as highly sealed surfaces, mostly sealed, partially sealed, and mostly unsealed. was categorized based on the imperviousness Density high-resolution layer (IMD HRL) from the Copernicus program (Union, 2022). The selected species included Acer platanoides, Robinia pseudoacacia, and Tilia cordata, representing different functional traits and tolerances.
Tree dimension, diameter at breast height (DBH), crown diameter, crown volume, and tree height were recorded for each tree species. Linear regression models were used to analyze species-specific growth responses across the different sealing categories. To evaluate physiological performance, transpiration rate, chlorophyll content (SPAD), stomatal conductance, and midday leaf water potential were measured between 10:00 and 14:00 h on clear days without clouds to minimize the diurnal variation. Measurements were replicated over two growing seasons (2021-2022) to account for annual climatic variability.
Linear mixed-effect models were used to test the effects of sealing percentage, species, and other climatic factors that could influence the results (e.g., precipitation, air temperature) on physiological traits. Other random effects were used to account for repeated measures and individual variability.
Ecosystem services were assessed by estimating aboveground biomass using species-specific allometric equations. Microhabitat abundance and diversity were recorded using a field guide for tree-related microhabitat and quantified using the Shannon diversity index. Microclimatic regulation was assessed by measuring surface temperature reduction over both grass and asphalt, using a laser infrared thermometer under sun-exposed and shaded surfaces.
Main findings
Species-specific responses to different sealing surfaces were evident and contrasting across structural and physiological parameters. R. pseudoacacia demonstrated strong allometric relationships even in highly sealed surfaces (e.g., DBH and tree height), A. platanoides showed moderate allometric correlations, with reduced performance under medium and partial sealing surfaces. T. cordata performed better in less sealed areas, with a weaker relationship under high sealing surface R2= 0.33. Crown diameter also shows a strong relationship with DBH across species. R. pseudoacacia, T. cordata and A. platanoides exhibit high R2, particularly in all sealing categories, except in A. platanoides, where R2 in highly sealed surfaces was very low.
Physiological responses varied significantly. Increased surface sealing reduces stomatal conductance (p= 0.008) and transpiration (p= 0.033). R. pseudoacacia maintained higher stomatal conductance than A. platanoides (p 0.001). Leaf water potential differed among species (R. pseudoacacia, P < 0.001; T. cordata, p= 0.005), but the sealing categories did not significantly affect. Chlorophyll content was positively correlated with DBH and precipitation, but also did not show a significant relationship with different sealing categories.
Aboveground biomass was significantly influenced by DBH and species (p < 0.001). R. pseudoacacia accumulated the most biomass, followed by T. cordata, while A. platanoides have the lowest values. Surface sealing had no significant effect on biomass (p = 0.231),
Microhabitat diversity and abundance were highly influenced by both sealing and species. T. cordata supported the highest number of microhabitats, particularly in partially sealed and unsealed areas. A. platanoides has high Shannon diversity values in unsealed areas, while R. pseudoacacia showed relatively low diversity across all sealing categories. Microhabitat presence was significantly higher in less sealed environments, with a 228 % increase in occurrence (IRR = 3.28) compared to highly sealed surfaces.
The microclimate regulation, grass surfaces beneath canopy shade showed significantly lower temperature in highly sealed areas compared to unsealed areas (estimate -0.28, p = 0.036), while the asphalt cooling effect did not vary significantly.
Discussion:
This study reveals distinct interspecific strategies for selecting with urban sealing. R. pseudoacacia demonstrated both physiological and structural resilience, making it suitable for planting on highly sealed surfaces. A. platanoides showed greater sensitivity to sealing and was better suited to less compacted areas. T. cordata displayed adaptive plasticity, supporting both ecological functions and microhabitat provision under different surface conditions. The results showed the importance of considering both trees and environmental conditions when planning urban trees.
In conclusion, the resilience and multifunctionality of urban forests can be improved by careful species selection and site-specific planting design.
Keywords: urban forestry, soil sealing, tree physiology, ecosystem services, species-specific responses
