Spatial and temporal dynamics of the leaf area index (LAI) of selected tree species in urban environments

Knowledge about a tree's leaf biomass, leaf area or leaf area index (LAI) are important prerequisites for deriving precise estimates of its ecosystem services. However, data availability on LAI is limited especially for urban trees, and influences including site conditions, tree size and vitality or seasonal changes are hardly known and usually not considered. The aims of this study are (i) to present a comprehensive database of LAI data over tree size for 15 urban tree species, and (ii) to evaluate seasonal LAI data for over 100 trees of six species in relation to species traits and location with hemispherical photography. Further, (iii) data on leaf emergence were analyzed and compared with data from the German Weather Service (DWD) to derive broader ecological trends. Our results showed that LAI increases with size. The highest LAI was observed at maximum stem diameter, with Tilia cordata showing the highest (maximum LAI 4.7) and Gleditsia triacanthos the lowest values (maximum LAI 2.4). Furthermore, we found significant differences in seasonal LAI development and leaf emergence, influenced by species traits like light requirements, sprouting type, wood anatomy and foliage density. Leaf patterns followed the typical course of a steep increase from branch area index (BAI) values of 0.3 in spring to a maximum LAI of 4.0 in summer and autumn/winter values of 0.5 BAI/LAI. The influence of soil sealing on LAI was less pronounced, albeit statistically significant. The study highlights that the hemispheric image method can be applied easily to individual urban trees and can support a precise calculation of ecosystem services. However, the data processing has some weaknesses, and the results should be adjusted with a correction factor.