Estimation of leaf area for open-grown urban trees in Great Britain using HemiView

Abstract

Ecosystem service provision by urban trees depends on a range of factors including the tree total leaf area, which itself is related to canopy size and health. Consequently, a quantification and valuation of ecosystem services delivered by urban forests is more accurately made if based on correct leaf area values. The measurement of leaf area using direct methods is however unrealistic for urban trees; hemispherical photography offers an alternative. We photographed the canopies of fifty-one open-grown Acer platanoides, Fraxinus excelsior and Platanus x hispanica trees of different sizes, growing in public urban greenspaces in southern Great Britain (GB) and estimated their leaf area using HemiView 2.1 (Delta-T devices). For each tree, photographs were taken from four cardinal points and the subsequent individual leaf area estimates were compared to assess the variance associated with photographing a canopy from different orientations. This variance was highest for larger trees than for small ones, particularly those with estimated leaf area >400 m2; still, it remained always under 10% of the mean leaf area. The relationships between the tree’s leaf area estimate and its biometric variables were also found. All relationships tested were significant, especially those using tree height and crown height. Finally, leaf area estimates achieved using HemiView were compared to estimates achieved using published allometric equations and found to be significantly related to those for two of the three species. Differences between estimates (HemiView vs allometric equations) tended to be considerable for larger trees, but for most small trees estimates were broadly approximate. Leaf area estimated using HemiView or other indirect methods should be undertaken in future through comparison to empirical leaf area measurements.