Do leaves make urban trees structurally more complex?

Details

Trees have a fractal-like branching architecture that determines their structural complexity. Researchers used terrestrial laser scanning to investigate the role of foliage in the structural complexity of urban trees. They sampled 45 trees of three deciduous species—Gleditsia triacanthos, Quercus macrocarpa and Metasequoia glyptostroboides—on the Michigan State University campus.

They calculated a box-dimension metric (Db) from terrestrial laser scanning point clouds of the trees in leaf-on and leaf-off conditions. They also applied an algorithm to the leaf-on point clouds to separate foliage from woody material, artificially remove the leaves and recalculate Db.

Across all three species, Db was significantly greater in the leaf-on point clouds than in the leaf-off point clouds. The leaf removal algorithm introduced bias into estimates of leaf-removed Db for G. triacanthos and M. glyptostroboides. An index of leaves’ contribution to structural complexity—leaf-on Db divided by leaf-off Db, minus one—was negatively correlated with branch surface area and different measures of path length through the branch network, indicating that the contribution of leaves decreases as branch network complexity increases. After artificial leaf removal, underestimation of Db for G. triacanthos was related to maximum branch order.

The results help disentangle the contributions of leaves and woody structures to tree structural complexity. They also highlight important methodological considerations when studying tree structure with and without leaves using laser-derived point clouds.