Close Encounters

Tree crowns avoid colliding in 3D

New metric quantifies the “puzzle-shape-ness” of tree crowns.

Forests are dynamic environments where plants are competing for light and nutrients. The sizes and shapes of tree crowns are of fundamental importance in ecology. Sometimes, there are gaps in the tree tops that are likely to be formed by wind moving the canopy and causing abrasion between trees of a similar height. Since the 1920s, scientists have observed that the crowns of a few tree species (black mangrove, lodgepole pine) grow in a jigsaw puzzle manner that try to avoid overlapping with other trees. This phenomenon is called crown shyness and it is extremely tricky to quantify and study for scientists. 

Jens van der Zee, a graduate student student at Wageningen University and Research (WUR), supervised by Drs Alvaro Lau at WUR and Alexander Shenkin from the University of Oxford proposed a new measurement of crown shyness. The researchers found that a metric, called surface complementarity that is normally used for predicting the formation of protein complexes, can quantify canopy avoidance in 3D. This research was part of Jens van der Zee’s masters thesis. Dr Lau previously quantified the tree biomass from terrestrial laser scans in Guyana and Dr Shenkin recently applied Metabolic Scaling Theory to understand tree crown size and shape variability across different ecosystems.

Canopy shyness leads to a tree top jigsaw puzzle. Source: MAHIM BHAT/WikimediaCommons

In 2017, researchers used terrestrial LiDAR (Light Detection and Ranging) scanning of over 100 trees in Guyana. van der Zee and colleagues selected 14 tree pairs which were positioned closely enough to detect crown shyness and produced 3D point clouds. They measured tree slenderness by measuring the tree heights and diameter at breast height. The researchers computed pairwise shape complementarity and segmented the canopy interactions zones. Then, they compared the complementarity between overlapping and non-overlapping trees and investigated the relationship between slenderness and crown shyness.

Overlapping crowns of sycamore trees from top (A) and side (B) views. Source: van der Zee et al., 2021

Van der Zee and colleagues successfully applied the surface complementarity metric to quantify crown shyness. Half of the tree pairs had overlapping crowns and the metric was low (0.267) whilst the non-overlapping crowns had higher values (0.647). 

“Until recently, being able to observe crown shyness was reserved to structurally simple and flat canopies, where the backlighting of the sky reveals the gaps between tree crowns,” van der Zee and colleagues wrote. 

The researchers also found that the more slender the trees were, the more the canopies appeared to avoid overlapping. Previous studies found that slender trees grow smaller trees as those would sway more in the wind.

“[O]ur results suggest they also grow crown shapes that complement those of their neighbours. By doing so, trees optimize available growing space while minimizing damage from collisions.”

“This study serves as an example of the value of 3D tree modeling for expanding our understanding of canopy interactions as it helped both visualizing and quantifying an interesting canopy dynamic in unprecedented ways.”

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