Towards Tree Green Crown Volume: A Methodological Approach Using Terrestrial Laser Scanning

Zihui Zhu,Nils Nölke,Christoph Kleinn

doi:10.3390/rs12111841

Abstract

Crown volume is a tree attribute relevant in a number of contexts, including photosynthesis and matter production, storm resistance, shadowing of lower layers, habitat for various taxa. While commonly the total crown volume is being determined, for example by wrapping a convex hull around the crown, we present here a methodological approach towards assessing the tree green crown volume (TGCVol), the crown volume with a high density of foliage, which we derive by terrestrial laser scanning in a case study of solitary urban trees. Using the RGB information, we removed the hits on stem and branches within the tree crown and used the remaining leaf hits to determine TGCVol from k-means clustering and convex hulls for the resulting green 3D clusters. We derived a tree green crown volume index (TGCVI) relating the green crown volume to the total crown volume. This TGCVI is a measure of how much a crown is “filled with green” and scale-dependent (a function of specifications of the k-means clustering). Our study is a step towards a standardized assessment of tree green crown volume. We do also address a number of remaining methodological challenges.

Highlights

Crowns are important tree components, as they produce oxygen, offer habitats for many taxa, filter out dust and other pollutants, generate shadow and do largely determine the scenic beauty of trees and forests
While commonly the total crown volume is being determined, for example by wrapping a convex hull around the crown, we present here a methodological approach towards assessing the tree green crown volume (TGCVol), the crown volume with a high density of foliage, which we derive by terrestrial laser scanning in a case study of solitary urban trees
We introduce terrestrial laser scanning (TLS)-based k-means clustering as a proxy for a new 3D crown variable named tree green crown volume (TGCVol)

Summary

Introduction

Crowns are important tree components, as they produce oxygen, offer habitats for many taxa, filter out dust and other pollutants, generate shadow and do largely determine the scenic beauty of trees and forests. Tree crowns and the variables characterizing them are difficult to define and measure; that holds for any crown variable, be it wood volume, leaf area, crown volume, crown projection area, etc. All these crown variables are among those forest mensurational variables for which “true values” are virtually impossible to determine (at the standing tree and non-destructively). The crown, while being the most important part of the tree (engine), poses fundamental challenges for forest and tree mensuration. In forest definitions, canopy cover derived from individual trees’ crown cover is frequently used as a core criterion [5]

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