A three-dimensional spatial mapping approach to quantify fine-scale heterogeneity among leaves within canopies

Abstract

Premise of the study: The three-dimensional structure of tree canopies creates environmental heterogeneity, which can differentially influence the chemistry, morphology, physiology, and/or phenology of leaves. Previous studies that subdivide canopy leaves into broad categories (i.e., "upper/lower") fail to capture the differences in microenvironments experienced by leaves throughout the three-dimensional space of a canopy.

Methods: We use a three-dimensional spatial mapping approach based on spherical polar coordinates to examine the fine-scale spatial distributions of photosynthetically active radiation (PAR) and the concentration of ultraviolet (UV)-absorbing compounds (A₃₀₀) among leaves within the canopies of black mangroves (Avicennia germinans).

Results: Linear regressions revealed that interior leaves received less PAR and produced fewer UV-absorbing compounds than leaves on the exterior of the canopy. By allocating more UV-absorbing compounds to the leaves on the exterior of the canopy, black mangroves may be maximizing UV-protection while minimizing biosynthesis of UV-absorbing compounds.

Discussion: Three-dimensional spatial mapping provides an inexpensive and portable method to detect fine-scale differences in environmental and biological traits within canopies. We used it to understand the relationship between PAR and A₃₀₀, but the same approach can also be used to identify traits associated with the spatial distribution of herbivores, pollinators, and pathogens.

Keywords: Avicennia germinans; UV-absorbing compounds; spatial mapping; spherical polar coordinates.

Fig. 1.

Visual depiction of the spherical polar coordinate system used to quantify the spatial location of each leaf sampled. ρ is the direct distance of a point from a fixed origin, θ is the zenith angle measured from a fixed vertical direction, and $\varphi$ is the azimuthal angle, measured as the angle relative to a fixed direction on a reference plane that passes through the origin and is orthogonal to the vertical direction.

Fig. 2.

Schematic illustrating radial canopy depth (R_D) and the trigonometric approximation of vertical canopy depth (V_D).

Fig. 3.

Three-dimensional spatial distribution of (A) photosynthetically active radiation (PAR) and (B) UV-absorbing compound concentration as indicated by the absorbance at 300 nm (A₃₀₀) within the canopy based on the combined leaf measurements from four black mangrove (Avicennia germinans) trees on a unitless Cartesian scale. Orientation of the composite canopy relative to the cardinal directions is displayed along the x-y plane.

Appendix 1.

Zenith angle (θ) protractor construction template. Appropriate cuts and their respective orders are indicated by the dashed lines and numbers. Image reproduced from https://commons.wikimedia.org/wiki/File:Rapporteur.svg#filelinks (Rapporteur.svg, Autiwa) under a CC BY-SA 3.0 license (https://creativecommons.org/licenses/by-sa/3.0/legalcode). Overlaid images by J. D. Patterson.

Appendix 2.

Azimuthal angle ($\varphi$) protractor construction template. Appropriate cuts and their respective orders are indicated by the dashed lines and numbers. Image reproduced from https://commons.wikimedia.org/wiki/File:Rapporteur.svg#filelinks (Rapporteur.svg, Autiwa) under a CC BY-SA 3.0 license (https://creativecommons.org/licenses/by-sa/3.0/legalcode). Overlaid images by J. D. Patterson.

Appendix 3.

Schematic illustrating the arrangement of the zenith (θ) protractor and azimuthal $\left(\varphi \right)$ protractor at the base of the canopy.