Canopy structure

F. Chianucci

Topics

  • Concepts and definitions

  • Measures of canopy structure

  • Digital canopy photography

  • Training applications

Concepts and definitions

Definitions

  • The forest canopy is a structurally complex and ecologically relevant component of the ecosystem

  • “The aggregate of all crowns in a stand of vegetation, which is the combination of foliage, twigs, fine branches, epiphytes as a well as the interstices (air) in a forest” (Parker 1995)

  • Many other definition exists, also varying from the discipline. ‘Canopy biology’ was proposed as trans-disciplinary field of study.

  • Key concept: canopy is a continuum (e.g. a stand), crown is individual(s) (e.g. tree(s))

Units

  • LAI is “half the total area of green elements of the canopy per unit horizontal ground area” (Chen and Black 1992)

Ritter 2024

LAI

  • LAI is basically influenced by:

    1. Leaf traits: shape, SLA, habit (evergreen/deciduous)
    2. Leaf orientation: random, clumped
    3. Leaf inclination angle distribution

Other canopy attributes

Canopy (foliage or crown) cover (CC): “the ratio of the forest floor covered by the vertical projection of crowns”

Canopy closure (CL): integrated measure of “amount of sky hemisphere covered by a vegetation when viewed from a single point”

Processes

Energy exchange

  • Canopy is the interface between plant and atmosphere

Lawrence et al. 2011

Energy exchange

  • Canopy is the interface between plant and atmosphere:
    • Light interception (overstory) and transmittance (understory)
    • Photosynthesis (PAR, absorbance), respiration, evapotranspiration
    • Rain interception, thorughfall, transpiration: direct effect
    • Evaporation, infiltration runoff: indirect effect
    • Litter production, carbon and soil mineralization

Microclimate

  • Canopy structure regulates microclimate in many ways

Lembrechts et al. 2020

Microclimate

  • Forest structure regulates microclimate

https://asknature.org/strategy/canopies-enhance-plant-diversity/

“The values of light intensity and quality, temperature, wind, moisture content, and concentrations of various gases and aerosols are strongly modified by canopy structure in several ways . . . The canopy offers its occupants a wide variation in water, light, and nutrient regimes compared to the understory and the forest floor, and this variety undoubtedly contributes to arboreal plant diversity.”

BOOK SECTION

Chapter: Forest Canopies, Plant Diversity

Encyclopedia of Biodiversity, Volume 3 | 2001 | Nalini M. Nadkarni, Mark C. Mewin, and Jurgen Niedert

Radiation

  • Canopy structure is influenced, and influences, the radiation regime
  • All optical theory hinges on the response of vegetation to incident light (PAR)
  • Incident light (\(I_0\)), light transmittance (\(\tau\)), light absorbance (\(\alpha\)) and reflection (\(\rho\))

Kattenborn 2018

Radiation

Key concepts

  • Canopy Structure: The arrangement of leaves, branches, and gaps determines how light penetrates and scatters within the vegetation.

  • Leaf Optical Properties: Pigments (chlorophyll, carotenoids) and leaf surface characteristics influence how much light is absorbed, transmitted, or reflected.

  • Radiative Transfer Models: Optical theory to describe how radiation propagates through plant canopies, predicting light distribution and energy balance.

Radiation

  • Radiative transfer for realistic light interception modeling…
  • …up to real 3D LiDAR representations

Applications

Monitoring

  • Crowns and canopy react more promptly to disturbances than any components

    • Phenology

    • Forest long-term monitoring

    • Forest recovery after extreme events

    • Global change

    • Forest management

Monitoring phenology

Monitoring phenology

https://data.mendeley.com/datasets/gkr667jvhx/1

Monitoring forest management

  • Forest management

Cutini et al. 2015

Monitoring canopy recovery

  • Post-storm recovery

Xiong et al. 2022

Monitoring canopy recovery

  • Post-fire recovery

Filicetti et al. 2021

Earth Observation

Roman and Ursu 2016

Earth Observation

From Landsat to Sentinel…

… up to Google Earth Engine

Biodiversity

  • Forest canopy is the most global species-rich environment (the last biotic frontier Erwin 1983)
  • Estimated host 50% terrestrial species
    • About one quarter of invertebrates are canopy-specialists
    • Overstorey - understorey interactions
    • Epiphytic canopy dwellers
  • Still poorly understood

The last biotic frontier

Plant biodiversity

  • Relationship overstory - understory

Zhou et al. 2022

Bat biodiversity

  • Relationship canopy - bats

Oliveira et al. 2019

Butterfly biodiversity

  • Relationship canopy - butterflies

Lu Wang et al. 2022

Beetles biodiversity

  • Vertical canopy structure - beetles

Haack et al. 2021

Measurements

LAI

We recall that LAI is basically influenced by:

1.  Leaf traits: shape, SLA, habit (evergreen/deciduous)
2.  Leaf orientation: random, clumped
3.  Leaf inclination angle distribution (G)

Optical measurements

  • Optical methods infers LAI from radiation transmittance:

\[L=-\frac{log(P(\theta))cos\theta}{G(\theta)\Omega(\theta)}\]

  • \(P(\theta)\) gap fraction
  • \(G(\theta)\) foliage orientation
  • \(\Omega(\theta)\) foliage clumping

It is known as Beer-Lambert law

Itakura and Osoi 2019

Ground optical methods

  • Passive sensors (imagery):

  • Active sensors (LiDAR):

Proximal sensing

  • UAV & aerial data

Remote sensing

  • Satellite imagery:
  • Airborne or satellite LiDAR:

Canopy photography

A cheap optical canopy method

  • Digital photography is widely used for field measurements of canopy:

    • Cheap and permanent record

    • Many open-software and tools for image analysis

  • Multiple-view (fisheye) vs Single (vertical) photography

Fisheye photography

  • Pros: most information at the largest footprint

    • Ecologically-relevant info (LAI, Light regime)

  • Cons:

    • specific equipment (lens)

    • sensitive to image acquisition (sky and exposure settings)

Cover photography

  • Pros: simplicity and flexibility

    • Lower sensitivity to sky conditions and processing
    • Forest inventory attribute (cover) and Remote sensing applications

  • Cons:

    • Limited, mainly vertical sampling
    • Require assumption for LAI

Canopy Photography monitoring

  • Phenology monitoring

Chianucci et al. 2021

Canopy Photography monitoring

  • Field and UAV monitoring:

Chianucci et al. 2016

Canopy Photography and the city

  • Urban forest monitoring

Chianucci et al. 2015

Canopy Photography and biodiversity

  • Which method?

von Meijenfeldt et al. 2025

Canopy Photography and biodiversity

  • Hemispherical photography is better suited for ecology-biodiversity applications

    • Largest footprint -> radiation regime, ‘sessile’ organisms

von Meijenfeldt et al. 2025

Canopy Photography and biodiversity

  • Cover photography hold potential for continuous and phenology biodiversity monitoring

Data not published

Assignment

  • Try to use canopy photography in R

  • Basic steps:

    1. Import a digital canopy image
    2. Classify it into gap and canopy pixels
    3. Estimate angular gap fraction (\(P\theta\))
    4. Apply formula relating LAI to \(P\theta\)

Canopy R packages

  • Package to use:

Thanks