GapFraction

GapFraction is a metric used in forest ecology and canopy science to describe the proportion of sky or background that is visible through a vegetation reference, typically from a fixed point within or under the canopy. It is commonly defined as the ratio between the sky area and the total area in a hemispherical view, often expressed as g = A_sky / A_total, and can be resolved as a function of viewing direction, g(θ,φ). The overall or global gap fraction is the average sky fraction across all viewing directions.

Measurement methods: Hemispherical photography with a fisheye lens is the standard method. Digital images are processed to classify pixels as gap or vegetation; the sum of gap pixels divided by total pixels yields the gap fraction. Other methods include laser scanning (LiDAR) and radiative transfer models that estimate gap fraction from leaf clumping and canopy structure. Directional gap fractions g(θ,φ) give angular variation, used to estimate light interception and the leaf area index (LAI) when combined with a radiative transfer model.

Applications: Gap fraction informs estimates of light transmittance, canopy photosynthetically active radiation, and energy balance. It is an intermediate variable in methods for calculating LAI and for validating remote-sensing products.

Limitations: Accuracy depends on image quality, sky conditions, and canopy anisotropy; sun flecks and moving leaves introduce errors; calibration and standardization are needed for comparisons across sites.

See also: Leaf area index, hemispherical photography, radiative transfer, sky view factor.