Leaf Area Index (LAI)
VC-MET-010 — the one-sided leaf area per unit ground area, the structural measure of canopy density that sets light interception, transpiration capacity, and the saturation ceiling of NDVI.
Definition
Leaf Area Index (LAI) is the dimensionless ratio of one-sided green leaf area to the ground area beneath it, in m²·m⁻². An LAI of 4 means four square metres of leaf overlie each square metre of bench. At GreenWorld Sector-7 it is retrieved from the Spectral Canopy Array by gap-fraction inversion of below-canopy transmittance combined with the structured-light point cloud, giving the structural density that NDVI greenness alone cannot resolve once the canopy closes.
Why it matters
LAI sets the canopy's light-harvesting and transpiring surface, so it governs both productivity ceiling and water demand. Crucially, NDVI saturates above LAI ≈ 4 — once the canopy is closed, NDVI stops responding while LAI keeps rising or falling — so LAI is the channel that disambiguates a flat NDVI. CHLORA uses it to set planting density, pruning targets, and per-unit irrigation scaling, and to interpret the structural state behind a CVI reading.
Formula
LAI = − ln(T) · cos(θ) / G(θ) (gap-fraction inversion) where: T = canopy transmittance (below/above PAR ratio) θ = sensor zenith angle G(θ) = leaf-projection function (cultivar G ≈ 0.5 spherical) fused estimate: LAI = w_g · LAI_gap + w_p · LAI_pointcloud with w_g = 0.6, w_p = 0.4 LAI_pointcloud from voxelized leaf-surface reconstruction of the structured-light pass. smoothing: 4-pass (24-h) moving median. Reported to 0.1 m²·m⁻².
Inputs
| Symbol | Quantity | Reference Band | Source |
|---|---|---|---|
| T | Canopy PAR transmittance | 0.01 → 0.20 | Spectral Canopy Array / PhytoSense PAR |
| G(θ) | Leaf-projection function | ≈ 0.5 | Cultivar profile |
| LAI_pc | Point-cloud LAI estimate | per cell | Spectral Canopy Array |
| Stage | Phenological stage (band select) | veg / flower / mature | Cultivation Records |
Units & Scale
m²·m⁻², reported to 0.1. Nominal band is stage- and cultivar-dependent; the 3.5–6.5 envelope is the closed-canopy vegetative target. Aggregated per cell, then ground-area-weighted to a facility value. Confidence degrades when transmittance falls below the 0.01 instrument floor (very dense canopy, inversion ill-conditioned) or when point-cloud and gap-fraction estimates diverge by more than 1.5.
Sampling & Source
- Retrieved every 6 hours from Spectral Canopy Array transmittance + point-cloud passes.
- Smoothed over a 24-h (4-pass) moving median.
- Used to flag NDVI saturation (LAI > 4) for correct CVI interpretation.
- Stale handling: flagged STALE after 2 missed passes; divergent fused estimates flagged low-confidence.
Thresholds
OK
Healthy closed canopy. No action.
WARN
Thinning canopy; CHLORA advisory, investigate leaf loss.
CRIT
Sparse canopy; auto-dispatch, major leaf loss.
Recent Trend
Facility ground-area-weighted LAI, last 14 passes:
Interpretation Guidance
| LAI Band | Reading | Likely Driver | Action |
|---|---|---|---|
| 5.0–6.5 | Full closed canopy | Mature vegetative density | Watch for over-density > 7. |
| 3.5–4.9 | Healthy | Normal closed canopy | None; normal range. |
| 2.5–3.4 | Soft thinning | Young planting or mild leaf loss | Confirm stage; check senescence (VC-MET-011). |
| 1.5–2.4 | WARN thinning | Leaf loss or defoliation | Cultivation investigation. |
| < 1.5 | CRIT sparse | Major dieback or pest | Auto-dispatch; candidate for isolation. |
Related Metrics
NDVI
Saturates above LAI ≈ 4.
VC-MET-009Relative Growth Rate
Structural growth partner.
VC-MET-011Senescence Fraction
Leaf loss that lowers LAI.
VC-MET-001Canopy Vitality Index
Structural context for CVI.
VC-MET-007Sap Flow Rate
Transpiring area scales with LAI.
VC-MET-103Inner-Canopy Humidity
Over-density mildew risk driver.
Related SOPs
- SOP Library — gap-fraction PAR sensor placement & G-function assignment.
- Canopy density & pruning advisory triage — Operations SOP set in the SOP Library.
- Planting-density planning & alarm escalation — Monitoring Systems.