Light Distribution in Discontinuous Canopies: Calculation of Leaf Areas and Canopy Volumes Above Defined ‘Irradiance Contours’ for use in Productivity Modelling

Abstract

Plant canopies can be considered as assemblages of leaves, stems and fruits growing in zones of differing irradiance demarcated by contours of mean irradiance as measured on a horizontal surface. The following general equations have been derived to calculate the leaf area (L_I) and the canopy volume (CV_I) in zones external to any chosen contour of mean irradiance: (1) L_I = ((1nl)/(−K)(I−T_f) or leaf area index (LAI) if this is less (2) CV_I = L_I/(leaf area density m² m⁻²), where I is the specified value of irradiance (horizontal surface) expressed as a decimal fraction of that above the canopy, K is the appropriate extinction coefficient and T_f is the proportion of the total of available radiation which, if the canopy is discontinuous, would reach the ground by passing through gaps between the discrete canopy units. Where the canopy is continuous T_f is zero so expression (1) simplifies to L₁ = 1n I/−K (or LAI if this is less). For a range of model hedgerow orchards of varying dimensions, spacings and LAIs, it has been shown that the use of these equations gives very similar results to those obtained by detailed calculation of light penetration. They therefore seem to be of potential use in calculating both potential dry-matter production by discontinuous canopies of any type and, in the case of orchard fruit crops, the potential effect of changes in tree size, leaf area density, spacing etc. on the canopy volume in which irradiation is adequate for fruit bud initiation and fruit colour development.