Inclined Point Quadrats

Abstract

With 5 figures in the text) SUMMARY 'Relative frequency' recorded by point quadrats measures not the actual area of foliage but the area projected in the direction in which the quadrat lies. Accordingly the relative frequency varies both with the slope of the foliage and also -when inclined quadrats are used -with the inclination of the quadrat. A theoretical study reveals that variation in relative frequency resulting from difi^erences in foliage angle is greatest for vertical quadrats, is considerably reduced when (as suggested by Tinney, Aamodt and Ahlgren) quadrats are inclined at 45°, and is least when quadrat inclination is 32.5°. Accordingly the usual, vertical position for point quadrats is the worst possible one, since it results in the most erroneous estimates of percentage contribution (area basis); while with quadrats inclined at 32.5° errors are greatly reduced and are of an order acceptable in general survey work.

Review of ground-based methods to measure the distribution of biomass in forest canopies

“…It is defined as total leaf area per canopy volume (Welles and Cohen 1996). The foliage density, defined in Koike (1985) as the expected value of leaf number penetrated by a straight line within a unit distance, is identical with the relative frequency or percentage frequency in Wilson (1959Wilson ( , 1960Wilson ( , 1965 or the density of foliage in MacArthur and Horn (1969).…”

Estimating canopy structure of Douglas-fir forest stands from discrete-return LiDAR

“…In the first method, the vertical distribution of material within the canopy was measured by means of a foliage height profile (MacArthur and Horn 1969) using the camera point-quadrat method developed by Aber (1979a). This point-quadrat sampling method was first described by Warren Wilson (1960Wilson ( , 1963 who utilized a series of vertical transects from the top to the bottom of the canopy. Along each vertical transect the height of each intersecting leaf is recorded.…”

“…Less commonly used field-based methods to determine the distribution of both vertical and horizontal canopy elements include: the point-quadrat method (Warren-Wilson 1960, 1963, Aber, 1979a, which measures the interception of canopy elements in a series of vertical transects throughout the stand, and inventory-based foliar profiles estimated from measurements of crown dimensions and assuming a uniform foliage density within crown volumes (Walter and Hopkins 1990).…”

PAR extinction in shortgrass ecosystems: effects of clumping, sky conditions and soil albedo

“…Warren Wilson, 1960, Saeki, 1963, Miller, 1967, Cowan, 1968, Monteith, 1969. In most cases, a random leaf dispersion is assumed, and a Poisson model is used.…”