ANALYSIS OF THE SPATIAL DISTRIBUTION OF FOLIAGE BY TWO‐DIMENSIONAL POINT QUADRATS

Summary ‘Relative frequency’, yielded by point quadrat analysis of vegetation, is a measure of the area of the foliage in vertical projection. Because this projected area varies according to the foliage angle (i.e. the angle between foliage and horizontal), values of relative frequency fluctuate as attitudes change with environmental variation; also, misleading estimates are obtained when comparisons are made between species of different habit. By using horizontal as well as vertical point quadrats it is possible to estimate the foliage angle and the foliage denseness (i.e. total area of foliage per unit volume of space). If the frequencies of contacts with foliage per unit length of point quadrat are F90 and F0 for vertical and horizontal quadrats respectively, foliage angle (α) is given by tan α/2(F0/F90) and foliage denseness (F) by F=F90 sec α. The vegetation is analysed within horizontal layers. An apparatus is described which can give the required data, and which avoids the errors inherent in point quadrat methods that use needles of appreciable thickness. The total area of foliage per unit area of ground can be obtained by summation of F for all layers. Trials show that the method is practicable in the field, and that it gives values of α and F which reflect the character of the vegetation. Values of F obtained by this method agreed with direct estimates for the same vegetation within 2.5%. Some assumptions made by the method are discussed; provided that certain precautions are taken, the errors resulting from these are small or none.