Assessing the geometric structure of a white clover (Trifolium repens L.) canopy using 3-D digitising

A method was developed for assessing the three dimensional (3-D) geometric structure of white clover canopies. 3-D co-ordinates of pre-defined points on leaves, petioles and stolons were measured using a Polhemus Fastrak elec tromagnetic 3-D digitiser. Digitising progressed downwards from the top of the canopy and plant parts were removed after they have been digitised. Lea flets were treated as four quarter-ellipses, and petiole and stolons were t reated as cylinders. Leaf dimensions and areas calculated from 3-D co-ordin ates were within about 5% and 20% of direct measurements made with a ruler and a planimeter, respectively. Special software and freeware POV-Ray were used to reconstruct a virtual canopy from digitiser records and to calculat e canopy characteristics such as leaf area index (LAI), petiole intersectio n area, and profiles of leaflet areas and inclinations with height. It took between 3 and 7 h to digitise 10 x 10 cm stands of clover and the resultin g information was considerably more comprehensive and accurate than could h ave been obtained by the alternative 'point quadrat' or 'stratified clippin g' methods. (C) 2000 Annals of Botany Company.