HERBAGE YIELD IN AGROFORESTRY SYSTEMS AS A FUNCTION OF EASILY MEASURED ATTRIBUTES OF THE TREE CANOPY

Data from an experiment that measured rates of herbage yield below dif ferent tree canopies have been used to investigate the relationship be tween annual herbage growth and readily estimated parameters of the ca nopy structure of evergreen conifers. Canopy structure data were colle cted by destructive sampling of trees removed from the experimental ar ea immediately prior to its establishment, and from subsequent measure ments on growing trees during the following 2 years. In New Zealand, i t was found that relative pasture yield (the ratio of growth below a c anopy to open pasture growth) was linearly related to green crown leng th in the case of Pinus radiata canopies. More recently, a family of r elationships between relative pasture yield and green crown length, in dexed by the mean height of trees forming the canopy has been derived. For our data, the regression of annual herbage yield on green crown l ength is good (R2 = 92.5%) but shows systematic variation in the resid uals. Hence, other related explanatory variables were investigated to see if any gave a more satisfactory fit to the data. By assuming that individual trees have a canopy in the shape of a regular cone, the are as of the projections of the tree canopies at different inclinations t o the vertical were calculated. If it is further assumed that the cano pies are of even density, the projections can be used to estimate the proportion of direct light which is incident on the herbage under the trees as a function of the angle of the sun above the horizon. A regre ssion using both vertical and horizontal projections in varying propor tions indicated that the best fit to the data was obtained using the h orizontal projection alone (R2 = 94.0%). These results indicate: first , that the horizontal projection of the crown gives a good, simple pre diction of annual herbage yield; and secondly, that the horizontal com ponent of incident light is most important for herbage growth at Scott ish latitudes.