Neighbourhood interference amongst Pinus rigida individuals

(1) Recent yearly bole growth of individual trees, as estimated from height and annual growth ring measurements, is considered as a function of the number, distance and size of neighbours in a young Pinus rigida stand in New Jersey. (2) To measure the annual increase in tree bole volume, an allometric model of tree bole growth was developed. In the model, the cross-sectional area of annual growth rings is constant along the length of the bole, which is constructed as concentric ellipsoids. A complete ring profile of one individual tree is consistent with this model. (3) Significant correlations between individual plant growth rate and several measures of local interference demonstrate that interference is occurring. (4) The size of neighbours, estimated from height and girth measurements, was the most important single variable in the regressions on individual plant growth; the number and distance of neighbours was significant but of less importance. The angular dispersion of neighbours within 2 m did not make a significant contribution to the variation in individual tree growth. (5) The results are consistent with a model in which the growth of an individual is inversely related to the total effect of interference, and the contribution of each neighbour to this effect is additive in proportion to its size and inversely proportional to the square of its distance. (6) While the results show, as expected, that the effect of a neighbour decreases with its distance, they do not allow one to distinguish between alternative formulations with confidence. However, a modified version of the model in which the effect of a neighbour decreases with its distance always resulted in a slightly improved fit over the original formulation in which a neighbour's effect decreases with the square of its distance.