The zone of influence of paddock trees and the consequent loss in volume growth in young Eucalyptus dunnii plantations

Abstract

Old paddock trees, retained in young eucalypt plantations for biological conservation, can be assumed to have a circular zone of influence within which the growth of the young trees is reduced. We used a set of non-linear simultaneous equations to model the radius of the zone of influence of old paddock trees and the consequent loss in stand volume growth of young trees in 4-year old Eucalyptus dunnii plantations. For paddock trees with crown radii of 5, 10 and 15 m and a suppression intensity of 0.5, the predicted radius of the zone of influence was 9.04, 23.8 and 42.0 m, and the predicted loss in stand volume growth was 1.3, 8.7 and 27.0% per hectare, respectively. The reduction in volume growth is expected to increase with time because the discrepancies in size between trees in and outside the zone of influence will increase as stands grow and competition for resources intensifies. The reduction in merchantable stand volume is expected to be even greater since some trees within the zone of influence are not expected to reach merchantable size by the end of the rotation. To minimise the loss of productivity while maintaining biodiversity, other alternatives such as offsetting removal of paddock trees by planting more trees in designated retention areas may need to be formulated.