Nutrition and productivity of Pinus radiata following harvest disturbance and fertilization of coastal sand dunes

Abstract

The effects of harvesting intensity and urea fertilizer on the nutrition and above- and below-ground biomass of a second-rotation Pinus radiata stand were studied for 5 years in a sand dune forest in New Zealand. Treatments were: (1) whole-tree harvest and forest floor removal; (2) whole-tree harvest; (3) stem-only harvest; (4) stem-only harvest plus extra slash. Initial site nitrogen reserves in these treatments ranged from 869 to 1677 kg N ha −1. Nitrogen availability was increased by residue retention, but by the age of 5 years, all unfertilized stands were considered nitrogen deficient. Residue removals were associated with reduced foliar boron concentrations. Foliar concentrations of boron and phosphorus were lower in urea-fertilized trees owing to biomass dilution. Additions of 200 kg urea-N per ha-year aggravated the shortages of boron and phosphorus in stands with the forest floor removed, and resulted in reduced uptake of these nutrients. Forest floor removal reduced stand dry matter production by 24%, but retention of single or double quantities of slash have not increased productivity in the first 5 years above that observed after whole-tree harvesting. Urea additions were adequate to amend reductions in site productivity caused by forest floor removal. Neither harvesting treatments nor urea fertilization altered a basic branch mass to branch diameter ratio, or the ratio of crown mass to the stem diameter at the base of the live crown. However, fertilizer increased the proportion of dry matter allocated to branch biomass, and reduced the proportion allocated to stems. Fertilizer increased standing crops of foliage and fine roots, but did not alter the shoot to fine-root ratio.