Response of radiata pine forests to residue management and fertilisation across a fertility gradient in New Zealand

Abstract

New Zealand law requires sustainable forest management. International definitions of sustainability typically include maintaining the productive capacity of forest soils, and decision tools are needed for managing harvesting and site preparation practices. Trials were established between 1986 and 1994 in different physiographic regions of New Zealand to determine the effects of management on the second-rotation performance of Pinus radiata. Main harvesting and residue treatments at three North Island sites include: whole-tree harvest plus forest floor removal, whole-tree harvest, and stem-only harvest retaining a single layer of slash. These treatments were installed in a random block split-plot design (with and without fertiliser). Additional treatments were added at specific sites (e.g., double slash addition at Woodhill; topsoil removal at Tarawera) to test hypotheses that harvesting intensity is negatively correlated with second-rotation growth, that inadequate nutrition is the reason for the negative correlation, and that fertiliser additions can compensate for the negative impact of harvesting on nutrition. Stands were thinned-to-waste to reduce stocking by 50% at Woodhill (age 7 years), Tarawera and Kinleith (age 5 years). On recent coastal sand dunes at Woodhill, foliar analyses suggest that nutrient availability to trees of N, P, K, B, Mn, and Zn were reduced during critical periods of stand establishment by forest floor removal, and improved by retention of thinning slash. Diameter at 5 years declined as forest floor C:N ratio increased. Diameter after 11 years was reduced by forest floor removal, and positively related with residue retention. On approximately 100-year-old basaltic tephra gravel at Tarawera, foliar concentrations of K and B were positively related with slash and soil nutrient retention, however, the effect of harvest removals on tree nutrition and growth was substantially less than observed on recent sand dunes due to greater native fertility of basaltic gravel. Forest floor C:N ratio at Tarawera explained less variation in tree growth than at Woodhill. On the N-rich Taupo sandy loam of Kinleith Forest, there was no correlation between forest floor C:N ratio and tree diameter to age 5 years. Fertiliser additions have increased tree diameter at all sites, and can ameliorate harvesting-related reductions in growth. However, fertiliser additions caused significant reductions in foliar concentrations of most nutrients studied except N. These trials have been useful for identifying the benefits of residue retention among sites with varying levels of native fertility. Based on the results of this study, we recommend organic matter retention during harvest and site preparation on N-deficient sites such as Pinaki Typic Sandy Recent Soil.