M.F. Skinner

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

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.

The role of forest productivity in defining the sustainability of plantation forests in New Zealand

New Zealand has signed an international agreement that commits it to report on progress towards sustainable forest management as measured by indicators grouped within seven criteria. This paper evaluates the role of forest productivity in defining the sustainability of plantation forests in New Zealand. A key factor dictating the extent of the plantation forest industry in New Zealand is profitability. Therefore, forest productivity is an important criterion of sustainability because of its important relationship with economics and profitability. Another important issue is whether plantations could be grown for an indefinite number of rotations without adversely affecting the sites capacity for biomass production. Where management practices lead to reductions in productivity, some form of amelioration is required and, as long as this is economically viable the practice is still sustainable. Forest productivity is not a good indicator of soil quality because of the confounding effects of plantation management. While improved modeling techniques may help to overcome this problem, measurement of soil- or tree-based indicators may provide a more sensitive measurement of soil quality. For a given soil type, if the effect of management practices on the soil indicator and the effect of the soil indicator on forest productivity were known, then an assessment could be made of the impact of various management practices on productivity (or other sustainability criteria). This would allow the establishment of management guidelines, constraints, and ameliorative requirements necessary to maintain or enhance soil quality.

Potential for the use of GIS and spatial analysis techniques as tools for monitoring changes in forest productivity and nutrition, a New Zealand example

A scheme for combining forest data (growth, foliar nutrition) and soil and site information in a predictive spatial system is proposed to address the following questions: Can geostatistics be used to produce maps of forest sustainability indicator variables such as tree growth or foliage and soil properties to show spatial and temporal trends? Can soil map units be used to stratify forests according to certain criteria and what are the implications of such a stratification on sampling needs? Is a GIS the logical tool for analysis and presentation of trends? We concluded that geostatistics is a valid tool for estimating and interpreting spatial trends in growth (site index) and foliar data (foliar P), but that in the study area available soil and foliage data was too sparse for confident use of this technique. Soil map units were used to stratify the study forest for growth, foliar P nutrition and soil P concentration, but at the selected map scale this only led to sampling efficiencies for the foliar N data. A modelled example of how the GIS and soil map units could be used to illustrate predicted changes in productivity and P status over space and time was presented. Soils were ranked according to their resilience to changes in P status based on initial soil P concentration, soil volume, and P fixing character. Likely changes in foliar P concentration in P. radiata stands on such sites were estimated over two rotations and changes in overall forest productivity predicted. In this study, based on existing available forest data, it became apparent that monitoring systems for site quality will not be able to depend on such data alone, and structured explicit sampling designs will be needed to address the requirements of long-term monitoring programmes. Tree-based indicators will be easier and cheaper to monitor than soil-based indicators, but a mix of both indicator types is likely in future schemes. # 1999 Elsevier Science B.V. All rights reserved.

A strategy for the correction of boron deficiency in radiata pine plantations in New Zealand

Abstract Boron deficiency, which occurs sporadically within New Zealands radiata-pine plantations, is a damaging deficiency leading to permanent stem malformation. It appears to occur most frequently in droughty coarse-textured soils in a low-rainfall environment. Young crops are the most susceptible. Early management centred on correction with soluble sodium-borate fertilisers giving very short-lasting protection. We developed a strategy based on a detailed consideration of the interaction between crop and environment, and then tested that strategy in a designed field trial. We found that coarse-ground Ulexite and Colemanite (calcium borate) were greatly superior to the soluble B fertiliser in providing a steady moderate supply of B to the trees, and were ideally formulated for aerial application.

Scaling up or scaling down: the use of foliage and soil information for optimising the phosphate nutrition of radiata pine

Fertilising with phosphate is important in New Zealand radiata pine plantations. Phosphate fertiliser trials have provided functions for determining potential growth gains from stands based on foliar P status. We used the national foliage database to determine the proportion of forest stands likely to be deficient in NZ. This gave a statistical representation but without a spatial reference. We used a nested design to test the spatial representation of foliar nutrient status and responsiveness at varied map scales. First we used the national soil map (1:10 6 scale) to show where P deficiency, and hence response, was most likely to occur. It was apparent that this representation was not acceptable for any operational scale planning. General trends in foliar status could be seen but there was no way of defining where foliage sampling should be concentrated within soil units, leading to inefficient sampling strategies. We then concentrated on the pumice, allophanic and podsol soil orders in the central North Island and used foliage data associated with 1:100,000 scale soil series map units. The spatial variation in likelihood of P deficiency and responsiveness was much clearer at this scale and more efficient foliar sampling programmes could be designed. At the most intensive scale (1:10,000) there were further gains to be made in foliage sampling efficiency and identification of responsive sites in some, but not all cases, and this depended on the soil pattern. We then tested whether spatial analysis of foliage data without reference to soil was better than using soil series as a defining class. Semivariograms constructed for foliar P concentration indicated that errors involved with predicting foliar P concentration were no less than if soil series was used as a predictor of foliar P status and responsiveness. We concluded that 1:100,000 soil series maps were adequate for general nutritional management planning and definition of areas likely to be responsive. However, for really precise work 1:10,000 maps would give additional definition for P fertilisation operations and allow us to refine fertiliser prescriptions at the within stand scale in some cases. # 2000 Elsevier Science B.V. All rights reserved.

Carbon and net nitrogen mineralisation in two forest soils amended with different concentrations of biuret

Abstract Biuret is a known contaminant of urea fertilisers that might be useful as a slow release N fertiliser for forestry. We studied carbon (C), net nitrogen (N) mineralisation and soil microbial biomass C and N dynamics in two forest soils (a sandy loam and a silt loam) during a 16-week long incubation following application of biuret (C 23.3%, N 40.8%, O 30.0% and H 4.9%) at concentrations of 0, 2, 10, 100 and 1000 mg kg−1 (oven-dried) soil to assess the potential of biuret as a slow-release N fertiliser. Lower concentrations of biuret specifically increased C mineralisation and soil microbial biomass C in the sandy loam soil, but not in the silt loam soil. A significant decrease of microbial biomass C was found in both soils at week 16 after biuret was applied at higher concentrations. C mineralisation declined with duration of incubation in both soils due to decreased C availability. Biuret at concentrations from 10 to 100 mg kg−1 soil had a significantly positive priming effect on soil organic N mineralisation in both soils. The causes for the priming effects were related to the stimulation of microbial growth and activity at an early stage of the incubation and/or the death of microbes at a later stage, which was biuret-concentration-dependent. The patterns in NH4+-N accumulation differed markedly between the two soils. Net N mineralisation and nitrification were much greater in the sandy loam soil than in the silt loam soil. However, the onset of net nitrification was earlier in the silt loam soil. Biuret might be a potential slow-release N source in the silt loam soil.