Soil-landscape modelling and soil property variability for forestry land evaluation in Longwood Forest, Southland. Phase 1: soil-landscape model development

Large scale, quantitative information about the variability of target soil properties is required for forest management. This project is attempting to determine whether or not the New Zealand Soil Classification system (NZSC), when used in combination with a soillandscape model, adequately communicates this information. In the first phase of this project a soil-landscape model was developed and a pilot variability study conducted. The soils in the study area, located in the W oodlaw Block of the Longwood Range, are formed from either Permian andesite or greywacke on moderately steep to steep hill slopes under a moist cool climate and a vegetation cover of beech and podocarp forests. The soil-landscape model was developed using the land systems approach. The model consists of predictive relationships between topographic features and soil classes. There is a clear relationship between slope steepness, the abundance of surface boulders and the gravel content of the soil. A soil-landscape unit map showing the distribution of predicted soil classes has been produced. The results of the pilot variability study have showed that the soils sampled are acidic and have moderate to high P-retention values. An analysis of variance indicated that both of these properties are significantly variable between sites and between horizons. There appears to be a relationship between land component type and the magnitude and variability of these properties. The clay mineralogical analysis revealed that the dominant clay minerals present in all the soils sampled are chlorite-vermiculite, kaolinite, sepiolite, and allophane. The presence of allophane and kaolinite may be related to the moderate to high P-retention values

A soil-landscape model for southern Mahurangi Forest, Northland

Exotic plantation forestry has a productive area of about 75 000 ha in Northland (L. Cannon, personal communication). Forestry is thus an important land use of both economic and environmental significance in Northland as well as elsewhere in New Zealand. Therefore, it is of considerable importance that forestlands be managed sustainably by employing approaches such as site-specific management. The establishment of site-specific forest management practices requires information regarding the distribution of key soil properties (Turvey and Poutsma, 1980). Quantitative modelling to predict key soil properties of sustainable forestry from observable landscape features may be a cost-effective approach to mapping forestlands. We are investigating the efficacy of such an approach within Mahurangi Forest, Northland

A soil-landscape model for Mahurangi Forest, Northland, New Zealand

Exotic plantation forestry is an important land use of both economic and environmental significance in Northland and elsewhere in New Zealand. It is therefore of considerable importance that forestlands be managed sustainably by employing approaches such as site-specific management. The establishment of site-specific forest management practices requires information regarding the distribution of key soil properties (Turvey and Poutsma, 1980). Quantitative modelling to predict key soil properties from landscape features may be an effective approach to mapping forestlands. A study investigating the efficacy of such an approach is being conducted within Mahurangi Forest, Northland, New Zealand. As a pilot to the study, a detailed qualitative soil-landscape model was developed in order to gain a greater understanding of the soil-landscape relationships and soil pattern of the area. The qualitative soil-landscape model developed in the pilot study is presented here

Soil and foliar phosphorus as indicators of sustainability for Pinus radiata plantation forestry in New Zealand

We investigated how multiple-crop forestry has influenced the magnitude and variability of soil and plant phosphorus (P) fertility and site disturbance. Kinleith Forest, on Mamaku Plateau, covers >100,000 ha and comprises mainly plantation Pinus radiata. Three study areas in the forest were chosen to represent natural state (native forest), first crop of P. radiata (24 years growth), and second crop of P. radiata (4 years growth of second crop). The adjacent areas have similar relief and climate, and the soils are all the same age, being predominantly Andic Haplohumods developed in 1770 calendar-year-old non-welded tephra (Taupo Ignimbrite, ca. 0.5–0.8 m in thickness) and overlying a buried paleosol on earlier tephric material. Soil properties were compared using a random geometric sampling scheme stratified in a 40-m grid. Soil samples (0–20 cm) were taken at 1.5, 4.5 and 13 m spatial intervals in random directions away from each primary node, providing 192 sample sites for each study area. Additionally at selected sites, samples of the current year's foliage from the upper crowns were collected, the thickness of Taupo Ignimbrite (i.e. depth to buried paleosol) was recorded by augering, and site disturbance was assessed using a new six-point scale based on change relative to a modal soil profile. Geostatistics and geographical information systems (GIS) were used to assess variability and effects of forest management on the measured properties. Soil Bray-2 P concentrations were below guidelines for satisfactory growth (12 mg kg⁻¹) at all sites, and no differences were recorded between the different management areas. However, the amount of within-site variability in Bray-2 P increased with the number of crops. Foliar P concentrations were only marginally deficient in both the first and second crops, indicating that P is currently not significantly limiting growth. The lack of difference in foliar P between first and second crops indicates no crop-to-crop decline in foliar P status and suggests that no site P fertility decline has occurred. The soils have an unusual ability to continue releasing P through successive sequential extractions in the Bray-2 P test, indicating a strong buffering capacity, and this may explain the apparent lack of deficiency even with Bray-2 P values of <12 mg kg⁻¹. The site disturbance index increased and the spatial distribution of P data became increasingly variable with crop rotation. GIS, inverse-distance weighting and kriging proved useful in illustrating the trends between crops. The spatial variability of results indicated that there was no obvious pattern to the variability and that more site-specific forest management in the region would be difficult. However, there was some evidence that less disturbance during harvesting may minimise variability of soil P supply