Nutrition of tropical hardwood species in plantations in the south-western Pacific

• Ministry of Agriculture, Sugar and Land Resettlement, Fiji
• Kolombangara Forest Products Ltd, Solomon Islands
• Ministry of Agriculture, Forests, Fisheries and Meteorology, Samoa

It is estimated that more than 95% of world production of sawlogs, veneer logs and cabinet timbers originates from logging of natural rainforests. However, this resource is dwindling rapidly, due to unsustainable logging practices and/or the encroachment of shifting and permanent agriculture. Harvesting of these products from plantations on previously degraded lands in the humid tropics is a viable, long-term option for ensuring their continued supply. Such an industry would also provide long-term employment opportunities, export earnings and environmental advantages for many countries in the Asia-Pacific region.
Maintenance of adequate plantation nutrition is one factor critical for successful, long-term timber production from degraded soils in the tropics. Previous research supported by ACIAR demonstrated that widespread, severe nutritional deficiencies were limiting establishment and productivity of high-value timber species in many of the soil types available for plantation forestry in northeast Australia and the southwest Pacific. This project built on those results and sought to increase understanding of aspects of the nutrition of broadleaf tropical trees pertinent to their sustainable production in plantations.

Project objectives were: to undertake a comprehensive study of nutritional deficiencies in important broadleaf species; to develop environmentally-sound and economically feasible nursery and field fertiliser strategies for forest plantings in the humid tropics; to estimate the amount of nutrient capital lost through removal of logs during harvesting operations; to develop fertiliser strategies for plantations in Fiji and the Solomon Islands; to provide relevant training to project staff in partner countries.

The project studied five species important to the region: Swietenia macrophylla (mahogany), Flindersia brayleyana (Queensland maple), Tectona grandis (teak), Gmelina arborea (white teak/ white beech) and Eucalyptus deglupta. The scientists compared nutrient requirements and developed fertilisation strategies to maintain forest nutrition, focusing on the nutrients nitrogen, phosphorus, potassium, calcium and zinc (identified previously as limiting in the major soil types on which these species are grown in the south-western Pacific).
The scientists undertook the following studies - (1) residual value in the field of fertilisers applied in the nursery; (2) fertiliser placement (in relation to requirements and ability of the young tree to capture nutrients) and type (whether to use quick-acting, soluble fertilisers or slow/controlled release) during plantation establishment in the field; (3) mid-rotational fertiliser requirements (examined in field plantings in Australia, Fiji and Solomon Islands).
The fertiliser strategies were assessed in terms of increases in plantation productivity, technical feasibility and appropriateness, and economic viability. The scientists also studied harvesting effects on long-term site productivity.

Rapid diagnostic tests for nitrate and phosphate sufficiency were tested and found suitable for use in nursery stock and field plants. Omission trials were used in the glasshouse to determine the most likely nutrient limitations to tree growth in a number of representative soils. Plant deficiency symptoms were also described. Photos were used to illustrate these deficiencies and compiled for publication in three books.
The scientists found that applications of high levels of slow-release fertilisers in the potting mix, which remain with the seedling at out-planting, resulted in more effective fertiliser uptake than with the normal broadcast application. The largest commercial plantation forestry company in Solomon Islands, Kolombangara Forestry Products Ltd, now uses the technique routinely for Tectona grandis and Eucalyptus deglupta. Other potential applications are for new Swietenia plantations in Fiji, and for other plantations in Samoa and north Queensland.
The project studied mid-rotation fertilisation for several species through three trials established in Queensland and Samoa, using foliage analysis as a means of determining nutrient deficiencies.
The scientists used project results to devise a simple, robust framework for identifying the likelihood and severity of occurrence of specific nutrient deficiencies. This framework is applicable to new sites on similar soil types/parent materials through the Asia-Pacific region.
There is little information on nutrient distributions in plantation hardwood species of major importance from anywhere in the humid tropics. The scientists sampled 22 logs from each of three species to assess nutrient distribution along and across logs and between individual trees within and between species. The results provided good estimates of log volumes and nutrient contents for E. deglupta.
The scientists established nutrient site budgets that track the amount of nutrient taken off site by logging and the inputs made during growth, to ensure that over time there is no net nutrient depletion from repeated harvesting of the timber. This process has satisfied forest stewardship authorities sufficiently to enable 'Green label' certification of the timber. This attracts a premium price on European markets.
On Kolombangara Island the researchers developed a potting medium for plantation species, using decomposed coconut shells. This medium was superior to forest soil, and seedlings grew much faster. Producing the medium has created a small industry for village women, who grate up the composted coconuts through a metal screen to produce the final product. The women are also paid to plant up the seedlings in the composted mixture and transplant them to the plantations. Because the coconut medium is much lighter than soil, the women can carry and plant more seedlings in a day, increasing their overall earnings.