Mixed species plantations of high-value trees for timber production and enhanced community services in Vietnam and Australia

• Forest Research Centre, Vietnam
• Queensland Department of Primary Industries and Fisheries, Australia

Forest cover in Vietnam declined from 44 per cent in the early 1940s to 24 per cent by 1983. Similar declines have occurred in most other tropical countries, including Australia. In Vietnam, the decrease in the supply of goods and services from forests has had a profound effect on rural communities and the nation as a whole. Supplies of building timbers and fuelwood have declined along with the availability of many non-timber forest products such as herbs and medicinal plants. Erosion has increased, causing siltation of rivers, irrigation canals and dams. There has also been a decline in soil fertility in many areas. Inevitably, forest fragmentation and loss has diminished the country's overall biological diversity. Reforestation is therefore a high priority, and the Government is now preparing the Five Million Hectare Reforestation Program (5MHRP), which aims to lift the forest cover of Vietnam back to 43 per cent over 10 years. This reforestation will more likely be carried out by smallholders rather than government agencies.
Much of the previous reforestation effort in Vietnam used exotics such as Eucalyptus and Acacia, which are fast-growing species that tolerate poorer soils. While these will continue to have an important role, there is also considerable interest in using more indigenous and higher value species. The habitat preferences and silvicultural requirements are known for only a few of these species. This represents a serious impediment to the 5MHRP, because if species are planted at inappropriate sites the reforestation effort may fail.
A further difficulty is that many indigenous species are now being planted in ad hoc mixed-species plantations. Mixed plantations can offer some significant advantages, but only if the species concerned are complementary. Random assortments are unlikely to provide advantages and are more likely to reduce productivity or even fail. Research is therefore needed to define the attributes of complementary species and to identify those situations where mixtures can be advantageous (or where they should not be used).

The project aimed to develop silvicultural techniques so that greater use could be made of indigenous and other high-value tropical tree species in reforestation projects in Vietnam and Australia.

The project involved five main areas of study. The first was a socioeconomic survey of existing high-value timber tree planting by farmers. This survey also explored the financial profitability of various species and plantation designs.
The project scientists examined of the performance of various high-value species previously planted at sites across the northern highlands of Vietnam, and evaluated their particular site preferences. Other activities included an examination of the competitive abilities and tolerances of various species in older plantations, with the intention of helping to formulate guidelines for identifying species that can form complementary plantation mixtures.
The scientists also undertook a comparison of the performance of species in young plantation monocultures and mixtures, to determine the benefits or disadvantages of these alternative plantation designs for particular species and to suggest ways of identifying complementary species.
In Australia, work concentrated on more detailed studies of existing older mixed-species experiments in Queensland. Results from these studies were used to help predict outcomes in similar experiments in Vietnam and to interpret field data from studies in Vietnam.

Surveys of existing tree plantations established by farmers in three northern Vietnamese provinces found they grew more than 64 difference species. These include both native and exotic species being grown on short and long rotations, and most of these trees are less than15 years old (established following the period of doi moi or de-collectivisation in the late 1980s). Estimates were made of future productivity using various growth projections found that most indigenous species were less productive than some of the more productive eucalypts or acacia species used in Vietnam. But it may be possible to significantly increase these growth rates because a number of soil nutrient deficiencies exist across the region, most especially phosphorus. Trials suggest all trees (native and exotic) are also inhibited by the acid pH values of the soils being used for plantation establishment.
Most of 90 small rural sawmills surveyed reported difficulty in finding sawlogs, and revealed that prices of logs had increased substantially over the previous three years. There are substantial differences in the prices of logs of different species with most of the native species attracting substantially higher prices than the faster-growing exotic species. It was clear from the farmer survey that few farmers were aware of these prices and the market opportunities they represent.
The project team developed a list of priority species for further silvicultural research, based on farmers' preferences, species (current) growth rates and the market prices provided by rural sawmillers. Attempts to define site preferences for these species were inconclusive, in part because of insufficient knowledge of soils and climatic conditions across the region and also because of the patchy distribution of plantations of these species.
The team developed 10 different silvicultural systems to incorporate these species in plantations that might suit a range of different economic and ecological situations. These plantations involved fast- (e.g. Acacia, Eucalyptus) and slow-growing species as well as timber and multi-purpose species. Some were monocultures and others were mixtures. Financial analyses of each of these model systems, using data on timber prices from the sawmill survey and estimates of the productivity of different species from the plantation survey, determined that the highest net present values and rates of return arose from the fast-growing exotic species in successive short rotations.
Seven experimental plantings were established at four sites to test some of the principles upon which the 10 model silvicultural systems were based. These plantings were at three sites in the north and one site in the central highlands (Pleiku). A fifth site at an existing mixed species plantation on the central coast south of Hue was also used. These plantings explored species-site relationships using various species mixtures, and examined the differential competitive abilities of some of the key plantation species. Though still comparatively young, they have already provided evidence of the performance of different species under carefully prescribed and well-managed conditions (including which species not to plant at certain sites).
None of the mixtures showed greater (or lesser) productivity than the monocultures. However, there is evidence of a financially attractive mixture involving a short rotation of Eucalyptus urophylla grown with a longer rotation of Michelia mediocris. The same mixture appears to also improve the stem form of the Michelia. The fifth study site on the central coast investigated the performance of native trees established with the assistance of an Acacia overstorey or nurse tree. Studies showed that the Acacia was initially needed to facilitate the establishment of several native species, but growth was inhibited when the Acacia overstorey lasted beyond seven years.
Four years is too short to resolve all the scientific questions involved in developing financially profitable silvicultural techniques for these species or working out how these might be incorporated into a farm business plan. However, significant progress was made and the work already has a number of practical implications. These include:
Not all native species are equally useful.
Many farmers are unaware of current log prices.
There are a variety of silvicultural systems including multi-species mixes by which trees of different species can be grown in plantations.
There is a need to make more widely known the existing silvicultural knowledge (from this project and other sources) - by using existing field experiments and relevant farm plantings for field days to demonstrate the performance of different species, and by fostering learning networks among key farmers who are especially interested in tree growing.
There should be more demonstration plantings across the region to test survival and growth in different field situations.
More work is needed to extend the new knowledge on the actual productivity of various native species .
Soils at most of the sites available for reforestation in the region are very infertile.
Farmers are planting many native trees that will not maximise the returns on this investment unless they are better managed - further research is needed to develop prescriptions for individual species.
There is a need to explore the genetic variation in the key species.
Better information is needed about market chains - based on more accurate financial models and developed especially for growers living in more isolated areas and unable to sell to industrial users.
Small growers will need assistance when selling their trees if they are to obtain optimal prices.
Reforestation in the recent past is transforming the Vietnamese landscape, but more research is needed to determine how different types of reforestation affect ecological processes and functions.