The Role of Acacia Hybrids in the Reforestation Program in Vietnam
Prof. Dr Le Dinh Kha
Research Centre for Forest Tree Improvement
Forest Science Institute of Vietnam
Tuliem, Hanoi, Vietnam
Tel: +84 4 8347813
Fax: +84 4 8362280
Email: rcfti@netnam.org.vn


The forest cover in Vietnam has decreased from 43% in 1945 to 29% at present. Wars and over exploitation of natural forests have both contributed to the reduction. In 1998 the Vietnamese government embarked on a national reforestation program. Under this program 2 million ha of production forest and 1 million ha of protection forest will be established over the period 1999 - 2010.

Selection and breeding of new tree species with high productivity and adaptability will make a significant contribution to the implementation of the national planting program. Hybrids of Acacia mangium and A. auriculiformis are among many species selected for reforestation work. The occurrence of these hybrids in Vietnam and their important features are described.


A 5-month old acacia hybrid plantation in Ha Tay province, Vietnam

Occurrence of acacia hybrids in Vietnam
In Vietnam, natural hybrids of A. mangium and A. auriculiformis have been detected in A. mangium plantations derived from seed collected in species trials at Ba Vi (Ha Tay province), Trang Bom (Dong Nai province) and Tan Tao (Ho Chi Minh City). The hybrids at Ba Vi were a result of natural crosses between Daintree provenance (Queensland Australia) of A. mangium and Darwin provenance (Northern Territory, Australia) of A. auriculiformis. The hybrids at Trang Bom were of Mossman provenance (Queensland) of A. mangium, but their male parents came from an unknown provenance of A. auriculiformis.

Important characteristics
Observation conducted by the Research Centre for Forest Tree Improvement of the Forest Science Institute of Vietnam in cooperation with other research units has shown that there are 2-3 hybrid trees in each hectare of A. mangium plantations. The hybrid trees grow fast and their crowns emerge over the canopy of A. mangium plantations. Stem volume of acacia hybrids is 2-3 times greater than that of A. mangium and 3-4 times greater than that of A. auriculiformis of the same age. Typical features of phyllode morphology, number of veins, colour of flowers, shape and size of pods and seeds as well as bark colour of the hybrids are intermediate between their parental species. The wood density of acacia hybrids is also intermediate between the two parents although it is higher in some clones than their parents. Physical and mechanical properties such as shrinkage, moisture absorption, static bending strength, bending and rupture strength, slide and split resistance of the wood of the hybrids are of intermediate nature between the parents. The root system of the hybrids have also been found to be deeper and therefore they are rarely blown down by strong wind.

A very important feature of acacia hybrids is the higher pulping efficiency and especially, the paper produced has better mechanical strength, such as pulling and folding strength being markedly greater than the paper produced from A. mangium or A. auriculiformis wood. The folding resistance after and before bleaching of paper produced from wood of acacia hybrids is 790 and 1300 times while that from A. mangium is 305 and 440 times, and A. auriculiformis 417 and 820 times. The cellulose content of acacia hybrid wood is also markedly higher than that of Eucalyptus urophylla, E. camaldulensis, and some native tree species such as Styrax tonkinensis and Manglietia glauca. These properties make the wood of acacia hybrids very suitable for paper production.

Acacia hybrids have the number and weight of Rhizobium nodules 2-4 times higher than its parental species. This will improve the capability of acacia hybrids in soil improvement. It is due to this strong point combined with high productivity that acacia hybrids are now preferred by farmers for tree planting in Vietnam.

Research on indices of evaporation intensity, osmotic pressure of leaf cell fluid and wilting point of soil moisture of 3-month old saplings showed that some selected acacia hybrid clones were more drought tolerant than the two parent species.

Clonal test of acacia hybrids
The occurrence of acacia hybrids is from natural hybridisation between A. mangium and A. auriculiformis trees that differ markedly in growth and stem form. That is why the trees of F1 generation, although have some similar basic features, are very different in growth and stem form. Thus, it is essential to embark on a selection program that will be followed by clonal tests. From selection of many hybrid trees of best phenotypes the Research Centre for Forest Tree Improvement has succeeded in developing 20 clones and after 5 years of clonal test, has selected 6 best clones. These clones have stem volume twice that of the poorest clones and have the best stem quality indices such as strong apical growth, straight bole and light branches.

Some of the 6 best clones have very high rate of pulping efficiency at alkali ratio of high rates 20% and 22% (the rates widely used in paper mills in Vietnam). Folding and pulling strengths of the paper produced from the wood of selected hybrid clones are all significantly higher than that of A. mangium and A. auriculiformis.

Need for vegetative propagation of acacia hybrids
Seed collected from the hybrid trees of F1 generation has been used to raise seedling for forest plantations. Trees in plantation established from these seedlings are very variable in growth and form. This means that using seed of F1 trees for establishment of plantation will result in a marked segregation and regression. Therefore for plantation forestry of acacia hybrids, it is necessary to employ vegetative propagation methods by cuttings or meristem tissue culture must be used. This is a propagation based on mitosis in which there is no change in the number and structure the chromosomes in the process of cell divisions; the new cells remain the exact genetic quality of the original cells, and the ramet remains the exact quality of the ortet.

The Research Centre for Forest Tree Improvement has organised training courses in acacia hybrid cutting propagation for forestry staff from all provinces. More than one million of acacia hybrid cuttings are being produced by various production units in Vietnam every year. Total area of acacia hybrid plantations has already reached several thousand hectares. The current annual planting rate of the hybrids is estimated to be 500-700 ha. In addition, hundreds of thousands of scattered hybrid trees are planted around farms, homesteads, road and villages