Improving reforestation success of high-value and key forest species by direct seeding in Southeast Asia and Western Australia

Thea So

Worldwide, natural forests have been decreasing in area at an alarming rate. In Cambodia, the annual deforestation rate was 127 000 ha year-1 from 2005 to 2010 and this seriously threatens biodiversity and the livelihoods of rural communities. Therefore, there is an urgent need to reforest or establish forest plantations to meet two main objectives: economic development and biodiversity conservation. This thesis concerns the promotion of early survival and growth of planted or direct-seeded seedlings to overcome the harsh conditions of reforestation sites in tropical and mediterranean-type ecosystems, with special attention given to threatened high-value timber species of Southeast Asia. Worldwide, there is an increasing effort to reforest degraded forests and old agricultural lands. However, reforestation of degraded lands is often difficult and is usually expensive; therefore, direct seeding is an alternative to conventional tree planting. Some of the major constraints facing reforestation efforts were reviewed, and opportunities that could be useful for promotion of early establishment and growth of seedlings were explored. These were then used to define reforestation field studies in Australia, Thailand and Cambodia. In a harsh mediterranean-type ecosystem in Western Australia, effects of microorganisms (mycorrhizal fungi and N2-fixing bacteria) and planting material (seed and seedling) on survival and growth of two key post-disturbance colonizing species, Eucalyptus gomphocephala and Acacia saligna, were investigated. For E. gomphocephala, survival at 13 months was higher for out-planted seedlings (81%) than from direct seeding (7.5%). Inoculation with ectomycorrhizal fungal spores was not beneficial. For A. saligna, survival at 13 months was also higher for seedlings (84%) than for seeding (42.5%). Nitrogen-fixing bacteria from crushed root nodules of A. saligna did not promote survival or growth of the species nor did a mixed commercial mycorrhizal inoculum. In Southeast Asia, the leguminaceous rosewoods, Afzelia xylocarpa and Dalbergia cochinchinensis, are threatened throughout their range by habitat loss and over exploitation for their extremely highly-prized timber. The species have been promoted for reforestation in Cambodia for economic development and genetic conservation. The current conservation status of A. xylocarpa and D. cochinchinensis in Cambodia was examined, and information on silviculture, trade and current conservation measures applied in that country was drawn together. Some important steps in the development of domestication strategies, including testing and improving silvicultural practices and increasing the supply of genetically superior seeds from seed production areas and seed orchards, were outlined. This information was then used to help select species for the trials in Thailand and Cambodia. Many high-value timber species of continental Southeast Asia, including some rosewoods, have been promoted in reforestation programmes. However, the slowgrowing habit at the early stage of development is a challenge for promoting these species in tree plantings. Therefore, effects of beneficial microorganisms and fertilizer on establishment and growth of direct-seeded seedlings of D. cochinchinensis and Xylia xylocarpa were investigated in a trial on former agroforestry land in Thailand and compared to Acacia mangium, an exotic fast-growing plantation species. After 20 months, a mixed inoculum of arbuscular mycorrhizal (AM) + ectomycorrhizal (ECM) fungi and a mixed inoculum of AM fungi + N2-fixing bacteria (crushed root nodule) improved survival of D. cochinchinensis by 15 and 17%, respectively. The co-inoculation of AM with ECM also improved diameter growth of the same species by 43%. A second field trial explored the effects a water retention polymer and fertilizer on direct seeding of A. mangium, Afzelia xylocarpa, D. cochinchinensis, Eucalyptus camaldulensis, Sindora cochinchinensis and X. xylocarpa. The combination of polymer and fertilizer increased height growth of all six tree species by 40%. The effect of the polymer and fertilizer was further investigated in Cambodia with direct seeding of the same indigenous species. There, the combination of the polymer with fertilizer increased seedling establishment only by 7%. As selected strains of compatible N2-fixing bacteria were not available, crushed root nodules were used in some of the field trials. In order to improve the technology in the future, three strains of N2-fixing bacteria were isolated from root nodules of D. cochinchinensis grown in Cambodia and then tested under glasshouse conditions and a seed coating technique was employed as a means to deliver bacteria to seeds along with broth culture. After 16 weeks, one of the three isolates increased total dry weight of D. cochinchinensis seedlings by ca. 150% over the uninoculated control that was not fed inorganic nitrogen. The effective strain was identified as Bradyrhizobium elkanii after the DNA was amplified by polymerase chain reaction using RPO1 primer (5'-AAT TTT CAA GCG TCG TGC CA-3') and then partial 16S rRNA nucleotide gene sequences were compared with the Gene Bank database. Both methods of delivering of bacteria (seed coating and broth culture) were equally effective. The effectiveness of B. elkanii was explored under field conditions in Cambodia with direct seeding of D. cochinchinensis, but no effect on seedling establishment or growth was obtained after six months of trial establishment. Competition from indigenous bacteria was suggested as one of the reasons for the ineffectiveness of the introduced strain. The main finding of this thesis was the suitability of the four high-value timber species in reforestation by direct seeding in tropical regions, in former agricultural land where proper site preparation and intensive weeding were provided, as well as on land previously under degraded forest where minimal site preparation was undertaken. Also, the application of a water retention polymer promoted establishment and growth of seedlings under tropical conditions. This should be evaluated further in a wider range of reforestation sites including sandy soils. More work should be undertaken to identify effective symbionts for the tropical rosewoods. These and other symbionts should be evaluated in reforestation trials in which more attention is paid to site characteristics and populations of indigenous beneficial organisms.

Improving reforestation success of high-value and key forest species by direct seeding in Southeast Asia and Western Australia

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