Some tropical N 2 -fixing trees exhibit specific characteristics for phosphorus (P) acquisition and utilisation that contrast with the large nitrogen (N) fluxes in their litterfall. To investigate differences in N and P cycling in N 2 -fixing plantations, litterfall and fresh leaf quality of a N 2 -fixing Acacia mangium plantation were compared with that of a non-N 2 -fixing Swietenia macrophylla plantation and a coniferous Araucaria cunninghamii plantation. The N concentration in the A. mangium litterfall was higher than that in the litterfall of the two other species, whereas the P concentration in the A. mangium leaf litterfall was 0.16 mg g -1 , which was only 12-22% of that of the other species. The P concentration in the reproductive parts of A. mangium was markedly higher (16.1 mg g -1 ) than those in the other fractions. The N:P ratio was higher in the leaf fall (81) compared to the fresh leaves (29) of A. mangium, in contrast to the N:P ratios in the leaf samples of the other two species. An analysis of a global litterfall dataset of tropical plantations indicated that N:P ratios in litterfall were significantly higher in N 2 -fixers than in non-N 2 -fixers, and those of A. mangium were high among species in the N 2 -fixer group. These results indicated that A. mangium efficiently retranslocated P in contrast to very large N cycling, under field conditions. These differences may be related to other physiological characteristics of A. mangium.

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To investigate soil amelioration effects by older tropical fast-wood plantations, we examined the fine litterfall and accompanying nutrient flux of a 20-year-old Acacia mangium site over 3 years under a wet tropical climate in Sabah, Malaysia. The litterfall of a Swietenia macrophylla site and an Araucaria cunninghamii site was also examined for comparison. Annual nitrogen (N) flux through litterfall (kg N ha -1 ) was larger in A. mangium (207-223) than in S. macrophylla (126-153) or A. cunninghamii (72-94), whereas annual phosphorus (P) flux through litterfall (kg P ha -1 ) was considerably smaller in A. mangium (2.7-3.4) than in S. macrophylla (7.5-15.6) or A. cunninghamii (7.8-9.2). N flux through litterfall, forest floor N, and N concentration in topsoil (0-5 cm) were in the order of A. mangium [ S. macrophylla [ A. cunninghamii, but other element fluxes were not related to concentrations in soils. Our findings suggest that topsoil N increased because of a large N flux from litterfall. We conclude that these plantation trees, including A. mangium have the potential to produce a N flux in litterfall for the rapid return of organic N to soils larger than or equivalent to that in adjacent primary forests. However, the litterfall of a single species may lead to deficits of a particular element and cause nutrient imbalances. Using a mixture of fertilizer tree species or applying mixed litter might be a better solution.

Plantations of single tree species which are usually set up to reafforest cleared lands in the tropics, have often been criticized for being associated with a low level of diversity in the ecosystems. However, our study of understorey plant species in various plantations of single exotic or indigenous species has shown that more plant species grew within stands near a natural forest than within grasslands in Sakaerat, northeastern Thailand. These forests contribute to the acceleration of the secondary succession from grassland, which before reafforestation was achieved by recurrent fires. In the natural regeneration of understorey tree species in planted forests, large tree species in the natural forest seem to be the major common seed source among the stands studied. The small leaf biomass in these planted forests generated enough shade to prevent the invasion of 2 fire-adapted competitive grasses and to provide a suitable light environment for seedling establishment at an early stage of forest development. These factors facilitated the establishment of other species within the plantations of single tree species. Faster-growing exotic species accumulated larger amounts of understorey biomass than indigenous species. The most suitable species was Acacia mangium. The association of forests planted with single tree species with a high species richness in the ecosystems may enable to achieve a high biological diversity at the landscape level, which could bring various benefits to the forests of Sakaerat.Discipline: Forestry and forest products Additional key words: catalytic effect, forest restoration, plantation, plant species richness This study was conducted as a part of the after

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