Modulation of the molecular size of humic substances by organic acids of the root exudates

Foliar phosphorus content and mycorrhizal infection of dipterocarp seedlings growing in a 61-year-old forest plantation (site A) and a selectively logged forest (site B) were determined. Mycorrhizal infection levels were high–83.7% forShorea bracteolata and 95.0% forS. leprosula at site A, and 77.8% forS. leprosula and 86.3% forS. curtisii at site B. The seedlings all had similar foliar phosphorus contents although available soil phosphorus at site B was half that at site A. Significant correlations were obtained between foliar phosphorus content and percentage mycorrhizal infection at site B only.     

Sapling structure and regeneration strategy in 18 Shorea species co-occurring in a tropical rainforest

BACKGROUND AND AIMS: Inevitable trade-offs in structure may be a basis for differentiation in plant strategies. Juvenile trees in different functional groups are characterized by specific suites of structural traits such as crown architecture and biomass distribution. The relationship between juvenile tree structure and function was tested to find out if it is robust among functionally and taxonomically similar species of the genus Shorea that coexist sympatrically in a tropical rain forest in Borneo. METHODS: The sapling structures of 18 species were compared for standardized dry masses of 5 and 30 g. Pairwise simple correlation and multiple correlation patterns among structural traits of juveniles (0.1-1.5 m in height) of 18 Shorea species were examined using Pearson's correlation and principal component analysis (PCA), respectively. The correlation was then tested between the PCA results and three indices of shade tolerance: the net photosynthetic rate, the wood density of mature trees and seed size. KEY RESULTS: The structural variation in saplings of the genus Shorea was as large as that found in sets of species with much more diverse origins. The PCA showed that both crown architecture and allocation to leaves are major sources of variation in the structures of the 18 species investigated. Of these two axes, allocation to leaves was significantly correlated with wood density and showed a limited correlation with photosynthetic rate, whereas crown architecture was significantly correlated to seed size. CONCLUSIONS: Overall, the results suggest that an allocation trade-off between leaves and other organs, which co-varied with wood density and to a certain extent with photosynthetic capacity, accounts for the difference in shade tolerance among congeneric, functionally similar species. In contrast, the relationship between the architecture and regeneration strategy differed from the pattern found between functional groups, and the function of crown architecture was ambiguous.     

The response of seedlings of two dipterocarp species to nutrient additions and ectomycorrhizal infection

An experiment was conducted to investigate the effect of ectomycorrhizal infection on growth and nutrient uptake, especially of P and K of dipterocarp seedlings.Hopea helferi (Dyer) Blanco andHopea odorata Roxb. seedlings were grown in a sandy loam soil given a basal dressing. Nutrient treatments were unamended soil (NIL), amended soil with the addition of P and K (F), amended soil without P but with K (-P), amended soil without K but with P (-K), amended soil without P or K addition (-PK). Seedlings grown in the amended soil treatments showed foliar symptoms suggestive of calcium deficiency. Ectomycorrhizal infection appeared to improve shoot Ca concentration and relieved the foliar symptoms. Ectomycorrhizal infection inH. odorata plants increased shoot P concentration and increased shoot and total dry weight to the same or greater extent than those of uninfected plants growing on P amended soil.H. helferi showed a positive response to ectomycorrhizal infection in shoot, root and total dry weight in all nutrient treatments but no response to the nutrient treatments themselves.     

Mass flowering of dipterocarp forests in the aseasonal tropics

At irregular intervals of 2 to 10 years the aseasonal tropical rain forests in west Malesia come into heavy mass flowering, followed by mast fruiting. During a heavy flowering almost half the mature individuals and over 80% of the canopy and emergent tree Species in a forest may flower. This involves over 200 tree species in a forest flowering over a short period of 3–4 months. The pollination needs during a mass flowering appears to be overcome in several ways. A rapid increase in the number of pollinators seems to occur in the forest. This is partly caused by the migration of pollinators from the fringes of the forest to forage on the superabundance of flowers. At the same time, some groups of plants which share common pollinators appear to reduce pollinator competition by flowering in interspecific sequence. Many members of the family Dipterocarpaceae have evolved sequential flowering too. They also share unique pollinators, common flower thrips which appear to build up rapidly in numbers by feeding and breeding on the millions of dipterocarp flower buds which are present several weeks before the flowering. The environmental cue for this irregular, but widespread mass flowering can be traced to a small dip of about 2° C below mean night-time temperature for 4 or 5 nights. The conditions for such temperature drops occur during El Nino events.     

Logging,exotic plant invasions,and native plant reassembly in a lowland tropical rain forest

Habitat modification and biological invasions are key drivers of global environmental change. However, the extent and impact of exotic plant invasions in modified tropical landscapes remain poorly understood. We examined whether logging drives exotic plant invasions and whether their combined influences alter understory plant community composition in lowland rain forests in Borneo. We tested the relationship between understory communities and local‐ and landscape‐scale logging intensity, using leaf area index (LAI) and aboveground biomass (AGB) data from 192 plots across a logging‐intensity gradient from primary to repeatedly logged forests. Overall, we found relatively low levels of exotic plant invasions, despite an intensive logging history. Exotic species were more speciose, had greater cover, and more biomass in sites with more local‐scale canopy loss. Surprisingly, though, exotic species invasion was not related to either landscape‐scale canopy loss or road configuration. Moreover, logging and invasion did not seem to be acting synergistically on native plant composition, except that seedlings of the canopy‐dominant Dipterocarpaceae family were less abundant in areas with higher exotic plant biomass. Current low levels of invasion, and limited association with native understory community change, suggest there is a window of opportunity to manage invasive impacts. We caution about potential lag effects and the possibly severe negative impacts of exotic plant invasions on the long‐term quality of tropical forest, particularly where agricultural plantations function as permanent seed sources for recurrent dispersal along logging roads. We therefore urge prioritization of strategic management plans to counter the growing threat of exotic plant invasions in modified tropical landscapes.     

Estimation of the Outcrossing Rate on Dryobalanops aromatica Gaertn. f. in Primary and Secondary Forests in Brunei, Borneo, Southeast Asia

Abstract Dryobalanops aromatica is an emergent canopy tree species with hermaphroditic bee-pollinated flowers. This species is distributed in the lowland dipterocarp forests of Malaya, Sumatra, Rhio Archipelago and Borneo. The flowering of many different dipterocarp species was observed in some parts of Brunei in 1991, and all over Brunei in 1992. The outcrossing rates of D, aromatica were estimated in 1991 and 1992 in the secondary forest, and in 1992 in the primary forest. Selective harvesting of D. aromatica has occurred in the secondary forest about 20 years ago. Four enzyme systems with a total of six isozyme loci were used to estimate the single locus and the multilocus outcrossing rates. The multilocus outcrossing estimates ranged from 0.794 to 0.856, with a mean of 0.816. These estimates are similar to those for other tropical tree species. Although the flowering density of D. aromatica in the secondary forest was lower than in the primary forest, the multilocus outcrossing estimates were not significantly different. The variation between years in the secondary forest was also not significant. These findings may be largely the result of the high flowering tree density of this species, even in the secondary forest. In this study, the outcrossing rates of D. aromatica were maintained by high flowering density regardless of the size structure or the topography of the habitat.