Mycorrhizas in relation to Malaysian forest practice : a study of infection, inoculum and host response
Surveys on the status of mycorrhizal infection and VAM propagules were carried out in 3 representative forest areas of Jengka in the Lowland Forest of Malaysia. VAM spore numbers found in these forest soils were low and well below those recorded in other tropical soils. However, levels of root infection in soil cores and infections in individual plant species were comparatively high. The assessment also confirmed the taxonomic dominance of VAM tree species in Malaysian forest. Regardless of the contrast between spore numbers and mycorrhizal root colonisation, evidence showed that current forest logging practices influenced significant changes in the infective VAM propagules. Propagules were reduced by 30 - 50&'37 when forest soils were severely disturbed through heavy soil mechanical compaction, exposure and erosion. Bioassay experiment using legume tree species suggested that the indigenous VAM propagules were infective but infection was closely related to the propagule density. Using Parkia speciosa and Albizia falcataria as test plant species it was further confirmed that low rates of infection by indigenous VAM fungi was a result of low inoculum density and doubling this inoculum increased infection levels. Between the indigenous VAM inoculum and the mixed introduced VAM inoculum, the latter proved to be more effective. For equal levels of infection of indigenous and introduced VAM, the mixed introduced VAM inoculum promote better plant growth in terms of plant height and shoot dry weight. With Intsia palembanica , the plant can be infected by either VAM or EM fungi or by both infections in the same root system. The species displayed a range of typical ectomycorrhizal structures which were similar to the etomycorrhizas of Dipterocarpaceae. Suitable inoculum for EM infection was widespread, even in sites where EM trees were absent. Even the Dipterocarp mycorrhizal fungi were able to initiate infection and growth response in Intsia plants. Inoculation experiments in Intsia , with EM and VAM fungi showed that both VAM and EM fungi increased P uptake initially. However, when the P in soil became exhausted, only VAM plants showed evidence of exploiting soil P, which was apparently not available to EM plants. In a subsequent pot trial, it was confirmed that mycorrhizal fungi enhance the growth of Albizia, Parkia and Intsia . These species were highly dependent on mycorrhizal fungi at seedling stage. P application however, showed negligible effect in growth enhancement, except at an optimum level of 4g P/pot. At this level of TSP applied, the interaction effects of mycorrhizal inoculation and TSP application were significant.