In situ and Ex situ Conservation of Commercial Tropical Trees - ITTO

462Ectomycorrhizal associations in dipterocarp forestsAn important feature of the dipterocarp family is its close association withectomycorrhizal fungi during nutrient uptake. In addition to expanding the rootsystem through mycelial nets, these symbiotic fungi allow nutrients to be absorbedin organic forms. Nutrient acquisition without inorganic transformation controlsnutrient loss through leaching and fixation.Similarly, indigenous vegetation should be adapted to these rainforestsites. As indigenous species, Dipterocarpaceae seem best suited to theseconditions. Their growth rate is relatively low as it often occurs under nutrientdepletedconditions. The foliage contains relatively high contents of polyphenolsand lignin, which are less degradable, thereby avoiding nutrient loss throughrapid decomposition when returned to soils as litter-fall. This mechanism mayalso allow utilisation of nitrogen via protein-polyphenolic complexes mediatedby ectomycorrhizae, similar to infertile agro ecosystems that mimic polyphenolrichforests, as reported by Northup et al. (1998) and by Hattenschwiler andVitousek (2000) where, in particular ecosystems, ectomycorrhizal fungi playan essential role during nutrient acquisition.Ectomycorrhizal diversity in dipterocarp forests has attracted severalstudies. In general, initial abnormal tree growth is usually associated with anabsence of ectomycorrhizal association due to forest burning, abandoned loggedoverareas leading to grassland encroachment indicating mycorrhizaldependence. This also occurs during introduction to exotic regions. This is truefor Pinus merkusii, a pine species indigenous to humid tropical regions. Usuallythis group of fungi has a narrow-host range, and therefore specificity orcompatibility is one aspect to be considered.Despite their ability to be vegetatively propagated in laboratory media,forced association between vegetative inoculum of ectomycorrhizal fungi anddipterocarp hosts have not been successful. Unlike ectomycorrhizal associationsin sub-tropical or temperate regions, clonal association between host andsymbiont is still absent in the ectomycorrhizal fungi found on dipterocarps. Thistechnical barrier limits the advancement of studies on dipterocarp –ectomycorrhizae associations. So far, those studies have been restricted to:inoculation effects (using non-vegetative inoculants), morphological diversity /taxonomy, ecology, and nutritional aspects. These contrast with studies in thesub-tropical or temperate regions where techniques to synthesize mycorrhizalformation using vegetatively defined inoculant are routine protocols, allowingthe physiological and molecular aspects of the associations to be addressed. Inthe latter situation, an appreciation of the fungal diversity is attained.