A review of dipterocarps - Center for International Forestry Research

Root Symbiosis and Nutrition
is actively being pursued in Southeast Asia. However, the
same cannot be said of research into dipterocarp mineral
nutrition requirements. With the present interest in
establishing plantations of dipterocarps, fertilisers are
being applied with the hope of producing enhanced or
more rapid growth without a clear understanding of
dipterocarp mineral nutrition requirements. This very
important aspect of dipterocarp silviculture needs to be
studied in much more detail. This is reflected in the
current state of knowledge discussed above and in the
identification of research priorities discussed below.
A word of caution before discussing future research
priorities: results of many of the dipterocarp mycorrhizal
studies carried out in this region, for example, the BIO-
REFOR proceedings, are often difficult to interpret or
not verifiable because of poor experimental design, lack
of statistical analysis, or incomplete monitoring and
reporting. Experiments need to be more carefully
planned, controlled and monitored, to ensure that the
observed effects are genuinely due to the inoculated
ectomycorrhizal fungi and not from other contaminants.
In view of the multi-faceted and some yet unknown
aspects of dipterocarp mycorrhizas and nutrition, and the
current efforts to establish dipterocarp plantations in the
region, the following research priorities have been
identified. Many paraphrase the recommendations made
by Malajczuk et al. (undated) in their Annex 1 -
Recommended Research Programme on Mycorrhizal
Management, as these are found to be very relevant to
dipterocarp mycorrhizal research. The following should
be the future research priorities for dipterocarp
mycorrhizas and nutrition:
1. There is a need for more integrated studies on
dipterocarp mineral nutrient requirements and
mycorrhizal infection for seedling/cutting
establishment in the field.
Most fertiliser trials carried out thus far have ignored
the role of mycorrhizas. They have a significant role
to play in plant mineral uptake and are being
considered in some quarters as possible fertiliser
substitutes/supplements. Results from pot
experiments have limited applicability in field
conditions especially if plants in the field are
interconnected by mycorrhizal links. These intact
mycelial networks constitute the main source of
inoculum when seedlings are grown near an
established tree (Newton and Pigott 1991b,
107
Alexander et al. 1992, Yasman 1995) as is likely to
occur in reforestation of selectively logged
dipterocarp forests.
2. The mycorrhizal dependency of dipterocarps for
reforestation should be determined for each species
at various ages in various habitats (different light
regimes, soil nutrient levels, water retention, organic
substrates).
Mycorrhizal fungi like vascular plants may vary in
their ecological and physiological requirements and
under given circumstances, some fungi may benefit
particular hosts more than others. The ability of a
particular mycorrhizal fungus to enhance the foliar
nutrient content of the host may not be indicative of
the isolate’s ability to improve seedling growth and
subsequent outplanting performance (Mitchell et al.
1984). Surveys and identification of ectomycorrhizal
fungi associated with dipterocarps should be
continued and the results shared among workers in
the region.
3. Field studies should be conducted to determine the
influence of nutrition and mycorrhizal infection on
dipterocarp seedling survival, and their roles in
determining forest composition.
It has been suggested that the ‘nursing’ phenomenon
(Read 1991), i.e. regeneration of seedlings in the
vicinity of parent trees whereby they become
incorporated into a mycelial network, reduces tree
species diversity (Alexander 1989). It is believed that
because mycorrhizal fungi have a great influence on
plant survival in new and reclaimed sites, tree health
and site quality, they are the cornerstone to proper
establishment of functional forest ecosystems
(Malajczuk et al. undated).
4. Isolation and pure culture of indigenous
ectomycorrhizal fungi should be intensified, and
species associated with the desired host plant species
both in unlogged and logged over forest requiring
rehabilitation should be determined.
There is evidence that some of the easily manipulated
exotic mycorrhizal fungi such as P. tinctorius may
be out competed by indigenous (co-evolved)
mycorrhizal fungi in the field (see Chang et al. 1996).
Moreover, fungi which are beneficial to the host in
the natural forest may not be adapted to the degraded
site where reforestation will be carried out. It has
been suggested that successful establishment of
indigenous ectomycorrhizal trees is limited to areas