A review of dipterocarps - Center for International Forestry Research

Root Symbiosis
and Nutrition
S.S. Lee
At present dipterocarps are gaining much attention, this
volume being testimony to it. Since large tracts of
dipterocarp forests in tropical Asia have become
overlogged and/or degraded, interest in planting
dipterocarps either in plantations or by underplanting in
poor forests has gained momentum. With this move,
research on mycorrhizas and their association with
dipterocarps has gained a high profile.
Mycorrhizas are the symbiotic association between
specialised root-inhabiting fungi and the roots of living
plants. Harley and Smith (1983) recognise seven
mycorrhizal types but only two, the ectomycorrhizas and
the vesicular-arbuscular mycorrhizas (VAM) (now more
popularly referred to as arbuscular mycorrhizas) occur
in the Dipterocarpaceae. Dipterocarps are predominantly
ectomycorrhizal but a few species have been reported
to form both ectomycorrhizas and VAM (Table 1). Unlike
some members of the Leguminosae, the dipterocarps are
not symbiotic with nitrogen fixing bacteria.
Typical dipterocarp ectomycorrhizas are short,
pyramidal or racemously branched and variously
coloured (e.g. brown, black, white, yellow). A fungal
sheath (mantle) characteristic of the fungal partner
surrounds the host root. Underneath this sheath lie the
often radially elongated epidermal cells between which
are located the hyphae of the Hartig net (Alexander and
Högberg 1986, Lee 1988). The surface of the sheath may
be smooth but often bears hyphae or hyphal strands which
radiate out into the substrate.
The role of mycorrhizas in increasing the absorptive
efficiency of roots is well known. The growth of
mycorrhizal hyphae into the surrounding soil effectively
shortens the distance over which the slowly diffusible
ions, such as phosphate, must travel before being
absorbed and the association has proven particularly
Chapter 6
beneficial to the host in soils of low available phosphorus
concentrations. Ectomycorrhizas are also seen to play a
role in minimising nutrient losses from the nutrient cycle
through leaching (Read et al. 1989). The production of
a potent acid carboxypeptidase by some ectomycorrhizal
fungi such as Amanita and Boletus (Read 1991) indicates
that these fungi have the potential to mobilise the plant
growth limiting nutrient, nitrogen, from protein. This
implies that such ectomycorrhizal infected trees are no
longer dependent upon the activities of a separate group
of decomposer fungi for the release of nitrogen in the
form of the ammonium ion for plant uptake.
Ectomycorrhizas are also known to be able to increase
the tolerance of trees to drought, high soil temperatures,
organic and inorganic soil toxins, and very low soil pH.
The sheath has been shown to have important storage
functions, not only for phosphorus but also for other
absorbed nutrients and carbon. The sheath also protects
the root from pathogens, and is thought to be able to
reduce water loss and allow rapid rewetting, thus
lengthening root life and thereby increasing mineral
uptake and retention (Janos 1985). It has also been
suggested that the key role of the mycorrhizal symbiosis
under natural conditions is to enable seedling persistence
rather than rapid growth (Abuzinadah and Read 1989).
The presence of ectomycorrhizas in the
Dipterocarpaceae has led to several hypotheses regarding
the role they might play in dipterocarp biology. Ashton
(1982) suggested that the clumped distribution of the
dipterocarps might be reinforced by their
ectomycorrhizal associations as the mycelia persist and
gradually spread with the ever dispersing and coalescing
clumps of the dipterocarp trees themselves. He suggested
that his observation of the association of two different
groups on soils of different soil phosphorus levels could