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