One major difference between tropical and temperate
forests is that in tropical forests the vast bulk of the
rapidly cycling minerals is contained in the living
plants, the biomass. Many researchers have shown that
most of the calcium, magnesium, and potassium in a
rain forest is located not in the soil but in the living
plant tissue. For example, in a study Rafael Herrera
performed near San Carlos de Río Negro in Venezuela,
the distribution of calcium was as follows: 3.3% in
leaves, 62.2% in wood, 14.0% in roots, 3.1% in litter
and humus, and only 17.4% in soil.
Mycorrhizae
Throughout the tropics as well as most of the temperate
zone, there is an intimate mutualistic (mutually
beneficial) association between tree roots and a diverse
group of fungi collectively termed mycorrhizae. Up to
80% of all land plants contain mycorrhizae either on
or inside their roots; a single gram (0.035 oz) of soil
may contain 100 m (328 ft) of mycorrhizal filaments.
Mycorrhizae are ubiquitous components of soils
throughout most terrestrial ecosystems.
Part of the mycorrhizal mycelium is inside a plant
root, and part extends out into the soil. The fungus
uses some of the plant’s photosynthate as food. In
this regard, the fungus would appear parasitic. But
though mycorrhizae take food from the plant, they are
essential to the plant’s welfare, as they greatly facilitate
the uptake of minerals from the forest litter. When the
soil is nutrient poor, the benefit to the plant outweighs
the energy cost it pays to the fungus. The mutualistic
plant- fungus relationship is win- win. Many trees
dependent on mycorrhizae have poorly developed root
hairs; the fungal strands substitute for the missing root
hairs.
Mycorrhizae are essential in uptake of phosphorus, a
nutrient that tends to be of limited availability in rain
forest soils (it is discussed more below). Mycorrhizae
may also have a role in direct decomposition and
cycling, moving minerals from dead leaves into
living trees without first releasing them to the soil, in
essence taking a shortcut through the usual pathway
of recycling. Mycorrhizae may also affect competitive
interactions among plants, thus influencing the
biodiversity of a given forest.
Though mycorrhizae abound in the tropics, it should
not be forgotten that not all fungi are mutualistic
with vascular plants. Various studies have shown that
many fungi are important destroyers of seeds, having a
greater negative effect than insects and rodents.
Fungi are of vast importance in the functioning of
a tropical forest, but most of their work goes unseen.
Visitors who wander the trails usually see only the
reproductive bodies, such as the many forms of
mushrooms, which are often of remarkable beauty
(plates 6- 5– 6). These mushrooms produce the nearly
microscopic spores that, of course, are the essential
components of fungal reproduction.Where Do the Dead Leaves Go?
Stop along the trail and look at the leaves. No, not the
ones on the trees; look at the ones on the ground. There
is surprisingly little accumulation of dead leaves and
wood on rain forest floor, making for a generally thinPlates 6- 5 and 6- 6. Two examples of the fruiting (spore-
producing) bodies of the many kinds of fungi found in
Neotropical forests. Photos by John Kricher.84 chapter 6 essential dirt: soils and cycling