Other Nutrient- Retention
Adaptations
Some tropical plants have root systems that grow
vertically upward, from the soil onto the stems of
neighboring trees. These apogeotropic roots can grow
as fast as 5.6 cm (2.2 in) in 72 hours. The advantage
of growing on the stems of other trees may be that the
roots can quickly and directly absorb nutrients leached
from the trees as precipitation flows down the stem.
This unique system, thus far described only for some
plants growing on poor- quality Amazon soils, results
in recycling without the minerals ever entering the soil!
Both epiphytes and understory plants, especially the
wide crowns of certain palms (nicknamed “wastebasket
plants”), catch litter as it falls from the canopy. This
arrested litter subsequently decomposes aboveground,
enriching the mineral content of stem flow and thus
having the effect of fertilizing the soil in the immediate
vicinity of the wastebasket plant.
Canopy leaves play a direct role in taking up
nutrients. Algae and lichens on the surface of leaves are
nutrient scavengers, adsorbing nutrients from rainfall
and trapping nutrients on the leaf surface. When the
leaf dies and decomposes, these nutrients are taken up
by the root mat and returned to the canopy trees.
Some trees both in temperate and tropical regions
have what are termed canopy roots. These adventitious
roots, similar in structure to subterranean roots, grow
into the thick litter and epiphyte layer that accumulates
on the surface of thick branches far from the forest
floor. This adaptation enables trees to tap into nutrients
far above the forest floor and is confined to forests
where epiphyte density is high.
Nitrogen Fixation in the Tropics
Approximately 79% of Earth’s atmosphere is gaseous
nitrogen, a form of nitrogen not used in routine
metabolism. But some organisms convert gaseous
nitrogen into ammonia and nitrate, readily useable not
only by themselves but also by other organisms, a process
termed biological nitrogen fixation. This process is familiar
in the temperate zone and is prominent throughout the
tropics. It is the likely reason nitrogen is generally not a
limiting nutrient in tropical soils. In some parts of the
tropics the availability of nitrogen may exceed the demand
for it, thanks to the prevalence of nitrogen fixation.
Two kinds of biological nitrogen fixation are recognized,
symbiotic and free- living. Symbiotic nitrogen fixation is
most closely associated with plants of the legume family
(Fabaceae). Free- living nitrogen fixation occurs in soil
with nitrogen- fixing genera such as Azotobacter and is
also associated with epiphyllous microbes and lichens.
Because of the distributions of legumes and free- living
microbial nitrogen fixers, nitrogen fixation in the tropics
extends vertically from canopy to soil.
Plants of the huge and diverse legume family, abundantly
represented in biomass and biodiversity throughout the
global tropics, typically engage in symbiotic nitrogen
fixation. Nitrogen is acquired through nodules in legume
root systems. The nodules contain bacteria formerly
called Rhizobium but now recognized to belong to four
main genera and about nine others. Both the plant and
the bacteria benefit from their interaction, as the bacteria
convert free nitrogen into ammonium and in turn are
supplied with potential energy from the plant, an obligate
mutualistic association.
Free- living nitrogen fixation also occurs abundantly
in tropical forests. Certain epiphyllous lichens convert
gaseous nitrogen into useable form for plants in a
manner similar to that of leguminous plants. Studies
have shown that leaf- surface microbes (microbial film)
and liverworts facilitate uptake of gaseous nitrogen.
Nitrogen fixation also occurs in termites, brought
about by the metabolic activities of microbes in termite
guts. Because of the abundance of termites throughout
the tropics, these insects may contribute a substantial
amount of nitrogen to the soil.
Rates of nitrogen fixation both in soil and canopy
vary widely. Though nitrogen is usually not consideredPlate 6- 11. In Amazonian areas where soils are old and
depleted of minerals, the soils are characteristically white and
sandy. These areas drain into blackwater rivers, clear waters
darkly colored by only the tannins and phenols from plant
decomposition. Photo by John Kricher.88 chapter 6 essential dirt: soils and cycling