He was correct, though he had underestimated the
actual age of the site. Stephen Hubbell and Robin Foster,
in a later study, estimated that forest at BCI is actually
between 500 and 600 years old and agreed with Knight
that it is not yet in equilibrium (the term equilibrium
is sometimes used by ecologists to mean that there is
no net change occurring in an ecological community).
They concluded that though initial succession is rapid,
stochastic factors as well as climatic changes, periodic
drought, and biological uncertainty from competitive
interactions act to prevent establishment of a stable
equilibrium. This means, in essence, that the forest is
always changing to some degree; it is a dynamic state.
This is probably the norm for tropical rain forests.
Forest Gaps: Why They Are
Important
Rain forest trees are not immortal, and each will
eventually perish. Some remain in place, becoming
dead, decaying snags, homes for woodpeckers,
toucans, and night monkeys (Aotus). Others topple
to the ground, some bringing their root systems to
the surface as they fall, some snapping off somewhere
along the trunk and thus leaving their roots in the
ground. A tree may be brought down by windthrow,
or topple when weakened by termites, epiphyte load,
or old age. Landslides may bring down groups of trees.
Large branches commonly break off and drop. Indeed,
one of the often- heard sounds during a walk through
rain forest, especially in rainy season, is the sudden
crack of a falling tree or large branch. When a rain
forest tree or significant part of a tree falls, it creates a
canopy opening, a forest gap. In gaps, light is increased,
causing microclimatic conditions to differ from those
inside the shaded, cooler, closed canopy. Air and soil
temperatures as well as humidity fluctuate more widely
in gaps than in forest understory.
Gaps vary in area (plates 7- 10– 11). The general pattern
in most wet forests is many small gaps and few large
gaps. Large gaps— defined as having an area in excess
of 300 or 400 m^2 (3,200– 4,300 ft^2 )— though fewer, have
greater total area, and thus make up a large percentage of
total gap space within a forest. An emergent tree, should
it fall, may bring down several other trees, creating a
large gap. Lianas (woody vines) that connect several
trees together increase the probability of multiple tree
falls. When one tree goes, its liana connections to others
can result in additional trees falling, or large branches
being pulled down. Lianas are thus causal factors in tree
mortality, as several studies have shown.
Tree falls correlate with seasonality. On Barro
Colorado Island, tree falls peak around the middle of
rainy season, when soils as well as the trees themselves
are saturated with moisture and strong gusty winds
blow. At La Selva in Costa Rica, most gaps occur in
June– July and November– January, the wettest months.
In parts of the Caribbean and Central America,
hurricanes have periodically leveled hundreds of
hectares of forest, a giant gap indeed. For example,
when Hurricane Hugo struck Puerto Rico in 1989 it
opened large areas of El Verde rain forest to high lightPlate 7- 10. Gaps may be large or small. This photo of a small
gap shows how sunlight illuminates the plants, stimulating
growth. Photo by John Kricher.
Plate 7- 11. Gaps that open from a tree fall allow for high levels
of sunlight, stimulating both germination of seeds in the
soil bank as well as growth spurts among saplings already
present. Photo by John Kricher.100 chapter 7 if a tree falls . . . rain forest disturbance dynamics