How (and Why) to Notice Disturbance
When you first experience being in rain forest you
are sure to be impressed by the density of the plant
growth. Everything appears so lush, so dense, so green.
But perambulate slowly through the forest, and look
around carefully. Take in the full picture, from ground
level to canopy. As you move from one place to another,
you will see that the forest is not uniform in structure.
There are scattered fallen branches and toppled trees
that have opened the canopy, allowing sunlight to
illuminate part of the forest floor. Youthful, riotous
plant growth typifies such openings. Whole sections of
forest may be composed of denser and smaller trees.
Tropical forests are patchy, not uniform. And consider
too the vegetation you see as you drive the roads. The
trees and shrubs along roadsides and fields are largely
different from those inside the forest. Yet even some of
these species may be present inside forest, but only if
there is an opening where light is abundant.
Tropical humid forests are composed of vegetation
patches of varying ages that result from the local
disturbance history of the site. You can see this once you
start to notice such things as differences in the density
of trees in various forest areas, in the thicknesses of tree
boles, and in the amount of light reaching the ground.
Changes in species assemblages occur at various scales
of space and time because of periodic and generally
unpredictable natural or human- caused disturbance.
Disturbances are relatively frequent in tropical forests,
more the rule than the exception. Indeed, a common
sound within rain forest is that of a big tree branch
crashing to the ground.
The magnitude of disturbance is variable. A disturbance
may range from a single branch that falls from the canopy
to the forest floor to a major hurricane that flattens
hundreds of hectares (1 ha = 2.47 ac) of forest. Small
disturbances, such as tree falls, are far more frequent than
large- scale devastation. In fact, they are normal.
Local disturbance history results in a dynamic mosaic
of small and large vegetation patches. These patches
may not loom large to the human visitor but they do
to the plants and associated animals. Like the various-
size craters evident on the moon, disturbance patches
sometimes overlap, adding to the spatial heterogeneity
of the site.
Disturbance frequency is variable and strongly
stochastic. This results in what ecologists like to term
a shifting mosaic of local patch histories throughout a
forest. The forest is a living, changing patchwork quilt
of greenery, where each patch is somewhat different in
size and age. This is what disturbance does to a lowland
humid forest.
Forest clearance for logging (either selective or clear-
cutting), agriculture, and pasturage represent human-
induced disturbances that vary from tiny clearings to
hundreds of cleared hectares (plate 7- 1). Anthropogenic
disturbance is different from natural disturbance, though
both may initiate an important ecological process called
ecological succession (discussed below).
Local areas of natural disturbance are called gaps, and
the pattern of plant growth that follows the creation of
a gap is called gap dynamics. Gap dynamics unfold in
temperate as well as tropical forests. Once you develop
an eye for spotting gaps, you’ll notice that they are
common features of tropical forests.
What happens when a gap is created? That’s pretty
much what this chapter is about, but in a nutshell, gaps
support an array of light- tolerant and light- demanding
plant species that typically exhibit rapid growth thanks
to the abundance of sunlight. Soon a jungle of trees,
shrubs, and vines covers the gap, as plants compete for
access to light.
The vegetation development following a disturbance
event is termed ecological succession. The word succession
implies an orderly process, and as you will see, that
is somewhat accurate. It is possible to predict which
groups of species will first appear in gaps where sunlight
abounds. But there is much variation from site to site.
Heavy rains in tropical lowland forests are often
accompanied by strong winds that bring down large
branches and entire trees, creating gaps. Natural fire
and landslides are other agents of disturbance (plate 7-
2). Human- caused disturbance is different in that land
is normally cleared of forest, the slash typically burned
on site, and the vegetation replaced by agriculture or
pasture. In the case of logging, forests are penetrated by
logging roads and trails, and collateral damage to the
forest ensues as select trees are removed (chapter 18).
A diverse community of rapid- growing, shade-
intolerant, sun- loving plant species quickly becomes
evident in a disturbed area. Over time these species are,Chapter 7. If a Tree Falls . . . Rain Forest Disturbance Dynamics
95