falling branch, or a single fruit, or whole tree, or
perhaps buried beneath a fallen palm frond or some
other large leaf. Herbivores also occasionally consume
seedling and sapling leaves to the point of killing the
tree. For Dipteryx panamensis, a common canopy
species, seedlings ranging in age from seven to 59
months experienced a 16% mortality rate from litter
fall alone. Mortality rates are consistently highest in
juvenile plants, declining steadily as the plants age.
Furthermore, many seeds never germinate because
they are destroyed by a diversity of seed predators as
well as fungal pathogens.
For most of a tree’s life cycle, light acts as a significant
limiting factor. Growth rates of trees in shaded interior
forest are much lower than rates of those in more
lighted, open areas. Dipteryx panamensis, like many
tree species, shows extremely slow growth in low light
conditions but grows quickly taller and wider in a gap.
For this reason, growth rates tend to fluctuate during
the typical life cycle of a tree. Gaps open, close, and can
reopen, so that any given tree might experience several
periods of rapid growth (when in gaps), alternating
with periods of extreme slow growth (under a fully
closed canopy). As described earlier, a substantial
majority of tropical forest tree and shrub species show
high levels of shade tolerance, with an accompanying
high degree of growth plasticity— that is, the ability to
survive in very low light levels of the forest understory
and grow rapidly in gaps.
The existence of emergent trees has long been
recognized as a characteristic of rain forests. Of what
possible benefit is it to a tree to invest additional energy
to grow above the majority of other trees in the canopy?
Added light availability is certainly one possibility. But
in a study of five emergent tree species at La Selva,
these trees showed significantly lower adult mortality
rates than non- emergent trees. Perhaps emergents are
more protected from being damaged by other falling
trees, given that their crowns rise above the rest.
The Dynamics of Drought
Drought was discussed in chapter 5. Drought often
results from El Niño/Southern Oscillation (ENSO)
effects. Beginning in 1980, a 50 ha (125 ac) permanent
plot was established at Barro Colorado Island in Panama.
Under the direction of Richard Condit, all woody plants
at least 1 cm dbh were identified to species, measured,
and mapped, which is a lot of work. Censuses were done
in 1982, 1985, and 1990. Over the three censuses, 310
species were recorded in the plot, and data were taken
on 306,620 individual stems. In the brief time frame of
this study, weather was an unexpectedly strong factor.
An unusually protracted dry season coincident with a
strong El Niño brought a severe drought to BCI in 1983.
It has been well established that an ENSO, particularly
a strong one, will have significant effects on patterns of
secondary succession.
Tree mortality rates were strongly elevated in the
years immediately following the drought. For trees
with >16 cm (6.3 in) dbh, mortality was elevated fully
50%. The increased death rate among shrubs and trees
was attributed entirely to the drought. Approximately
two- thirds of the species in the plot experienced
elevated mortality from 1982 to 1985.
Those plants surviving the drought showed elevated
growth rates. For example, growth of trees with 16–
32 cm (6.3– 12.6 in) dbh was more than 60% faster in
1982– 85 than from 1985 to 1990. While this result may
seem surprising at first, it is really to be expected. The
death of so many trees permitted much more light into
the forest (the gap effect) and reduced root competition
for water and nutrients among plants. Total gap area on
the plot increased after the drought but returned to its
pre- drought level by 1991, an indication of how rapidly
surviving plants responded to the added influx of light.
Many species’ populations experienced changes in
abundance during the period of the study, 40% of them
changing by more than 10% in the first three years of the
census (Hubbell and Foster 1992). Ten species were lost
from the plot and nine species migrated into the plot
from 1982 to 1990. Nonetheless, there was remarkable
constancy in the number of species and number of
individuals within the plot at any given time:- 1982: 301 spp., 4,032 individuals
- 1985: 303 spp., 4,021 individuals
- 1990: 300 sp., 4,107 individuals
The drought killed many trees but created opportunities
for additional growth, with the result that the deceased
plants were quickly replaced. The speed of the replacement
process was a surprise to the researchers.
The analysis of the BCI data suggests some important
conclusions: - The forest reacts to short- term fluctuations caused
by climate. - The forest as a whole remains intact, though many
species undergo population changes.
106 chapter 7 if a tree falls . . . rain forest disturbance dynamics