plants persist for very long periods in the darkened
understory. Taken together, they represent over 95% of
all trees greater than 1 m (3.3 ft) tall four years after gap
formation. They are the first beneficiaries of the gap.
Next comes seed germination and sprouting, which is
also common in many tree species in small gaps.
Once a gap opens, plant biomass increases rapidly.
In Panama, Fakhri Bazzaz and Steward Pickett found
that biomass at a successional site increased from 15.3
to 57.6 dry tons per hectare (2.47 ac) from year two
to year six. This rapid growth reduces soil erosion,
as vegetation blankets and secures the soil, as well as
moves minerals from the soil into plant biomass.
Because of the density of competing plants in
successional plots, the leaf area index (see sidebar) may
reach that of a closed canopy forest within six to 10
years, although the vegetation is still relatively low and
the species composition at that time is not what it will
be as the site returns to closed forest.
The rate of photosynthesis in early successional
plants is higher in full sun than in partial or full shade;
these plants are obviously adapted to grow quickly.
Some early successional plants that grow in both shade
and sun develop significantly thicker leaves in full sun,
an adaptation that aids in protection from water loss.
Successional species also allocate considerable energy
to root production, facilitating rapid uptake of soil
nutrients.
During early succession, many plant species, termed
colonizers or pioneer species tend to be fast growing
and of small stature and produce many- seeded fruits.
In later succession, most plants are larger, grow more
slowly, and have fewer seeds per fruit; these are shade-
tolerant plants adapted to persist in the closed canopy.
Shade- tolerant species should be thought of as exactly
that— they tolerate shade well, though their rate of
growth in shade is usually slow. Virtually all shade-
tolerant species grow fastest in well- lit conditions.
It is their physiological ability to persist in the dark
understory until conditions favor their growth that is
their key adaptation.
Dennis Knight, in a classic study on Barro Colorado
Island (BCI) in Panama, found that plant species
diversity in successional areas increased most rapidly
during the first 15 years of succession. Diversity
continued to increase, though less rapidly, until 65
years. Following that, diversity still increased, but even
more slowly. Remarkably, Knight concluded, the forest
of BCI was still changing after 130 years of succession.Leaf Area Index
Ecologists measure leaf density as LAI, leaf area index,
the leaf area above a square meter of forest floor. In
other words, if you look could through a 1 m^2 (10.75 ft^2 )
hollow tube directly up from a point on the forest floor
to the canopy, LAI is the measure of all the layers of
leaves between you and the topmost leaves (plate 7- 9).
The LAI is indicative of the productivity of the forest.In a mature temperate forest in New England, LAI is
nearly 6, meaning that the equivalent of 6 m^2 (65 ft^2 )
of leaves occurs above one square meter of forest
floor. For tropical rain forest at Barro Colorado Island in
Panama, the LAI is about 8. Typically, LAI in the tropics
ranges from about 5.1 on a dry shrubland on poor soil
(called Amazon caatinga) to highs of 10.6 to 22.4 where
there is lush forest on rich soil. In forests with high
LAI, it is probable that the intensity of shading is so
great that many, if not most, understory leaves do not
approach optimum net primary productivity (chapter
5) because they are severely light- limited.Plate 7- 9. Leaf area index measures the density of leaves
from ground to canopy. The leaf area index of this well-
shaded Amazonian forest would be high. Photo by John
Kricher.chapter 7 if a tree falls . . . rain forest disturbance dynamics 99