The combination of frequent fire and exotic grass
invasion results in a grass– fire cycle that stabilizes in
the occupancy of the exotic grass, arresting normal
succession. Experimental studies performed in Panama
found that the exotic grass Saccharum spontaneum,
a tall (2.5 m/8.2 ft) species native to sub- Himalayan
valleys, will invade and persist in successional areas,
particularly when fires are frequent. The combination
of an aggressively growing invasive grass species and
frequent fire basically stopped succession in its tracks,
leaving a grassy field in its stead. Arrested successions
lead, of course, to diminished biodiversity and thus are
considered a serious conservation concern (chapter 18).
Summarizing Succession
Succession is a continuum of multiple life history
characteristics spread among many species of plants
that adapt them to grow and reproduce at various times
on a patch of habitat as it ages. Succession pathways are
situation dependent. Because life history characteristics
vary among species, and because all sites are subject
to stochastic factors, species composition on a given
site changes in response to changing abiotic and biotic
conditions, which, of course, are in part caused by
the plant species themselves. Natural selection selects
for suites of traits: seed dispersal rates, establishment
requirements, shade intolerance or tolerance, growth
rates, longevity, size at maturity, and so forth. Seeds
vary in how long they will reside and remain viable
in soil seed banks. Soil conditions vary among sites.
Exotic species and fire may interact to alter conditions
and prevent normal patterns of succession. There are
lots of variables affecting the outcome of any tract
undergoing succession.
To reiterate, the general pattern is that light-
demanding species typically appear first. These are
followed by non- colonizing light- dependent species.
These species are eventually replaced by shade- tolerant
species. All of these groups may be present early in
the succession, but they grow at different rates and
to a degree they replace one another. There are many
more shade- tolerant species than light- dependent tree
species, and many shade- tolerant species are present
from the outset of succession, though they are not
evident at the time.
Second- growth forests may be as species rich as
old- growth forests, though the two forest types may
maintain species assemblages that are largely different.
This is often due to long- lived pioneer species persisting
in the canopy even after shade- tolerant species have
attained full canopy stature.Forest Demographics
How long do rain forest trees survive? How long does
it take for a canopy giant to grow from seedling to
adult? Does most growth occur in rainy season or dry
season? How do short- term climatic fluctuations such
as occasional droughts influence forest dynamics? What
forces determine the probable survivorship of any given
tree? The study of rain forest demographics comprises
these and many other questions. To answer these
questions, it is necessary to initiate long- term, detailed
studies of specific tracts of forest, literally monitoring
the fate of each tree. This takes a lot of work but produces
some amazing results.The Life History of Typical Tropical
TreesConsider the Almendro (Dipteryx panamensis), a
common emergent tree that favors alluvial soils. What
might its longevity be? A Great Basin Bristlecone
Pine (Pinus longaeva) atop the cold, windswept
White Mountains of the Great Basin Desert in the US
Southwest might live in excess of 4,000 years. Tropical
trees show no comparable longevity. Ecologists have
studied forest turnover, gauging the average time that
a given tree (defined within a certain size range) will
survive in a particular spot. If, at La Selva Biological
Station in Costa Rica, you randomly select a tree at
least 10 cm (4 in) in trunk diameter, how long before
it is somehow destroyed or dies? The answer is known:
the rate of local disturbance is sufficiently high that the
entire forest is estimated to turn over approximately
every 118 years, plus or minus 27 years, and 6% of
the primary forest is in young gaps at any one time.
One study, conducted from 1970 to 1982, indicated
an annual mortality rate of 2.03% for trees and lianas
of at least 10 cm diameter. More recent work at La
Selva provides good estimates of tree ages and forest
dynamics. Trees ranging from gap specialists (such
as Cecropia obtusifolia) to emergent canopy species
(Diperyx panamensis) were studied.104 chapter 7 if a tree falls . . . rain forest disturbance dynamics