TROPICAL MOIST
K.D.
Holl. 2002. Tropical Moist
Background
Tropical
forests cover 7% of the terrestrial surface of the planet, but provide habitat
for more than 50% of all plant and animal species.
TMF
(tropical moist forests) have:
High rainfall (>1700 mm/yr)(68 in)
Even distribution of solar radiation
over the year
Constant high temperatures (monthly
mean >24oC)
Lack of frost
Rainforests
have equal rain throughout the year; other tropical forests have distinct dry seasons.
TMF
are primarily found on highly weathered soils with:
High clay content
Low phosphorus
High Fe, Al
Located
in
Within 23.5o N&S of
equator
More than half is in
Rationale for Restoration
TMF
are being cleared at an alarming rate.
Less than one third remains in
Causes
of clearing:
To create pasture for cattle grazing
Logging
Firewood
Commercial and subsistence agriculture
Population growth/resettlement
programs
Fire
Accelerating
feedback loop:
Fires to clear cattle pastures (are)
increasingly likely to spread (because
of)
decreases in rainfall (due to)
regional deforestation.
Fires will likely be worse in future
because of climate change.
Stable
TMF affects:
Carbon cycling. 23% of anthropogenic
CO2 emissions are from tropical forest clearing.
Soil stability and the hydrologic cycle.
Erosion control maintains good water quality.
Most nutrients are in living biomass.
50% of rain that falls is transpired by
plants. Clearing changes both hydrologic and nutrient cycling. Much less rain will fall.
Biodiversity. Extinction rates are now 100 to 1000 times
pre-human levels.
Agricultural
land in tropics loses productivity quickly and is abandoned. Restoration is now being considered an option
because of this productivity loss, and:
subsidies encouraging ag conversion
are being cut,
impacts on conservation and well-being
of indigenous people are rippling through society,
global CO2 strategies are
being recognized as being important.
Factors limiting recovery
Slash
and burn agriculture has long been practiced.
On abandoned farm-land, 15-60 years was required to regain biomass and
native spp.
Recovery
is slower on more disturbed sites:
Many more wind dispersed tree species
appear.
Some places will recover quickly,
others slowly, depending on site, intensity of disturbance,
and local species present.
Recovery
requires:
Dispersal of seeds
Avoidance of predation
Germination of seeds
Survival and growth of seedlings
These processes may be impacted by a
number of factors: seed predation is affected
by herbivore abundance, which is influenced by vegetative cover.
Seed
Availability
In
TMF, lack of seed is a primary factor limiting recovery (principally due to
poor dispersal).
Most
tropical seeds have very short period of viability.
Because
they are “recalcitrant” (will not tolerate drying like orthodox seeds), they
are absent from soil seedbanks in pasture land.
In ag
land that has been used for any length of time, roots have been destroyed and
are not available for resprouting.
The
majority of TMF seeds are animal dispersed:
Many birds, bats and other mammals
avoid cleared land.
“Seed rain” declines rapidly within a
few feet of forest edge.
Most seeds falling in a pasture are
from trees already in the pasture, or from a few small-seeded pioneer spp.
Activity is generally concentrated
under remnant trees or shrub patches.
High
rates of seed predation are the result of small mammals, ants, bruchid beetles.
Summary:
Few forest seeds arrive in pastures.
Those that do are highly patchy in
distribution.
Seed
Germination
There
is a wide range of germination rates for forest trees in abandoned pastures.
High rates for some species.
No germination for other species.
Presence
of pasture grasses may create a beneficial microclimate.
Germination
is variable, but is probably not the limiting factor in restoration.
Competition
with Existing Vegetation
Seedling
establishment in pastures is limited primarily by lack of dispersal, then
secondarily by predation and by failure of germination in some species.
Then,
other biotic and abiotic factors limit:
Competition with aggressive vegetation
Stressful microclimate
Fire
Lack of soil nutrients
Reduced mycorrhizal inoculum
Herbivory
Aggressive
existing vegetation is a major factor:
Exotic pasture grasses (Axonopus, Brachianaria, Melinus, Panicum,
Paspalum).
Dense ferns.
Existing
vegetation suppresses forest seedling growth by:
Providing shelter for seed and
seedling predators (rodents, leaf-cutter ants)
Competing for water, nutrients, light
Increasing fuel load for fires
Allelopathy.
Diversity
is higher where pasture grasses are cleared, though the grass may provide a
better germination microclimate.
Microclimate
and Fire
Compared
to forests, temperatures in pastures are higher; humidity and soil moisture are
lower.
Seedling
water stress is greater; available light is in excess of that needed for
optimal photosynthesis.
Climate
change leads to more fire.
Tropical
tree seedlings not fire-adapted.
Grass
not impacted by fire.
Soil
nutrients and microbial communities
Soils
are phosphorus poor in pastures.
Many
forest soils are nutrient poor; plants are adapted to this.
There
may be a mycorrhizal limitation; many forest trees are obligate mycorrhizal.
There
are, however, lots of fungal spores in pasture soils.
Herbivores
May be
a major factor in some pastures
Leaf cutter ants (Atta sexdens) can cause 30-80% defoliation in days.
Rabbits can also decimate sapling
populations.
Strategies for Accelerating
Recovery
The
state of tropical restoration
Restoration
efforts are a recent phenomenon (5-10 yrs).
It is
a trial and error process.
Many
efforts have been by small, grass-roots groups that have not widely
communicated their results.
Tropical
forests host a large number of species.
Species-by-species re-introductions do not look like an easy task.
Facilitating succession is the major
strategy.
This can work as long as there is a
large element of native forests near restoration projects to provide propagules, organisms.
Some
species of conservation interest are introduced (mahogany, Swietenia macrophylla).
Planting
Native Tree Seedlings
Most
common strategy:
plant seedlings,
of a few native tree species,
that are fast-growing, drought
resistant, able to grow in low-nutrient soils.
Seedling
planting results in:
improved understory diversity,
better soil structure and nutrient
availability,
ameliorated temperature and soil
moisture,
perches for bird and bat dispersal.
Until
recently, degraded TMF was planted with pinus,
eucalyptus, and tectona (teak).
Now,
many native trees are being tested for their ability to quickly colonize poor,
degraded sites:
Seeds can be collected locally.
Are generally grown out in nursery
from 3 mo to a year.
Direct
seeding is an option, but predation rates are high.
Starting
and maintaining seedlings is difficult in the tropics:
1. Seed collection is difficult: trees
do not set seed every year, tree identification is hard, tree seeds are up in the canopy, and some must be
collected from tree to insure
germination.
2.
Tropical tree seed-germination information is limited.
3.
Seeds of many trees die if dried, therefore they will not store.
4.
In dense grass, biomass must be cleared away from seedlings every few months until trees overtop grasses.
5.
Seedling herbivory is high.
In
spite of difficulties, studies have shown that some species have survival rates
in excess of 80%, and will grow as much as 2.5 m per yr.
But there are some sites where
survival and growth are much less, due to:
Slope, aspect
Soil nutrients
Mother tree seeds collected
from
An
existing canopy will increase chance of seedling survival.
You
may manage a site:
Add fertilizer
Fence to suppress herbivory
Apply fungicides to suppress
leaf-cutter ant colonies
There
are downsides to active management; fertilizer encourages weeds, fences are
expensive, fungicides may have residual effect.
You
may use live stakes instead of planting seeds or seedlings:
Living fences are common in the
tropics.
Trunk stakes seem to work better than
branches.
Air-rooting
works:
Plastic bag, soil, growth hormone,
incision in cortex.
Non-native
trees as nurse trees
Why
would you use non-natives?
Lack of nurseries for natives.
Slow growth and low survival of
natives.
Non-native
tree plantations:
Shade out pasture grasses
Increase soil nutrients
Enhance seed dispersal
Provide source of income for
landowner.
Must
be carefully matched for their:
Aggressiveness
Potential to spread
Potential to alter soil chemistry.
Remnant
Trees and Planting Patches of Trees
Remnant
trees:
Increase seed dispersal
Ameliorate microclimate
Increase soil nutrients
Result
is better growth below the remnant trees.
Patches:
Patches
may be cost-effective (Robin Hood in reverse theory)
They will spread
They will add to spatial diversity
Seeding
Shrubs
Shrubs
may provide the same advantages as trees:
Microclimate
Increasing nutrients
Shading out grasses
Perches for dispersers.
Shrubs
can, however, suppress tree seedlings.
Shrubs
can accelerate recovery:
They produce copious seeds, easily
collected.
May be directly seeded, even with high
seed predation, because you can collect so many
seeds that seed predators cannot eat all of them.
Artificial
Perches, Slash Piles, Logs
Perches
offer an advantage:
In pastures with no remnant trees.
In areas where majority of woody
plants have adaptations for bird dispersal.
Birds
still prefer real trees.
Bats
do not use them; bats are important dispersers of larger-seeded spp.
Grass
needs to be cleared from beneath perches for tree seedling survival.
Slash
piles create seedling microsite improvement, and serve as perches.
Clearing
Existing Vegetation
Burning
grasses
There is a risk of fire escaping into
forest.
Herbicides
Grasses require repeated treatments.
Expensive
Potential soil and water pollution.
Clearing
may have negative side effects
Increases erosion
Creates a stressful microclimate.
Initial
clearing during first year of seedling growth may be best strategy.
A good
shade producer that shades out grasses is the best solution.
Fire
Prevention
Fire
kills woody species:
After restoration begins, fire should
be suppressed.