TROPICAL MOIST FOREST RESTORATION

 

K.D. Holl.  2002.  Tropical Moist Forest.  Ch. 23 in M.R. Perrow and A.J. Davy (eds.) Handbook of Ecological Restoration, Vol. 2, Restoration in Practice.  Cambridge University Press.

 

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 Latin America, Africa, SE Asia

          Within 23.5o N&S of equator

          More than half is in Latin America. Most in northern S. America; less in C.      America and Caribbean.

 

Rationale for Restoration

 

TMF are being cleared at an alarming rate.  Less than one third remains in SE Asia and Africa.  Less than 50% remains in Central America.  14% of the Amazon basin is deforested; infrastructure is in place to clear 40% in next two decades.

 

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.