Rainforest Ecology

• If a tree falls in the rainforest and no one is around, does it make a sound?

• It makes a GAP!!Who Cares!!

Rainforest Ecologyforest gaps

• Gaps have a different environment• More light, changes humidity and temp• The range is gap size is widespread (3-

400m2), but probably follow logarithmic • Large trees, particularly emergents, can

take out several others• Lianas increase the likelihood of a

multiple tree fall

Rainforest Ecology forest gaps

• When? – Tree falls peak around the middle of the

rainy season (extremely wet with relatively strong winds)

• Landslides can create long swaths of openings

• How frequent?– 4-6% of any forest may be comprised of

recent forest gaps

Rainforest Ecology forest gaps

• Gaps are typically shaped like an inverted cone, where it is larger the higher you get into the canopy

• This adds a nice component to structural diversity (which is going to influence biodiversity)

• Rapid growth and reproduction

Rainforest Ecology forest gaps

• Measurements made at La Selva in Costa Rica indicate that gaps of ≈300m2 experience 8.6-23.3% full sunlight, compared with interior forest understory, which receives only 0.4-2.4% full sunlight (50x more), which is appropriate for photosynthesis

Rainforest Ecology forest gaps

• Of 105 canopy species studied as saplings at La Selva, about 75% are estimated to depend at least in part on gaps to complete their life cycle

Rainforest Ecologyforest gaps

• Denslow (1980) suggested that rainforest trees fall roughly into 3 categories (with respect to gaps)– 1) large-gap specialists whose seeds

require high temps of gaps to germinate and seedlings are not shade tolerant

– 2) small-gap specialists whose seeds germinate in shade but whose seedlings require gaps to grow to mature size

– 3) understory specialists, don’t need gaps

Rainforest Ecologyforest gaps

• In reality, species don’t fit into these 3 packages

• Many shade-tolerant tree species show no gap association but rather demonstrate high levels of growth plasticity growing in shade but accelerating growth in sunlight

Rainforest Ecologyforest demographics

• How long do trees survive?• How long does it take a seedling to

reach the top of the canopy?• Is growth even during the seasons?• What is the impact of short-term

climatic fluctuations?• What factors influence tree survival?

Rainforest Ecologyforest demographics

• To answer these questions, we need long-term, detailed studies tracking the fate of individuals trees

• La Selva and BCI

Rainforest Ecologyforest demographics: La Selva

• Longevity-for any given tree (>10 cm), it can expect to live 118 ± 27 yrs; estimate of disturbance rate resulting in complete turnover of the forest (although this would be rare with an average much shorter)

• Another study suggests annual mortality of 2.03% for ‘large’ trees

• Other Amazon studies, 1.2%, 1.13%, 1.5%

Rainforest Ecologyforest demographics: La Selva

• Remember, adults survive much better than seeds or seedlings

• For Dipteryx panamensis seedlings (7 to 59 mo) experienced 16% mortality from litter fall alone

• Study of 6 sp of seedlings survival ranged 3-19%/yr

Rainforest Ecologyforest demographics: La Selva

• Because growth rate is so strongly influenced by sunlight, growth is not consistent throughout an individuals life

• Why do emergents emerge?– Sunlight– Escape other falling trees– They do have higher survival rates

Rainforest Ecologyforest demographics: BCI

• 50 ha plot (1980) all plants >1cm surveyed, then again in ’82, ’85, & ’90

• El Niño brought drought in ’83• From ’82-’85, >8cm plants had a

mortality rate of 3.04% / yr• 10.5% in shrubs, 18.6% in understory

trees, 19.3% in subcanopy trees and 31.8% in canopy trees

• DBH > 16cm 50% mortality

Rainforest Ecologyforest demographics: BCI

• Those that survived showed elevated growth rates (e.g. 60% faster than ’85-’90)…why?

• Imagine all the gaps created…• That was short lived as by 1991 the

number of gaps returned to predrought estimates

Rainforest Ecologyforest demographics: BCI

• Turnover…what is it?

Rainforest Ecologyforest demographics: BCI

• Turnover• Many species changed abundance• 10 sp were lost and 9 were gained from

’82 to ’90• Overall, rather consistent• E.g. 1982=301 sp, 4,032 ind, 1985=303

sp, 4,021 ind, 1990=300 sp, 4,107 ind• Why?

•There are always winners and losers

Rainforest Ecologyforest demographics: BCI

• Conclusions:– 1) sort-term fluctuations can have

relatively dramatic effects on forest composition and structure

– 2) there is long-term change in species composition (precipitation has declined dramatically over past 70 yrs…14%)

• 20-30 moisture-sensitive sp may be lost and rare sp were more (-) impacted

Rainforest Ecologyforest demographics: BCI

• “the BCI forest suffered a sever drought, yet the overall structure of the forest bounced back. There is a regulating force at work here: remove a tree and a tree grows back. But this force only preserves the forest as a forest, not the diversity of tree species it contains”

• Condit et al. 1992

Rainforest Ecologydisturbance and ecological

succession• In many places the forest is relatively

open

Rainforest Ecologydisturbance and ecological

succession• But in others, it can be extremely

dense

Rainforest Ecologydisturbance and ecological

succession• When the forest gets disturbed

enough, it can properly be called ‘jungle’

• Oxford: “land overgrown with tangled vegetation, especially in the tropics”

• Jungles are representative of disturbed rainforest (which is everywhere)

Rainforest Ecologydisturbance and ecological

succession• Think of a forest being cleared• After the land is abandoned, some

things start to colonize (which ones can be random to predictable)

• Eventually, it will likely to likely come back to the pre-disturbance state (albeit with some changes)

• The speed at which this occurs resilience

Rainforest Ecologydisturbance and ecological

succession• The sequence of events can also be

predictable• E.g. the first species are going to

those that do well in light, temp variability, and have a fast growth rate

• Next sp that tolerate the shade better and may be stronger competitors eventually come in and displace the first set of pioneer sp

Rainforest Ecology disturbance and ecological

succession• Bare soil• Sedges and grasses• Herbs, shrubs and vines

(epiphytes rare) • Trees quickly appear

(many which reproduce quickly)

• More shade tolerant sp compete for light

Rainforest Ecologydisturbance and ecological

succession• Within Amazonia, a typical square

meter of soil is estimated to contain 500-1000 seeds. This does not take into account the number of seeds deposited on the surface on a relatively regular basis

• The result is a crowded group of plants (e.g. vines, shrubs, and quick growing palms) all competing for sun

Rainforest Ecology disturbance and ecological

succession• Is the composition of early success

plots very consistent?• The ‘species pool’ is generally too

large to predict which species will establish themselves, although we may be able to describe some of their general attributes

Rainforest Ecology disturbance and ecological

succession• Species appearing early (colonizers)

grow rapidly and produce many seeded fruits

• In later succession, most plants tend to be larger, grow more slowly, and have fewer seeds per fruit (equilibrium sp)

Rainforest Ecology disturbance and ecological

succession• Depending upon the frequency of

disturbance and the quality of soil, there may be significant changes

• E.g. on poor soil the repeated displacement, colonization of rapid growing plants, may result in a general long-term drain on minerals resulting in a shift from forest to savanna

Rainforest Ecology disturbance and ecological

succession• Successional forest can be relatively

productive (biomass and LAI) often mimicking older forests, although composition is not similar– E.g. 11 mo after burning, one plot had a

dense mixture of veg attaining 5m• By 15 yrs, it may resemble a older,

closed-canopy forest (again, not in composition)

Rainforest Ecology regeneration pathways

• The regeneration pathways are going to be dependent on the scale and intensity of the disturbance

• Some possible pathways include:– 1) from seedlings and saplings already

present in the forest understory (advance regeneration pathway)

Rainforest Ecology regeneration pathways

• Some possible pathways include:– 2) from vegetative sprouting from stem

bases and/or roots (which remain after trees are disturbed)

– 3) from recolonization by germination of seeds already present in soil (seed bank)

– 4) from the arrival of new seeds brought by wind or animal dispersal

Rainforest Ecology regeneration pathways

• On small scale disturbances (0.01 -10 ha), the advance regeneration pathway dominates– There is a huge storehouse of small

saplings (10-20 per m2) waiting their turn– They will represent >95% of all trees

>1m tall 4 yrs after gap formation

• ‘sprouting’ is a common approach of some individual trees in small gaps

Rainforest Ecology regeneration pathways

• In large scale disturbances (1-100,000 km2) death to the understory trees is common (e.g. fire, flooding)

• Regeneration is from a combination of vegetative sprouting plus germination of seeds in the soil, plus import of seeds by dispersal mechanisms

Rainforest Ecology disturbances

• Fires in Amazonia appear to relatively frequent in recent (6,000 yrs) time

• Probably occurred during dry periods

Rainforest Ecology disturbances

• Pastures abound in the tropics• Relatively good evidence that much

(light to moderately grazed) of converted rainforest can convert back from pasture into rainforest

• The speed of the resilience will depend upon the intensity of grazing

• The composition of reclaimed forest was very different than original forest

Rainforest Ecology disturbances

Rainforest Ecology disturbances

Rainforest Ecology regeneration pathways

• Tikal practiced intensive agriculture and had relatively extensive urbanization (i.e. deforestation)

• Having been abandoned, it was ‘reclaimed’ by the jungle

• Figs, palms, and mahoganys all sat on the main square

• Given enough time, most gaps can be closed!

Rainforest SuccessionalPlants

• Heliconia: one of the most conspicuous early colonizers

• Sun loving• 150 sp • Hermit hummingbird• Bloom

sequentially• Birds disperse seeds (28 sp)

Rainforest SuccessionalPlants

• Piper is a common early successional plant (and understory) with 500 sp

• Most are shrubs• Pollinated by many sp. of bees,

beetles and fruit flies• Fruit dispersal

by Carollia bats

Rainforest SuccessionalPlants

• Mimosas and Legumes (acacias)

• Probably most diverse family of tropical plants (everywhere in tropics)

Rainforest SuccessionalPlants

• Cecropias are abundant and are readily found in gaps

• Seeds remain viable (2 yr)• In Surinam 73/m2

• Grow rapidly (2.5 m/yr)• Short-lived• Moderate in size• Easy to recognize

Rainforest SuccessionalPlants

• Cecropias have separate sexes• Female can producee 900,000 seeds• 48 sp of animal use Cecropia

obtusifolia• 33 bird sp feed on its flowers & fruits• Hollow stems (why?): biting ants

(Azteca spp) live inside feeding on extrafloral nectaries

Rainforest SuccessionalPlants

Rainforest SuccessionalPlants

• Ceiba, Silk Cotton, or Kapok Tree

• Very typical traits (e.g. flat crown, buttress, smooth bark)

• High light requirement, common rivers, edges, disturbed areas

• Rapid growth• Lose leaves prior to

flowers (why?)

Rainforest SuccessionalPlants

• May flower once every 5 or 10 yrs• 500-4,000 fruits with 200+ seeds per

(thus 800,00 seeds per yr)• Each seed is surrounded by silky,

cottonlike fibers called ‘kapok’• May aid in seed dispersal• Leaves not well protected, lots of

insect damage. Is this a problem?

Rainforest SuccessionalPlants