Tropical forests Flashcards
Main limitations
Canopy level
Nitrogen availability
Torrential rainfall
Canopy limitation
The ground level PS is 100 times less than the top. Need a tree to fall for enough light for a sapling to grow.
Availability of N
Temperature is ideal for rapid recycling of nutrients (decomposition). Plants have to be very efficient in making use of it.
Torrential rain
High rainfall causes leeching of nutrients
Where to tropical forests occur?
- High mean T and PPT (all months)
- High solar irradiance (all year)
- Rapid N recycling (leaf litter decomp = 0.5 years)
- Acidic clay soil (in conjunction with leeching)
Major formations
- Neotropics = 50% of tropical vegetation
- African tropics = 20%
- Indo-Malesian = 30%
Subdivisions
- Lowland rainforest
- Montane forest - smaller trees
- Semi-deciduous forest - more seasonal PPT
- Swamp forests - seasonal PPT
- Mangroves in intertidal zones
Diversity in the tropics
- 40% of global flora is found in the tropics
- Diversity is particularly in respect to trees
e. g. Malaysia - 176 species/hectare, 1/2 are trees
Why are the tropics so diverse?
Several arguments:
- Climatic stability
- Pleistocene ice ages
- Complex structure
Explain the effect of the pleistocene ice age on diversity
Glacial advances and retreats moved the vegetation band further north, and then back to its original position
Glaciation > fragmentation>isolation>speciation
What are the 5 main strata?
(ABC) Trees - Emergent discontinuous, dense stratum below, shortest at bottom.
(D) Palms, ferns, tall herbs
(E) Herbaceous plants and seedlings
What are the two additional strata?
Climbers and epiphytes
Describe climbers
Fast-growing, light-demanding plants.
High leaf to stem biomass.
Attach to another plant - support via tendrils, hooks or twining around host.
Leaves protected by phototoxins.
Vulnerable to drought and nutrient deficiency (poorly developed root stock).
Describe epiphytes
Plants that grow on other plants (not usually harming host)
Moisture available through stemflow
- conducted along branches and stems
- put out root structures
Describe hemi-epiphytes
These begin as a climber but finish as an epiphyte as their twine or roots decay. Some develop aerial shoots to obtain oxygen.
How old is the tropical forest?
Several hundred years although tree rings are not distinctive.
Describe the leaves present
Smooth with tip drips to remove water
- discourages epiphytes (impair PS)
- Water impairs absorption of sunlight
Why may these plants have shallow roots?
Nutrient (N) are recycled very quickly. Plants need to trap nutrients in the upper soil level efficiently (competition). This is problematic for taller plants. Buttresses provide additional support.
Why might these plants need deep roots?
When there is a dry season, trees are more susceptible to drought if they have shallow roots. Deep roots can search lower soil levels for moisture.
How long do leaves last? And during dry season?
13 months. In dry seasons they shed them more quickly, particularly at the top of the canopy.
What is phenology?
Anything to do with seasons
Describe flowering
- continuous throughout the year
- increases in the dry season
- synchronised within species, unsynchronised between species (to limit competition for pollination)
Why does flowering increase during the dry season?
There is a reduction in leaf cover so more available to pollinators
Seed dispersion is synchronised…
Explain.
Within and between species - to try and reduce predation of the seeds. Idea is to overwhelm animals with a surplus of food so they have to store it somewhere.
Interplay between organisms.
The main method of pollination
Insects. There is not much wind flow.
Birds, Mammals and bats also important.
Why are bats important for pollination?
They pollinate during the night as well
Why is C monitoring so important?
Nearly half of PS on land occurs in the tropics (only makes up 10-20% global vegetation)
What is GPP?
Gross Primary Product (measure of photosynthesis
Release of C into the atmosphere is known as…
Autotrophic respiration
What is NPP?
Net primary product - the bit thats left for other organisms (accumulates at the land surface)
How is NPP calculated?
GPP - Ra
What happens to NPP?
Respiration within the leaf litter (fungi, bacteria). They return it to the atmosphere as CO2 - Heterotrophic respiration
What is NEE?
Net ecosystem exchange
NEE = GPP - Re
What would NEE be in a balanced ecosystem?
Zero.
What is Re?
Ecosystem Respiration = Ra + Rh
Where does carbon monitoring take place?
Isolated areas away from anthropogenic sources (e.g. Mauna Loa)
Why do we monitor Carbon exchange?
We want to know how vegetation will respond - whether it will become a PS-dominated system (bad) or a Resp.-dominated system (good)
What is FLUXNET?
Existed since 90s, measure exchange of CWE.
How does FLUXNET work?
It exists 15-20 m above the canopy and measures climatic variables and eddy covariance. NOT photosynthesis, just the net exchange.
What is eddy covariance?
based on the turbulent transport theory.
Measures energy fluxes from the covariance between vertical wind speeds and gas concentration and known heights - yields NEE
Why does FLUXNET monitor at night?
There is no photosynthesis so it measures purely Re. This can be used to calculate CO2 drawdown (GPP-Re)
How are EC sites distributed?
Globally but biased towards developed countries - problematic because the powerhouse of PS is in the tropics.
Why is the tropics underrespresented?
Economics reasons and because of limited access for construction and maintenance
What are the goals of FLUXNET?
- Constant and consistent monitoring of CWE (ideally across all PFTs)
- Understand and compare current ecosystems
- Predict what will happen to CWE exchange under increasing atmospheric CO2 and global warming
What are the inaccuracies of FLUXNET?
- Unlikely that we will be able to measure all C exchange (e.g. requires certain meteorological conditions)
- Underestimation of C flux is 20%
- Insufficient sampling in the tropics
How much sunlight [SW] is reflected back into the atmosphere [SW’]?
10-20% (albedo)
What happens to the energy absorbed by vegetation?
Heats the vegetation - warmth. Some returns as thermal radiation [LW’]
Where does downwelling thermal radiation [LW] come from?
Gas molecules in the atmosphere have thermal radiation
What is latent heat of evaporation [LE]?
Produced when water is turned from liquid into vapour (a lot of energy is required). This means water vapour given off acts as a major source of energy exchange.
What is sensible heat [H]?
Involves conduction and convection - transport of parcels of air back into the atmosphere
Explain water exchange
- Arrives as PPT
- Large portion released through ET
- The rest is
- stored in the soil
- Underground drainage into rivers
Using a diagram, explain energy exchange
[SW] down, [SW’] up, [LW] down, [LW’] up, [LE] up, [H] up
What did Cox et al’s study suggest?
With warmer and drier climate, the rainforest would start to die back. Plants have evolved to have enough water all year.
Describe Baker et al’s study
Based on FLUXNET data:
During dry part of the season, PS dominates. When its dry, they conserve water.
Explain the results of Baker et al’s study
- Shallow rootstock to collect nutrients quickly
- Upper soil dries out in dry season
- Bimodal root stock
What is a bimodal root stock?
Shallow roots for nutrients, tap roots for moisture.
