gap dynamics and regeneration in tropical forests (lecture 3) Flashcards
1
Q
What are the three main stages of the gap mosaic model?
A
- proposed by Aubreville in the 1930s
1) gap creation immediately following a tree fall or gap opening event
2) building or regeneration phase characterised by competition for light/other resources leading to regeneration/growth
3) mature phase where high emergent forest canopy is re-established
2
Q
How are gaps created?
A
- rainforest trees eventually die & fall - e.g. uprooted/snap/blown down/struck by lightning
- trees often bring down more trees as crowns meshed together with lianas
- large branches continually break off from trees and fall
- gaps vary in size
- damage to vegetation below varies with composition of surrounding vegetation & type of forest
3
Q
What is the effect of gap opening allowing light into the forest?
A
- creates diverse microclimates affecting light, moisture, temperature and wind conditions
- light is limiting abiotic factor in rain forest understory
- full sunlight 200μmol per metre squared
- la Selva, Costa Rica
- gaps of approximately 300 m2 experience 8.6%-24% of full sunlight
- understorey which received 0.4%-2.4% full sunlight
- quality of light changes with an increase in photosynthetically active radiation (PAR), light with wavelength 400-700nm
4
Q
What are the microclimate characteristics of a small gap in semi-deciduous seasonal tropical forest in Brazil?
- Portes et al., 2007
A
- light in forest gap reached 1600µmol per metre squared per second
- less that 25µmol per metre squared per second in understorey
- air vapour pressure deficit (VPDair)
- gap conditions 2.8kPa in June and 4.0 kPa in August
- understorey maximun 1.5kPa in June, 2.8kPa in August
- values observed at midday when air temperatures were high
5
Q
What are the stages of succession after a gap forms?
A
- gap opens
- competition for light & nutrients is intense & rapid growth and reproduction take place
stages of succession:
- rapid colonisation by shrubs, vines, lianas, pioneer tree species seedlings
- rapid growth short-lived light-demanding species form a canopy over 10-30 years
- slower growing shade tolerant climax tree species grow in biomass & species richness below the pioneer canopy and become taller
- eventually short lived species die and shade tolerant climax trees emerge and reestablish the tall canopy (75-150 years)
6
Q
What is the concept of pioneer and climax species?
A
- broad categorisation based on characteristics (functional traits) influencing growth and survival strategies
- pioneer-climax classification represent extremes on a continuum
7
Q
What are pioneer species seed, environment and taxanomic characteristics?
A
seeds:
- lots of small seeds
- dispersed widely by wind/animals
- can lay dormant in soil seed bank until a gap opens producing favourable conditions for germination
- require light to germinate
- gap dependent
- seedlings persist in well lit gaps, are rarely found in understorey
- pioneer genera widely distributed but species poor
- species belong to few genera/families
- good dispersal reduces development of distinct population
8
Q
What are pioneer species growth characteristics?
A
- establish and grow rapidly whilst light/nutrients/water available
- high rates of photosynthesis and respiration
- rapid growth under high light
- leaves with high hydraulic conductance
- transpire rapidly allowing high rates of photosynthesis
- water use efficiency low
9
Q
What are pioneer species leaf characteristics?
A
- crowns open or branched to capture light maximally
- leaves of pioneer species usually quite large for light capture
- short lifespan and turnover quickly
- little investment in defence against pathogens/herbivores
- trade off between growth and defence
10
Q
What are pioneer species wood & root characteristics?
A
- low wood density due to rapid growth rates
- highly branched roots for efficient acquisition of nutrients
- can be mycorrhizal
11
Q
What are common genera of pioneer species and where are they found?
A
- cecropia (neotropics)
- musanga (africa)
- trema (pantropical)
- macaranga (paleotropics)
12
Q
What are climax species seeds/seedling characteristics?
A
seeds:
- large
- produced annually or less frequently (mast fruiting)
- short range dispersal, diverse mechanisms
- few species in soil seed bank
seedlings:
- germination in understorey
- species may persist as seedling banks
- survival in understorey is essential
- can grow very little for years waiting for gap to open
- large variation in degree of shade tolerance
13
Q
What are climax species growth characteristics?
A
- low rates of photosynthesis and respiration compared to pioneer species
- low rates of transpiration
- high water use efficiency
- great variation/continuum between climax species in growth-survival trade offs e.g. ability to utilise light when a gap opens
- continue to grow and become emergent canopy trees
- can live 100 years
14
Q
What are climax species leaf characteristics?
A
- more branches than pioneer species
- relatively small leaves
- long-lived
- slow rate of turnover
- tough with chemical defences to deter herbivores
- adapted for survival at expense of rapid growth
15
Q
What are climax species wood and root characteristics?
A
- high wood density (slow growth)
- mycorrhizal roots
