gap dynamics and regeneration in tropical forests (lecture 3) Flashcards
What are the three main stages of the gap mosaic model?
- 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
How are gaps created?
- 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
What is the effect of gap opening allowing light into the forest?
- 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
What are the microclimate characteristics of a small gap in semi-deciduous seasonal tropical forest in Brazil?
- Portes et al., 2007
- 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
What are the stages of succession after a gap forms?
- 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)
What is the concept of pioneer and climax species?
- broad categorisation based on characteristics (functional traits) influencing growth and survival strategies
- pioneer-climax classification represent extremes on a continuum
What are pioneer species seed, environment and taxanomic characteristics?
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
What are pioneer species growth characteristics?
- 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
What are pioneer species leaf characteristics?
- 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
What are pioneer species wood & root characteristics?
- low wood density due to rapid growth rates
- highly branched roots for efficient acquisition of nutrients
- can be mycorrhizal
What are common genera of pioneer species and where are they found?
- cecropia (neotropics)
- musanga (africa)
- trema (pantropical)
- macaranga (paleotropics)
What are climax species seeds/seedling characteristics?
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
What are climax species growth characteristics?
- 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
What are climax species leaf characteristics?
- 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
What are climax species wood and root characteristics?
- high wood density (slow growth)
- mycorrhizal roots
Does disturbance promote diversity?
- forest gap dynamic fit with evidence disturbance promotes diversity
- species may be adapted to exploit different stages of succession from gap opening to closing
- generates diversity in species number & ecological strategies
Do light gradients (or other abiotic variables) lead to niche partitioning across gaps?
- thought to contribute to coexistence of rain forest tree species and promote species diversity
- three underlying premises:
- gradient in light availability at forest floor
- differential distribution of tree species with respect to light
- trade off in species performance that explains position along light gradient
Is there a gradient in light availability at the forest floor?
Poorter et al., 2003
- gradient of light on forest floor
- canopy openness ranged from 0.2 - 10.6%
- sites with high canopy openness were in gaps & relatively rare
- deep shade also relatively rare
What is the crown illumination index?
Poorter et al., 2003
- crown illumination Index (CII)
- visual estimate of amount of light received by tree crowns
- ordinal scale
- high CII values indicating high light levels.
1 - no direct light
1.5 - low amount of lateral light
2 - medium amount of lateral light
2.5 - high amount of lateral light
3 - parts of crown receives direct overhead light
4 - whole of crown receives direct overhead light
5 - crown completely exposed
Do different species occupy different light environments?
Poorter et al., 2003
- relative frequency distributions of crown illumination index for saplings of 10 rain forest species (1997)
- species occurred in similar environments but differed in their crown illumination index
Is there a trade off in species performance that was related to position on the light gradient?
Poorter et al., 2003
- proportion of seedlings in low light negatively correlated with successional position
- light demanding species had a low number of saplings in low light conditions
- light demanding species had higher sapling mortality in low light conditions
- higher growth response to light
What were the conclusion of the study on light gradients and niche partitioning?
Poorter et al., 2003
- three premises of the study were met
- clear gradient in shade tolerance within this group of non pioneer species
- species showed a subtle variation in plant strategy
- may have contributed to the packing of species along a small range of light environments
- light partitioning may have contributed to the co-existence of these species in conjunction with other mechanisms
Do forest gaps help explain high tree species diversity?
Hubbell (1999)
What did they do?
- Barro Colorado Island, Panama
- 1200 gaps of varying size in 50 hectare old forest plot studied over 13 years
- all woody plants with stem diameter at breast height (DBH) <1cm tagged, measured, mapped, identified to species level
- censuses in 1982, 1985, 1990, 1995
species divided into three categories:
- gap tolerant pioneer species
- shade tolerant species
- intermediate species
Do forest gaps help explain high tree species diversity?
Hubbell (1999)
What did they find?
- no correlation between species richness and gap disturbance
- gaps had greater seedling establishment & higher sampling densities per quadrat than control areas
- species richness higher in gaps than control areas
- much variation in species composition between gaps
- species richness per stem was no different in gap sites/control areas
