Lecture 4: Gap Dynamics Flashcards
In the 1930’s Aubreville proposed ___
‘gap mosaic’ model in which the forest cycles through 3 main stages
3 main stages of ‘gap mosaic’ model
1) gap creation: immediately following a tree fall or gap opening
2) Building or regeneration phase: which is characterised by competition for light & other resources leading to regeneration & growth
3) Mature phase: where the high emergent forest canopy is re-establsihed
Gaps are a ___ part of forests dynamics
natural
Forests are a mosaic of __ &__ at different stages of ____
gaps & trees and different stages of regeneration
how are gaps created?
- rain forest trees will eventually die and fall, they may be uprooted or snap
- trees may be blown down
- struck by lightning& fall
how does one tree falling cause so much destruction:
- when one tree falls it often brings many more as crowns are meshed together by lianas
- Large branches continually break off from trees and fall
- Gaps vary in size
Damage to vegetation below varies with
gap size, composition of surrounding vegetation, type of forest
gap fraction is defined as
the fraction of sky visible through the canopy
Gap characteristics of different rain forest types ___
VARY
- Gap size & Gap fraction
gaps create ___ microclimates affecting _______
diverse microclimates affecting light, moisture, temperature & wind conditions
__ is the most limiting factor in the understorey of rain forests
Light
full sunlight is approximately ___ micromoles/m^2/s on a sunny day
2000 micromoles/m^2/s
At la Selva, Costa Rica gaps of approximately 300m^2 experience ___% of full sunlight compared to the understory which received ___ full sunlight
gaps = 8.6-24% understory = 0.4-2.4%
Quality of ___ also changes with an increase in Photosynthetically Active Radiation (PAR) (400-700nm)
LIGHT
Gaps receive differing levels of irradiance from…
full sun in the centre to shade at the edge
Vapour Pressure Deficit, or VPD, is the
difference (deficit) between the amount of moisture in the air and how much moisture the air can hold when it is saturated
once a gap opens competition for ___ & ____ is intense &____ take places
once a gap opens competition for light and nutrients is intense and rapid growth and reproduction take place
there are different stages of succession within gaps:
-rapid colonisation by shrubs, vines, lianas & seedlings of PIONEER TREE species
-rapid growth of short lived light-daminding species which form canopy over 10-30years
-Slower growing shade tolerant (CLIMAX TRESS) species grow in biomass and species richness below the pioneer canopy & become taller. This phase transitions into the next phase when
-shorter lived species die & the more shade tolerant climax trees emerge & re-establish the tall canopy (can take 75-100 years )
HOWEVER LOT OF VARIATION IN THESE PATTERNS
classification for pioneer and climax species:
- broad categorisation based on plant characteristics (functional traits) that influence growth & survival strategies
- pioneer-climax classification represent extremes on a continuum
pioneer species typically produce:
lots of small seeds
seeds of pioneer species are dispersed __
widely –> wind / animals
pioneer genera are ___ distributed but species ___ as good dispersal tends to reduce the development of distinct populations
widely
poor
pioneer: seeds can ____ in the soil seed bank until a gap opens producing favourable conditions for germination
lay dormant
most pioneer species require __ to germinate and are ___
light and are gap - dependant
pioneer species growth rates:
- typically establish and grow rapidly whilst light nutrients and water are available
- high rates of photosynthesis & respiration that allows rapid growth under high light
- leaves have a high hydraulic conductance’ they transpire rapidly allowing high rates of photosynthesis
- water us efficiency is low
pioneer crowns are usually
open branched to capture light maximally
leaves of pioneer species are usually quite
large for good light capture
pioneer species leaf life span
short life span and turnover quite quickly
pioneer species don’t invest greatly in
defence against pathogens and herbivores
pioneers hold a trade off between
growth & defence
wood density of pioneer species
= low, have rapid growth rates
pioneer species roots:
highly branched for efficient acquisition of nutrients
pioneer can be m___
mycorrhizal
how long do pioneer species typically live for?
10-30 years
most pioneer species belong to
a few genera and families
common genera of pioneer species:
- Cecropia
- Musanga
- Trema
climax species: Seeds & germination
- seeds often large, produced annually / less frequently
- often no dormancy
- dispersal is usually short
- germination occurs in understorey
- soil seed bank contains few species
- species may persist as seedling banks
where do climax species usually germinate, establish and persist:
in shade below the canopy
climax: survival in the understory is
ESSENTIAL
climax seedlings can maintain themselves for
many years without putting on much growth waiting for a gap to open
climax species have __ rates of photosynthesis and respiration compared to pioneer species
low
climax species and transpiration
low rates of transpiration so they had a high WUE
climax species often have _____ number of branches compare to pioneer species
greater number
climax species leaves:
relatively small, long lived and with a slow rate of turnover
climax and herbivores;
leaves are tough and contain chemical defences to deter herbivores
climax species are adapted for
survival at the expense of rapid growth
climax species and wood density
is high
climax species roots
are mycorrhizal
once the pioneer species die..
the climax species continue to grow and become the emergent canopy trees
climax trees can live for
100 years
evidence suggests that disturbance and diversity are related: Gap dynamics
‘Gap dynamics’ in forests fits with evidence that disturbance promotes diversity i.e.
- Species may be adapted to exploit different stages of succession from gap opening to closing
- This generates diversity in species number and ecological strategies
what is the crown illumination index
(CII) is a visual estimate of the amount of light received by the tree crown
Ignore
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