Gap Dynamics Flashcards
What are the stages of Aubréville’s gap model?
Gap creation, Regeneration phase, Mature phase
How are gaps created?
Trees die and fall, are blown down, struck by lightning. Crowns are meshed together by lianas. Large branches continually break off.
Gap characteristics of different rain forest types
Vary in average gap size, in gap size range and in gap fraction
What happens as a result of a gap opening?
Gaps create diverse microclimates, light, moisture, temperature and wind conditions vary. In costa rica gaps experience 8.6%-24% of full sunlight compared to understorey which received 0.4%-2.4% (typically receives 1%). Quality of light changes too (photosynthetically active radiation)
What did Portes et al’s 2007 study show?
In the forest gap maximum PPFD was ~1600 µmol m-2 s-1 and, in the understorey, it < 25 µmol m-2 s-1.
Air vapor pressure deficit (VPDair) under gap conditions reached 2.8 and 4.0 kPa in June and August,
In the understorey maximum VPDair was 1.5 kPa in June and 2.8 kPa in August.
Gap regeneration and canopy closure
Intense competition for nutrients and light. Rapid growth and reproduction take place.
Different stages of succession within gaps
Rapid colonisation by shrubs, vines, lianas, and seedlings of pioneer tree species.
Rapid growth of short lived light-demanding species which form a canopy over 10 – 30 years.
Slower growing shade tolerant (climax tree) species grow in biomass and species richness below the pioneer canopy and become taller. This phase transitions into the next phase when…
Eventually the shorter lived species die and the more shade tolerant climax trees emerge and re-establish the tall canopy (can take 75 – 150 years)
Pioneer species characteristics
- lots of small seeds
- widely dispersed
- widely distributed but species poor due to good dispersal
- gap-dependent species
- can have dormancy in seed bank
- seedlings persist in open well lit situations
- establish and grow rapidly in gap
- high rates of ps & respiration
- leaves have high hydraulic conductance, high transpiration
- water use inefficient
- crowns are open branched to capture light
- leaves are large
- short life span
- little investment in defence
- low wood density
- highly branched roots
- can be mycorrhizal
- live for 10-30 years
- Cecropia, Musanga, Trema genera
Climax species characteristics
- Seeds are often large, produced annually or less frequently (mast fruiting)
- Often no dormancy (recalcitrant)
- Dispersal is usually short range (diverse mechanisms)
- Germination occurs in the understorey
- Soil seed bank contains few species
- Species may persist as seedling banks
- Climax species usually germinate, establish and persist in shade below the canopy.
- Large variation in the degree to which these species can persist in the deep shade
- Survival in the understory is essential.
- Seedlings can maintain themselves for many years without putting on much growth waiting for a gap to open.
- low rates of photosynthesis and respiration
- Low rates of transpiration so they have a high WUE.
- There is a great variation/continuum amongst the climax species with respect to their ability to utilize light when a gap opens and consequently growth rates vary.
- Climax species often have a greater number of branches compare to pioneer species
- Leaves are relatively small, long lived and with a slow rate of turnover.
- Leaves are tough and contain chemical defences to deter herbivores.
- Adapted for survival at the expense of rapid growth
- Wood density is high
- Roots are mycorrhizal
- Once the pioneer species die the climax species continue to grow and become the emergent canopy trees.
- They can live for 100 years
Disturbance and diversity are related
Species may be adapted to exploit different stages of succession from gap opening to closing
Why is niche partitioning controversial?
the light partitioning hypothesis:
-Is there is a gradient in light availability at the forest floor
-Tree species show a differential distribution with respect to light
-There is a trade-off in species performance and survival that explains their different positions along the light gradient.
Poorter & Arets (2003) study :
- there was a gradient of light on forest floor, deep shade relatively rare
-Species occurred in similar environments but differed in their crown illumination index
-There was a trade off in species performance that was related to position on the light gradient
Why is niche partitioning controversial?
the light partitioning hypothesis:
-Is there is a gradient in light availability at the forest floor
-Tree species show a differential distribution with respect to light
-There is a trade-off in species performance and survival that explains their different positions along the light gradient.
Poorter & Arets (2003) study :
- there was a gradient of light on forest floor, deep shade relatively rare
-Species occurred in similar environments but differed in their crown illumination index
-There was a trade off in species performance that was related to position on the light gradient
But does this affect species diversity?
Do gaps help explain high tree species diversity?
No,
HUBBELL (1999):
- No correlation between species richness and gap disturbance
-Hubbell concluded that recruitment limitation was the major factor determining local species richness and species composition in the BCI forest.
What determines the species composition of regenerating gaps?
- size of gap, topography and microclimate
- degree of damage to existing vegetation following tree fall
- composition of soil seed bank
- re-sprouting of existing vegetation (lianas)
- natural seed dispersal into gap
- recruitment limitation
- JC hypothesis
Adaptations to light
Plasticity (response to changes in light availability) and acclimation (response to changes in irradiance)
