adaptations to light in tropical rainforests (lecture 4) Flashcards Preview

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Flashcards in adaptations to light in tropical rainforests (lecture 4) Deck (18)
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1
Q

How has light shaped tropical rainforests?

A
  • light powers tropical rainforests via photosynthesis
  • plants evolved various mechanisms to optimise use of sunlight/survive in deep shade
  • can exploit contrasting habitats e.g. understorey vs gap
  • can acclimatise to changes in irradiance
2
Q

What is plasticity and acclimation in tropical forest plant species?

A
  • many plants show great plasticity in response to changes in light availability within a particular habitat
  • acclimation - process of adapting to new environment
  • potential for acclimation to irradiance changes
  • enables plants to exploit more variables than those with narrow range of responses to light
3
Q

What are sunflecks? How do they relate to plant plasticity?

A
  • sunflecks = short bursts of irradiance
  • major source of energy for maintenance/growth in the understorey
  • can be 10-85% total daily light exposure in understorey plants
  • can enhance C gain by 60-70%
  • dynamic irradiance of sunflecks varies in frequency/intensity
  • plants must be adapted to optimise use of this energy when available
4
Q

What changes in irradiance do climate trees experience in their lifetime?

A
  • start life in the understorey
  • irradiance increases when gap opens
  • seedlings need to be able to adapt and take advantage of the light to grow
  • shade-tolerant tree growth initially occurs below pioneer canopy
  • climax tree crowns will emerge into full sunlight eventually
  • plasticity & ability to acclimate to both short and longer term changes in irradiance
  • important determinants of ability to compete/establish/survive
5
Q

How does Begonia erythrophylla maximise light interception into the understorey?

A
  • B. erythrophylla
  • epidermal cells behave like lenses, focus light onto chloroplasts
  • irradiance reaching chloroplasts 15x greater than incident light and lead surface
6
Q

How do anthocyanin pigments maximise light interception of understorey plants?

A
  • some monocots/dicots
  • possess red/purple anthocyanin pigments on underside of leaves
  • increases efficiency of light capture
  • reflects absorbed light into leaf
7
Q

How do blue irridescence maximise light interception of understorey plants?

A
  • few species of plants in taxa Selaginella, Melastomatacae, Begoniceae
  • blue iridescence
  • result of microscopic anatomical features
  • interfere with light, increase capture of PAR at end of spectrum
8
Q

How do tree form and leaf placement affect light capture efficiency?

A

understorey trees:

  • often have horizontally inclined branches (plagiotropic)
  • wide fairly spaced leaves
  • self-shading minimised by leaf shape size and angle

trees growing in sun:

  • more vertical growth (orthotropic)
  • small leaves
  • oriented further than horizontal to avoid damage from excess irradiance (photoinhibition)
  • can change form/leaf angle at different stages of their lifestyle
  • need to maximise irradiance as they grow beneath a canopy
  • but crowns eventually emerge into full sunlight
9
Q

How do tree form and leaf placement affect light capture efficiency in Macaranga gigantia?

A
  • large leaves near stem
  • expansion takes three weeks but
  • petioles extend for 91 days
  • allows aging leaf to extend beyond newly produced leaves
  • avoids shading them
10
Q

How do leaves of plants adapted for sun and shade differ?

A
  • adapted usually to growth into direct sunlight OR shaded conditions
  • same for leaves
  • leaves that grow under shade of other leaves are anatomically/metabolically different from those in expose canopies
  • sun plants have higher rate of light saturated photosynthesis (Pmax)
11
Q

What are key differences between cells of sun and shade plants?

A

sun plants:

  • large cells
  • small chloroplasts
  • low chlorophyll:rubisco ratio
  • high chlorophyll a:b ratio
  • high N content
  • high xanthophyll cycle pigments

shade plants:

  • small cells
  • large chloroplasts
  • high chlorophyll:rubisco ratio
  • low chlorophyll a:b ratio
  • low N content
  • low xanthophyll cycle pigments
12
Q

What are key differences between leavesof sun and shade plants?

A

sun plants:

  • small thick leaves
  • high stomatal density
  • high transpiration rate
  • high chlorophyll a:b ratio
  • low specific leaf area (area per unit mass)
  • high N content
  • high xanthophyll cycle pigments

shade plants:

  • large thin leaves
  • low stomatal density
  • low transpiration rate
  • low chlorophyll a:b ratio
  • high specific leaf area (area per unit mass)
  • low N content
  • low xanthophyll cycle pigments
13
Q

What are key differences between sun and shade plants?

A

sun plants:

  • vertical leaf orientation
  • leaf area index higher
  • more canopy layers
  • short leaf lifespan
  • high leaf turnover
  • high photosynthetic capacity

shade plants:

  • horizontal leaf orientation
  • leaf area index lower
  • fewer canopy layers
  • long leaf lifespan
  • low leaf turnover
  • low photosynthetic capacity
14
Q

What are characteristics of sun in contrast to shade leaves?

A
  • typically thick
  • high N content per unit leaf area
  • higher chlorophyll a:b ratio
  • greater leaf rigidity to reduce wilting and drought susceptibility
  • thicker leaf cuticle contributing to rigidity
  • higher stomatal density, more CO2 into leaf
  • sun leaves are usually smaller/narrower than shade leaves
  • higher transpiration rates
  • large amounts of carotenoid xanthophyll pigments which protect from excess irradiance damage
15
Q

Does seedling ecology reflect differences in ability of photosynthetic apparatus of two climax dipterocarps to acclimate to different light environments?

Zipperlen and Press, 1996

  • tree characteristics
A

S. leprulosa

  • light hardwood
  • faster growth rate
  • needs higher light to regenerate
  • seedlings don’t persist long in deep shade

D. lanceolata

  • medium hardwood
  • seedlings survive in a wide range of light conditions
  • deep shade to canopy gaps
16
Q

Does seedling ecology reflect differences in ability of photosynthetic apparatus of two climax dipterocarps to acclimate to different light environments?

Zipperlen and Press, 1996

  • experimental protocol
A
  • seedlings reared from seed
  • neutral density shade at 30% irradiance
  • 3 months
  • seedlings planted in danum valley
  • three contrasting light environments: undisturbed primary forest (low light)/ five year old secondary forest (medium light)/ one year old secondary forest (high light)
  • irradiance varied 2.4-2.45mol of photons per metre squared per day
  • plants measured after 15 months
17
Q

Does seedling ecology reflect differences in ability of photosynthetic apparatus of two climax dipterocarps to acclimate to different light environments?

Zipperlen and Press, 1996

  • results
A

D. lanceolata:

  • greater ability to acclimate to shade
  • greater net carbon assimilation rate at low instantaneous photon flux densities (PFD)
  • attributable to lower light compensation point & greater mean assimilation rates at low PFDs
  • less branch growth, no height growth but
  • proportionally greater carbon allocation to few long branches
  • total leaf area increased horizontally
  • better in understorey environments

S. leprosula

  • greater ability to acclimate to high light
  • greater assimilation rates at high PFDs in all environments
  • higher height/branch growth rate
  • forages for light vertically
18
Q

Does seedling ecology reflect differences in ability of photosynthetic apparatus of two climax dipterocarps to acclimate to different light environments?

Zipperlen and Press, 1996

  • conclusions
A
  • both climax species
  • L. leprosula relatively high light demander
  • superior growth in high light
  • low survivorship low light
  • D. lanceolata
  • shade plant longevity characteristics
  • more limited ability to increase growth in high light
  • may allow for niche differentiation with respect to timing of disturbance events that increase light availability