adaptations to light in tropical rainforests (lecture 4) Flashcards
How has light shaped tropical rainforests?
- 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
What is plasticity and acclimation in tropical forest plant species?
- 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
What are sunflecks? How do they relate to plant plasticity?
- 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
What changes in irradiance do climate trees experience in their lifetime?
- 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
How does Begonia erythrophylla maximise light interception into the understorey?
- B. erythrophylla
- epidermal cells behave like lenses, focus light onto chloroplasts
- irradiance reaching chloroplasts 15x greater than incident light and lead surface
How do anthocyanin pigments maximise light interception of understorey plants?
- some monocots/dicots
- possess red/purple anthocyanin pigments on underside of leaves
- increases efficiency of light capture
- reflects absorbed light into leaf
How do blue irridescence maximise light interception of understorey plants?
- 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
How do tree form and leaf placement affect light capture efficiency?
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
How do tree form and leaf placement affect light capture efficiency in Macaranga gigantia?
- 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
How do leaves of plants adapted for sun and shade differ?
- 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)
What are key differences between cells of sun and shade plants?
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
What are key differences between leavesof sun and shade plants?
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
What are key differences between sun and shade plants?
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
What are characteristics of sun in contrast to shade leaves?
- 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
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
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
