Download 5-Solar radiation

TREE AUTECOLOGY: SOLAR RADIATION
A. The nature of solar radiation and its variation in time and space.
electromagnetic radiation
solar (shortwave) vs. terrestrial radiation (longwave)
solar: ultraviolet, visible, and the near infrared
PAR = photosynthetically active radiation
Spatial variations in intensity and quality of solar radiation due to:
1. Atmospheric influences (clouds, water vapor, CO 2, etc.)
2. Vegetation influences
important role of sunflecks
leaf absorptance, reflectance and transmittance varies with wave lengths
quality of light varies beneath different species
B. Effects of variations in the intensity of solar radiation on photosynthesis.
Photosynthesis is the production of carbohydrates using water, carbon dioxide, and light
energy; oxygen is produced as a by-product. Photosynthesis is the light-dependent process in
which the rate of photosynthetic fixation of both CO 2 and solar energy is largely dependent upon
light intensity.
Respiration is the opposite of photosynthesis; carbohydrate is broken down and
combined with oxygen to yield carbon dioxide, water and chemical energy.
Rate of photosynthetic fixation initially increases with increased light intensity.
Light compensation point (CP) = the light intensity at which there is no net increase in
CO2 because the rate of CO2 loss in respiration is balanced by rate of CO2 gain from
photosynthesis.
At light intensities above the compensation point, rate of photosynthesis continues to
increase until the saturation point (SP) is reached. Beyond the SP there is no further increase in
net CO2 fixation. At very high light intensities, net fixation may decline because of damage to the
photosynthetic apparatus or for other reasons. Expressed graphically, this is the photosynthetic
light saturation curve.
Temperature has an important influence on net photosynthesis. With increasing
temperature gross photosynthesis initially increases rapidly but levels off whereas respiration
continues to increase as temperature rises.
C. Effects of variations in the intensity of solar radiation on plant morphology
Heliophyte vs. sciophyte
Shoot/root ratios and etiolation: under low light levels tree seedlings have higher shoot/root
ratios. In the shade, root growth and stem diameter growth are reduced to permit greater height
growth.
Shade leaves vs. sun leaves:
Leaves grown in deep shade are normally larger, thinner, less lobed, and have fewer layers of
palisade cells (with chloroplasts). They usually also have a thinner cuticle and fewer cuticular hairs
than sun leaves.
Leaves grown in full sunlight are usually thicker, more deeply lobed, have a smaller surface
area to weight ratio, have more and smaller stomata, more hairs, thicker cuticle, leaf blades are less
horizontal to the ground, and have less intercellular space within the leaf. Many of these traits are
adaptations to prevent excessive moisture loss (to be discussed later).
D. Shade tolerance.
Shade-intolerant (light demanding) vs. shade-tolerant plants.
1. Understory tolerance: apparent shade tolerance often reflects a better ability to compete
for soil-related resources as demonstrated by classic trenching experiments.
2. Shading causes an allocation of new growth so that shoot/root ratios increase, usually
making tree seedlings more susceptible to damage from wind, animals, and snow.
3. Some plants alter their leaf morphology greatly to optimize photosynthesis under low light
intensities (i.e. produce shade leaves that are horizontal to the ground, large leaves, thin
cuticles, high ratios of photosynthetic to non-photosynthetic biomass).
4. Shade tolerant trees often have large seeds with abundant energy reserves to aid initial
growth of the seedling.
5. Shade tolerant trees typically are more resistant to fungal pathogens which tend to be more
common in the humid understory of a forest.
E.
Effects of temporal variations in solar radiation
Photoperiodism = the response of the plant to changes in the relative length of the day and
night during the course of a year.
Photoperiodic influences on flowering, growth cessation (onset of dormancy), and breaking of
dormancy.
Short-day plants
Long-day plants
Day-length indifferent (day-neutral) plants
Determinate tree species (mid- and high latitudes) set a terminal bud
and cease to grow during the unfavorable season.
Indeterminate tree species are able to grow in height whenever
conditions are suitable for photosynthesis (i.e. even during winter).