SOLAR RADIATION

A. The nature of solar radiation

electromagnetic radiation

solar (shortwave) vs. terrestrial radiation (longwave)

solar: ultraviolet, visible, and the near infrared

PAR = photosynthetically active radiation

B. Spatial variations in intensity and quality of solar radiation due to:

1. Atmospheric influences (clouds, water vapor, CO_2, etc.)

2. Vegetation influences

relative illumination = % of full sunlight (i.e. in an open area) that reaches the ground surface beneath the vegetation

important role of sunflecks

leaf absorptance, reflectance and transmittance varies with wave lengths

quality of light varies beneath different species

C. Importance of solar radiation to plants for:

1. Photosynthesis-- production of carbohydrates using water, carbon dioxide, and light energy; oxygen is produced as a by-product.

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.

Compensation point = 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 is reached.

2. 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 (short-day vs. long-day plants)

growth cessation and onset of dormancy

breaking of dormancy and growth initiation

SOLAR RADIATION AND PLANT CHARACTERISTICS

Heliophyte vs. sciophyte

Shade-intolerant vs. shade-tolerant plant ("understory tolerance")

SOME GENERALIZATIONS:

1. Larger seeds for shade plants.

2. Tree morphology of shade plants:

leaves more perpendicular to solar radiation

fuller crown in shade

greater crown depth (in contrast to "umbrella crown" of heliophyte in a dense stand)

3. Shade leaves:

thinner cuticle, fewer cuticular hairs, larger surface area, thinner, fewer layers of palisade cells (with chloroplasts) and less lobed than sun leaves.

4. Sun leaves = more sclerophyllous (or scleromorphic):

thicker, more deeply lobed, smaller surface area to weight ratio, more and smaller stomata, more hairs, thicker cuticle, less horizontal leaf blades, less intercellular space within the leaf.

5. Shoot/root ratios:

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.

SOME GENERALIZATIONS ABOUT SHADE TOLERANCE

C Shade tolerant species have a better ability to compete for soil moisture and nutrients under shaded conditions (i.e. Aunderstory tolerance@)--trenching experiments.

C Effect of shading on allocation of new growthC

Low light levels may result in smaller root systems that are disadvantageous in competition for moisture and nutrients; high shoot/root ratios make trees susceptible to windthrow.

C Pathogen resistance:

Shade tolerance often correlates with resistance to fungal pathogens favored by humidity of the understory.