Ecology: Chapter 4
The Terrestrial Environment
 

1 nanometer = 1 billionth of a meter ( 0.000000001 meter)

 1 nanometer = 0.001 micrometers (Ám)

1Ám = 1000 nanometers

 

 

UV-B: 280 - 320 nm (99% absorbed by ozone, very damaging)

UV-A: 315 - 380 nm (not as dangerous as UV-B but still dangerous)

Visible light: 400 - 740 nanometers - collectively known as Photosynthetically Active Radiation (PAR)

Reflected IR: 760 - 3000 nanometers

Thermal IR: 3000 - 30,000 nanometers

 

Atmospheric absorption of UV:

  • Most UV radiation is absorbed by ozone.
  • Some UV is absorbed by clouds

 

Ground measurements of UV-B are greatest at the equator where the ozone layer is thinnest

UV-B increases 14-18% per 1000m increase in elevation

In addition to variations in wavelength visible light has three other characteristics:

  • intensity (sun angle)
  • duration (length of day)
  • directionality (reflection)

Light can be:

  • absorbed
  • reflected
  • scattered
  • transmitted

Absorption of Light

 

In general, a leaf:

  • reflects about 70% on incoming infrared energy
  • reflects only 6-12% in the visible
  • green light: 10 to 20%
  • red and orange light: 3 to 10%
  • only about 3% of UV is reflected

 

 

Much of the visible light is absorbed. Plants have a low albedo in the visible

 

 

A cut-away illustration of the internal structure of a leaf. Shows indicating red, gree, blue, and infrared light energy interact with the leaf structure. The Red and Blue light is absorbed, green light is reflected by the top layer of the mesophyll and the infrared energy is reflected off the bottom layer.

 

 

 

Spectral Signatures of Vegetation

 

 

 

A graph showing wavelengths in nanometers on the x-axis and percent reflectance on the y-axis. Snow, ice and clouds show a high reflectance across all wavelengths. Dry soil, wet soil, turbid water and clear water all seem to reflect similar values in the blue and green wavelengths, but have very different value closer to red and infrared where soils reflect more than water. Vegetation reflects more in the green and infrared than in the blue and red.

 

 

 

A small fraction of light is transmitted through the leaf depending on:

  • leaf thickness
  • leaf structure

In grasslands and forests, the leaf canopy intercepts most incoming solar radiation

 

Different wavelengths are filtered out -- the light becomes attenuated

The degree of attenuation depends on:

  • density of the canopy
  • optical properties of the leaves
  • number and size of gaps in the canopy

Blue and green are the most highly attenuated wavelengths

Green and far infrared wavelengths pass through relatively unaltered

These changes in spectral quality vary among forests

  • for example, beneath a canopy of deciduous forest on a clear day, far infrared radiation dominates with lesser amounts of blue, green and red giving an observer the impression of standing in reddish-green shade
  • In the depths of the forest, only far infrared wavelengths make it to ground level
  • In more open pine forest, little differences exist in the transmission of various wavelengths

 

 

Leaf Area Index (LAI) and Extinction Coefficient

  • The amount of photosynthetically active radiation (PAR) reaching the earth's surface depends upon:
    • length of day
    • sun angle

 

 

  • Leaf Area Index (LAI) a ratio that characterizes plant canopies

     

                       Total leaf area

    • LAI =  ---------------------

                         Projected ground area

 

 

 

 

 

 

 

 

 

 

 

 

The arrangement of leaves on a plant also influences the amount of light that reaches the surface

 

 

 

 Forests and Grasslands

  • Only 1% to 5% of the light that strikes the canopy of a typical temperate deciduous forest (LAI = 3 - 5) in the summer reaches the forest floor.
  • More light travels through a stand of pines trees (LAI = 2 - 4), about 10% to 15%
  • In a tropical rain forest, (LAI = 6 - 10), only 0.25% to 2% gets through

 

 

 

Seasonal Changes in Amount of Light Reaching Ground

  • In temperate regions many trees are deciduous
  • The amount of light that penetrates the stand varies with the season
  • In early spring 20% to 50% of the incoming light may reach the forest floor
  • Spring flowers flourish
  • When less than 10% of light reaches the floor, flowering is over
  • In the fall, when leaves begin to drop, there's another surge of flowering

 

 

 

 

  • Determining LAI:
    • Direct Methods
    • Indirect Methods

 

 

 

Hemispherical Photo of Forest Canopy

 

 

 

 

 

Leaf Area Index - July 2014 - TERRA/MODIS
(
MOderate Resolution Imaging Spectroradiometer)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Leaf Area Index - January 2014 - TERRA/MODIS