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Orono, Maine 04469-5755
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www.forest.umaine.edu
www.umaine.edu
College of Natural Sciences, Forestry,
and Agriculture
School of Forest Resources
Unifying Themes and Forests
Light and Temperature Interactions in the Forest
June 23, 2009
by Bill Livingston
School of Forest Resources
University of Maine
Light is the source of all energy for trees and the ecosystem. Red and blue light can be absorbed
by the leaves during photosynthesis to produce sugars. The remaining light can get reflect or
absorbed by surfaces resulting in increased temperatures. Without leaves, all the light energy that
is not reflected gets converted to heat, not sugars. In this exercise, you will measure the amount
of light under full sun and the amount of light that can make it past the leaves. This light is all
that is left for the plants under the trees and to warm the soil.
Objectives


Examine how leaves affect the amount of light that reaches the forest floor compared to a
field
Relate the amount of light to the temperature of the air and soil.
Materials
Light meter
a) Visible range sensor, or
b) PAR sensor (photosynthetically active radiation)
Stand for meter (tripod or ring stand with clamp)
Thermometer probe
Vernier temperature probe connected to a computer or LabQuest will work
Procedure
1.
2.
3.
4.
5.
6.
Go to the open field.
Choose an area that is representative of the field.
Place thermometer probe 2 cm into soil.
Set up the stand with light sensor - place over thermometer - make sure sensor is level.
Attach sensor to meter and turn meter on. Keep sensor above ground vegetation.
Record the light value. If the readings aren’t steady, then take 3 readings at 5 seconds
apart.
7. Record soil temperature after a steady value is reached.
8. Remove the temperature sensor from the soil and hold it about 4 ft above the ground
9. Record the air temperature after a steady value is reached.
10. Go to a forested area and repeat the procedure.
11. Go to an area with asphalt and repeat the procedure.
page 2
Data
Units of measure for the light meter = __________________
This is usually in lux or µmoles of photons/m2/sec. 1 µmoles = 6.02 X 1020 photons.
Approximates 54 µmoles of photons/m2/sec = 1 lux in sunlight.
(http://www.egc.com/useful_info_lighting.php)
Asphalt
Light
(a) ___________
(b) ___________
(c) ___________
If multiple readings
needed,
the average value is:
___________
Field
Forest
Light
(a) ___________
(b) ___________
(c) ___________
If multiple readings
needed,
the average value is:
___________
Light
(a) ___________
(b) ___________
(c) ___________
If multiple readings
needed,
the average value is:
___________
Temperature
Surface: ________ºC
Temperature
Soil: __________ºC
Temperature
Soil: __________ºC
Air: __________ºC
Air: __________ºC
Air: __________ºC
Percent light reaching the forest floor
% = 100*(Forest Light/Field Light)
______________
page 3
Class Results Table
Asphalt Light
Measure
units =
_______
Average
Field Light
Measure
Forest Light
Measure
% Light
reaching forest
floor
Asphalt
(ºC)
Surface
Field
(ºC)
Air
Soil
Forest
(ºC)
Air
Soil
Air
page 4
Questions
1. Why do trees grow tall?
2. Which environment consistently had the warmest surface/soil temperatures:
____________. Why? Refer to all possible factors that affect light transformation to
energy.