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Biosphere 2 Center

Exercise 6: Environmental Controls over Photosynthesis — Direct Measurement in the B2 Rainforest Biome
Module 6: Producers — The Basis of Ecosystems

T. Kittel and A. Hoylman

“[The sun] furnishes the fuel required for the maintenance of all terrestrial processes – whether organic or inorganic.”   — K. Stumpff, Planet Earth

Your Questions

  1. How does photosynthesis vary with changing environmental conditions?
  2. What can you infer from this about how plant primary production varies across the landscape?  By biome? 
    • What resources are most limiting for different biomes, or at different positions on the landscape?
  3. How does photosynthesis vary with time of day?  Why?  Is this the same for different environments?
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Background

Measurement of Photosynthesis

Infrared gas analyzers (IRGA) are a class of instruments that measure CO2 concentration.  Some of these instruments measure changing levels of CO2 in a confined chamber (cuvette) and are portable for use in the field.  An example of one such instrument is made by LI-COR.  When applied to a leaf, LICOR's measure the flux of CO2 into the leaf and so estimate rates of photosynthesis per unit leaf area.  LICOR's can also measure how these change with environmental conditions such as temperature, light, and CO2 level outside the leaf.  For information on these instruments, see the following sites:
http://env.licor.com/products/gas.htm
http://env.licor.com/products/li6400/6400.htm

Biosphere2

You are already familiar with the wide range of vegetation growing under the domes in the Biosphere2 domes and outside.  These domes attempt to partially represent conditions and the vegetation of selected biomes.  For this assignment, we will be able to work in the Rainforest Biome, provided we use non-destructive protocols.  You can review information on the B2 biomes at: 
http://www.bio2.edu/virtualtour/lower_rainforest.htm
http://www.bio2.edu/Research/res_entry.htm
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Your Assignment

Your assignment is to evaluate environmental controls over photosynthesis (psn) in a controlled system, and to infer from these results how plant production varies over time and within and among different ecosystems.

This is an one-day lab involving the following tasks: 

  1. Formulate testable hypotheses and create a robust experimental protocol. 
  2. Implement experimental protocol in Biosphere domes
    • Measure CO2 fluxes (and H2O fluxes, depending on instrumentation and time available) with a LICOR.
  3. Analyze results:
    • Analyze LICOR data for psn (and transpiration) rates vs. controls (response curves).
    • Evaluate hypotheses based on results. 
  4. Write-up and discuss results
    • Compare results with those of other ecosystems as data are available in the literature and on the web. 
  5. Presentations
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Objectives

  1. Be able to address the questions: What is the ultimate driver of ecological processes?  And how is this expressed – i.e., by what mechanisms?
  2. Understanding of plant production and how it varies with lifeform, landscape unit, and biomes.
  3. Determination of controls over plant production.
  4. Familiarity with techniques to measure photosynthesis.

    5.  
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Key Skills

  1. Increased competence implementing the Scientific Method
  2. Dexterity using complicated lab/field instruments.

    3.  
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Timetable

  1. Total elapsed time to perform the experiment: 1 day.
  2. Total elapsed hands-on time :  8-9 hours
    • Instructions and formulate testable hypotheses and create a robust experimental protocol. (1 hr) 
    • Implement experimental protocol:  (3 hrs)
    • Analysis of  results: (3 hrs)
    • Write-up, discuss, and present results (1.5 hr).
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Procedural Notes 

  1. Exercise is in B2 biomes.  Need to allow time for class to enter/exit via air lock.

    2.  
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Materials Needed

  1. Portable Photosynthesis Analyzer (LI 6400 or other) or CO2/H2O Analyzer (LI 6262).
  2. Graphic output from B2 control center of biome ambient conditions (T, RH, CO2)
  3. Computer and printer 
  4. Software
    • Excel or some other data base and statistical analysis package 
    • eBiome
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All Materials Copyright © 2001 by T. Kittel and A. Hoylman 
All Rights Reserved. 

Rev 6/9/00, 4/26/01