Biosphere2

Exercise 6: Environmental Controls over Photosynthesis — Direct Measurement in the B2 Rainforest Biome
Module 6: Producers CO₂ —The Basis of Ecosystems

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Background Lecture

• Introduction to ecosystem material and energy flow
• Role of producers
• What are producers? – green plants, chemotrophs
• What do they do? – convert solar (or inorganic chemical) energy and C02 to carbohydrate.
• Photosynthesis – reaction equation (vs. respiration)
• CO₂ + H2O  — [+SR]   O₂ + (CH2O)n, (SR=solar radiation)
• Limitation:  psn= f(CO₂, H2O, SR, T, Rubicose[=fn(N,P,S)])
• Chemical reactions are T limited: show generalized exponential response plot
• Biologically-mediated reactions are facilitated by enzymes, so occur at faster rate at lower T's than would otherwise.  Response curve has a T(optimum) because proteins change shape at high T's (before denature).
• Can get response fn for each fast-acting limiting factor (CO₂, H2O, SR, T) by careful manipulation of psn under controlled conditions. 
• This is a lead in for the Abiotic Section (Ex. 9-11)
• How is psn measured — one method: IR Gas Analyzer (IRGA)
• CO₂ absorbs Infrared light (IR) – IR activation of CO₂ molecules: vibration, rotation, etc. 
• IRGA looks at absorption spectrum to determine how much CO₂ is in a gas sample from the cuvette, thus monitoring CO₂ flux from plants.  Same principle for measuring CO₂ flux from soil (decomposition)
• LICOR
• cuvette = controlled environment
• generates CO₂, PAR, T response curves (PAR=photosyntetically active radiation)
• [need pressure bomb to evaluate H2O availability/response curve]
• Plant morphology enabling psn – leaf structure in vascular plants
• C3, C4, CAM structures – optimize CO₂ assimilation while limiting H2O exchange (transpiration).
• Plant use of photosynthate – maintenance respiration, growth, etc. (allocation)
• GPP vs. NPP
• Geographic distribution of terrestrial and marine production.
• relate to limiting factors
• Leibig's Law of the Minimum.

Hypotheses

Independent variables in the experiment

• Photosynthetic rate
• Possibly, transpiration rate

Dependent variables that could be used in experimental design

1. T, SR, CO₂
2. Different spp. 
• Deep shade plants
• Banana plants at perimeter of Rainforest biome

Activity notes

However, the generation of a single response curve takes on the order of 20-30 minutes (if I recall correctly from this exercise in 2000).  So practically speaking, it is only feasible to generate a few curves during the exercise.   If, in future years, LICOR's are available at the other SEE-U sites, it would be possible to compare response curves from other biomes/lifeforms through cross-site data sharing.

The most feasible objectives for this activity is for students (1) to become acquainted with the LICOR and how it works and (2) to contrast a limited number of response curves.  Our experience in 2000 was that students gained a lot from exposure to such high tech instrumentation for taking ecophysiological measurements.  In addition, Barb Watson was a great teacher, giving students confidence that they could not only operate such equipment (if at a rudimentary level) but understand what it was doing.

In 2000, we completed 3 response curves, working only in the Rainforest Biome:

1. a CO₂ response curve for a deep shade plant (species=Pothos)
2. 2 light response curves — one for Pothos (deep shade plant) the other on a banana leaf (banana are planted along the high light perimeter of the dome). 

Results from the exercise in 2000 are in the Excel file: Ex6_all_data2000_modified501.xls.

Note that some of the 'field' time is needed to introduce the LICOR to students and to let them take individual measurements.  Barb Watson lead the demonstration and directly oversaw the students in their use of the LICOR.  She provided 2 LICOR's. 

After sampling was completed, Barb downloaded the data off the machines back in her lab and emailed the output tables to the instructors later in the day.  These tables were easily imported into Excel. 

Preparation before course

1. Permission to use B2 dome vegetation is needed.
2. Also need to inquire as to availability of instruments for class for the day of the exercise.  Instruments available at B2 are given at: http://www.bio2.edu/Research/fac_analytic.htm
3. Need technical help for setting up instruments and for programs to analyze machine outputs. 
• In 2000, Barb Watson, B2C staff, led the class in the use of a LICOR in the Rainforest Biome.  Also possible to use in Savanna and IFB (Agroforestry biome) with her, but in 2000 Rainforest was all we had time for.
4. Graphic output from B2 control center of biome ambient conditions (T, RH, CO2).  The Rainforest is also called the 'Wilderness' biome on those plots.
5. This lab would be an appropriate time for a guest lecture from someone involved in B2 ecophysiological research.
6. Portable?  Tasks are all inside B2 or class computer lab
7. Need to discuss with B2 staff what LICOR runs are possible, given that creating a response curve is time consuming, and so to evaluate how much students will be able to accomplish in time allotted. 
8. Need to locate: 
• psn response curves for other sites/biomes in the literature. 
• Maps and tables of psn, NPP, evapotranspiration by biome in the literature and on the web.
9. SEE-U students and instructors need to exercise caution when handling the LICOR's — they're very expensive (~$10K?)

Student evaluation

• Students will give short (10 min max/group) presentation to share with the class their findings. 
• Participation in discussion

More extended activity ideas

The class has a wide range of 'vegetation' growing inside B2 and plant communities outside on the B2 campus.  Comparisons of psn rates can be made at several levels:

• A macro-level — measuring CO₂ uptake for tropical forest, savanna, and desert systems in the domes, and the desert outside.  At other SEE-U sites (lacking the variety offered by the dome facility), the exercise could  be rephrased to look at plants of different lifeforms within a given plant association or on different landforms and so might serve the same question, if at a much finer scale.  If LICOR measurements are implemented by other SEE-U classes, coordination would permit comparisons among these biomes, as noted above.
• Along a local gradient (moisture, light, CO₂) — 
• The Ag-forestry dome is setup for experiments with 3 levels of elevated CO₂.
• Effect of light in and outside of the tropical canopy in the dome environment is another gradient that students could evaluate.  This gradient was used in 2000 by looking at the light response curve for a deep shade plant and a perimeter banana plant.
• Along a moisture gradient in the desert outside the domes, such as (1) for the same sp. in and out of a drainage, and (2) within a drainage comparing shrubs that are phreatophytic vs. shallow rooted.  (i) This would show moisture control, and (2) would show that different lifeforms in identical environments differentially exhibit water stress. 
• Diurnally – students could measure psn on a single plant during the course of a day (or, e.g., at least from sunrise to noon) as light increases, until stomata close down due to heat stress. 

• Questions:
• How does primary production (as reflected by instantaneous, but standardized photosynthesis measurements) vary by plant lifeform – for lifeforms that are characteristic of different or the same environments?
• How does resource availability (e.g., moisture, light, CO₂) affect primary production? – a lead-in for the Abiotic Section.
• Approach is for students to come up with specific questions to test from these and to choose sites based on them in order for then to explore how psn varies.
• Prep Discussion:  Ample time should be set aside to introduce the use of  LICOR's and methodology, with the assistance of a B2 researcher or technician.
• Students are expected to confer with their lab mates in devising hypotheses and experimental protocol, in selecting sites once in the field and the domes.
• This activity should be set up for the students to direct the experiment themselves, but with strong technical assistance in running the LICOR's and analyzing the LICOR numbers.
• Statistics can be used in analysis if time has permitted getting multiple samples for each site/life form.
• Hypotheses to test 
• Photosynthetic rates vary by B2 biome.
• Psn varies less / more by lifeform than by biome type (i.e. comparing similar lifeforms across biomes, vs other lifeforms in same biome) – controlling for moisture availability and time of day?
• Psn varies by time of day, at first controlled by available light and then by available moisture.
• Near drainages, plants that can tap into subsurface water (phreatophytes) will not show signs of water stress on psn as soon in the day as more shallow rooted plants.
• Psn varies by availability of moisture, light, CO2 – controlling for time of day.
• Independent variables in the experiment
• Time of day
• Moisture availability – inferred from landscape position, biome type
• Light
• CO2
• Lifeform
• Biome type
• Dependent variables that they could use in their experimental design.
• Psn, as measured from CO2 flux Li-Cor output
• Transpiration, depending on instruments

Data from Previous Years

An Excel spread sheet with the leaf LI-COR data from Biosphere 2, SEE-U 2000 is available. If prompted, click on No to keep the existing information.