Lab: Photosynthesis, Carbon Dioxide Cycles, and Holdridge Life Zones

Part I: Photosynthesis and Carbon Dioxide Cycles in Biosphere 2

Biosphere 2 Background

Greenhouse in Arizona desert N of Tucson

• Enclosed biomes representative of desert, savannah, ocean, rainforest & cropland

• Area of base = 13 thousand square meters

• Volume of greenhouse = 170 thousand cubic meters

• Tour of Biosphere 2

Photosynthesis by green plants and respiration by all biota in Biosphere 2

• Closed system causes CO₂ to decrease when photosynthesis occurs

• Closed system causes CO₂ to increase when respiration dominates

• Pattern of daily changes in CO₂ closely linked to cycle of sunlight

Diurnal cycle of carbon dioxide in Biosphere 2

• Data are from January 1996: ( Fig 1 , Fig 2 )

• Amplitude larger during summer than winter

• Average carbon dioxide concentration in Biosphere 2 substantially above free atmosphere levels (about 350 ppm)

• Amplitude of carbon dioxide diurnal cycle in Biosphere 2 much larger than for free atmosphere (see for example Mauna Loa CO₂ record, Fig 3)

• Approximately 1 mole of O₂ is produced for each mole of CO₂ removed from the atmosphere during photosynthesis,

  CO₂+ H₂O = Ch₂O + O₂

• PAR = photosynthetically active radiation.

NOTE: All graphs should have the axes appropriately labeled and meaningful units assigned.

Excel Help - If you are unfamiliar with Excel, work through these Excel help pages.

Biosphere 2 CO2 data

1. Plot the first 24 hours of light and CO₂ data vs. time. Print out this plot and answer the following questions (include all units):

   1. What time did the sun rise on January 6, 1996? What time did it set on the same day?

   2. What was the maximum atmospheric CO₂ concentration on this day? What time did it occur?

   3. What was the minimum atmospheric CO₂ concentration on this day? What time did it occur?

   4. What was the average in the CO₂ concentration?

   5. By how much does the CO₂ concentration vary throughout the day? Answer this question by calculating the standard error of the mean (SEM) CO₂ concentration using the following equation in Excel:

      SEM = STDEV(#:#)/SQRT(n)

      where n is the number of data points and "#:#" is the column of CO₂ data.

2. Now plot the light data vs. time for the entire month of data. On a separate graph, plot the CO₂ data vs. time for the entire month. How do light and atmospheric CO₂ change over the course of the month?

   1. Fit a trendline to both plots.

   2. What is the temporal trend in the maximum light levels through the month of January? Why?

   3. Is there a trend in the CO₂ data? Is it positive, negative or zero? Explain why.

3. Plot the Biosphere 2 light response using all the data.

   1. Make a scatter plot with CO₂ on the y-axis and light on the x-axis.

   2. Describe how the CO₂ concentration changes with changing light levels during the day. Label dawn, morning, noon, afternoon, dusk and night on your graph and include each one in your answer.

4. The previous plot shows how the atmospheric CO₂ changes with light levels. Now predict the photosynthetic response to varying light levels (or in other words, how the rate of carbon uptake by plants changes with light level).

   1. Describe your prediction and rationale (i.e. how does photosynthesis change with light and why?)

   2. Draw a free hand plot of your prediction.

5. The net system carbon exchange (or the rate of carbon uptake/release) by the vegetation in Biosphere 2 can be calculated using the information given above.

   1. Convert the volume of Biosphere 2 into moles of gas molecules. Assume 1 mol = 22.4 l (liters), and there are 1000 liters per m³.

   2. Calculate the carbon exchange rate as

      A = ([C_b-C_f]V)/(DT*SA)

      Where

      A is the carbon exchange rate in µmol CO₂ m^-2 surface area s^-1.
      C_b is the micromolar fraction of CO₂ in air at the beginning of the measurement.
      C_f is the micromolar fraction of CO₂ in air at the end of the measurement.
      V is the volume of Biosphere 2 in µmoles.
      DT is the time between consecutive measurements in seconds.
      SA is the surface area in m^2.

      NOTE: CB and CF should be dimensionless (in "parts"; they are originally in "parts per million").

   3. Plot the rate vs. light using all the data, and interpret the graph. Does this match your prediction in part C? Why or why not?

   4. What was the maximum rate of carbon uptake and what time of day did this occur?

Part II: How Climate Influences Vegetation - Holdridge Life Zones

The relationship between climate and photosynthesis has led to the system of classification known as the Holdridge Life Zones. Holdridge, a botanist considered temperature and rainfall to prevail over all other environmental factors in determining vegetation form. His scheme incorporates the biological effects of climate on vegetation. Temperature and rainfall interact to define humidity provinces. Each humidity province is further subdivided by the ratio of potential evapotranspiration to precipitation. Potential evapotranspiration is in turn a function of temperature. Therefore the humidity provinces relate temperature and rainfall to the water relations of plants in a way that is meaningful.

Climate modelers use this information to predict how the worlds vegetation could change under different scenarios of climate change. Let's take a look:

Take a look the map of today's distribution of vegetation types. Summarize the major trends. Compare today's map of vegetation to the predicted vegetation in an increased CO₂ climate. What factors besides increased CO₂ could alter the global distribution of vegetation types?

Related map view

Difference map of the eastern part of North America

Lab Report Instructions

1. Write a lab report report (as per the Lab Report Format) summarizing the major findings of your investigation.