SEE-U 2000: Black Rock Forest - Module 6:Exercise 6

Exercise Contents:

• | Your Questions
• | Background
• | Your Assignment
• | Objectives
• | Key Skills
• | Timetable
• | Procedural Notes
• | Materials Needed

Exercise Contents:
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# Black Rock Forest ## Exercise 6: Producers—Measuring Photosynthesis in Forest Ecosystem Module 6: Producers: The Basis of Ecosystems
Printer format ### Your Questions How does resource availability affect primary production and plant community diversity? ### Background The process of photosynthesis is the cornerstone of all life and the basis of all food chains (http://buglady.clc.uc.edu/biology/bio104/photosyn.htm). Through photosynthesis, plants convert light-energy (sunlight) to chemical-energy (sugars). The basic reaction in photosynthesis is as follows: Carbon dioxide + water + light energy glucose (high energy molecule) + oxygen 100 billion metric tons of sugar is produced annually in terrestrial systems and 50 billion metric tons in the seas. The energy fixed in photosynthesis is termed gross primary production (GPP) while net primary production (NPP) accounts for the energy lost by plant respiration (estimated to be 50-70% of the energy fixed). Primary production varies over the Earth’s surface with the highest production found in the tropical rainforests and coastal shelves. Light, water, nutrients and temperature limit photosynthesis. Photosynthesis in terrestrial plants is strongly tied to water availability and temperature while light and nutrients largely limit aquatic systems. To learn more about the importance of photosynthesis, read “Why Study Photosynthesis?” (Devens Gust), and a summary of the chemical reactions in “The Photosynthetic Process.” Knowledge of the physico-chemical process of photosynthesis is essential for understanding the relationship between living organisms and the atmosphere and the balance of life on earth. In addition, human activities such as the burning of fossil fuels are changing the earth’s atmosphere and increasing CO2 concentrations. Understanding what effect elevated CO2 levels will have on plant productivity is an important research area. Photosynthesis can be estimated from changes in CO2 concentrations around a leaf using a gas analyzer. Infrared gas analyzers (IRGA) are a class of instruments that measure CO2 concentrations. Some of these instruments are designed to measure changing levels of CO2 in a confined chamber and are portable for use in the field. When applied to a leaf, the flux of CO2 with time can be used to estimate rates of photosynthesis per unit leaf area and how these change with time. An example of one such instrument is a Li-Cor (e.g., http://env.licor.com/products/li6400/6400.htm and http://weather.nmsu.edu/Teaching_Material/soil698/Student_Material/Photosynthesis/, which also measures fluxes of water vapor (transpiration) from leaves. A less direct way of measuring photosynthesis is to estimate the total plant carbon, which represents the end product of photosynthesis, that is plant biomass. In today’s activity we will estimate total plant carbon in dominant tree species along a moisture gradient in Black Rock Forest. ### Your Assignment Your assignment is to evaluate how photosynthesis (psn) varies along a resource gradient by calculating total carbon of tree species, and to infer from these results effects on plant community diversity. This is a one-day lab involving the following tasks: (8 hours total) Lecture introducing photosynthesis and carbon cycling Hike to Glycerine Hollow. (1 hr) Field measurement of height and diameter of three tree species along a transect, as well as understory plant community diversity: (4 hrs) Analyze results and estimate total carbon (2 hrs) Write-up and discuss results (1 hr). Evaluation: By group, orally summarize your analyses. Participate in discussion of other group presentations. Based on your analyses, do you agree or have additional insights ### Objectives Understanding plant production and how it varies with resource availability. Determination of controls over plant production. Familiarity with techniques to measure photosynthesis. ### Key Skills Increased competence collecting field measurements and plant diversity data. Measuring plant height and diameter and calculating total plant carbon. ### Timetable This activity involves a lecture introducing photosynthesis and carbon budgets and field measurements of tree height and diameter along with understory vegetation analysis. Total elapsed time to perform the experiment: 1 day. Total elapsed hands-on time : 8 hours Lecture = 1 hr Implement field measurements: (4 hrs) Analysis of results: (2 hrs) Write-up and discuss results (1 hr). ### Procedural Notes The field location at BRF is an active research site and care must be taken not to disturb instruments (watch for flagging) At BRF, a demonstration of a Li-Cor 6400 is possible but not a very effective teaching tool given there is only one instrument with limited visual effects. A Li-Cor costs approximately $30,000. Will need technical help for setting up instruments and for programs to analyze machine outputs. This lab would be an appropriate time for a guest lecture from someone involved in ecophysiological research. Not very portable: Rainy weather would limit field part of exercise. ### Materials Needed DBH tape, tape measurements, Biltmore sticks or Hoggett Instrument to measure tree height