Atlantic Forest

Exercise 6: Producers—Measuring Photosynthesis in Forest Ecosystem
Module 6: Producers: The Basis of Ecosystems

Your Questions

1. How does resource availability affect primary production and plant community diversity?
2. How would this diversity of production affect the rest of the biotic community in the area?
3. How would this diversity of production affect other abiotic factors in the area?
4. What are the assumptions that we make as we do this study? (this is given below - don't read that text until you have first tried to answer this question)

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. We won't be using these materials today, but will be using a less direct way of measuring photosynthesis.

A commonly used 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 Intervales State Park.

Your Assignment

Your assignment is to evaluate how photosynthesis varies along a resource gradient using the total carbon present in the tree species, and to infer from these results effects on plant community diversity.

This is a one-day lab involving the following tasks: (7 hours total)

1. Lecture introducing photosynthesis and carbon cycling (1 hr, Sunday afternoon)
2. Hike to the top of a hilly area within Intervales that has the same species present throughout the trail. (1 hr)
3. Field measurement of height and diameter of three tree species along a transect, as well as understory plant community diversity: (3 hrs)
4. Analyze results and estimate total carbon (1 hrs)
5. Write-up and discuss results (1 hr).

Evaluation:

1. By group, orally summarize your analyses.
2. Participate in discussion of other group presentations. Based on your analyses, do you agree or have additional insights?
3. What is/are the resources that are leading to the changes in the carbon present along the transect?

Objectives

1. Understanding plant production and how it varies with resource availability.
2. Determination of controls over plant production.

Key Skills

1. Increased competence collecting field measurements and plant diversity data.
2. Measuring plant height and diameter and calculating total plant carbon.
3. Familiarity with forestry techniques to measure photosynthesis.

Timetable

This activity involves a lecture introducing photosynthesis and carbon budgets and field measurements of tree height and diameter along with understory vegetation analysis.

1. Total elapsed time to perform the experiment: 1 day.
2. Total elapsed hands-on time : 7 hours
   • Lecture = 1 hr (given Sunday afternoon before leaving IPÊ for Intervales)
   • Implement field measurements: (4 hrs)
   • Analysis, write-up, and discussion of results: (2 hrs)

Procedural Notes

1. The field location at Intervales is an active research site and care must be taken not to disturb instruments (watch for flagging)
2. Portable: Rainy weather should not limit field part of exercise.
3. Assumptions of this study are:
   • Standing total plant carbon (C) is that part of photosynthesis (psn) that is not used for respiration (maintenance) and not allocated to already shed deciduous parts of the plant (flowers, fruit, leaves) over the years.
   • The measurement of tree bole volume is estimating only the C in the bole. Biomass of below ground (woody and fine roots) and other above ground (limbs, leaves, etc) parts are omitted. Allocation ratios along the gradient may not be constant, if the ages of the forest or the species composition change.
   • Our measurements along a gradient assume that the average age of trees is the same at each point on the gradient. So that the total C measured, when divided by age of the forest gives some rate of wood production. Ages may vary based on disturbance history (wind throw or when trees are knocked down by wind, fire, land use, etc.) or if trees have shorter lifespans (senesce faster) on one end vs. the other of the gradient. Then again, depending on the question we ask, this assumption may not be relevant, as it may itself be the question!
   • With respect to the second assumption above, the same species of tree may alter allocation of C to above vs. below ground components along a strong moisture gradient. If moisture is truly limiting at the dry end of the gradient, then more C may go to roots - so a higher portion is hidden. Looking at above ground C only would then overestimate differences in total C along the gradient. Also, if the spacing of trees changes along the moisture gradient (e.g. more open on the dry end), then the allocation of C to limbs vs. bole might change. In more open forest, there might be more allocation to limbs if there is more light coming in from the side.

Materials Needed

1. DBH tape, tape measurements, calipers, Biltmore sticks or Hoggett Instrument to measure tree height

Rev 7/23/02