Biomass Accumulation Model:
Predicting
Net Primary Productivity and Biomass using Temperature and Precipitation
Purpose
- To understand how a simple model can describe emergent properties of a complex system (in this case, how the growth rate of ecosystems varies with broad-scale patterns of temperature and rainfall).
- To allow students to use the computer model to draw some conclusions about how variation in model parameters affects the resulting predictions.
- For students to recognize the utility and limitations of a simple model. (The model provides is a good approximation of growth rates over a range of ecosystems, but there are many other factors that can influence growth at a particular location that are not included.)
Overview
Scientists use computer models to help them understand the behavior of complex systems and to predict outcomes that cannot be measured directly. Models can be written at many levels of detail, from the very simple to the very complex. Not surprisingly, there is often a tradeoff between the ease with which a model can be understood and used and the number of properties that can be reliably predicted. In this activity, students will learn to use a simple model that predicts the biomass and average growth rate of ecosystems (Net Primary Productivity) based on annual temperature and annual precipitation. The model is very simple in that it only responds to variation in these two climate variables and all forms of biomass (leaves, wood, roots, etc.) are lumped into a single box, known as the biomass pool or standing stock.
The model is based on the work of H. Leith and R. Whitaker who realized that, despite the high degree of local variability that can be exhibited by ecosystems, at broader spatial scales, biome distribution and rates of productivity follow predictable patterns with mean annual temperature and mean annual rainfall (Figures 1 and 2). Simple mathematical equations describing these patterns form the basis of the model used here. Although there are other important factors the model does not include (e.g. nutrients, soil type, insect pests, seasonal climate variation), it provides a firm foundation for examining one important part of the global carbon cycle. It has been used in both science and education and can help introduce students to modeling and begin to test hypotheses about the behavior of ecosystems.
Essential Question(s)
- What do the terms Net Primary Productivity and biomass mean with regard to the carbon cycle?
- How do precipitation and temperature inputs influence the 1-box NPP-Biomass model results, including the biomass stock?
- How does turnover rate relate to biomass? How does this change with biome type?
- Using the model, what does a 100-year run indicate about carbon storage in a particular geographic location?
Student Outcomes
- Students will work with a computer model and gain the ability to read model output graphs and tables.
- Students will understand biomass and relate it to carbon storage.
- Students will understand how inputs are related to model results by changing variables one at a time.
- Students will compare model run results for their own location to at least one other biome and note differences.
- Students will understand the basic modeling concepts of flow (inputs and outputs from growth and death), standing stock (biomass), turnover rate (flow ÷ stock), residence time (stock ÷ flow) and equilibrium.
Science Concepts
Time
60 minutes to introduce model terms, model storyline step-through.
60 minutes Part 1: Learning to run the model and understanding the dynamics of biomass accumulation.
60 minutes Part 2: NPP predictions for other biomes around the world.
30 minutes Part 3: Predicted NPP under changing climate conditions.
60-120 minutes Part 4: Small group research project
Level
Middle & High School (AP/IB classes)