Modeling Carbon and Nitrogen in Terrestrial Ecosystems

In the late 1970s and early 1980s, Ecosystems Center researchers collaborated with Berrien Moore from the University of New Hampshire to develop a model that showed the impact of humans on carbon in land (terrestrial) ecosystems. Researchers called this model the Terrestrial Carbon Model (TCM). Center scientist Skee Houghton was especially interested in the impact of historical deforestation on carbon dioxide emissions. Although wood burning releases some carbon, most of the impact of deforestation is from the transformation of forests into grazing or agricultural land. All of the scientists involved in the Center’s Global Carbon Cycle project contributed to the TCM research and publications.

The Terrestrial Carbon Model used data on rates of deforestation and the amount of carbon stored in different types of vegetation from 1860 to 1970. Researchers collected these data from the Food and Agricultural Organization and a variety of historical sources that contained records of deforestation. The TCM simulated the flow of carbon, in different forms, between the atmosphere and terrestrial biota and soils. The model showed that deforestation created a net release of carbon dioxide into the atmosphere. Agriculture, the harvest of wood, and the decay of plant products were the main sources of carbon dioxide. The Terrestrial Carbon Model showed that deforestation released more carbon dioxide than fossil fuel burning from 1860 until about 1960, when fossil fuel burning started to rapidly increase. Researchers called the Terrestrial Carbon Model a “bookkeeping” model because it described the flow of carbon based on historical data. The limit of this model is that it only calculates the amount of carbon dioxide released by terrestrial ecosystems.

Scientists at the Ecosystems Center were not only interested in keeping track of how much carbon plants consumed and released. They also wanted to study the behavior of plants in response to changes in environmental nutrient levels. Some scientists predicted that plants would respond to the rise in carbon dioxide levels by growing more quickly and thus transferring carbon from the atmosphere into plants and the soil. Short-term experiments also indicated that plant ecosystems would compensate for increased carbon dioxide by increasing their rate of photosynthesis and turn carbon dioxide into organic carbon. To test this hypothesis, starting in the mid-1980s Ed Rastetter and Gus Shaver developed a mathematical model. They based this model on the availability and replenishment rates of nitrogen and carbon in an ecosystem, and the assumed rates of plant uptake of nutrients. They named this the Multiple Element Limitation model.

The Multiple Element Limitation model was a simple model of plant acclimation to changes in nitrogen and carbon in their environment. Plants rely on a variety of nutrients for their growth, and if they run into a shortage of one nutrient, they will spend energy trying to gather more of that nutrient, such as growing more roots and therefore investing less energy in leaf production. Rastetter and Shaver created different scenarios for the amounts and replenishment rates of available carbon and nitrogen in the plants’ environment. The US National Science Foundation and National Aeronautics and Space Administration funded this project. Rastetter and Shaver reached two main conclusions: 1) plants’ short-term response to nutrient changes in the environment would not predict the plants’ long-term response, and 2) the most important factor that determines plants’ long-term response to change in nutrients is the replenishment rate of the less-available nutrient. These results contradicted what many scientists had assumed about plant responses to climate change, which was that increases in carbon dioxide levels would create more plant growth, as plants remove carbon dioxide from the atmosphere and use it in photosynthesis to fuel their growth. The Multiple Element Limitation model showed that the replenishment rate of nitrogen in the soil would ultimately control how plants responded, in the long-term, to a rise in carbon dioxide levels. This model did not rely on experimental data, but it laid the groundwork for future experiments on how plants respond to multiple nutrient limitations. In 1994 Rastetter and Shaver collaborated with visiting researcher Göran Ågren, from the Swedish University of Agricultural Sciences, to include soil processes and nutrient recycling in the model.