Case Study: Carbon Storage in a Northeastern Forest

Through the process of photosynthesis all plants use CO2 from the atmosphere in combination with water to create carbohydrates. Carbohydrates are simple and compound sugars, such as cellulose that make up the main plant structure. This process of taking CO2 out of the atmosphere and converting it to carbohydrates actually locks up the carbon in plant tissues, storing it until that plant dies. If you were to dry a plant, 45-50% of the plant mass, also called biomass, is made up of carbon. In the case of trees that can grow for hundreds of years and get very large, significant amounts of carbon are temporarily removed from the global carbon cycle and stored in the tree's stem, bark and branches. *A note on biomass and growth: Growth is equal to the increase in biomass over time.

In some places forest growth can be calculated using direct measurements taken on trees, however there are many locations where access to the forest is not a possibility. When this is the case we may need to use a proxy, some other related variable, to estimate forest growth. In the Northeastern United States the amount of nitrogen in the leaves (foliar nitrogen) increases at a consistent rate as forest productivity increases. Therefore we can use this relationship to predict growth when data are not available.

Given that trees play such a large role in the global carbon cycle it is important to understand how forests grow and change over time. In science the most common way for researchers to understand the future conditions of a system is to build a model. Models use current knowledge including mathematical relationships between important variables to predict values under a range of scenarios. Models then allow scientists to ask questions such as: What will happen in the future? How will the outcome change if the model inputs change? What do current conditions suggest about the past?

In this chapter you will assess how well forest biomass, predicted by a STELLA computer model, matches biomass data measured at the Hubbard Brook Experimental Forest. To do this you will need to download data from the Hubbard Brook Experimental Forest to calculate biomass, use the Foliar Chemistry Database to determine a foliar nitrogen input value to run the STELLA Biomass Accumulation model, graph model output with HB calculated biomass, and assess differences in data considering topics including, biomass, carbon storage, and change over time.