CARBON BUDGET OF UNITED STATES FORESTS

Q: What is a carbon budget?

A: A carbon budget (sometimes called carbon balance) shows the inventory of C in C pools and the balance of exchange between the pools. The rate of exchange between the pools is called C flux. The most direct method of tracking the amount of C in ecosystems for policy analysis is the calculation of C budgets based on biomass inventories. For forests, this generally involves multiplying inventory data collected at different times for trees, woody detritus, leaf litter, understory, and soil by conversion factors to express all units in terms of weight of C. Common units are million metric tons (MMT=Teragrams=1012 grams), megagrams (Mg=106 grams), and billion metric tons (petagrams=1015 grams) Therefore, we can represent the pool of C in forests as:

Vt = Tt+WDt+Lt+Ut+St,

where Vt=total C in the forest, Tt=the amount of C in trees, aboveground and belowground, WDt=C in woody detritus, Lt= the amount of C in the leaf litter layer, Ut= the amount of C in the understory, and St=the amount of soil C in the forest, all at time t. Carbon flux would be calculated as Fp=Vt-Vt-1, with Fp=C flux for period p, expressed on an annual basis by dividing Fp by length of period.

Carbon budgets or balances are often also calculated for individual plants in physiological terms, including photosynthesis, respiration, and allocation (which refers to the relative amount of C stored in specific organs) using time steps on the order of hours or daily. Generally, the models producing these budgets are called process models, as they describe the process underlying the system under study. The models are quite useful for investigating certain aspects of C budgets, but they are generally less accurate within observed limits and more expensive for policy analysis than empirical models based on biomass.

SOURCE: This text is more fully developed in Heath, L.S., and L.A. Joyce. 1997. Carbon sequestration in forests as a national policy issue. In: Communicating the role of silviculture in managing the national forests: Proceedings of the National Silviculture Workshop; 1997 May 19-21; Warren, PA. General Technical Report NE-238. U. S. Department of Agriculture, Forest Service, Northeastern Forest Experiment Station: 29-36.