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). 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.