Research on the role of forests and other terrestrial ecosystems in the carbon cycle was stimulated especially by the papers of Woodwell et al., Broeker et al., Houghton et al., Tans et al. and Siegenthaler and Sarmiento. Forests are the most likely candidates as carbon sinks, as their wood is an immense store of carbon while their growth rates are known to be increasing. In this case the pool of carbon in the soil would be constant and the fluxes of carbon into the system would be matched exactly by the total respiratory fluxes from the ecosystem, arising from both heterotrophic and autotrophic respiration. For estimating carbon accumulation rates, some researchers use an 'expansion factor', which is simply the multiplier required to find the total carbon accumulation rate from the rate of bole growth. The storage of carbon as woody debris and as soil organic matter is substantial in cold climates.