Alternatively, these changes can be calculated by the stock change method as the change in stocks between two consecutive inventories. In NFIs, changes in growing stock are often quantified in terms of the volume of stem wood (merchantable). For the Greenhouse Gas Inventory, this change in volume is multiplied by constants (biomass expansion factors) to convert from stem wood volume to whole tree biomass and then CO2 equivalents

(e.g., see Formula (5)). Another approach is to directly estimate the biomass per tree fraction by applying biomass regression equations (BiEqs) to sample trees and then converting the biomass to CO2 equivalents by scaling (see, for example, Formula (1); Somogyi et al., 2007). When estimating changes in living biomass at a national scale, it is usually difficult to obtain a reliable value for selleck chemicals the whole tree biomass

from the stem volume because stem proportion increases with tree size at the expense of branches, foliage, stump and roots (Fig. 1). Hence, the use of biomass expansion factors (BEFs) may http://www.selleckchem.com/products/at13387.html lead to biased estimates because BEFs vary with tree size (age, etc.) and tree populations change over time (e.g., Satoo and Madgwick, 1982, Albrektson and Valinger, 1985 and Pajtík et al., 2011). When using the stock change method, to reduce the risk of bias BEFs should reflect the actual change in stock by incorporating the accumulation of growth per tree fraction with the effects of harvest and natural thinning patterns in one constant. Such BEFs can be derived but need to be updated if the allocation of growth and harvest patterns change. For practical reasons, instead of representing the actual change in stock, BEFs are often derived for the standing stock, which introduces an unknown bias into the estimates. To reduce the risk of bias, age-dependent (e.g., Lehtonen et al., 2004, Lehtonen et al., 2007 and Tobin and Nieuwenhuis, 2007) or volume-dependent (e.g., Schroeder et al., 1997 and Fang et al., 2001) BEFs have been developed, which enable the ratio of whole tree biomass to stem volume

to change with tree size. Levy et al. (2004) performed Loperamide regression and variance analyses of BEFs and found that tree height was a better predictor than age. Therefore, in summary, there is a growing body of evidence that estimates based on BEFs are not constant but vary with tree, site and stand conditions (e.g., Jalkanen et al., 2005 and Guo et al., 2010). Currently, BEFs are frequently used for greenhouse gas reporting because the volumes of growing stock and stem-wood growth are usually the most reliable estimates in traditional forest inventories. However, only a few investigations have assessed the magnitude of potential error that may be introduced if the BEFs are incorrect (e.g., Lehtonen et al., 2007 and Albaugh et al., 2009).