Gaps in this methodology exist because sufficient data are not available to quantify all of the pools and fluxes of greenhouse gases in settlements. Obvious gaps include:
• Methodology for estimating emissions of non-CO2 greenhouse gases (N2O and CH4);
• Detailed methodology to account for carbon stocks other than live biomass and soils (specifically, dead wood and litter);
• Discussion of carbon stocks and fluxes from turfgrass and turf management;
• Discussion of carbon stocks and fluxes from gardens and other herbaceous plants; and
• A generalized methodology to account for different classes of settled lands, with different amounts of woody and non-woody vegetation and different types of management.
Non-CO2 greenhouse gases. While some evidence exists to support the idea that nitrous oxide fluxes may be enhanced in urban areas relative to the native condition (Kaye et al, 2004), this result likely depends on the native condition (i.e., the climate and region in which the settlement is located) and the management regime typically applied in that settled area. Additional data are required before conclusions about the impact of settlement on non-CO2 greenhouse gas fluxes can be drawn.
Dead wood and litter. Dead wood is a class variously composed of fallen or pruned branches or trees, or dead standing trees not yet replaced with live individuals. This dead wood may be burned or disposed of as solid waste, used for composting, left to decay either in-site or off-site. This material is treated in this methodology as a loss from the live biomass term. Because dead wood is likely to be carried off-site in settlements (rather than left on-site to decay as in forests), a more detailed methodology developed in the future might account for the proportion of dead wood taken to landfills, disposed of in compost piles, burned, or left on-site to decay. The portion taken to landfills or composted might be treated as harvested wood products (HWP) or as waste, both of which are treated in other sections of the Guidelines.
Turfgrass and turf management. Turfgrass biomass consists of roots, stubble, thatch, and above-ground components. Though estimates of turfgrass productivity have been published (Falk, 1976; Falk, 1980; Qian et al., 2003), grass decomposes quickly and there is little information about the overall accumulation of biomass in the longer-lived components of turf biomass. Turfgrass allocation to the above-ground and below-ground components also depends on the management and mowing regime. Because of the lack of generalizable information on this topic, as well as the lack of activity data quantifying the area covered by turfgrass in settlements, there is currently no detailed methodology describing carbon removed by turf systems. A more detailed methodology would require additional information on turf productivity, turfgrass turnover, and allocation to different plant components as it varies with management regime. Of course, the activity data required to implement this methodology would include information on management regimes and the proportion of settlements covered by turfgrass.
Gardens and other herbaceous plants. Similar to the situation with turfgrass, information does not exist describing the annual biomass accumulation and allocation of garden plants to different above-ground and below-ground parts. Similarly, information is not available describing the variation in plant productivity with management regime. Activity data required to implement a more detailed methodology would include information on management regimes and the proportion of settlement area covered by this type of vegetation. These are mainly garden plants, so sampling them in private gardens presents the additional problem of their likely disturbance and consequent denial of access to them (cf. Jo and McPherson, 1995).
Land classes. A more detailed methodology would benefit from a consistent set of definitions of land classes within settlements, that could be applied to any country regardless of its climate, native vegetation, or typical settlement regime. This would make settlements parallel to other land uses - Forest Land, Grassland, Cropland,
Wetlands - which are easily defined based on a set of measurable and objective parameters. Some research has been applied in this direction (Theobald, 2004), but current classifications are inconsistent. While the rate of carbon sequestration per unit of tree crown cover is fairly consistent, for example, the overall rate of carbon storage per unit of settlement area depends entirely on the relative amounts of tree and turfgrass cover within that settlement. This land classification would be part of the set of activity data collected by countries, and the detailed methodology could be developed and applied consistently based on those land cover data. This type of land-use classification would also enable countries to account for changes in carbon storage resulting from management changes within areas broadly classified as settlements. For example, when vacant plots are developed, the adventitious vegetation remaining in the non-built areas might be replaced with landscape species differing in ability to store carbon.
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