What methods are available for quantifying, characterizing, and cross-checking terrestrial carbon stocks over differing timescales and spatial scales?
Workshop participants noted that the landscape is incredibly heterogeneous due to variables such as topography, climate, soils, land use (both current and historical), and vegetation type and age distribution, and carbon dynamics are strongly dependent on these heterogeneities. Fire and other disturbances can cause rapid changes to carbon stocks on the timescale of hours, followed by years to centuries of recovery. Because most relevant biological processes are not additive, models based on average values of important underlying heterogeneities are likely to be highly inaccurate for long-term predictions.
Several presenters emphasized that multiple methods and investigations, including seemingly redundant cross-checks, are important to quantify biophysical and biogeochemical constraints on carbon storage. Multiple methods may include using trace gas inventories (through a “top-down” approach), biospheric inventories (through a “bottom-up” approach), field-scale experiments, and process studies (including process-based modeling that links together phenomena operating at varying spatial and temporal scales). For example, some biomass maps are available that could be combined with remote sensing data to produce a much better estimate of land use changes.
Workshop participants noted that there is a large and growing set of data important for characterizing carbon stocks and fluxes and the mechanisms involved over varying spatial and temporal scales, and these measurements are increasing in extent, resolution, and reliability. Nevertheless, several data needs were highlighted to improve the assessment of terrestrial carbon stocks. Some important factors identified include knowing the area over which the particular land use change is occurring and the time frame for the initial land use disturbance and recovery. Rates of regrowth are also not well understood. Methods proposed for obtaining such data include (1) high-resolution remote sensing to classify and map disturbance types, ages, and land-cover changes and (2) satellite lidar products to classify and map forest height and age. Comprehensive inventories of the tropical region were also suggested, with the greatest emphasis placed on vulnerable areas. To better understand changes in tropical forest carbon stocks, a workshop participant encouraged more research to identify historical land use patterns and quantify forest degradation and fragmentation, using spatial models to further identify drivers of forest degradation. Formal network design studies could develop efficient inventory systems with known statistical properties and protocols for monitoring and remote sensing. Workshop participants also suggested that policies be better able to accommodate future improvements in measurement technologies.