Assessment of biomass and carbon dynamics in pine forests of the spanish central rangea remote sensing approach

Resumen

Forests play a dynamic role in the terrestrial carbon (C) budget, by means of the biomass stock and C fluxes involved in photosynthesis and respiration. Remote sensing in combination with data analysis constitute a practical means for evaluation of forest implications in the carbon cycle, providing spatially explicit estimations of the amount, quality, and spatio-temporal dynamics of biomass and C stocks. Medium and high spatial resolution optical data from satellite-borne sensors were employed, supported by field measures, to investigate the carbon role of Mediterranean pines in the Central Range of Spain during a 25 year period (1984-2009). The location, extent, and distribution of pine forests were characterized, and spatial changes occurred in three sub-periods were evaluated. Capitalizing on temporal series of spectral data from Landsat sensors, novel techniques for processing and data analysis were developed to identify successional processes at the landscape level, and to characterize carbon stocking condition locally, enabling simultaneous characterization of trends and patterns of change. High spatial resolution data captured by the commercial satellite QuickBird-2 were employed to model structural attributes at the stand level, and to explore forest structural diversity. Aboveground biomass (AGB) was calculated retrospectively at specific dates (1990 and 2000) with modelled temporal trajectories of spectral indices, and maps were produced with 30 m spatial resolution depicting biomass, biomass change, and measures of uncertainty, from which carbon budgets were calculated. Results indicate that the area occupied by pines in the Central Range of Spain fluctuated during period 1984-2009, with a final net increase of 40%. A global activation of carbon pools was observed, being the area intermittently covered by pines strongly involved in processes of C exchange, while the permanent pines had a near to neutral net C character. The temporal character of local carbon fluxes was identified and summarized by date, showing that in 2000 there was a maximum of 33% of the area in a process of net C accumulation. A widespread trend to accumulate biomass was confirmed, with 18% of initial biomass accrued by the pine stable area between 1990 and 2000. On average, these pines accumulated 0.65 t ha-1 y-1 of carbon in the form of AGB between 1990 and 2000, equivalent to 2.38 t ha-1 y-1 fixation of CO2. Remote sensing supports and enhances the value of forest inventories based on sample plots for assessment of biomass and C budgets, complementing rather than substituting essential field work. Access to archived historical and contemporary images of high and consistent quality provides opportunities to develop methods for unveiling information related to the development of forest ecosystems that would otherwise remain incomplete or unknown. In addition to international archives of medium spatial resolution images providing data free of cost, like the US Geological Survey and the European Space Agency, the Spanish Plan Nacional de Observación del Territorio acquires national coverage of high spatial resolution imagery annually, that constitutes an opportunity to support the evaluation of the national forest resources for planning and decision making.