Dinâmica dos limites floresta-savana ao norte da Amazônia e suas implicações no acúmulo de biomassa e no estoque de carbono

Abstract

Some coupled carbon/climate models are predicting globally changes in Amazon carbon and water cycles for the near future, with conversion of forest to savanna like vegetation, in a process called savannization . Early signals of this process are predicted to happen primarily in ecotonal areas between forests and savannas ecosystems, favored by factors as fire regime intensification, climate and land use changes, among others. Protected areas seem to play an important role in maintaining the climate-vegetation balance, reducing the probability of savannization occurrence in Amazonia. In this study we combined different methodological approaches to investigate the forest-savanna boundaries fluctuations in a Brazilian northern Amazon climatic and biogeographical boundary area and, its implications to vegetation and soil biomass accumulation and carbon stocks. Comparing savanna enclaves within a strictly protected nature reserve (ESEC Maracá) and its outskirts non-protected incrusted forest islands within continuous savanna matrix we found out that (i) the forest-savanna mosaic dynamics in Northern Amazonia was influenced by rainfall changes and conservation status; (ii) with distinct origins and dynamics patterns, different savanna type did not respond the same way to climate change and fire events neither in the last decades nor during the Holocene; (iii) vegetation structure changes from disturbance events have directly influenced biomass accumulation and carbon stocks. After 20 years of protection, a potential total carbon stock increase from forest encroachment into savanna intensified in higher rainfall regime, showed ESEC Maracá effectiveness in act as carbon sink. In the other hand, its protected savanna enclaves proved to be more resilient to past climate changes keeping unchanged since its establishment (at least 5,350 years BP) indicating an origin from non-anthropogenic edaphic conditions. Comparatively, non-protected mosaics outside ESEC Maracá revealed dynamic equilibrium in regional scale, suggesting stability in 20 years. Forest encroachment predominance in augmented rainfall years (1994 2006), in non-protected areas, pointed climate effects prevalence over disturbance effects, while the savannization occurrence in reduced rainfall periods (1986 1994) in this area showed evidences that the future changes in extension and directions of forest limits will be affected, disrupting ecological services as carbon stock and local biodiversity, if confirmed the increase of dry events frequency predicted by climate models for this region. Evidences of past climate changes influence were demonstrated to savanna matrix outside ESEC Maracá through forest shrinkage of at least 70 m occurring between 10,000 e 780 years BP, by the presence of typical forest isotopic sign in its depth soils. A better understanding of forest-savanna boundaries dynamics and its biomass and carbon stock variations of little knowledge in northern Amazon transitional areas here presented, should be considered a first approximation to assist discussions about the values of different conservation and management strategies in politic context of reducing emissions from deforestation and forest degradation (REDD) in Amazonian forest edges areas.