Propriedades físicas do solo e seus efeitos sobre a estrutura da floresta determinam os estoques de necromassa na Amazônia Central

Abstract

Necromass is an essential component in tropical forest ecosystems and presents great variation in different forest landscapes. Relationships between necromass, soil, forest structure, and other environmental factors were analyzed to understand the drivers of necromass variation in different soil types across Central Amazonia. To estimate necromass stocks and density of dead wood, 79 plots of 0.5 ha were assessed along a transect spanning ~700 km in undisturbed forests from north of the Rio Negro to south of the Rio Amazonas. Soil physical properties were evaluated by digging 2 m deep pits and taking auger samples. Vegetation data were obtained from permanent plots. Soil physical properties were the best predictors of necromass. Soil anoxia and soil depth explained the most variation in necromass (35% and 30%, respectively). Necromass stocks on physically non-restrictive, deep, unsaturated soils (33.1 Mg ha-1) were twice those on soils with restrictive physical properties (16.0 Mg ha-1). A topographic index, which describes the spatial distribution of soil moisture, also explained significant variation in necromass stocks. Vegetation parameters, notably average biomass per tree, were modulated to soil conditions which had strong influence on local necromass stocks. Average biomass per tree alone explains about 20% of the variation in necromass. However, when soil anoxia was included in regression models, vegetation parameters were no longer significant, with this suggesting that, despite of only an indirect effect, there is a strong link between soil physical properties and necromass stocks. Seasonal soil anoxia and restrictive rooting depth in some regions are likely to influence forest structure and dynamics which in turn decreases necromass production and stocks. Substantial variation in necromass may be estimated over large scales through soil physical properties, topographic index, and forest structure.