Soil–vegetation relationship in base-deficient premontane moist forest–savanna mosaics of the Venezuelan Guayana

Natural vegetation mosaics of heterogeneous forests, shrubs and savanna are characteristic for the thinly populated Venezuelan Guayana. Vegetation studies and 14C data suggested that the mosaic represents a stage in a long-term process of savannisation, which is originally triggered by forest fires. In the paper, we are looking for the possible role the soil plays in this process. Vegetation structure and soil chemical conditions were analyzed in 74 plots. We further studied the composition of soil solution or water extract, fine root distribution, aboveground biomass and total nutrient stores in selected sites. The soils are humic ferralsols and under woody vegetation, always covered by an organic layer (8–114 t ha−1). The mineral soil is characterized by high organic matter content (0–50 cm: 113–165 t C ha−1), a low cation exchange capacity of the mineral soil (0.94 cmolc kg−1 of clay), and a dominant Al saturation of the exchange complex. Under forest, Ca concentrations on the exchange complex, and in the soil solution/water extract, rank below that of NH4, K, Mg and Fe. Ca-deficiency is considered the main element of the chemical stress responsible for the extremely shallow rooting (<30 cm), which renders the forest systems sensitive to drought stress. NO3 in the soil solution showed high variability and often occurred below the rooting zone, which may indicate disturbance of fine roots. Status of vegetation development and soil properties are not consistently related. Selected case studies show that decreasing aboveground biomass result in changes of soil properties (C, base cation stores), but with a time lag. Forest instability is supposedly conditioned by a combination of constant soil chemical and episodic drought stress.