Nutrient fluxes in incident rainfall, throughfall, and stemflow in adjacent primary and secondary forests of the Gran Sabana, southern Venezuela

Abstract

Rainfall partitioning into throughfall and stemflow was studied during a year in a primary forest (tall forest) and in two adjacent secondary forests with contrasting structure (medium and low forests) in the Gran Sabana. Mean annual concentrations of nutrients in throughfall and stemflow were significantly higher ( P < 0.05) than those in incident rainfall, particularly for K. Concentrations of N were very low not only in rainfall but also in throughfall and stemflow, indicating absence of N leaching in these forests. Concentration of P in rainfall was under the detection limit, and in throughfall and stemflow was very low. With very few exceptions, the mean annual concentrations of nutrients in throughfall and stemflow did not differ significantly ( P > 0.05) between the forests, which seem to indicate that differences in structure and floristic composition do not play an important role in the nutrient dynamic at the canopy level. Throughfall and stemflow amounts represented 71–77% and 2–8% of the annual incident rainfall, respectively. Annual volumes of throughfall were quite similar in tall, medium and low forests, while the annual volume of stemflow decreased from tall to medium and low forests. With some exceptions, the annual inputs of nutrients in throughfall tended to be quite similar between the studied forests. Stemflow input tended to decrease with decreasing tree density from tall to medium and low forests, which is more related to the decrease in the volume of this flow from tall to medium and low forests than with changes in the element concentration of the stemflow among the forests. We concluded that incident rainfall is an important means of nutrient transfer from the forest canopy to the soils, and that a decrease in the tree density affected negatively the inputs of nutrients in stemflow but not in throughfall.