Forest floor water dynamics and root water uptake in four forest ecosystems in northwest Amazonia

Abstract

A common feature in the undisturbed forest ecosystems in the Middle Caquetá (Colombian Amazonia) is the presence of a thick litter layer with abundant fine roots over mineral soils which are highly weathered and very low in available nutrients. In these situations, the litter layer or the forest floor (FF) may play an important role in the forest water balance, controlling water fluxes and nutrient cycling. We investigated the forest floor water dynamics in four representative forest ecosystems in the Middle Caquetá, Colombian Amazonia (tertiary sedimentary plain, high terrace, low terrace and the rarely inundated flood plain). Meteorological conditions were measured on hourly basis, FF water storage capacity, TDR water content in the FF and litterflow or FF drainage were measured daily over a two year period. A dynamic model was developed to simulate FF water storage, root water uptake and drainage to the mineral soil. The four-parameter model was calibrated applying a step-wise procedure. Analysis of collected data showed that FF water content was generally lower in the sedimentary plain than in the other ecosystems, whereas FF water storage was the highest due to a high FF mass. The average storage capacity per unit FF thickness was 1.23 mm cm −1. The sensitivity analysis and calibration of model parameters highlighted the relevance of storage capacity as the most sensitive parameter for the FF water dynamics. For the validation period, there is a good agreement between predicted and measured FF water storage and especially between predicted total drainage from the forest floor and measured litterflow. Model predictions indicate that water uptake from the FF’s during the validation period (190 days) differed between ecosystems, ranging from 15 to about 28% of the reference transpiration. This seems to be related to the fraction of fine roots in the FF in each ecosystem and to the water availability. On the other hand, total drainage to the mineral soil was very similar among ecosystems, except for the sedimentary plain, where total drainage was the lowest with about 87% of incoming throughfall.