Plant Water Sources in a Tropical Forest of the Central Amazon Basin in Brazil: A Stable Isotope and Soil Physic Approach

Abstract:

As a consequence of climate change, it has been expected an increase in the intensity and duration of Amazon droughts. Despite great amount of studies performed, it is still unknown how upland Amazon forest will respond to these extreme events. Some authors have reported forest growth while others reported increase of mortality rates in the same conditions. Crucial to this process seems to be the linkage between atmospheric demand from water and its provision by soil moisture. Stable isotopes have been extensively used as indicators of the plant-water relationships. Here we present results for a small basin in Amazon, 80km distant from Manaus. In July and October 2015 – from middle to the of the dry season of one El Niño drought - soil, plant, river, ground - and rainfall water samples were collected for stable isotopes analysis. Soil samples from a profile from a 15m deep pit were sampled for texture and water retention curves. Leaf and bark samples of 13 species were collected and analyzed in relation to the plant water use strategies. Results showed great variability of ¹⁸O depletion in the soil profile. The great part of the species presented isotopic signal very similar to the shallowest layer (20-80cm) isotopic signal. A small number of species, named Eschweilera coriaceae, Lecythis prancei and Scleronema micranthum, on the other hand, presented isotopic signal similar to the 100-140cm or 200-380cm soil layers. The most resistant specie to cavitation, Dypterix cf odorata seems to be to capture water from deeper soil – below 400cm. Despite the great homogeneity of the soil profile in terms of their textural class, the soil layer between 160 and 640cm presents higher values of residual soil moisture and microporosity, being able to store more water and retain it more strongly than the others. These findings can give us the preliminary idea that the local plants use water mainly from the layers above 80cm, even in the dry season. However, some species seems to have the ability to capture water from deeper layers to meet their water requirements during the dry season. Except for the Dypterix cf odorata, the great part of these species presented greatest vulnerability to cavitation, which could make them to search water from deeper layers.