2.1. Study site
The study was conducted at the Amazon Tall Tower Observatory (ATTO) within the PELD-MAUA (PELD is the acronym in Portuguese for Long-term Ecological Research) experimental plots. This experimental site is in the Uatumã Sustainable Development Reserve (USDR), about 150 km northeast of the city of Manaus (S 02 08.9° W 059 00.2°), in central Amazonia (Fig. 1). The climate is tropical humid, with mean annual temperature and precipitation of 28ºC and 2376 mm, respectively, and is marked by a pronounced rainy season from November to May and a drier season from June to October (Andreae et al. 2015). The reserve covers 4244 km² with a mosaic of dense non-flooded upland forest vegetation, dense non-flooded forest upon ancient river terraces, and shrubland/closed-canopy vegetation on white sands (Yáñez-Serranoet al. 2015). These three forest types are characterized with differences in soil and vegetation attributes. In the upland forest, soils are classified as ferralsols, which are highly weathered and well-drained (Chauvel, Lucas & Boulet 1987). Soils in the white-sand forests are classified as arenosols, with characteristic properties of high water permeability, low water holding capacity, low specific heat capacity, and often low nutrient contents that are mostly held on organic matter (Quesada et al. 2011). Also, white-sand forests can be subject to extremes of flooding and drought at different times of the year. Through intense leaching, Fe, Al, Mg and other compounds are deposited in the lower layers of the soil forming a hard layer that can block water drainage. Thus, in the dry season the vegetation can suffer severe water deficit, and in the rainy season water-logged soils or even superficial inundation of the root system for months (Kubitzki 1989). Soils in the ancient river terrace forests are classified as alisols, which represent a more recent pedogenetic status compared to the ferralsols from the upland forest and therefore have greater capacity to supply nutrients, as it was observed with higher total phosphorus and higher total reserve bases (Andreae et al. 2015). However, some ancient river terrace soils show signs of anoxia (mottling) in deeper horizons, which may have an influence on forest structure (Quesadaet al. 2012; Emilio et al. 2013) and dynamics (Cintraet al. 2013), and possibly restrict tree height and individual biomass storage (Martins et al. 2015) compared to upland forests. The vegetation across the three forest types presents differences in tree species richness, with the highest number of species on upland forest (137±5), followed by ancient river terrace forest (127±8) and white-sand forest (64±18) (Andreae et al. 2015). Carbon stocks in above-ground biomass followed the same pattern as species richness, by increasing from 79 ± 26 Mg ha−1 in the white-sand and 101 ± 13 Mg ha−1 in the ancient river terrace to a maximum of 170 ± 13 Mg ha−1 in the upland forests.  The ATTO site combines high alpha diversity with high beta diversity within a small geographic scale, where tree species diverge mostly to local edaphic conditions (Andreae et al. 2015).