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).