Edaphic Factors and Flooding Periodicity Determining Forest Types in a Topographic Gradient in the Northern Brazilian Amazonia

Abstract

The Brazilian Amazonia is a region covered by an extensive mosaic of tropical forests conditioned by different topographical and hydro-edaphic features. Although studies relating environmental determinants of structure and floristic composition are systematically evolving in the region, there is no doubt that there are still information gaps due to the lack of research in peripheric areas of the Amazonia. The seasonally flooded areas of the state of Roraima situated on rio Branco-rio Negro basin, northern Brazilian Amazonia, still are deprived of such information. In this way, this work had as objective determine the physical and soil chemical attributes, and the flooding periodicity that characterize different forest types dispersed in a topographic gradient located in an area on the north of rio Branco-rio Negro basin. Soil samples (0-60 cm) were collected along a 2.7 km transect (31.1-64.8 m a.s.l.) crossing three different forest types: (i) mosaic between treed and forested shade-loving (La+Ld), (ii) area of ecological tension between forested shade-loving and open ombrophilous forest (LO) and (iii) open ombrophilous forest (Ab+As). The results indicated different soil classes and flooding periodicity for each forest type observed: Entisols Fluvents (La+Ld, 3-4 months flooded), Entisols Quartzipsamments (LO, 1-2 months) and Yellow Ultisols (Ab+As, no flooding). All analyzed soils were defined as nutrient-poor areas, especially those located on low altitude, characterized for higher hydrological restrictions (seasonal flooding) aggregating forest types of lower structural patterns (e.g. La+Ld). Soils on low altitude were also characterized as those with the highest percentage of fine sand and silt, while soil free of seasonal flooding (Yellow Ultisols) presented the highest levels of clay and coarse sand, always associated with the ombrophilous forests (higher structural patterns). These results improve our understanding of the environmental factors conditioning different forest types in this peripheral region of Amazonia, suggesting that ecosystems with higher hydro-edaphic restrictions are a strong indicator of forest types with lower structural patterns.

Environmental filters and biotic interactions drive species richness and composition in ecotone forests of the northern Brazilian Amazonia

“…Our study indicated that the altitude (a topographic proxy for risk of seasonal flooding) can be seen as an important predictor for determining the species composition in the ecotone forests of eastern Maracá, because it has a synergistic effect with both fertility and soil texture, especially Clay content (Appendix 1). Different types of environments characterized by seasonal flooding are seen as important factors that support distinctions in the tree species composition in the Amazon (Assis et al, 2015;Alemán et al, 2019). However, the seasonal flooding should not be seen as a single environmental filter predicting tree species composition in Maracá.…”

Above‐ and below‐ground biodiversity responses to the prolonged flood pulse in central‐western Amazonia, Brazil

“…For example, dry season length, clay content, soil phosphorus (P), calcium (Ca), magnesium (Mg), and potassium (K) cation exchange capacity (CEC) appear to be important determinants of woody plant community composition (Sombroek, 2000; Toledo et al, 2017). In areas that flood, flood duration (i.e., the hydro‐period) also seems indicative of woody plant community composition and alpha diversity (Alemán et al; Arias et al, 2018; Assis et al, 2015a; Junk et al, 2012; Targhetta et al, 2015). Given the demonstrated implication of below‐ and above‐ground community interactions for ecosystem functioning elsewhere (Wang & Ruan, 2008), it would be important to understand how Amazonian below‐ground biodiversity relates to changes in these abiotic factors and the above‐ground community.…”