Impact of sedimentary processes on white-sand vegetation in an Amazonian megafan

Abstract

Abstract:Amazonian white-sand vegetation has unique tree communities tolerant to nutrient-poor soils of interest for interpreting processes of adaptation in neotropical forests. Part of this phytophysionomy is confined to Late Quaternary megafan palaeo-landforms, thus we posit that sedimentary disturbance is the main ecological factor controlling tree distribution and structuring in this environment. In this study, we characterize the topographic trend of one megafan palaeo-landform using a digital elevation model and verify its relationship to the forest by modelling the canopy height with remote sensing data. We also compare the composition and structure (i.e. canopy height and diameter at breast height) of tree groups from the outer and inner megafan environments based on the integration of remote sensing and floristic data. The latter consist of field inventories of trees ≥ 10 cm dbh using six (500 × 20 m) plots in várzea, terra firme and igapó from the outer megafan and 20 (50 × 20 m) plots in woodlands and forests from the inner megafan. The unweighted pair group method with arithmetic mean (UPGMA) and the non-metric multidimensional scaling (NMDS) were applied for clustering and dissimilarity analyses, respectively. The megafan is a sand-dominated triangular wetland with a topographic gradient of < 15 cm km−1, being more elevated along its axis. The outer megafan has a higher number of tree species (367), taller canopy height (mean of 14.1 m) and higher diameter at breast height (mean of 18.2 cm) than the white-sand forest. The latter records 89 tree species, mean canopy height of 8.4 cm and mean diameter at breast height of 15.3 cm. Trees increase in frequency closer to channels and toward the megafan's axis. The flooded and nutrient-poor sandy megafan substrate favoured the establishment of white-sand vegetation according to the overall megafan topography and morphological heterogeneities inherent to megafan sub-environments.

The influence of late Quaternary sedimentation on vegetation in an Amazonian lowland megafan

“…The megafan paleomorphology constituted an environment favourable to the colonization of grassland and shrubland vegetation, which is in sharp contact with surrounding rainforest. A previous publication also demonstrated that forest patches within the megafan consist of tree communities with a lower number of species, lower canopy height and lower diameter at breast height (DBH) than those from the surrounding rainforest (Cordeiro et al ., ). The megafan deposits are moderately to poorly‐sorted, subrounded to subangular, very fine to coarse‐grained, kaolinized sands locally interbedded with silts and muds of Late Pleistocene–Holocene ages (Rossetti et al ., ) (Figures and ); sands typically have dispersed granules of angular to subangular quartz pebbles.…”

“…However, other areas with such phytophysiognomy were related to residual landforms reflecting depositional environments deactivated in a relatively recent geological time (e.g. Rossetti et al ., ; Amaral et al ., ; Rossetti et al ., , ; Cordeiro et al ., ).…”

“…Horton and DeCelles, ; Chakraborty et al ., ; Weissmann et al ., ; Fontana et al ., ). The close association between white‐sand vegetation and megafan paleolandforms in northern Amazonia has been already documented at a regional scale (Rossetti et al ., , ; Cordeiro and Rossetti, ; Cordeiro et al ., ). However, what controls the distribution of structurally different vegetation formations on these megafans needs to be clarified, and the context of the megafan sedimentary history can contribute to answering this question.…”

White sand vegetation in an Amazonian lowland under the perspective of a young geological history

“…For comparison, our analysis also includes adjacent areas of relatively recent (Late Pleistocene-Holocene) or significantly older (Plio-Pleistocene or older) geological history. An earlier publication has shown that the diversity of tree species in the patches of white sand vegetation from this megafan is lower than that in the surrounding rainforest (Cordeiro et al 2016). We unify these data with additional tree plots to better detect floristic differences between forests on the megafan sediments and older basement rocks.…”

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

“…Recent works have demonstrated that the ecosystems which form the mosaic of landscapes in this region present a direct integration between the plant cover and the physical, chemical and biological attributes of the soil [23] due to essential processes related to the biogeochemical cycles, water Table outcrops, accumulation and decomposition of organic matter [16,24]. This is a strong indication that environmental conditions associated with temporal flooding, sediment drag, and nutrient leaching may have high importance in the formatting of different forest types in this peripheral region of the Amazonia [25][26][27].…”