Structure, floristic composition, and distribution of swamp forests across a white-water flood-plain in the Colombian Amazon

Abstract

Questions

The fluvial dynamics of meandering white-water rivers of Amazonia drive vegetation primary succession. Directional successional processes have been recorded for the seasonal várzea forests that occupy well-drained soils on levees and point bars across the spatial gradient of the flood-plains. However, the types of forests occupying the swampy depressions interspersed between the point bars and their distribution along the flood-plain's spatial gradient are poorly understood. Here, we aimed to unravel the spatial patterns of swamp forests along the sequence defined by an axis perpendicular to the river and the relationship with edaphic and spatial factors.

Location

The flood-plains of the middle Caquetá River basin, Colombian Amazonia.

Methods

Forest types were identified by cluster analysis performed on 42 square plots (33 m × 33 m) set in poorly drained depressions of the flood-plain. Floristic composition and vegetation structure as response data, and edaphic and spatial variables as predictors, were analyzed through partial redundancy analysis (pRDA). The effect of geographic position was included by using the first two axes of a principal coordinates of neighbor matrix analysis as conditional factors in the pRDA.

Results

The three identified swamp forest types were not arranged along a directional spatial pattern. Permanent várzea forests, closest to the river, showed the greatest diversity and alluvial sediment input. Oxandrales, furthest from the river, dominated by Oxandra polyantha, showed the greatest tree density, basal area and soil sand content, and received additional flooding from black-water streams. Cananguchales, dominated by Mauritia flexuosa, exhibited the highest dominance and soil organic matter layer thickness. Distribution of the permanent várzea and oxandrales was relatively constrained by the distance to the river, whereas that of the cananguchales was not.

Conclusions

Flooding dynamics delay vegetation development of the swamp forests in permanent várzea and oxandrales. Cananguchales keep accumulating organic matter, becoming ombrotrophic peats after isolation from the river flooding influence. The swamp forests across these flood-plains are far from being arranged along a linear sequence.