Vertical distance from drainage drives floristic composition changes in an Amazonian rainforest

Abstract

Background: Plant composition changes with topography and edaphic gradients that correlate with soil-water and nutrient availability. Data on soil water for the Amazon Basin are scarce, limiting the possibility of distinguishing between soil and soil-water influences on plant composition.

Aim: We tested a new proxy for water table depth, the terrain height above nearest drainage (HAND), as a predictor of composition in trees, lianas, palms, shrubs, and herbs and compared HAND to conventional measures of height above sea level (HASL) and horizontal distances from nearest drainage (HDND).

Methods: Plant-species composition in 72 plots distributed across 64 km² of lowland evergreen terra firme forest was summarised using non-metric multidimensional scaling (NMDS). NMDS scores were regressed against estimates of HAND, HASL and HDND.

Results: Plant composition was highly correlated with the vertical distance from water table, capturing up to 82% of variation. All life forms showed highest turnover rates in the zone with seasonally water-saturated soils, which can extend 350 m from stream margins.

Conclusions: Floristic composition is closely related to water table depth, and HAND appears to be the most robust available topographical metric of soil-water gradients. Brazilian conservation laws protecting 30-m-wide riparian buffers are likely to be too narrow to encompass the full zone of highest floristic turnover and may be ineffective in safeguarding riparian plant diversity.

Keywords:

• beta diversity
• height above nearest drainage
• distance from stream
• plant species composition
• soil hydrology
• Shuttle Radar Topography Mission
• terra firme forest
• topography
• tropical rain forest
• water table

Acknowledgements

Data used in this paper were obtained from data repositories maintained by the Brazilian Programme for Biodiversity Research (PPBio) and the National Institute for Amazonian Biodiversity (INCT-CENBAM). The field infrastructure was installed by the Programa Norte de Pós-Graduação (PNOPG-CAPES) and maintained by the Brazilian PELD (LTER) Program financed by National Council for Scientific and Technological Development CNPq. J.S. and T.E. were supported during this study by a CNPq fellowship. Part of this manuscript was developed during the workshops of PPBio/PPGECO-INPA at Reserva Ducke and RAINFOR (Gordon and Betty Moore Foundation)-UFAC workshop in Rio Branco, Brazil, 2011. We thank Laszlo Nagy, Toby Marthews and one anonymous reviewer for the positive insights and suggestions on the manuscript structure.