Understory palms are not canopy palms writ small: Factors affecting Amazonian understory palms within riparian zones and across the landscape

Abstract

Understory palms are subject to different ecological pressures than those experienced by canopy palms, but most studies do not distinguish between understory and canopy palms, or include only canopy palms. Much of the variation in the taxonomic composition of understory palms in the Amazon is concentrated in riparian zones. However, abiotic characteristics of the riparian zone that influence this variation are underexplored or have been modeled only through additive relationships. Our objectives were to determine (1) if the abundance of understory palms is related to the abundance of canopy palms, (2) how topographic and soil variables, and their interactions, affect variation in composition of understory-palm assemblages across the entire landscape, which includes riparian and non-riparian zones, and (3) how understory-palm assemblages vary in species composition along streams as a result of variation in abiotic factors and their interactions within the riparian zone. We sampled 40 riparian areas and analyzed the data in combination with existing data from 72 non-riparian plots. Plots (250 m × 4 m) were distributed across 64 km2 of a terra-firme forest (Reserva Ducke) covering two major drainage basins that differ in topography and soil characteristics. Abundances of understory and canopy palms have opposite relationships to the same environmental variables and, therefore, should not be treated as a single group. In addition, interactions among environmental factors at the mesoscale (entire landscape) and at the local scale (riparian zone), generate compositional turnover even among sites with only subtle environmental differences. Stream discharge and soil structure are among the first variables to be affected by human occupation of riparian zones, which indicates that a better understanding of interactions between environmental predictors and a landscape approach will be necessary to conserve Amazonian understory palms.