Fine-scale effect of environmental variation and distance from watercourses on pteridophyte assemblage structure in the western Amazon

Defining the factors that regulate the geographic distribution and abundance of plants in natural systems is still a major challenge. This challenge is even greater when non-arboreal plants are used as a model organism, as they have historically received less attention than trees. In this study, we evaluated how the richness and composition of pteridophyte species varies over a gradient of distance from small watercourses in the Amazon and how the environment can explain these variations. We installed eight sampling units consisting of six transects, 2 m × 150 m each, along a gradient of distance from the banks of small watercourses in the western Brazilian Amazon, totalling 1.44 ha sampled area. In these transects, we sampled all pteridophytes and counted bamboo culms (an especially important element in western Amazon forests), measured the litter layer thickness and analysed the soil chemical and granulometric properties. Higher species richness of pteridophytes was observed near the bank of the watercourses in relation to the most distant portions. Species composition also differed between portions closer and farther from watercourses and was influenced by soil factors, especially those related to nutrient availability. The results indicate that distance from the watercourses and variation in nutrient availability along this gradient are important predictors of variation in the structure of pteridophyte assemblages in tropical forests.