Reducing tree density affects interactions between trees and atmospheric Tillandsia species (Bromeliaceae)

Abstract

Disruptions in species interactions derived from forest fragmentation are a major driver of biodiversity declines. Atmospheric bromeliads, a group of extreme epiphytes in the Tillandsia genus, often thrive in anthropogenic landscapes, spreading beyond their usual distribution. Here, we investigate how distinct tree densities affect the interactions between tree traits and patterns of atmospheric bromeliad host selection, abundance, functional composition, and canopy distribution. We measured a total of 3130 atmospheric bromeliads hosted on 140 of the 384 trees sampled and found a greater contribution of tree traits on their host selection and abundance at low tree density. We observed that distinct tree traits were significantly correlated to atmospheric bromeliad traits under low and high tree densities, with most correlations positive under low tree density and negative under high tree density. Additionally, the canopy distribution of individuals under high tree density was less scattered and slightly restricted to outer canopy layers. Shading levels seem to be a strong environmental filter on atmospheric bromeliad fitness, limiting their spread in forests with high tree densities; however, major reductions in tree density may allow atmospheric bromeliads to spread to other canopy layers and increase their abundance, mainly boosted by traits related to tree size. Our results suggest that reductions in tree densities of forests may alter ecological interactions among plants, which could lead to the spread of opportunistic species.