Abstract
Local variation in the abundance and richness of vascular epiphytes is often attributed to environmental characteristics such as substrate and microclimate. Less is known, however, about the impacts of tree and branch turnover on epiphyte communities. To address this issue, we surveyed branches and epiphytes found on the forest floor in 96 transects in two forests (Atlantic rainforest in Brazil and Caribbean rainforest in Panama). In the Brazilian forest, we additionally distinguished between edge and core study sites. We quantified branch abundance, epiphyte abundance, richness and proportion of adults to investigate the trends of these variables over branch diameter. Branches <2 cm in diameter comprised >90% of all branches on the forest floor. Abundance and richness of fallen epiphytes per transect were highest in the Brazilian core transects and lowest in the Panamanian transects. The majority of epiphytes on the floor (c. 65%) were found attached to branches. At all three study sites, branch abundance and branch diameter were negatively correlated, whereas epiphyte abundance and richness per branch, as well as the proportion of adults were positively correlated with branch diameter. The relationship between branch diameter and absolute epiphyte abundance or richness differed between study sites, which might be explained by differences in forest structure and dynamics. In the Panamanian forest, epiphytes had been previously inventoried, allowing an evaluation of our surveying method by comparing canopy and forest floor samplings. Individuals found on the forest floor corresponded to 13% of all individuals on branches <10 cm in diameter (including crowns), with abundance, richness and composition trends on forest floor reflecting canopy trends. We argue that forest floor surveys provide useful floristic and, most notably, demographic information particularly on epiphytes occurring on the thinnest branches, which are least accessible. Here, branchfall acts as an important demographic filter structuring epiphyte communities.
Highlights
Vascular epiphytes are plants that grow on shrubs and trees, and thereby on a substrate distributed in three-dimensional space [1,2,3]
We addressed three hypotheses for epiphytes on the forest floor: 1) epiphyte abundance, 2) epiphyte richness and 3) proportion of adult epiphytes are positively correlated with branch diameter (Fig 1)
Using the branches on the forest floor, we addressed whether the assumption that branch abundance on the forest floor is negatively correlated with branch diameter was true
Summary
Vascular epiphytes are plants that grow on shrubs and trees, and thereby on a substrate distributed in three-dimensional space [1,2,3]. Abiotic conditions vary within inner-crowns and between tree species [6,7] These environmental gradients have been suggested to structure the distribution of epiphyte species [3,4, 7,8,9,10]. Apart from the abiotic environmental conditions, the dynamics of the substrate itself should influence epiphyte communities This is because trees are constantly growing, producing new and losing older branches, meaning that the substrate persists only for a limited period [4, 11]. Thin branches stand less mechanical stress by epiphyte load, wind force, rainfall, or arboreal animals [9] This might be important in the outer crown of overstory trees, but the crown of understorey trees can be disturbed by tree- and branchfall of large and emergent trees [14]. Branchfall may profoundly influence the distribution of epiphyte species within the canopy, contributing to niche partitioning in epiphyte communities
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