Branchfall as a Demographic Filter for Epiphyte Communities: Lessons from Forest Floor-Based Sampling

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.

维管附生植物（vascular epiphytes）的多度与丰富度的局地变异，通常被归因于基质、微气候等环境特征。然而，学界对树木与枝条更替动态对附生植物群落的影响仍知之甚少。为解决这一研究空白，我们在两片森林——巴西大西洋雨林与巴拿马加勒比雨林——的96条样带中，对林地表层的枝条与附生植物开展了调查。在巴西的研究样地中，我们额外区分了边缘样地与核心样地。我们量化了枝条多度、附生植物多度、物种丰富度及成株占比，以探究这些变量随枝条直径的变化规律。林地表层直径小于2厘米的枝条，占所有地表枝条总量的90%以上。每条样带中凋落附生植物的多度与物种丰富度，在巴西核心样带中最高，在巴拿马样带中最低。地表上的附生植物中，多数（约65%）均附着于枝条之上。在所有三类研究样地中，枝条多度与枝条直径呈显著负相关；而单根枝条上的附生植物多度、物种丰富度及成株占比，则与枝条直径呈显著正相关。枝条直径与附生植物绝对多度或物种丰富度之间的相关关系，因研究样地而异，这一差异或可通过森林结构与动态的不同加以解释。在巴拿马的研究样地中，此前已开展过附生植物的清查工作，这使得我们可通过对比冠层与林地表层的采样结果，评估本次调查方法的有效性。林地表层发现的附生植物个体数，对应直径小于10厘米（含冠层枝条）的枝条上总个体数的13%；林地表层的附生植物多度、物种丰富度及群落组成趋势，与冠层的变化趋势相一致。我们认为，林地表层调查可提供有价值的区系信息，尤为重要的是，可获取针对最细枝条上附生植物的种群统计数据——这类枝条通常最难直接取样。在此场景下，断枝掉落作为重要的种群过滤因子，塑造了附生植物群落的结构。