Environmental heterogeneity and its influence on fern diversity in a low-altitude mountain forest in central Taiwan

Environmental heterogeneity plays a crucial role in shaping the distribution and composition of natural vegetation, including understory ferns. This study investigated the influence of environmental variation on understory fern communities within a one-hectare permanent plot in a low-altitude mountain forest in central Taiwan. Twenty-two environmental factors, including topographic, soil, and biotic variables, were recorded. Multiple regression, cluster (two-way indicator species analysis, TWINSPAN), and ordination (detrended correspondence analysis, DCA, and canonical correspondence analysis, CCA) analyses were conducted. A total of 51 fern species (including Lycophytes) belonging to 20 families and 30 genera were recorded. Among these, 43 were terrestrial, and eight were epiphytic; however, only terrestrial species were analyzed because of the limited representation of epiphytes. Multiple regression analyses revealed that environmental variables significantly affected fern richness, abundance, and community composition. Specifically, stream distance and the importance value (IV) of the saplings significantly influenced fern richness; herb/vine IV affected abundance; and the carbon-to-nitrogen ratio (C/N), manganese concentration (Mn), and herb/vine IV impacted the first axis of the DCA. Furthermore, the elevation, curvature, slope, and topographic wetness index (TWI) significantly influenced the second axis of the DCA. In all the models, topographic variables-particularly stream distance-were one of the most influential drivers. TWINSPAN categorized the ferns into four distinct groups (Diplazium donianum var. donianum [DIPLDO], D. donianum var. aphanoneuron [DIPLAP], Blechnopsis orientalis [BLECOR], and Angiopteris lygodiifolia [ANGILY]), and CCA revealed that environmental factors structured the community compositions in line with the TWINSPAN grouping. The DIPLDO and DIPLAP groups were associated with ridges and upper slope habitats characterized by higher elevations and drier conditions. In contrast, the BLECOR and ANGILY groups were associated with lower elevations, stream proximity, steeper slopes, and higher humidity. This study highlights the role of topographic and soil C/N heterogeneity in structuring fern communities in fine-scale plots for future ecological monitoring in subtropical forest ecosystems.