Topographic gradient influences vascular epiphyte occurrence in a small watershed covered by a mature coniferous/broadleaf evergreen mixed forest in Japan

Aim

At fine scales (<10 km in spatial resolution), only a few studies have evaluated the relationships between abiotic factors and the horizontal occurrence of vascular epiphytes. At fine scales, variation in abiotic variables can be attributed to topographic heterogeneity. For example, air humidity and wind speed are likely to vary along ridge–valley gradients, and differences in slope aspect cause variation in light intensity as well as temperature gradient along an elevation. We tested the hypothesis that the horizontal epiphyte occurrence at fine scales is influenced more by topographic gradients related to abiotic factors than by host tree size and species. We also assessed air humidity variation along the ridge–valley gradient at our study site as a way to explore its possible correlation with the epiphyte occurrence.

Location

A coniferous/broadleaf evergreen mixed forest in a humid temperate zone of Kochi, southwest Japan.

Methods

We surveyed epiphyte occurrence on 310 trees and recorded host tree size and species within a 52-ha watershed with an elevational range of 378–777 m a.s.l. Epiphyte occurrence was modeled using three topographic variables (ridge–valley gradient, slope aspect differences and elevational gradient), host tree size and species. The effect sizes of each variable were then compared. Air humidity was measured on the valley bottom, slope and ridge throughout the year.

Results

Epiphyte occurrence probability increased toward the valley bottom and on larger host trees; it also varied among host tree species. Slope aspect differences and elevational gradient were less effective. The ridge–valley gradient had a greater effect than host tree size on epiphyte occurrence. Air humidity was highest at the valley bottom and lowest on the ridge.

Conclusions

Horizontal epiphyte occurrence was driven more by topographic gradient than by host tree size and species in a small watershed covered by a warm-temperate broadleaf forest. This finding suggests the importance of forests at valley bottoms to support epiphyte occurrence.