Epiphytes contribute substantially to the extraordinary high biodiversity in tropical and subtropical rain forests. Many epiphytes grow in association with other epiphytes but the benefit from such association has not been thoroughly examined. Asplenium nidus is a widely distributed epiphytic fern in Old World rain forests. The base of A. nidus is characterized by a large mat of canopy-soil. In northeastern Taiwan Vittaria zosterifolia is an epiphytic fern that often grows in association (hanging below) with A. nidus. We conducted a field survey and removal experiment to examine the effect of the association on the two epiphytic ferns.
We surveyed 50 A. nidus plants and 50 V. zosterifolia plants each along a trail in a subtropical forest in Taiwan and recorded the size and the presence or absence of the other species. Fifty-two A. nidus in association with V. zosterifolia were located and randomly and evenly assigned to four treatments, control (no manipulation), A. nidus fronds removal, A. nidus fronds and soil mat removal, and V. zosterifolia removal.
The field survey indicates that 66% of the V. zosterifolia plants grew in association with A. nidus, whereas only 36% of the A. nidus plants grew in association with V. zosterifolia. There was no difference in plant size (frond length and frond number) between A. nidus growing alone or in association with V. zosterifolia. On the other hand, the size of V. zoesterifolia in association with A. nidus was greater than those growing alone suggesting a positive effect (facilitation) of the latter on the former. Prior to the treatment, plant size of the two species did not differ among the treatments. One month after the treatment, the growth of A. nidus did not differ among the treatments. There were 40% fewer fronds of V. zosterifolia in the A. nidus removal treatments than in the control. The frond number of V. zosterifolia did not statistically differ between the A. nidus frond removal treatment and the control. Our results indicate that there is a one-way facilitation from A. nidus to V. zosterifoila. Because the removal of A. nidus fronds alone did not affect the growth of V. zosterifolia, the facilitation effect must occur via the soil mat. Epiphytic soil mats are not uncommon in rain forests. Further studies on its role in facilitating the growth and perhaps distribution of epiphytes in tropical and subtropical rain forests are in progress.