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Epiphytic ferns as potential indicators of changes in forest microclimate

Extracted from the paper “Studies on epiphytic ferns as potential indicators of forest disturbances”, by Edward Andama (Department of Zoology, Makerere University, Kampala, Uganda), Charles M. Michira (African Conservation Centre, Nairobi, Kenya) and Gebhard B. Luilo (Department of Chemistry, University of Dar es Salaam, Dar es Salaam, United Republic of Tanzania), submitted to the XII World Forestry Congress.

Epiphytic ferns live symbiotically on other plants (phorophytes) from which they obtain nutrients and moisture. They grow attached to the trunks and branches of trees and other plants like climbers, and some even grow on the surface of living leaves. Epiphytic ferns provide the chief and sometimes the only habitat for a rich fauna and flora and thus have an important role in the forest ecosystem. They accumulate masses of humus, which provide nesting sites for many species of arboreal ants and other invertebrates.

Epiphytic ferns are very sensitive to humidity and direct sunlight. Changes in the micro-climate of closed forests resulting from changes in canopy cover due to logging or ill health of trees are likely to influence the distribution of epiphytic ferns on the tree. These ferns may therefore have potential as early indicators that could alert forest conserva-tionists to the need to take action, for example to examine the state of tree health.

In the Amani Nature Reserve in the United Republic of Tanzania, which is a tropical rain forest, epiphytic ferns are part of the closed plant community. A study was carried out in this area to investigate factors that influence the occurrence of Asplenium nidus (chosen because it was the most dominant epiphytic fern in the area), its distribution on phorophytes and its potential as an indicator of forest environmental change. A total of 307 trees belonging to 47 species in more than 30 families were studied. Asplenium nidus occurred on many species of trees, most commonly Myrianthus holistii , Cephalosphaera usambarensis and Pouteria cerasifera .

It was found that the fern does not have preferences for particular host species; rather, morphological features of the host such as branching type, diameter at breast height (DBH) and canopy type were found to be influential factors in determining the host. Ferns were abundant in phorophytes with rough bark, DBH of 81 to 130 cm and acute branching angle with respect to the trunk. Asplenium nidus was also found to be more prevalent in trees with grooves or cuts on the trunk.

Moreover, A. nidus clumps were most prevalent in the subcanopy at a height of less than 20 m, and their prevalence decreased with increasing height. The preference for the subcanopy level is an adaptation to minimize water loss through evapotranspiration. To cope with the low light intensity at the subcanopy level, the fern has developed broad leaves to maximize light absorption. Where the canopy is more open, the fern develops a smaller leaf structure to reduce moisture loss on exposure to more sunlight. Epiphytic ferns may not survive as the forest becomes more open, and new clumps emerging after ferns die may have narrower leaves.

Monitoring of the ecological changes in an epiphyte community and of the fern’s population dynamics could thus provide clues on ecological changes taking place in the forest ecosystem, and might have a role in tropical rain forest management.

Asplenium nidus grows attached to a tree, from which it obtains nutrients and moisture

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