0129-B1

Studies on Epiphytic Ferns as Potential Indicators of Forest Disturbances

Edward Andama, E., Charles M. Michira and Gebhard B. Luilo^[1]

Abstract

World forests are endowed with diversity of communities of flora and fauna whose interaction is very complex. Some of the flora have symbiotic relationships with higher plants and they could be used as indicators of environmental change. Epiphytic ferns form an important closed plant community in the Amani Nature Reserve found in Tanzania. These ferns live symbiotically on other plants (phorophyte) from which they obtain nutrients and moisture. Being very sensitive to direct sunlight they could be used as indicators of forest disturbances. This study investigated factors that influence occurrence of Asplenium nidu, its distribution on phorophyte and its potential as an indicator of forest environmental change. A total of 307 trees belonging to 47 species were studied in Amani Nature Reserve. It was found that the fern was not host specific, instead morphological features such as bulk type, branching type, girth at breast height (GBH) and canopy type were found to be influential factors. Ferns were abundant in a phorophyte with rough bark and a GBH of 81-130 m and at acute branching angle to the trunk. Moreover, A. nidus clumps were most prevalent at a height of less than 20m and kept on decreasing with increasing height. It was also found that the leaf sizes of the fern were broader when they occurred at a lower height. A broad leafy structure at a lower height maximized little light, while thinner leaf structure is an adaptation to reduce evapo-transpiration upon exposure to more sunlight. Therefore, monitoring of epiphyte fern population dynamics in the thick forests could provide a good indication of forest ecological disturbances, as they cannot survive as the forest become more and more open.

Introduction

Epiphytes are autotrophic plants living symbiotically on other plants (phorophytes). 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 (Richards 1996). In closed rain forests the majority of epiphytes grow above the ground where relatively strong illumination compensates for lack of soil but some may be found growing on twigs and tree bases where the light is more favorable than on the ground. Rain forest epiphytes are mainly small plants although a few grow to several meters high (Richards 1996). They play a crucial role as they provide the chief and in sometimes the only habitat for a rich fauna and flora which play an important role in the forest ecosystem (Ursula et al. 1995). Epiphytic ferns collect masses of humus, which provide nesting sites for many species of arboreal ants and other invertebrates. The epiphyte's roots grow round the host (phorophyte) stem or branches, giving anchorage and contact with the stem and branch. Dissolved chemicals in the rainfall water are trapped by the roots as water runs down the bark of the host. The epiphyte roots obstruct the flow thus reducing the erosivity of the waters and thereby allowing accumulation of considerable quantities of debris.

In the East Usambara Mountains, ferns belonging to the family Aspleniaceae form a good proportion of the vascular epiphytes in, the tropical rain forests. Members of this family can be distinguished from their broad leaves and spore patterns on the underside of the leaves. Most epiphyte communities are living on other living plants but many species will survive even on the standing old trees with barks and branches where they can attach. Epiphytes are good indicators of environmental quality of an area since they are sensitive to changes in humidity due to opening up of forests due to logging. The study investigated on the influence of tree morphology on the distribution of the epiphytic fern, Asplenium nidus, in the Amani Nature Reserve in the East Usambara Mountains in Tanzania and its potentiality as indictors of human induced or natural ecological disturbances in close rain forests.

Methods

The study was conducted in Amani Nature Reserve found in the East Usambara Mountains in Tanzania. The area receives a mean annual rainfall of about 1900 mm that falls in two monsoons, the southeast (March to June) and the north east (October to December). The mean maximum temperature is 27.1 ^oC centigrade while the mean minimum temperature is 13.4 ^oC. Four transects (20 m x 20 m) were established in the closed natural forest surrounding the International Union for Conservation of Nature (IUCN) Hostels and National Institute of Malaria Research. All the trees falling within the plot with a girth at breast height (GBH) of over 30 cm were sampled. Features recorded were GBH, presence or absence of grooves, presence or absence of natural or man-made cuts, angles branches make with the tree trunk (branching angles less than 60^o, between 60 - 90^o, and above 90^o). Only branches with an estimated GBH of 10 cm were considered. The canopy type was recorded for each plant species as sub-canopy level (< 21 m) high), canopy level (21 - 0 m high) and emergent canopy level (above 40 m). Potential sites of attachment of the fern to host tree were on trunk and at branching angle (at an angle the branch makes with the main trunk, at an angle sub-branches make with the main branch that were recorded as primary branch, secondary branches and tertiary branches).

Result and Discussion

A total of 307 trees with girth at the breast height (GBH) of more than 30 cm were examined in the four transects covering an area of 6.4 km². A total of 47 tree species belonging to more than 31 families were recorded in this study. As shown in the Table 1 Asplenium nidus occurred on many species of trees. Of the common tree species in the study area, Myrianthus holistii, Cephalosphaera usambarensis and Pouteria cerasifera had the highest number of A. nidus clumps while Maesopsis eminii and Leptopychia usambarensis had none.

Table 1. The most common trees, which had more than 4 individuals, encountered in the study area.

In the family Anacardiaceae, which had 21 % of all the trees in the four transects, only 9% of them had ferns while the family Cecropiaceae, which accounted for 7% of the trees sampled, 63.6% of its individuals had clumps. Both families were represented by only one species each. Families, Guttiferae and Rhamnaceae, were represented by one species each and yet contributed 16.6% and 14.7% to the total tree samples recorded respectively. But only 15% of trees in the family Guttiferae and 0% of trees in the family Rhamnaceae had the clumps. This indicates that Asplenium nidus has no preference to any particular tree species. Asplenium nidus seemed to prefer some families to others. This is supported by the results from Chi-square test (X² = 126, df = 48, p < 0.0001). The morphological features of the plant are the ones, which matters most. After studying the various aspects of host tree morphology in relation to occurrence of the epiphytic fern, very interesting results were obtained.

The Girth at Breast Height (GBH)

The number of A. nidus clumps was found to increase with an increase in GBH up to 130 cm after which the number went down. (Figure 1). This finding is supported by Mann-Whitney test (U = 3916, p < 0.0001) that indicates that GBH is an important factor contributing to the occurrence of A. nidus clumps on the phorophytes. The GBH is usually related to the age of the plant. Therefore, young trees are usually under cover in the closed forest as result they receive limited light. As they grow the vegetative parts (trunk, shoots and branches) get larger enough to provide sites for fern to attach and at the same time leaf cover reduce strong light penetration to the sub-canopy levels. But when the tree becomes older it loses most of its vegetative parts, particularly leaves, allowing too much light for the fern to survive. As a result the amount of clumps decrease in number because epiphytic ferns become unable to withstand strong exposure to solar radiation.

Figure 1. Number of A. nidus clumps encountered on host trees with different GBH.

Points of Attachment of the Epiphytic Fern to Phorophyte

Trunk and branches

The trunk of the host trees had the highest number of clumps of the fern followed by the point of attachment of the branch to the host tree and the primary branch. The secondary and tertiary branches had the lowest number of fern clumps (Figure 2). In general, the type of branching in the host tree was observed to influence the occurrence of A. nidus. The Chi-square test showed that presence of both primary branch (X² = 33.9, Df = 4, p < 0.0001), and secondary branch (X² = 13.5, Df = 4, p < 0.008) had contributed to the occurrence of clumps of the A. nidus on the phorophyte. The trunks of trees are had the highest number the fern clumps may be due to the fact that tree trunk has higher water flow on it than on the branches. So, ferns are likely to get enough water and nutrient when attached on the trunk and also at the angle the branch makes with the trunk. The preference of the fern to the point of attachment of branches to the main trunk may reflect the fact that clumps are less likely to be firmly held at this position. Allanblackia stuhlmannii, which had a small percentage of its branches at acute angles to the main trunk, hosted very few fern on it despite the fact that A. stuhlmannii contributed to 16.6 % of all trees sampled in the study area and indeed was second to Sorindeia madagascariensis.

Figure 2. Distribution of A. nidus clumps in potential sites of attachment on host tree

Branching angle

In the previous section it has been observed that branching type had impacts on the number of fern hosted on the phorophyte. However, the stability of the clump of the fern at the branching angle size of the angle depends on the magnitude of the angle formed. This aspect of tree morphology was also investigated and the results showed that the number of A. nidus clumps occurring on branches decreased with an increase in size of the angle the branches make with the main trunk or main branch (Figure 3). These are supported by results from the Chi-square test (X² = 38.8, Df = 7, p = 0.0001) for angle less than 60 ^o; X² = 14.5, Df = 13, p = 0.01 for angle between 60^o and 90^o and X² = 2, Df = 4, p = 0.05 for the angle above 90^o). The branching angle below 60^o had the highest number of A. nidus clumps. At such an angle clumps are more stable than at any other branching angle above it.

Figure 3. Number of A. nidus clumps on the host tree with increasing branching angle.

Presence of grooves and cuts on the trunk

Presence or absence of groves on the main trunk recorded and their relationship to the fern was investigated. It was found grooves had influence on the occurrence of A. nidus on phorophyte (X² = 17, Df = 1, p = 0.001). For example, of the trees with grooves 52.3 % were found to host clumps while only 16 % of plants without grooves had clumps. 25 % of the trees with cuts had a clump of Asplenium nidus while only 8 % of those without cuts had clumps. This indicated that the presence of cuts is an important for attachment of clumps (X² = 14.7, Df = 1, p < 0.001).

Canopy type

Out of the 307 trees, 57 were found to host a total of 101 Asplenium nidus clumps. Of the 101 A. nidus clumps recorded, 83 % of them occurred at a height of less than 20 m and only 18 % occurred between 21 m and 40 m. This indicates that A. nidus preferred sub-canopy level to high canopy levels. It was observed in this study the sub-canopy had more clumps of A. nidus and the number of clumps decreased from sub canopy to canopy and to the emergent canopy (Figure 4). These observations were supported by results from the Chi-square test which proved that the differences were significant (X² = 28.5, Df = 13, p = < 0.04). The fact that most of the clumps were found in the sub-canopy level highlights significant ecological impacts. The preference to the sub-canopy level, indeed, is an adaptation to minimize water loss through evapo-transpiration and therefore maximize the little moisture trapped in the debris. Nevertheless, being in sub-canopy level is also disadvantageous because there is little light reaching there for maximum photosynthetic process. To cope with low light intensity in t the sub-canopy level, the fem has developed broad leaves so as to maximize light absorption. Whereas, the fern recorded at other canopies had relatively smaller leaves than that of the sub-canopy. This is adaptation to reduce lost of moisture as they are receiving strong illumination. Any felling of trees, whether logged or naturally falling, brings about a change in the microclimate created in the closed forests the forest may become open.

Figure 4. Occurrence of A. nidus clumps on host trees of different canopy levels.

This is likely to bring about a change in the distribution of any epiphytic ferns on the tree and in some cases may result into the disappearance of epiphyte ferns. Therefore, epiphytic fern, Asplenium nidus, could be used as an important indicator of the changes on the structure of closed rain forests as this one due to maturity, disturbance, etc., for they are very sensitive to direct solar radiation and change in climatic conditions (Richter 1991). The fate of epiphytes is directly related to the fate of the host tree (phorophyte) and hence there can be no epiphytes without trees. Went (1940) considered differences between epiphyte flora on different species of phorophyte and concluded that besides physical factors of the host tree chemical differences in the bark and in the stem flow water contribute to the presence or absence of the epiphyte.

Conclusion

The epiphytic fern, Asplenium nidus, is not host specific but the morphology of the host tree is probably are determining factors of its occurrence on the host plant though they may not be the only ones. Trees, with a high diversity of attachment sites, are more likely to contribute to the conservation of epiphytes in the forest. Thus, considering the crucial roles played by epiphytes in the rain forest ecosystem it is important to conserve epiphyte ferns for which you just need to conserve the plant ecosystem. There can be no epiphytes without trees and hence the whole lot of flora and fauna supported by epiphytes all depend on the host tree.

Keeping track of the changes in any epiphyte community through ecological monitoring is an important aspect in tropical rain forest management that could provide clues on ecological changes that are taking place with the forest ecosystem.

Acknowledgement

The authors of this paper wish to express their heartfelt thanks to the Tropical Biology Association (TBA) for sponsoring the study. We are also grateful Dr. Rose Trevelyan and Dr. Pierre Bingeli for the technical support. We are also thankful to Mr. Ahmed Mndolwa helping us in identification of tree species sampled.

References

Richards, P.W., 1996. The tropical rain forest, An ecological study, 2^nd edition, Cambridge University Press, UK, 599 p.

Ursula, H., H. Peter and G. Sergio, 1995. Epiphyte vegetation and diversity on remnant trees after forest clearance in Southern Veracruz, Mexico. Biological Conservation, 75:103-111.

Went, F.W., 1940. Soziologies der epiphyten eires tropischen Urwades. Annales Jardin Botanique Buitenzorg, 50: 1-89.