Richards (1952) gives a full review of the available information on these dependent synusiae, and there is a review of terminology in Grubb, Lloyd, Pennington, and Whitmore (1963). They are discussed here in so far as they bear on later chapters, for example, there are differences between different rain-forest Formations, and to draw attention to topics ripe for further investigation.
Epiphytes and climbers are stratified. Within each synusia two main groups can be recognized: a photophytic or sun-loving group, adapted morphologically and physiologically to the microclimate of the canopy top, and a skiophytic or shade-loving group, adapted to the cooler, darker, moister microclimates within the forest canopy; though the distinction is never absolute.
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Epiphytes, climbers and stranglers
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Epiphytes
Tree-crown epiphytes include many orchids and Ericaceae (especially Vaccinium species). A special group of this synusia is the hemi-parasitic epiphytes which are attached to the host tree by haustoria through which mainly water and mineral nutrients, and in addition some photosynthate, are taken up. All Loranthaceae are in this class, the mistletoe of Europe being the most familiar example. Far Eastern Loranthaceae have long, conspicuous, vivid red and yellow (lowers and are believed to he bird-pollinated. There are also several Santalaceous hemi-parasites, Dendrotrophe, Dendromyza, and their allies and including one genus, Phacellaria, which is hemiparasitic on other Santalaceous or Loranthaceous hemi-parasites (Danser 1939; Whitmore 1973h). A few Loranthaceae are hemi-parasitic on other hemiparasites, for example, Viscum articulatum.
Bole epiphytes range in their degree of adaptation to cool and dark conditions. It is sometimes useful to distinguish the vascular epiphytes ((lowering plants and pteridophytes) from the non-vascular ones, that is, Bryophyta, Algae (including the orange-red coloured Green Alga Trentepohlia) and lichens.
Ability to resist or endure periodic drought is important for epiphytes. Walter (1964, 1973) has drawn attention to the two major strategics in his two classes: homoiohydres, plants which avoid desiccation, and poikilohydres, plants whose protoplasm resists drying out, and which revive on rewetting; in this latter class are most non-vascular plants, filmy terns (Hymenophyllum and Trichomanes), and some dicotyledonous herbs, especially members of the Gesneriaceae. Went's work at Cibodas, demonstrating that at least some epiphytes are host has been briefly reviewed above.
Went mentioned in passing that the epiphytic fern Pyrrosia nummulariifolia, which frequently webs coffee twigs with its long, thin rhizomes, was believed by planters to contribute to the decay of the twigs. Ruinen ( 1953) was able to confirm this, and show that certain small orchids produce a similar effect. She coined the term ‘epiphytosis’ for what appears to be a form of mild parasitism. She also showed a similar influence for the tiny epiphytes, growing on leaves, the so-called epiphylls, which are a characteristic feature of the more humid rain forests.
In a recent massive study of the effects of high levels of radioactivity on lower montane rain forest on Puerto Rico in the Caribbean, Odum and his collaborators discovered that there is a very high retention of radioactive fallout by the epiphyll flora, which was also shown to fix atmospheric nitrogen (Odum 1970). It is now well established that considerable quantities of minerals are leached from the canopy, mainly from the leaves (for example, Kenworthy 1970). Odum suggested that his findings indicate an important role of the so-called `phyllosphere' in trapping minerals cycling in the forest ecosystem. The function of drip tips (the long drawn-out apices sometimes possessed by all or some of the leaves of many rain-forest species) is ripe for review, in correlation with studies on the establishment and role of an epiphyll flora. There has long been a controversy (imply reviewed by Richards (1952) as to whether drip tips increase the rate of run-off from the leaf lamina or not.
In the lowland rain forest of Kolombangara, an island in the Solomon archipelago, epiphytes were found to be consistently less luxuriant in an area believed to be first-generation regrowth from extensive clearings than in older forest (Whitmore (1974). On a regional scale, shade epiphytes are commoner in the lowland forest of the Solomons and Santa Cruz archipelagos than in the Malayan and Bornean forest, reflecting the extremely moist, tropical oceanic climate of these islands. In the dry-land lowland rain forests of Malaya, and probably of most other countries of the tropical Far Fast, epiphytes are inconspicuous. They are in general commoner in swamp forests and in particularly humid situations such as enclosed valleys. They are also in general commoner at high elevations rather than low, being characteristically abundant on mountain peaks subject to frequent fog.
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