6.1 The Conquest of Space:
Cryptogams and a Diversity of Life Forms of Vascular Plants
Perhaps epiphytism could be thought to be primarily the utilization of any possible surface for holdfast and establishment, i.e. a conquest of space with epiphytes found in aquatic and terrestrial habitats made up of various combinations of lower and higher plants. In aquatic habitats, i.e. lakes, rivers and the sea, there are always algae growing on each other. This not only applies to unicellular and filamentous forms and their colonies, but also to macroalgae like kelp and red algae. In the mesic terrestrial climate many lower plants are epiphytic, like mosses and lichens and also some forms of small pleurococcoid aerial green algae as well as cyanobacteria (blue green algae). In the tropics lower plants may constitute massive formations of epiphytic biomass, e.g. the bryophytes the biomass of which increases with altitude (Freiberg and Freiberg 2000) in upper montane cloud forests ("moss-forests", Fig. 6.1). Even the surfaces of leaves of plants in such forests may harbour a diverse flora or phyllosphere with bacteria, cyanobacteria, fungi, green algae, bryophytes and lichens and occasional seedlings of vascular plants (Ruinen 1961, 1974; Coley et al. 1993; Freiberg 1998) (Fig. 6.2).
Life forms of vascular plants that evolved in the conquest of space are lianas, hemi-epiphytes and the mistletoes, where the latter not only compete for space but also combine epiphytism with parasitism. In the temperate zone we have climbers and vines often especially in moist gallery forests, but if we exclude the parasites of the mistletoes among the vascular plants the fern Polypodium vulgare is the only known epiphyte, and moreover, it is only facultatively epiphytic. In contrast, the popular view of tropical rainforests is determined by the image of an abundant flora of epiphytes, vines and lianas, climbers with hanging and host-strangling shoots and curtains of aerial roots (Fig. 6.3).
This image, although intuitively correct, needs to be carefully differentiated. Growth of lianas, climbers and vines is particularly rich at the perimeter of forests, along rivers, roads and around clearings. They are often light-demanding plants. Epiphytes are often although not generally found to be more abundant in montane
rainforests and in cooler upper montane fog and cloud forests, where air moisture is always high, than in the hot lowland rainforests (Freiberg and Freiberg 2000).
Remembering that vascular plants evolved from aquatic ancestors during the conquest of land and that then they were subject to many new kinds of stress with respect to water and nutrient relations, it may not be surprising that there is very little
fossil record of epiphytism. Epiphytism must be a fairly recent development among vascular plants. Most epiphyte diversity dates from the Pliocene-Pleistocene (Benz-ing 1989a, 1990; Luttge 1989). Against this background, when plants may even live in the air, this may be considered a rather extreme case of the conquest of space. The
so-called atmospheric bromeliads constitute such life forms. They have given up any contacts with substrates supplying water and nutrients other than from the atmosphere. They may hang down from the branches of phorophytes (see Fig. 6.15D below) or may even get holdfast on wires of fences or telephone lines (Fig. 6.4).
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