Forest Vines to Snow Tussocks: The Story of New Zealand Plants
Sharing a need for much brighter light than is available on the floor of a closed forest, the majority of species of epiphytes, vines and parasites grow high in tree crowns. In this situation epiphytes alone face special problems. Vines are able to obtain soil, water and mineral nutrients via their stems; parasites can tap the supplies drawn up by their host trees; but epiphytes either have no connection with the ground throughout their lives or send roots down to it only after a period of some years. Soil does not form easily on trunks and branches and as they are sunnier, windier and better drained than the ground, they suffer both more frequent and more severe droughts. In response to these stressful conditions, many epiphytes have evolved modifications enabling them to store water and to reduce its loss by evaporation as well as to build up a layer of water-retentive soil. Water can be stored internally in special cells, whose presence confers fleshiness on the organs concerned, or externally in cavities formed by appropriately shaped and arranged leaves. Some epiphytes can get by with a minimum of mineral nutrients page 67and need little or no soil; others build up considerable quantities of dark humus, largely from the decay of their own old leaves and roots with a varying contribution of bark flakes and leaves from surrounding trees. Many other epiphytes which are unable to form soil themselves take advantage of those that can.
Now we have defined the epiphyte category, how rigorously do we interpret the definition in deciding whether or not a particular species should be included? Certainly not so rigorously that we exclude those species which, although normally epiphytic, are sometimes to be found on sunny, rock outcrops which provide conditions similar to those of tree tops. In fact, it might well be that there are no epiphytes, even those of tropical forests, unable to grow on the ground in suitable circumstances.
Going to the other extreme, should we include species which are normally terrestrial, but can occasionally grow on trees? In this case the answer is 'no' as, apart from reservations about stretching the definition so far, the number of species involved would be inconveniently large. In certain circumstances almost any plant is able to grow as an epiphyte. For example, in forests of high rainfall, particularly where frequent mists maintain high atmospheric humidity, seeds germinate just as readily on moist, moss and lichen covered trunks and branches as on the ground. A notable example of chance epiphytism in these circumstances in New Zealand is an occasional silver beech (Noth-ofagus menziesii) growing on a tree of the same species. Even in forests of average rainfall and atmospheric humidity, the branch systems of large, long-lived trees, such as the kauri, are available as habitats for so many centuries that quite unlikely species can sometimes be found as epiphytes on them. The intrepid Harrison-Smith32 found a 3 m kauri growing on a kauri, as well as several examples of other conifers — rimu, totara, kahikatea and several angiosperm trees. In most cases the occasional epiphytic plants of otherwise terrestrial trees and shrubs are small and do not grow to reproductive maturity.
Lichens, often followed by mosses, are generally the first epiphytes on trees in both temperate and tropical regions. In New Zealand small filmy ferns are frequently associated with the mosses. The thin layer of soil that these small epiphytes form is important for the establishment of most of the vascular epiphytes,41 which are the concern of this section.