Nest Epiphytes

Much more evident to the casual observer are the massive nest epiphytes perched high in tree crowns (Figs. 26, 39, 40). In New Zealand there are three long and narrow-leaved species belonging to two closely related genera of the lily family — Collospermum hastatum, C. microspermum and Astelia solandri. Two other species of Collospermum are found outside New Zealand, one each in Fiji and Samoa; both are epiphytes. In the much larger and more widespread Astelia found mostly in the southern hemisphere, only a few species are consistently epiphytic. The others are found in New Zealand in a variety of habitats — coastal cliffs, swamps, forest floors, alpine tussock grassland and a few reduced page 73turf-forming species in alpine bogs.

All three nest epiphytes usually establish among mosses and lichens in branch forks or on inclined or horizontal branches. As their stems, completely hidden by the leaf clusters, are short and more or less erect the plants are fixed in position; although when they branch to form additional leaf clusters the resulting massive clumps of foliage may be metres in diameter. The 'nests' are attached to their supports by extensive root systems and as the old roots and leaves die and decay, considerable depths of dark spongy soil are built up. The likely eventual fate of these large soil and plant masses is to fall to the ground. Heavy rain absorbed by the soil greatly increases the weight of the mass and if the rain is accompanied by wind, complete branches may crash to the ground under the weight of epiphytes.

Astelia solandri is more shade tolerant than the collospermums and so is often found below them in the lower crowns and on the upper trunks of trees. The silvery green leaves are in three ranks and are 1–2 m long, but only 2-3 cm wide. Their bases are tightly folded, forming a narrow ridge at the back. Astelia solandri is found in lowland forests throughout the North Island, near the northern coast and down the western side of the South Island to about 44° S.

Collospermum hastatum accompanies Astelia solandri through the North Island and to about 42° 30' in the South Island. Collospermum microspermum is restricted to the North Island and replaces C. hastatum in montane forests above about 300 m. C. hastatum has fan-like arrangements of black-based leaves (Fig. 41) which are somewhat shorter and much broader than those of Astelia solandri. The bases of the leaves are strongly rounded and enclose spaces or 'tanks' which become filled with water when it rains. Often sufficient water is contained by the tanks to provide a shower bath for the unwary when a fallen Collospermum nest is lifted and tilted. A species of mosquito has been described whose larvae always develop in the water stored by Collospermum hastatum⁴³ but this is a very modest fauna compared with the many insects and even frogs which inhabit the 'tanks' of the tropical American epiphytes of the Bromeliaceae (pineapple family). The water stored by C. hastatum is partly absorbed by roots which grow into the tanks, but the suggestion that it is also absorbed through the embedded multicellular bases of overlapping scales has not been confirmed. In some tropical bromeliads all of the stored water is absorbed through the bases of similar scales.

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Collospermum microspermum is equally specialised but its leaves are as narrow as those of Astelia solandri and dark-brown rather than black at the base.

The collospermums are the only known tank epiphytes outside the family Bromeliaceae, but as they form soil very efficiently, they are also nest epiphytes.