Forest Vines to Snow Tussocks: The Story of New Zealand Plants
Epiphytes on Tree Ferns
Epiphytes on Tree Ferns50
We will consider these separately, as tree ferns provide a substrate rather different from the bark of ordinary trees. To begin with, tree ferns do not branch so only the trunk is available for colonisation. Secondly, the trunks are built up from persistent leaf bases and adventitious roots, which provide a variety of surfaces according to the species, but none is quite like bark. Also the large crowns of leaves cast considerable shade so any epiphytes need to be shade tolerant, at least when young.
The different species of tree fern vary in their suitability for epiphytes. In our largest and most handsome species, the mamaku (Cyathea medullaris) which has trunks up to 20 m high and jet-black leaf stalks, the leaf bases decay down to hard leaf scars which collectively form an armour-like surface unsuitable for epiphytes. In the lower parts of the trunks masses of slender roots grow out, adding considerably to the diameter of the trunk, and these too form a hard dry surface. The related gully tree fern (C. cunninghamii) has similar trunk characteristics.
In the ponga or silver tree fern (Cyathea dealbata) the leaf bases decay more gradually and do not form well defined scars. As a consequence, soil forms readily in the interstices and a variety of epiphytes are able to establish. Wheki (Dicksonia squarrosa) has a trunk surface similar to page 87that of the ponga. In Dicksonia flbrosa (Fig. 54), Cyathea smithii and young plants of C. medullaris and C. cunninghamii, epiphytes are discouraged in the upper part of the trunk by the persistence of the old leaves as a 'skirt'.51 Thus epiphytes are most commonly found on the trunks of ponga and wheki.
In moist situations, lichens, mosses, liverworts and smaller and larger filmy ferns may be abundant on tree fern trunks. Climbing ratas and other climbers may also be present as well as a range of seedlings of trees and shrubs which die before reaching maturity. Here we will consider only the consistent and specialised vascular tree fern epiphytes. It is worth noting that the species concerned are different in the main from those occurring on ordinary trees.
Figure 53 (opposite left) Dead northern rata 'trunk' root, one of whose girdling roots still clasps a portion of the trunk of the original supporting tree. The smaller epiphytes are Still living showing that they are not parasites as they are clearly not dependent on their tree supports for nutriment. Kaitoke, near Wellington, southern North Island. Photo: M. D. King.
A lycopodium is quite commonly enountered which is smaller than Lycopodium varium, and with the leaves associated with the sporangia towards the branch tips often not reduced to scales. Some treat this lycopodium as a distinct species, L. novae-zelandicum; others regard it as a form of L. varium.
Asplenium flaccidum, common on trees, also inhabits tree fern trunks. The filmy ferns Hymenophyllum lyallii, H. ferrugineum and Trichomanes venosum are almost confined to tree fern trunks and others are frequently present. Several species of Tmesipteris remain largely restricted to such sites — T. elongata subspecies elongata throughout and T. lanceolata and T. sigmatifolia in the northern North Island.
One shrub and one tree frequently, and several other species less commonly, play this role on tree ferns.
Five-finger (Pseudopanax arboreus) is common as a terrestrial plant in shrubby forest regrowth, but in more mature forest it can be surprisingly frequent as a tree fern epiphyte, mostly on the ponga (Fig. 55), but also on wheki. The seedlings establish at the top of the trunk and, being fairly light-demanding, their leaves soon push between and above the fern fronds. The primary root begins to grow down to the ground, but soon gives off a branch root which grows horizontally around the trunk, sometimes returning to and fusing with the vertical root. It is thus comparable with the girdling roots of the puka and northern rata. The vertical root eventually reaches the ground and sometimes branches to enclose the tree fern trunk in a network of roots near the ground. In the meantime, the crown of the five-finger has continued to branch and grow upward with the tree fern crown following behind it. Raukawa (Pseudopanax edgerleyi) and Coprosma grandifolia may adopt a similar life style but less frequently.
Figure 55 (opposite) Five-finger (Pseudopanax arboreus) epiphytic on a tree fern (Cyathea dealbata). The root/stem junction of the five-finger is indicated with an arrow. Te Marua.
Photo: M. D. King.