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Dicksonia squarrosa
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This article was published in The Fringe, October 2015


In this time of national debate about flags and symbolism, the silver ponga fern (Cyathea dealbata) was an early leader. But what of our other tree ferns? Why are they so neglected?

There are about 10 species of tree ferns in New Zealand although some are rather spreading or even prostrate rather than having the singular vertical trunk we tend to think of as a tree fern. They are divided into two genera, Cyathea and Dicksonia, depending on whether they have hairs along the frond or scales.

The significant difference is that Cyathea have scales which are broad at the base tapering to a point, while Dicksonia have hairs, much like our own, a regular one cell thick along their length.

The best known Cyathea are C. medullaris (mamaku) and C. dealbata (silver ponga) while the two most common Dicksonia are D. squarrosa or wheki and the thicker, single-trunked D. fibrosa or wheki-ponga. They both occur throughout New Zealand although D. fibrosa is rare north of Auckland.

There is a third species, Dicksonia lanata, but it is scarcely a tree fern as it has two low-growing forms. The northland form’s rhizome is more like the single trunk of the upright forms, except it is a prostrate trunk. Further south the underground rhizome or trunk tends to divide and spread widely with fronds arising 1.5m straight out of the soil.

Wheki grows underneath regrowth plants such as manuka and establishes in thick swards which prevent other species establishing as the dense layer of falling fronds builds up a carpet of litter too thick for most seedlings to penetrate. Wheki also grows in deep forest but generally as a more widely scattered colony allowing space for other species.

Dicksonia squarrosa has a complex, but very successful structure. Initially it has a single trunk just like Dicksonia fibrosa, but then goes on to produce aerial buds which burst randomly directly out of the trunk and proceed to grow into secondary trunks under the canopy of the original fronds. This clever survival technique allows the lower growing points to take over if or when the top one is damaged or dies. Not content with this strategy, wheki then develops its underground rhizome which can spread 2m from the original before emerging and growing upright as a seemingly separate plant. Of course, just like all ferns, wheki can also spread widely as the wind disperses its spores which have been produced on the underside of fertile fronds.

Even cutting the tree fern down at ground level will not kill it and it is the plant that provides all those trunks you see forming fences in front of many houses. Look closely and you will see the trunks sprouting koru from the aerial buds. In addition, there is likely to be a host of opportunistic epiphytic species which have established on the trunks prior to the tree fern being cut down.

Perhaps the flag debate should not be for or against the silver fern but rather which tree fern merits the honour of being the national symbol. If resilience counts for anything, then perhaps there is no better candidate than the wheki tree fern.



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