Pacific cousins

May 15, 2018

On the occasion of my 40th birthday (during the previous year), my family and I travelled to the birthplace of my grandmother, Rarotonga, for a week that was far more concerned with spending time in the sea than it was with looking at plants. However, I set aside one day for the Cross-Island Track, which is of particular interest for the number of plants growing here that are tropical cousins of many New Zealand plants.

That said, one of the most interesting sights on the track is still the well-known boulder pictured below, which is cross-cut by an intriguing series of incisions that make one wonder whether their origin is natural or manmade (although I believe that they are natural).

Ascending from this point, beautiful fern meadows clothe the slopes of the mountain, including a species that extends as far south as the Kermadec Islands (Arachniodes aristata, as shown below), which we have found to be a very attractive and useful species for some urban projects within Auckland.

Rata is an exception to the southward drift that I mention herein, for some interesting research by Dr Shane Wright (and associates) demonstrated that Metrosideros (of which there are many species in the Pacific) probably evolved first in New Zealand before moving north. On Rarotonga, the one area in which I saw specimens of rata was at the summit of the Cross-Island Track, where gnarled trees of Metrosideros collina arch out from steep ground.

Kiekie is the only New Zealand member of the Pandanus family, and one of its relatives is a common feature of the understorey in parts of Rarotonga. Freycinetia arborea (pictured below) has smaller foliage and a more compact growth habit than our own Freycinetia banksii, which can clothe entire trunks of forest giants.

Meryta pauciflora (pictured below, left) also has slightly smaller (notably in terms of width) foliage than its New Zealand counterpart, M. sinclairii. The Rarotongan species is an attractive element within the subcanopy, where its bold leaves form a sharp contrast with the surrounding vegetation.

It stands to reason that some plants that are at the edge of their environmental tolerance in New Zealand are relatively rare on our shores. A small number of tropical ferns exhibit this, including the species shown above, right (Christella dentata), which is only known from one area in Northland (although an allied taxon occurs in geothermal areas). Despite growing very well in cultivation in northern New Zealand, it is understandably even more at home in the warmth of Rarotonga.

King fern (Ptisana salicina) is an impressive brute that endures within some forests in the north of New Zealand. Para, as it is otherwise known, also grows on Rarotonga, such as the small population near the summit of the Cross-Island Track, including the specimen pictured above.

Rather than tropical affiliations, this journal entry ends on the subject of slightly errant timing, with respect to the highly distinctive and attractive tree, Fitchia speciosa (shown above). Despite hoping to find it in bloom, I was not disappointed about missing its conspicuous orange flowers (the buds were developing), as the tree’s foliage, form and (most of all) prop roots were all interesting to be able to observe.

Despite not being mentioned herein, many other relatives of familiar plants are found on Rarotonga, including species of Pittosporum, Weinmannia, Elaeocarpus, Geniostoma and Coprosma. The most intriguing, however, is a plant (which I would very much like to see in full bloom some time) called po’utu’kava. Contrary to what one might think, this shrub from the coastal margin is actually a relative of kōwhai (Sophora), and its linguistic similarity to pohutukawa most likely has something to do with its growing station (right on the shoreline, before the crashing waves) rather than any familial ties.