Divaricating plants

Imagine the Cromwell basin, a thousand years ago, before the arrival of Māori and their fires, and the later arrival of Europeans with agricultural grasses, sheep and rabbits.  The major herbivores were moas, members of the ratite family, large flightless birds which originated on the supercontinent Pangea.  Still surviving are ostriches in Africa, emus in Australia, and rhea in South America.  The dry Central Otago climate allowed the growth of shrubs rather than trees, interspersed with dry-tolerant grasses.  Moas grazed the grasses and low-hanging leaves on the shrubs, and there was no competition with grazing mammals.  The landscape featured moderately to widely dispersed dense shrubs, often with berries which allowed their seeds to be spread in bird droppings.

Many of these shrubs exhibited divaricating branches; the high incidence of divaricating plants being one of the many unusual features of New Zealand's vegetation.  “Divaricating” means branches at angles of 90° or more.  Divaricating plants have masses of tangled criss-cross branches bearing tiny leaves with no dominating leading shoots, and the outer branches often form a protective shield around the plant. Some are shrubs such as Corokia cotoneaster while others are juvenile forms of trees such as Sophora microphylla (Kowhai) or Plagianthus regius (Ribbonwood).  One theory of why they are so common in Central Otago is that small-leaved plants with tough and wiry interlaced branches growing in a zig-zag pattern were resistant to grazing by moas (Greenwood & Atkinson, 1977: Evolution of divaricating plants in New Zealand in relation to moa grazing. Proc.NZ.Ecol.Soc. 24,21-32).  Some plants retain this habit as adults while others, once the juvenile plant has out-grown the reach of moa, revert to a more normal pattern of growth with larger leaves and straight branches.

The moa theory is supported by research on divaricating plants in Patagonia (McQueen, 2000: Divaricating shrubs in Patagonia and New Zealand. NZ.J.Ecol. 24, 69-80).  In New Zealand, where 11 different species of moa grazed both forests and dry-land vegetation, divaricates are common in forest as well as dry-lands, but in Patagonia they are mainly restricted to dry-land areas, grazed by Rhea and also by browsing mammals.  Furthermore, Patagonian divaricates are nearly all spiny, leading to the idea that they could resist both ratites and soft-lipped mammals.  The sole spiny divaricate in New Zealand is Discaria toumatou (Matagouri).  McQueen suggests that Patagonian divaricates evolved mainly to resist grazing by mammalian herbivores.  Examination of stomach contents shows that Rhea can insert their beaks to harvest berries and thus help in propagation.  In New Zealand, by contrast, divaricate evolution was driven by pressure to resist grazing by moa, both in forests and drylands.

For vineyard planting, divaricating plants may be chosen for their form (e.g. Plagianthus regius, Pseudopanax crassifolius (Lancewood)), for their flowers (Olearia, Discaria toiumatou (Matagouri), or to provide berries for birds and lizards (Corokia, Coprosma).  Coprosma acerosa  is a good ground cover on dry banks;  Mrysine divaricata, Corokia, and Coprosma virescens can be used in rows for hedging and wind-protection, and all the shrubs are attractive as individuals or mixed with other local natives such as Carmichaelia.  Their diversity of colours, e.g. red, brown and green for different varieties of Corokia, and the different patterns of foliage forms, make mixed plantings interesting and attractive.

Native plant suppliers list large numbers of different Corokia and Coprosma.  Another good point of reference is the Dunedin botanic gardens which have a comprehensive divaricating plant collection.  Among the small trees worthy of restorative planting, Olearia (tree daisies) deserve special mention, and will feature in a later installment of this series.

(Illustration) Matagouri in flower on the Māori Point riverbank.

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