Forest Vines to Snow Tussocks: The Story of New Zealand Plants
However, most theories about divaricating shrubs regard them as an example of parallel evolution in response to some environmental factor, but there is little agreement yet on what that factor is. The first to speculate late last century was Diels,107 who as a young man wrote a remarkable thesis on New Zealand vegetation even though at that stage he had not seen it. He based his account on herbarium and living plants available to him in Berlin, earlier publications and information from correspondents including Cockayne. Diels concluded that the divaricate habit was an adaptation to drought and suggested that shrubs with this form had evolved from forest ancestors at a time when the South Island axial ranges were higher than they are now and the rain shadow effect to the east of them, accentuated by drying winds, was much more extreme. Diels also suggested that coldness during glacials was a contributing factor in the evolution of this growth form, and Rattenbury9 later suggested that the influence of coldness could have been indirect by inducing physiological drought of the root systems. Cockayne100 accepted and extended Diels' interpretation, paying particular attention to trees with divaricating juveniles. He saw these as being ultimately derived from pre-ice age tree ancestors which completely lacked divaricate branching at any stage. During the cold and dry conditions in the east during glacials divaricating shrubs evolved from the trees and with page 142the return of warmer, moister conditions some of these regained their tree form, although only after passing through a small-leaved, divari cating juvenile phase. Cockayne regarded the last growth pattern as an example of 'ontogeny recapitulating phylogeny', that is the persistance of ancestral traits in the embryonic or, in this case, juvenile phase of a life cycle. In support of this evolutionary sequence he noted that ribbonwood (Plagianthus regius) has a divaricating juvenile on the main islands but not on the outlying Chatham Islands. He regarded the population on the latter group as the ancestral form, which survived in the milder, oceanic climate of a small island.
Wardle108 took a different view of trees with divaricating juveniles. Noting that divaricates generally are conspicuous in 'dryish' eastern forests, he suggested that divaricate juveniles might relate to existing climates which tend to periodic drought and that the changeover to the larger-leaved adult form takes place once a deep, efficient root system has developed. At times of greater aridity, perhaps during glacials, some divaricate juveniles would become 'fixed' flowering and fruiting divari-cate shrubs. Thus according to Cockayne the sequence would have been: non-divaricating tree — divaricating shrub — tree with divaricating juvenile; and according to Wardle: non-divaricating tree — tree with divaricating juvenile — divaricating shrub.
More recently, Wardle's hypothesis concerning microphyllous juveniles has been applied on Reunion and Mauritius Islands in the Indian Ocean where microphyllous, although not divaricating, juveniles are unusually common on the drier sides of the islands.26
With regard to Sophora, Cockayne held a view in conflict with his own theory but in accord with that of Wardle: the divaricate shrub Sophora prostrata is interpreted as a flowering and fruiting fixed juvenile of S. microphylla. Godley questions this view,109 pointing out that the leaves and stems of 5. prostrata and the divaricate juvenile of S. microphylla are quite dissimilar, as are the flowers and seed pods of the two species. He proposes an interesting and quite different explanation — the extremely variable Sophora microphylla (with everything from non-divaricate to strongly divaricate juveniles depending on locality) may derive from an ancient hybridisation between the non-divaricate and relatively largeleaved small tree S. tetraptera and the dwarf divaricate shrub S. prostrata. He further suggests that all trees with divaricate juveniles may have originated in a similar way.page 143
McGlone and Webb,110 in response to the 'moa theory', which we wall consider later, reaffirmed and refined the climatic explanation of the divaricating habit. They believe 'that the divaricating habit is an adaptation which enables the plant to resist damage from wind, frost and desiccation, while retaining enough flexibility to exploit a wide range of habitats'. They relate this growth habit to what they regard as New Zealand's distinctive climate: 'as a consequence of its position in the mid-latitudes and its isolation in a large area of ocean, New Zealand has a mild, generally humid, windy climate. However, it is also a very variable climate and the continental pattern of a cold winter and a consistently warm summer does not apply here.'
They point out that in many forest areas in New Zealand, night frosts may occur at any time of the year, often to be followed by quite warm days. Also in areas to the east of the mountain ranges, dry often very warm föhn winds may be frequent in spring and summer and are sometimes followed by cool or even frosty weather especially inland. In such circumstances deciduousness would seem an inappropriate adaptation, but McGlone and Webb believe that the mostly evergreen111 smallleaved divaricating shrub is well suited to survive the inclement and to take advantage of the favourable spells of weather. They see the balllike divaricating shrub, with its internal as well as external twigs, growing points and leaves, serving as a wind screen, frost screen and heat trap. With regard to wind they see three benefits for the internal leaves, which outweigh the disadvantage of their being partially shaded: the drying effect of the wind is reduced; abrasion by wind-borne particles is reduced; and the tendency of the interlaced mass of twigs to move as a whole means that twigs and leaves are less likely to hit each other and become damaged. With frost they have observed in a number of divaricating species that 'even in severe frosts in which exposed leaves are frozen the interior leaves of the shrubs are unaffected'. Finally, concerning the heat trap idea, they say, 'besides possibly acting as a frost screen, the protective network of branches may, on cold, but sunny days, act as a heat trap, raising the temperature of the air mass inside the shrub, and this may permit higher rates of photosynthesis'. With divaricate juveniles McGlone and Webb agree with Wardle that the changeover to the adult form may follow the development of a deep root system, but also suggest that the changeover may take place once I a height above ground frost level is attained.page 144
They suggest that the genera in New Zealand which have some divaricating species probably originated under at least subtropical conditions and had the opportunity to evolve cold-tolerant forms because New Zealand's isolation and lack of mountains during the Tertiary largely precluded the establishment of genera which had evolved in colder climates: 'we believe that the divaricating plant habit was one of the responses of this essentially subtropical flora to the onset of glacial climates.'
Indeed, evidence of trends in branching patterns and leaf sizes in tropical and subtropical forest genera in relation to decreasing temperature does indicate that the divaricating habit might represent the end point of such trends. In places in the world such as coastal Brazil,112 where rain forest extends from the equator to subtropical latitudes, a correlation has been observed between decreasing temperatures with increasing distance from the equator and a replacement of big-leaved, sparsely branched species with progressively smaller-leaved and more freely branched species. New Zealand's isolation when climates became colder would have enabled such a trend to continue to its ultimate extreme.