GREENWOOD AND ATKINSON: DIYARICATING PLANTS AND MOA BROWSING 21 EVOLUTION OF DIVARICATING PLANTS IN NEW ZEALAND IN RELATION TO MOA BROWSING R. M. GREENWOOD' and I. A. E. ATKINSON' SUMMAR Y: New Zealand appears to be the only country where spineless, small-leaved divaricating plants make up nearly 10% of the woody flora. Climatic explanations have been advanced to &ccount for the origin of these divaricating plants. We suggest that the divergent and interl~ced branching, the woody exterior and the tough stems of these plants are adaptations evolved in response to browsing by moas. Together with a few species of much smaHer birds, moas were the only browsing vertebrates in New Zealand prior to the arrival of man. Thq divaricate habit is probably only one of several strategies evolved by plants in response to moa browsing. However, because fioas fed in a different way from mammals there is little to support the idea that introduced browsing mammals have merely replaced moas as aQ ecological factor in New Zealand. MORPHOLOGICAL FEATURES OF NEW ZEALAND difficult. The most helpful keys are those of Bulmer DIY ARICATING fLANTS (1958) and Taylor (1961), which deal specifioolly The term Hdivaricating", indicating branching at with these plants. New Zealand species found in this a wide angle, is used in New Zealand to describe the investigation to be capable of divaricating are listed many species of small-leaved woody shrubs that in Table 1. In cases where there was difficulty in have closely interlaced bra,nches. Some are the deciding whether a species should be included in juvenile stages of trees that lose the divaricate habit the table the criteria used for inclusion were (i) as they grow taUer. The interlacing arises from 10% or more of the branches of a.t least some reduced apical growth associated with a continued individual plants of the taxon should diverge to glfowth of 1a.teral branches, either straight ones angles equalling or exceeding 900 and (ii) the inter- produced at widely divergent angles (often 90° lacing of branches should be three-dimensional, or more) or recurved ones bending downwards father than two-dimensional as in some prostrate or sideways in directions away from the plants. On the basis of the limi.ted number of main stems (Fig. J). In some species sympodial individual plants examined, some species listed by branching occurs in which a branch apex earlier authors as divaricating, have been excluded. ceases growth and lateral bijds produce new branches Thus Table I should be considered as a minimum from just behind the apex. The interlaced branch list of New Zealand divarioating plants. Many system may vary from stiff with frequent branching, species show considerable within-population and to flexible with thinner branches and less frequent between-population variation in the degree of branching. A cut branch is usually difficult to divarication, and there may be further species which, disentangle. unknown to us, include divaricating populations. Another common feature of New Zealand divaricating plants is ,that the outer branches often INCIDENCE OF DIYARlCATING PLANTS IN THE NEW have longer internodes with fewer and smaller leaves ZEALAND FLORA AND ELSEWHERE than the inner branches, a trend which becomes more A rernarkabJe feature of the New Zealand flora is marked with increase in the degree of divarication. its high incidence of divaricating plants. The 54 Often the stems of New Zealand divaTicating species listed in Table 1 belong to 20 genera and plants are very difficult to break being tougher than represent 16 families of angiosperms and one family those of their non-divaricating relatives. of gymnosperms. This number is nearly 10% of the The similarity in leaf and stem morphology among woody flora. * There are in addition at least 9 species divaricating plants belonging to taxonomically from 6 genera representing 4 additional families of unrelated families sometimes makes identification angiosperms that can be described as semi.divaricat. lApplied Biochemistry Division, DSIR, Palmerston * Percentage based on an unpublished estimate by North Mr A. P. Druce of 540 species of woody plants, 2Botany Division, DSIR, Lower Hutt excluding Hanes, in the flora. 22 PROCEEDINGS OF THE NEW ZEALAND ECOLOGICAL SOCIETY, VOL. 24, 1977 FIGURE 1. Divaricaling branch systems of Coprosrna rhamnoides (left) and PiUosporurn divaricatum (right) showing widely divergent angles associated with both straight and recurved branche:~. ing, e.g. juvenile Nothofagus solandri, Neomyrtus between outer and inner branches is not known. pedunculata, Teucridium parvifo/ium and juvenile Tucker (1974) gives a list of 53 species he considers Rubus squarrosus. as divaricating from the desert and chaparral Divaricating plants are apparently absent from communities of California and Arizona. Study of other Pacific islands but whether this is true of New individual species descriptions shows that at least Guinea, Australia and Tasmania is not known to 44 of these species have spiny branches or leaves and the writers. Coprosma quadrifida from Tasmania that their aVeI"4ge leaf size (ca. 1.5 em) is much and eastern Australia has numerous short spiny larger than that of most New Zealand divaricating branch lets with reduced leaf size and number on species. On present evidence there appears to be no the outer branches. However, representative country apart from New Zeala,nd where spineless, specimens examined, though frequently branched, small-leaved divaricating shrubs constitute anything lacked the sti.ff habit and widely divergent branch approaching a tenth of the woody flora. angles of a typical New Zealand diva,ricat.ing plant. Judged from photographs and descriptions in HYPOTHESES CONCERNING DIVARICATING PLANTS floras, divaricating plants do occur in some other That so many unrelated plant families show the countries, particularly in xeric environments. same parallel trend towards divarication indicates Examples are DecGryia madagascariensis (Didierea- environmental condi.tions peculiar to New Zealand. ceae) from Madagascar with zig-zag thorny branches Cockayne (1912) considered that the divaricate habit (Carlquist, 1965) and Condalia microphylla was a xerophytic growth form resulting from adapta- (Rhamnaceae), a thorny divaricating shrub from tion to an earlier "steppe-climate period" when Patagonia (Bartlett and Bartlett, 1976). Whether such conditions were more windy and drier than 'at plants have the combination found in New Zealand present. In discussing Sophora microphylla he divaricating species of interlaced branching, small suggested that the divaricating juvenile "arose leaf size, stem toughness and difference in leaf size during a probable period of drought on the east of GREENWOOD AND ATKINSON: DIVARICATING PLANTS AND MOA BROWSING 23 the Southern Alps at the time of the glacial period". of several different chromosome segments between Rattenbury (1962) considered Ithat the divaricate unrelated families. habit was an adaptation to drier or cooler conditions It was however a reading of Went's (1971) state- in. the Pleistocene and suggested that the compacted ment: "One might e.g. have assumed that the nature of the growth form, with leaves confined extreme degree of branching, combined with micro- mostly to the inner shoots, acts as an effective phylly, would be an adaptation against browsing windbreak. Wardle (1963) pointed out that there was animals, but New Zealand is the only extensive scarcely any development of a distinctive xeric flora geographical area without larger native herbivores. ." adap.ted to areas of New Zealand which experience that lead one of us in 1974 to consider a possible dry climates at present. He suggested that the connection between moas and divaricating plants. divaricate growth form was an adaptaJtion to still- Further enquiry showed that such a connection had existing conditions such as occur in drier forests or been considered at least ten years earlier. Denny shrub communities . (1964) had concluded that the divaricate habit was Several difficulties are apparent with climatic an adaptation to present-day climate, but she suggest- explanations for divaricating plants. As pointed out ed as a "remote possibility" that "grazing" by IDoas by Dawson (1963a), the conditions described are had had some effect on the shape of divaricating not peculialf to New Zealand but can be found in shrubs (p. 30). Livingstone (1974) speculated that other parts of the world where divaricating plants some of ,the leaf changes occurring between young are unimportant or lacking. If wind is emphasised, and adult forms of some trees were an adaptation then in some of the windiest parts of the country to browsing by moas. Carlquist (1974: p. 242) such as the Cook Strait coast, the Rangipo desert suggested that the tough tangled microphyllous east of Ruapehu and outlying islands such as the branches of many divaricating shrubs were function- Chatham group, divaricating plants are a very minor ing as "armor" devices with virtually the same component of the vegetation. Furthermore, in an effect as thorny branches, and Taylor (1975) raised archipelago such as New Zealand, wind is frequently the question of whether the "dry, unpalatable twigs" ass.ocia,ted with wind-carried salt. Divaricating plants of divaricating shrubs gave them an advantage during apparently have no particular adaptations to with- "grazing pressure" from "flocks of herbivorous stand salt such as waxy leaf cuticles or tomentose moas". Apparently none of these authors have leaf surfaces, although the woody exteriors of the explored the matter further. more extreme forms might reduce salt entry to The superorder Ratites, to which moas belong, are leaves. If on the other hand drought is emphasized, not re.stricted to New Zealand. There are the emus, then some simple experiments comparing water loss cassowaries and extinct dromornithids of Australia between divaricating plants and their large-leaved and New Guinea, the rheas of South America, the non-divaricate relatives have failed to demonstrate ostriches of Africa and the extinct elephant-bi~ds that the small-leaved species are mOJ'"e drought- of Madagasca.