Comparison of the flora and vegetation of the southern Andes and New Zealand

Abstract

The annual range of temperatures in the southern Andes is similar to that in New Zealand, although they are cooler at comparable latitudes. South of 45°S in Chile the precipitation regime is also similar to New Zealand, but northwards and also east of the Andes there is an increasing period of Summer dryness. There are corresponding similarities in vegetation, especially in evergreen Nothofagus forests, tussock grassland in eastern rain shadows, and cushion bogs. Deciduous Nothofagus species are absent from New Zealand, but dominate widely in drier and colder environments of the southem Andes. In the southern Andes deciduous Nothofagus forms the subalpine forest and krummholz although, in the northern part of the region, there is also Araucaria araucarna growing as erect, emergent trees. Volcanic eruptions may lower the tree limit so that it intersects with evergreen Nothofagus forest. In New Zealand, evergreen Nothofagus species, which are less cold‐resistant than the deciduous Andean species, form most tree limits, and a belt dominated by tall Chionochloa tussocks intervenes between forest and low‐growing alpine vegetation. The absence of very hardy tree species in the New Zealand mountains may be related to isolation and late uplift. Extrapolation to tree limit indicates southern Andean warmest‐month mean temperatures from around 7°C in northern localities to only 5.7°C at 55°S, compared with 10°C which prevails in New Zealand. Since tree limit altitudes in the Southern Andes exceed those of New Zealand by at least 250 m, this indicates that in temperature terms New Zealand tree limits are 550 m lower than those of the southern Andes, corresponding with summer isotherms 3.3°C warmer. Vascular species were listed from equivalent vegetation types in the southern Andes and New Zealand, including forest, rain‐shadow grassland, alpine, coastal, swamp, lake‐edge and bog vegetation, and also from communities near the upper tree limit without close equivalents, notably deciduous forest in the Andes and Chionochloa grassland in New Zealand. Phytogeographic categories were assigned to all species, genera, and families. These were grouped as shared‐austral, shared with wider distributions, ‘’realm‐endemic‘’ confined to either south America or Australasia, and non‐shared with wider distributions About 90% of the 465 species listed from the southem Andes and 522 listed from New Zealand are realm‐endemics. Forty species or closely related pairs of species are shared, nearly half of these being coastal. Among genera, realm‐endemics are the largest element overall and exceed 30% in forests of both regions and in New Zealand subalpine, grassland, alpine, successional, and bog vegetation. Shared genera that extend to north temperate regions form the next most important, reaching over 40% in southern Andean swamp and New Zealand coastal samples. Shared genera with austral distributions reach 21–32% in all New Zealand vegetation classes and southern Andean bog, compared with 5–20% in other southern Andean vegetation classes. They contribute disproportionately to the physiognomic similarities between the two regions, notably in Nothofagus forests and bogs. Non‐shared genera with austral or subtropical distributions are most numerous in forest samples from each region. Non‐shared genera with north temperate distributions constitute only 2% in New Zealand, contrasting with the southern Andes where they constitute 19% overall and 20–29% in subalpine, grassland, alpine, successional, and coastal vegetation. Distribution of families among phytogeographic categories is similar to that of genera, except that more are shared‐widespread and fewer are shared‐austral and realm‐endemic. Sequencing of the gene rbcL for pairs of taxa in shared‐austral genera and for populations of Hebe salicfolia yields divergence estimates ranging from 130 ± 75 to 7 ± 4 million years. Divergences among tree species in shared genera may predate the separation of Australasia from Antarctica and South America, but not the separation of New Zealand from Australia. Taxa of open habitats show the most recent divergences, and their presence in both regions demands some mode of transoceanic dispersal. Possibly as late as the Pliocene, this may have involved a partly vegetated Antarctica.