Effects of glacial climates on floristic distribution in New Zealand 2. The role of long-distance hybridisation in disjunct distributions

Abstract

In Part 1, it is proposed that coldintolerant disjunct species in southern and central centres of floristic richness occur as the result of post-glacial long-distance hybridisation with resident hardier species, followed by reconstitution of the less hardy species. For this to operate, the species involved must hybridise freely, pollen must be transported over long distances, and it must retain viability.Nothofagus species meet these conditions, and their hybrids have been found several kilometres from one parent. Hybrids between three other pairs of anemophilous species have probably arisen through pollen dispersal over hundreds ofkilometres. Early post-glacial conditions are likely to have been especially conducive to segregation of immigrant species from populations of fertile hybrids.

Te Waahipounamu: South‐west New Zealand World Heritage Area. Ecological research and conservation history†

“…The intriguing issue of Pleistocene survival of hard beech in South Westland, suggested by Mark & Lee (1985), was contested by Wardle et al (1988), who proposed that this relatively cold-intolerant disjunct species of Nothofagus could have been reconstituted during the Holocene through long-distance hybridisation with the resident hardier species, in this case mountain beech They cited comparable cases from the literature However, this hypothesis was subsequently dismissed by Haase (1992), whose gel electrophoresis and isozyme variation techniques indicated that the small South Westland populations of hard beech possess relatively low genetic diversity, without any indication of recent gene transfer from mountain beech Haase's results were interpreted as evidence for in situ survival of hard beech in South Westland during the glacial maximum But where 7 Landscape and habitat disturbances from various causes obviously are major ecological factors throughout the south-west, as in other regions of New Zealand This justifies continued research for both its basic ecological information as well as its application to human and ecosystem welfare Fortunately, such research is continuing in this region (Smith & Lee 1984, Duncan 1993, R P Duncan and G H Stewart pers comm 1995, P Wardle unpubl data), and new approaches are being attempted (Smith et al 1994) I am sure we all hope and trust that the excitement, challenges (Fig 29), and importance of Te Waahipounamu, the South-west New Zealand World Heritage Area, will be sufficient to maintain and increase the research investment that has been made to date …”

Cold climate plants in a warmer world

“…more commonly form hybrids in disturbed ground, open areas and at the treeline than in the forest core. This hybridisation has been linked with their survival through periods of climatic fluctuation in various disjunct areas (Wardle et al 1988). In the European Alps, the variation found in the treeline Krummholz pine (Pinus mugo) is particularly noticeable.…”

Environmental and floristic gradients in Westland, New Zealand, and the discontinuous distribution ofNothofagus

“…The third category comprises local endemics, mostly of the western Nclson-Paparoa region. Pseudopanax macintyrei appears to be limited to limestone outcrops (Wardle 1968(Wardle , 1988 which are lacking in central Westland. Most of the remaining species have wind-dispersed small seeds or plumed achenes and their limited geographical range is unrelated to their dispersal ability.…”

“…cliffortioides, and subsequent back-crossing and selection (Wardle et al 1988), necessitates a sequence ofunlikcly events and will not receive much support, unless verified by chemotaxonomic or genetic studies. Analysis ofchloroplast-and mitochondria-DNA and enzymes would give definite results, since these are only provided by the presumed maternal parent taxon (N. solandri var.…”

“…The most spectaculardisjunction occurs in central Westland, whereNothofagusiscompletely lacking between the Taramakau and upper KarangaruaRivers (160 km) and is only represented by scattered outliers for some distance to the north and south of the gap. A widely accepted theory the "glacial refugia hypothesis", states that the abs~nce of Nothofagus and a number of other plant species from large areas of the central South Island is the result of Pleistocene extinction, and that reimmigration from northern and southern refugia is still incomplete for some taxa (e.g., Willett 1950;Wardle 1963Wardle , 1964Wardle , 1988Burrows 1965). In the genus Nothofagus, the limited dispersal ability ofits seeds, which are not particularly adapted to either wind or.bird-dispersal, is believed to act as the major constramt on the rate of migration.…”