Nothofagus biogeography revisited with special emphasis on the enigmatic distribution of subgenus Brassospora in New Caledonia

Abstract

Dispersals versus vicariance events and the presence of subgenus Brassospora in New Caledonia are two riddles of Nothofagus biogeography, a genus also distributed in New Guinea, New Zealand, South America, Southeast Australia, and Tasmania. Within a cladistic framework using the software COMPONENT 2.0, we demonstrate that most parsimonious area cladograms (areagrams) sensu cladistic biogeography need not always be the most plausible explanation nor reflect alternative geological hypotheses. The most parsimonious Nothofagus history sensu historical biogeography is reconstructed where a minimum of dispersed taxa is hypothesized and vicariance events are identified. A fully resolved well-established Nothofagus phylogeny was reconciled with three geological hypotheses (geograms) of East Gondwana break-up: (a) the conventional view, (b) an Australian—New Caledonian relationship, and (c) a biotic interchange between New Guinea and New Caledonia. Fossils determined to subgenus were optimized to the predicted lineages in the reconciled tree. Due to extensive extinctions, a maximum of three vicariance events are inferred, all being basal in the subgenera, an indication of subgeneric diversification prior to the break-up of Gondwana. Two taxa, N. gunnii and N. menziesii, are hypothesized as being long-distance dispersed. The most parsimonious solution suggests a close relationship between New Guinea and New Caledonia, supporting a Brassospora colonization route, but this hypothesis fails to predict numerous extinct lineages observed in the fossil record and thus must be rejected. The traditional break-up sequence of Gondwana is not the most parsimonious solution, indicating one incongruent node, but causes no overall incongruence with the fossil record. Considering all parameters, the occurrence of Brassospora in New Caledonia is most parsimoniously explained as a single colonization event from New Zealand where the subgenus subsequently went extinct in the Pliocene.