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Changes in leaf physiognomy of New Zealand woody assemblages in response to Neogene environmental cooling

Reichgelt, Tammo, Lee, William G., Lusk, Christopher H., Kennedy, Elizabeth M.
Journal of biogeography 2017 v.44 no.5 pp. 1160-1171
Asteraceae, Ericaceae, Plantaginaceae, Rubiaceae, cooling, extinction, forests, fossils, immigration, leaves, New Caledonia, New Zealand, Queensland
AIM: To identify New Zealand Neogene leaf physiognomy change by comparing Miocene fossil floras to modern assemblages and assess the contributions of extinction, immigration and adaptation to leaf morphology evolution. LOCATION: New Zealand. METHODS: Woody angiosperm leaf assemblages were collected from modern forests throughout New Zealand and early/middle Miocene leaf assemblages (23–11 Ma) from New Zealand outcrops. All leaves were scored for 25 attributes of margin morphology, area, apex, base and general shape, and length:width ratio. Additionally, Miocene and modern Queensland leaf assemblages were compared because of their supposed climatic similarity. RESULTS: Modern New Zealand angiosperm leaves are generally smaller, and have rounder apex/base shapes than early/middle Miocene assemblages. The occurrence of leaf serrations and attenuate apices are markedly similar between modern and Miocene assemblages. Leaf size ranges of New Zealand Miocene assemblages appear to be similar to those of modern Queensland. Comparison of leaf sizes in plant families in Australia and New Caledonia with extinct and extant representatives in New Zealand, reveals that there is no consistent pattern of extinction or persistence based on leaf size. MAIN CONCLUSIONS: Late Neogene cooling in New Zealand has selected for smaller leaves with rounder apex/base shapes. Genus‐ and species‐level extinction of broad‐leaved plants contributed to leaf morphological change since the Miocene. However, it is unclear why certain broad‐leaved genera persisted while others from the same family did not. The most potent drivers of late Neogene physiognomic change are the persistence, adaptation and radiation of selected pre‐existing families to cooling (e.g. Rubiaceae and Ericaceae) and the arrival and diversification of cold‐adapted families in the late Cenozoic (e.g. Asteraceae and Plantaginaceae). Lack of change in other characteristics, such as entire margin occurrence, suggests that these traits were not under strong selection.