Canopy change though the Cenozoic in south eastern Australia

Abstract

Reconstructing canopy closure is difficult and has up until recently only been done through interpretation of cuticular morphology. However, along with the morphology, isotope characteristics preserved in the leaves have enabled the deduction of the “canopy effect” and thus the ability to reconstruct the closure of ancient forests. Australia has had rich and unique development since its separation from Antarctica and its flora has developed from closed canopy tropical rainforests in the Eocene to open arid hummock grasslands in the present day. To assess the canopy change from the Eocene through to the Miocene we employed the carbon isotopic data from leaf fragments from two sites, Anglesea (Victoria) from the Eocene and Kiandra (New South Wales) from the Miocene, and compared them to present day carbon isotopic data of open and closed canopy forests. There were two assessments conducted on the sites and individual leaf and a dispersed cuticle to validate whether dispersed cuticle is reflective of single leaf specimens for the deposit. The mean individual leaf values of the fossil sites show that they are significantly more depleted in 13C, resulting in the larger Δ leaf values. Anglesea has a larger mean value at 28.01 ± 0.52‰, with Kiandra having a lower value of 27.45 ± 0.51‰. The range of isotopic values for the two sites are 6.10‰ and 6.2%. An additional test at Anglesea showed that there is a large influence in the dispersed cuticle from gymnosperms. Modern closed canopy tropical rainforests have mean isotopic ranges of approximately 5.8‰, with mean values of 22.59‰. Whereas modern open canopy temperate forests have isotopic ranges of approximately 4.8‰, with mean values of 21.08‰. So, upon inspection, the Anglesea and Kiandra sites are both similar to tropical rainforests. An additional study was conducted of dispersed cuticle across the Eocene-Miocene interval for 13 sites. The results varied, showing a decrease in canopy structure during the Oligocene from the Eocene, followed by the closed canopy Miocene.