Calibrating new isotopic and morphological tools for palaeoecological forest reconstructions

Abstract

The ability to identify forest architecture in the geologic past has implications for our understanding of palaeoecological processes. The degree of canopy closure (or density of foliage) effects atmospheric circulation and hydrologic cycling, which in turn, can influence terrestrial temperature and rainfall patterns. Closed canopy forests are characterised by strong gradients in light intensity, which influence the chemistry and morphology of leaves. This study has used isotopic and morphological leaf traits from the modern closed canopy Daintree Rainforest in Queensland, which can also be measured from fossil leaves, to calibrate a multiproxy tool that characterises the spatial distribution of light intensity. Leaf area index (LAI) was used to quantify forest canopy closure. Changes in carbon isotope ratios (δ13C), leaf mass per area (LMA), undulation index (UI) and cell area (CA) all had linear correlations with increasing LAI from the canopy to the understory. Therefore, these traits can be used as a proxy for reconstructions. However, the magnitude of responses varies between species. A portion of species were unresponsive in UI. Therefore, this proxy may not be suitable for all species. Traits from leaves from beneath a gap in canopy closure and those from a drought experiment, did not deviate from the general gradient seen with LAI. The model used to predict LMA (petiole width2/leaf area) did not correspond with measured LMA and did not characterise the light gradient found within the Daintree rainforest. However, they did predict the average LMA of the rainforest. Investigations into inter-trait variations demonstrate that δ13C, CA and UI correlate with LMA, and CA correlates with UI. Results show several new leaf traits that can be used to identify the degree of shading in closed canopy forests in the fossil record and demonstrate how light gradients drive variation within a forest.