Carbon isotope discrimination in leaves of the broad‐leaved paperbark tree, Melaleuca quinquenervia, as a tool for quantifying past tropical and subtropical rainfall

Abstract

Quantitative reconstructions of terrestrial climate are highly sought after but rare, particularly in Australia. Carbon isotope discrimination in plant leaves (Δ leaf ) is an established indicator of past hydroclimate because the fractionation of carbon isotopes during photosynthesis is strongly influenced by water stress. Leaves of the evergreen tree Melaleuca quinquenervia have been recovered from the sediments of some perched lakes on North Stradbroke and Fraser Islands, south-east Queensland, eastern Australia. Here, we examine the potential for using M. quinquenervia ∆ leaf as a tracer of past rainfall by analysing carbon isotope ratios ( 13 C) of modern leaves. We firstly assess Δ leaf variation at the leaf and stand scale and find no systematic pattern within leaves or between leaves due to their position on the tree. We then examine the relationships between climate and Δ leaf for an 11 year timeseries of leaves collected in a litter tray. M. quinquenervia retains its leaves for 1-4 years; thus cumulative average climate data are used. There is a significant relationship between annual mean ∆ leaf and mean annual rainfall of the hydrological year for 1-4 years (i.e. 365-1460 days)prior to leaf fall (r 2 =0. 64, p=0.003, n=11). This relationship is marginally improved by accounting for the effect of pCO 2 on discrimination (r 2 =0.67, p=0.002, n=11). The correlation between rainfall and Δ leaf , and the natural distribution of Melaleuca quinquenervia around wetlands of eastern Australia, Papua New Guinea and New Caledonia offers significant potential to infer past rainfall on a wide range of spatial and temporal scales.3

Evaluating methods for extraction of α‐cellulose from leaves of Melaleuca quinquenervia for stable carbon and oxygen isotope analysis

“…These leaves were retrieved from the lakebed of Swallow Lagoon, a perched lake on North Stradbroke Island. The lagoon contains an accumulation of sub‐fossil leaves of the same species as are preserved in organic‐rich sediments spanning much of the Holocene . These leaves are also preserved in other perched lakes across southeast Queensland's sand islands, and have the potential to be found throughout their natural range across eastern Australia .…”

“…In particular, leaves preserved in lake sediments are valuable for palaeoclimate studies as they preserve a time series of vegetation. Therefore, measurement of the carbon visotope ratios of bulk leaf tissue holds promise as a technique for quantitative rainfall reconstructions . While carbon isotope ratios (δ 13 C values) of bulk leaf tissue are frequently measured, the oxygen isotope ratios (δ 18 O values) of bulk leaf tissue are generally not, as the oxygen in leaf tissues is highly susceptible to exchange with environmental waters and mixtures of different compound classes .…”

“…In these wetland environments, M. quinquenervia usually makes up the emergent layer of the canopy. The natural range of the species spans the eastern coastal region of Australia between 8 and 34° S, as well as Papua New Guinea and New Caledonia . Leaves of M. quinquenervia are of interest as they have been retrieved from a study site at Swallow Lagoon on North Stradbroke Island in southeast Queensland.…”

“…Leaves of M. quinquenervia are of interest as they have been retrieved from a study site at Swallow Lagoon on North Stradbroke Island in southeast Queensland. This lake preserves an accumulation of sub‐fossil leaves in sediments spanning much of the Holocene . This study's primary objective is to investigate a reliable permutation of the Brendel technique for delivering pure α‐cellulose products from the leaves of M. quinquenervia without significant changes to the stable isotopic composition of the cellulose.…”

“…Therefore, measurement of the carbon visotope ratios of bulk leaf tissue holds promise as a technique for quantitative rainfall reconstructions. 7 While carbon isotope ratios (δ 13 C values) of bulk leaf tissue are frequently measured, the oxygen isotope ratios (δ 18 O values) of bulk leaf tissue are generally not, as the oxygen in leaf tissues is highly susceptible to exchange with environmental waters and mixtures of different compound classes. 8,9 This is not the case for the oxygen isotopes in cellulose, as this oxygen does not exchange isotopes with the environment after synthesis.…”