Radiocarbon dating and wood density chronologies of mangrove trees in arid Western Australia.

Nadia S Santini,Catherine E Lovelock,Quan Hua,Nele Schmitz,Ben Bond-Lamberty

doi:10.1371/journal.pone.0080116

Abstract

Mangrove trees tend to be larger and mangrove communities more diverse in tropical latitudes, particularly where there is high rainfall. Variation in the structure, growth and productivity of mangrove forests over climatic gradients suggests they are sensitive to variations in climate, but evidence of changes in the structure and growth of mangrove trees in response to climatic variation is scarce. Bomb-pulse radiocarbon dating provides accurate dates of recent wood formation and tree age of tropical and subtropical tree species. Here, we used radiocarbon techniques combined with X-ray densitometry to develop a wood density chronology for the mangrove Avicennia marina in the Exmouth Gulf, Western Australia (WA). We tested whether wood density chronologies of A. marina were sensitive to variation in the Pacific Decadal Oscillation Index, which reflects temperature fluctuations in the Pacific Ocean and is linked to the instrumental rainfall record in north WA. We also determined growth rates in mangrove trees from the Exmouth Gulf, WA. We found that seaward fringing A. marina trees (∼10 cm diameter) were 48±1 to 89±23 years old (mean ± 1σ) and that their growth rates ranged from 4.08±2.36 to 5.30±3.33 mm/yr (mean ±1σ). The wood density of our studied mangrove trees decreased with increases in the Pacific Decadal Oscillation Index. Future predicted drying of the region will likely lead to further reductions in wood density and their associated growth rates in mangrove forests in the region.

Highlights

Mangrove forests are an important component of tropical and subtropical coastal ecosystems
We found significant but variable trends between rainfall and the Pacific Decadal Oscillation Index (PDO) in the Exmouth Gulf, Western Australia (WA) for the period between 1966 – 2008
Growth rates of A. marina expressed as increments in stem circumference per year ranged from 4.0862.36 mm/yr in the period between 1999 – 2008 to 5.3063.33 mm/yr in the period between 1952 – 1976, with an overall mean growth rate of 4.8462.66 mm/yr for the period between 1952 – 2008

Summary

Introduction

Mangrove forests are an important component of tropical and subtropical coastal ecosystems. The stature and productivity of mangrove forests is correlated with latitude [2,3] with trees tending to be larger and communities more diverse in tropical latitudes. Rainfall is likely to be an important factor determining the variation in productivity and diversity of mangrove trees in arid regions because it reduces the salinity of soil porewater (which limits water uptake and growth of mangrove trees) through dilution and recharge of groundwater and increases in soil moisture [7]. Projections of rainfall with climate change indicate an increasing drying with less rainfall and higher temperatures in WA and in parts of central Australia [9,10]. Examples of impacts of climate change in estuarine ecosystems are scarce [11,12]

Methods

Results

Discussion

Conclusion