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Ra isotopes in trees: Their application to the estimation of heartwood growth rates and tree ages
Author(s) -
Hancock Gary J.,
Murray Andrew S.,
Brunskill Gregg J.,
Argent Robert M.
Publication year - 2006
Publication title -
global biogeochemical cycles
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.512
H-Index - 187
eISSN - 1944-9224
pISSN - 0886-6236
DOI - 10.1029/2005gb002641
Subject(s) - hardwood , temperate climate , dendrochronology , softwood , radiata , environmental science , δ13c , annual growth % , botany , isotopes of carbon , ecology , biology , stable isotope ratio , paleontology , total organic carbon , physics , vigna , quantum mechanics
The difficulty in estimating growth rates and ages of tropical and warm‐temperate tree species is well known. However, this information has many important environmental applications, including the proper management of native forests and calculating uptake and release of atmospheric carbon. We report the activities of Ra isotopes in the heartwood, sapwood and leaves of six tree species, and use the radial distribution of the 228 Ra/ 226 Ra activity ratio in the stem of the tree to estimate the rate of accretion of heartwood. A model is presented in which dissolved Ra in groundwater is taken up by tree roots, translocated to sapwood in a chemically mobile (ion‐exchangeable) form, and rendered immobile as it is transferred to heartwood. Uptake of 232 Th and 230 Th (the parents of 228 Ra and 226 Ra) is negligible. The rate of heartwood accretion is determined from the radioactive decay of 228 Ra (half‐life 5.8 years) relative to long‐lived 226 Ra (half‐life 1600 years), and is relevant to growth periods of up to 50 years. By extrapolating the heartwood accretion rate to the entire tree ring record the method also appears to provide realistic estimates of tree age. Eight trees were studied (three of known age, 72, 66 and 35 years), including three Australian hardwood eucalypt species, two mangrove species, and a softwood pine ( P. radiata ). The method indicates that the rate of growth ring formation is species and climate dependent, varying from 0.7 rings yr −1 for a river red gum ( E. camaldulensis ) to around 3 rings yr −1 for a tropical mangrove ( X. mekongensis ).