Ra isotopes in trees: Their application to the estimation of heartwood growth rates and tree ages

摘要

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 Ra-228/Ra-226 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 Th-232 and Th-230 (the parents of Ra-228 and Ra-226) is negligible. The rate of heartwood accretion is determined from the radioactive decay of Ra-228 (half-life 5.8 years) relative to long-lived Ra-226 (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).