Tree-ring stable isotopes and historical perspectives on pollution - An overview

Hydrogen (delta H-2), carbon (delta C-13), oxygen (delta O-18) and nitrogen (delta N-18) isotopes of tree rings growing in field conditions can be indicative of past pollution effects. The characteristic delta C-13 trend is a positive shift generally explained by invoking closure of stomata, but experimental studies suggest that increased rates of carboxylation could also generate such trends. In many cases the delta O-18 and delta H-2 values decrease in trees exposed to pollution and exhibit inverse coinciding long-term trends with delta C-13 values. However, some trees exposed to diffuse pollution and experimental conditions can show an increase or no delta O-18 change even if delta C-13 values increase. These diverse responses depend on how stress conditions modify physiological functions such as stomatal conductance, carboxylation, respiration, and perhaps water assimilation by the root system. Recent studies suggest that delta N-18 changes in trees can be caused by soil acidification and accumulation of anthropogenic emissions with isotopic signals deviating from natural N. Crown Copyright (C) 2009 Published by Elsevier Ltd. All rights reserved.