Tree-ring stable isotopes and historical perspectives on pollution--an overview.

Hydrogen (delta2H), carbon (delta13C), oxygen (delta18O) and nitrogen (delta15N) isotopes of tree rings growing in field conditions can be indicative of past pollution effects. The characteristic delta13C 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 delta18O and delta2H values decrease in trees exposed to pollution and exhibit inverse coinciding long-term trends with delta13C values. However, some trees exposed to diffuse pollution and experimental conditions can show an increase or no delta18O change even if delta13C 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 delta15N changes in trees can be caused by soil acidification and accumulation of anthropogenic emissions with isotopic signals deviating from natural N.