Climatic significance of δD variations in a tropical tree species from India

Abstract

The climatic significance of the stable-isotope ratio of hydrogen (δD) in tree cellulose has been demonstrated using data mainly from coniferous trees that grew in temperate climates^1–4. In such regions, the mean annual δD of precipitation is linearly related to the mean annual temperature⁵, and trees thriving on precipita-tion preserve this temperature signal in the δD of the carbon-bound hydrogen of cellulose in their growth rings. Similar studies on tropical deciduous trees could yield useful information on climate, such as the monsoon variability over the past few centuries. However, because of the small summer-to-winter temperature con-trast, tropical trees grow throughout the year and hence do not lay down well-defined growth rings (even in the few species with clear rings, it is difficult to ascertain if they are annual⁶). Also, throughout the year tropical mean surface air temperatures are >20 °C, above which the linear relation between δD of precipita-tion and temperature breaks down⁷. Thus there is no temperature signal in the δD of precipitation for the trees to record. In spite of these problems, we report here a significant correlation (0.7) between δD variations in two individual teak trees from the west coast of India with the amount of rainfall and mean maximum temperature, as in the case of temperate trees⁴. We propose a mechanism whereby δD of teak tree cellulose is correlated with amount of rainfall.