Northern European trees show a progressively diminishing response to increasing atmospheric carbon dioxide concentrations

Abstract

In order to predict accurately how elevated atmospheric CO2 concentrations will affect the global carbon cycle, it is necessary to know how trees respond to increasing CO2 concentrations. In this paper, the response is examined over the period AD 1895-1994 of three tree species growing across northern Europe to increases in atmospheric CO2 concentrations using parameters derived from stable carbon isotope ratios of trunk cellulose. Using the isotope data, values of intrinsic water-use efficiency (IWUE) and intercellular CO2 concentrations in the leaf c (i)are calculated. The results show that trees have responded to higher levels of atmospheric CO2 by increasing IWUE whilst generally maintaining constant c (i) values. However, the IWUE of most of the trees in this study has not continued to rise in line with increasing atmospheric CO2. This behaviour has implications for estimations of future terrestrial carbon storage. The study illustrates, how trees world-wide are adapting to increasing levels of atmospheric CO2 in different ways. The reduction in sensitivity of IWUE to increasing Ca shown by most of the trees in this study may be related to the loss in climatic sensitivity of latewood relative densities in recent years. If a long-term or permanent reduction in sensitivity to atmospheric CO2 is established for a significant proportion of trees in temperate regions, the degree of future terrestrial carbon storage will have been greatly overestimated.