Dramatic water conserving effects of rising CO2 in European trees is revealed in 100 years of data

Tree cores reveal a wealth of information

Italian researcher(s) Tognetti et al. (2014) show that improvements in intrinsic water-use efficiency (iWUE) — the ratio between photosynthetic rate and stomatal conductance — “is recorded in the variation of the carbon isotope discrimination (Δ¹³C) of the annual growth rings that are laid down during each growing season.

As more CO2 has become available to trees they have studied the trees have been stimulated to breathe less and thereby lose much less water via evaporation and transpiration. The amount of water conserved is large amounting to upwards of a 35-40% less water being required by the trees. This of course translates into more water being available to other living members of the forest ecosystem and confirmation of the potent global greening effect of high and rising CO2. The study found similar effects in both Italy and Chile showing trees conserve water more and more as CO2 rises.

The research paper published in PLOS One cites the work of Duquesnay et al. (1998); and the authors additionally note, in this regard, that “elevated atmospheric CO2 concentration (Ca) is expected to reduce stomatal openings and increase assimilation rates,” as has been demonstrated by the work of Tognetti et al. (1998, 1999), which changes, in their words, “can alter the Ca to intercellular CO2 concentration (Ci) gradient and result in increased iWUE in the long-term, which will be recorded in the variation of this Δ¹³C.”

To obtain these data, as well as to develop an independent means of assessing tree growth, the five researchers used large-diameter borers to extract two increment cores at breast height from each of 16-20 dominant trees at eight sites in Italy and five sites in Chile. They say that for both sets of beech trees, “a continuous enhancement in isotope-derived iWUE was observed throughout the twentieth century,” a finding that “was common to all sites and related to changes in ambient CO2 concentration and secondarily to increases in temperature.”

More specifically, they report that “over the twentieth century, the iWUE increased 35.3 ± 1.4% in the sites of Southern Italy and 39.8 ± 1.3% in those of North-Central Italy” (see figure below), both of which sets of results are in good agreement with the 44% increase in water efficiency reported by Duquesnay et al. (1998) and Peñuelas et al. (2008).

100 years of decadal mean variation in intrinsic water-use efficiency of Fagus sylvatica and Nothofagus trees growing at the eight Italian and five Chilean sites over the period 1900-2010. Adapted from Tognetti et al. (2014).

References
Duquesnay, A., Breda, N., Stievenard, M. and Dupouey, J.L. 1998. Changes of tree-ring δ¹³C and water-use efficiency of beech (Fagus sylvatica L.) in north-eastern France during the past century. Plant, Cell and Environment 21: 565-572.

Peñuelas, J., Hunt, J.M., Ogaya, R. and Jump, A.S. 2008. Twentieth century changes of tree-ring δ¹³C at the southern range-edge of Fagus sylvatica: increasing water use efficiency does not avoid the growth decline induced by warming at low altitudes. Global Change Biology 14: 1076-1088.

Tognetti, R., Johnson, J.D., Michelozzi, M. and Raschi, A. 1998. Response of foliar metabolism in mature trees ofQuercus pubescens and Quercus ilex to long-term elevated CO2. Environmental and Experimental Botany 39: 233-245.

Tognetti, R., Longobucco, A., Miglietta, F. and Raschi, A. 1999. Water relations, stomatal response and transpiration ofQuercus pubescens trees during summer in a Mediterranean carbon dioxide spring. Tree Physiology 19: 261-270.