Reference
Duquesnay, A., Breda, N., Stievenard, M. and Dupouey, J.L. 1998. Changes of tree-ring d13C and water-use efficiency of beech (Fagus sylvatica L.) in north-eastern France during the past century. Plant, Cell and Environment 21: 565-572.
What was done
The authors studied the intrinsic water-use efficiency of beech trees growing under two different silviculture regimes in north-eastern France over the past century by analyzing the relative amounts of 12C and 13C (typically expressed as d13C) contained in their tree rings.
What was learned
During the past century, d13C values in beech tree rings declined, indicating that open pore space for CO2 diffusion into leaves also declined, which caused the water-use efficiency in these trees to increase. Specifically, in a high forest silvicultural system, intrinsic water-use efficiency increased by 44% during the past 100 years, and in a coppice-with-standards system, it increased by 23%.
What it means
Over the past century, the average water-use efficiency of two beech tree silviculture systems increased by approximately 33%, in agreement with the results of experiments that have reported similar increases in water-use efficiency in response to atmospheric CO2 enrichment. Hence, it is possible that the rising CO2 content of the air during the past century may have been the cause of the increased water-use efficiencies in these tree systems. If so, these stands should respond with even greater increases in their water-use efficiencies as the CO2 content of the air continues to rise ever higher.