Changes of tree-ring δ13C and water-use efficiency of beech (Fagus sylvatica L.) in north-eastern France during the past century
We investigated variation in intrinsic water-use efficiency during the past century by analysing δ 13 C in tree rings of beech growing in north-eastern France. Two different silvicultural systems were studied: high forest and coppice-with-standards. We studied separately effects related to the age of the tree at the time the ring was formed and effects attributable to environmental changes. At young ages, δ 13 C shows an increase of more than 1‰. However, age-related trends differ in high forest and coppice-with-standards. Changes in microenvironmental variables during stand maturation, and physiological changes related to structural development of the trees with ageing, could explain these results. During the past century, δ 13 C in tree rings shows a pattern of decline that is not paralleled by air δ 13 C changes. Isotopic discrimination has significantly decreased from 18.1 to 16.4‰ in high forest and varied insignificantly between 17.4 and 16.9‰ in coppice-with-standards. As a consequence, intrinsic water-use efficiency has increased by 44% in high forest and 23% in coppice-with-standards during the past century. These results accord with the increased water-use efficiency observed in controlled experiments under a CO 2 -enriched atmosphere. However, other environmental changes, such as nitrogen deposition, may be responsible for such trends.