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 delta(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, delta(13)C shows an increase of more than 1 parts per thousand. 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, delta(13)C in tree rings shows a pattern of decline that is not paralleled by air delta(13)C changes. Isotopic discrimination has significantly decreased from 18.1 to 16.4 parts per thousand in high forest and varied insignificantly between 17.4 and 16.9 parts per thousand 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 CO2-enriched atmosphere. However other environmental changes, such as nitrogen deposition, may be responsible for such trends.