CHANGES OF TREE-RING DELTA-C-13 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 i n north-eastern France. Two different silvicultural systems were studi ed: high forest and coppice-with-standards, We studied separately effe cts related to the age of the tree at the time the ring was formed and effects attributable to environmental changes. At young ages, delta(1 3)C shows an increase of more than 1 parts per thousand. However, age- related trends differ in high forest and coppice-with-standards. Chang es in microenvironmental variables during stand maturation, and physio logical changes related to structural development of the trees with ag eing, could explain these results. During the past century, delta(13)C in tree rings shows a pattern of decline that is not paralleled by ai r delta(13)C changes. Isotopic discrimination has significantly decrea sed from 18.1 to 16.4 parts per thousand in high forest and varied ins ignificantly between 17.4 and 16.9 parts per thousand in coppice-with- standards, As a consequence, intrinsic water-use efficiency has increa sed by 44% in high forest and 23% in coppice-with-standards during the past century. These results accord with the increased water-use effic iency observed in controlled experiments under a CO2-enriched atmosphe re. However other environmental changes, such as nitrogen deposition, may be responsible for such trends.