The study of growth trends in the French forests begun 10 years ago. Very large surveys have been done in several regions and for many species in order to study changes in radial growth of individual trees during the last 150 years. Up to now, the following species were studied: silver fir (Abies alba Mill.) in the Vosges and Jura mountains, Norway spruce (Picea abies (L.) Karst.) in the Vosges mountains, beech (Fagus sylvatica L.) in the Lorraine plain and Vosges mountains, oaks (Quercus robur L. and Q. petraea (Matt.) Liebl.) in the eastern part of France, mountain pine (Pinus uncinata Mill.) in the Pyrenees mountains and Corsican pine (Pinus nigra Arnold ssp. laricio var. corsicana) in western France. In each case, 500 to 1500 trees were sampled at the regional level.
In order to take into account the confounding effect between age and date, a peculiar standardisation method was adopted: standardisation was done at the regional level, instead of tree by tree. This was done either by construction of a mean regional age curve, either by analysis of variance of the whole bulk of data.
In all cases, a significant increasing growth trend appears during the last 150 years. In some cases, this trend is only apparent after elimination of competition effects. This trend varies between 50 % and 400 % for basal area increments, depending on the species and the location.
Careful analysis of possible bias during sampling and analysis have been done. Beside these bias, possible causes of such trends are discussed: silviculture changes, nitrogen pollution effects, CO2 fertilisation effects and climatic changes. Study of free growing trees (Pinus uncinata in the Pyrenees mountains) shows that, even without any silvicultural changes and at low competition levels, increasing growth trends are still very steep, strongly reinforcing the possible role of environmental changes. Comparison of level of growth between living trees and archaeological data are also consistent with previous results.
In order to assess the impact of such changes on the global carbon cycle, multilocal and multispecies approaches are needed. Territories still to be explored are tropical regions, mountain areas, boreal forests.
Key words: radial growth, long term trend, environmental changes, competition, sampling strategy.
Correspondence: Jean-Luc Dupouey, Forest Ecophysiology Unit, I.N.R.A., F-54280 Champenoux, France
Telefax: 33 83 39 40 69
E-mail: dupouey@nancy.inra.fr