Ann. For. Sci. 50 (1993) 235-246
DOI: 10.1051/forest:19930302

Le dépérissement observé en Isère sur sapin et épicéa : relations avec les caractéristiques physico-chimiques des écorces

I Legrand^a, J Asta^a and Y Goudard<sup>b

a</sup>  Laboratoire de Biologie Alpine, Université Joseph-Fourier, Grenoble BP 53 X, 38041 Grenoble Cedex
^b  ONF, 38000 Grenoble, France

Résumé - Dans le cadre d'une étude sur le dépérissement des forêts dans les Alpes du Nord (massifs de Belledonne, Vercors et Chartreuse), nous avons prélevé des échantillons d'écorce sur le tiers des 474 sapins et 682 épicéas faisant l'objet de notations sur leur état de dépérissement (épaisseur des 3 derniers cernes, pourcentage de perte d'aiguilles et pourcentage de couleur anormale du houppier). Les écorces récoltées ont été analysées en laboratoire (pH, conductivité et épaisseur de l'écorce). Des analyses statistiques ont mis en évidence des différences dans le degré de dépérissement entre sapins et épicéas, ainsi qu'entre massifs : si le massif du Vercors est moins touché par le phénomène, c'est en Belledonne qu'on observe la plus grande proportion d'arbres très dépérissants, le sapin semblant plus atteint que l'épicéa dans ce massif. L'analyse des données sur les écorces apporte des résultats intéressants sur les variations des caractéristiques physico-chimiques, aussi bien à l'intérieur d'une même essence qu'entre sapins et épicéas, mais également entre massifs, et nous tentons de donner une explication aux différences observées. L'analyse des relations entre caractéristiques physico-chimiques des écorces et critères de dépérissement montre que les arbres les plus dépérissants ont statistiquement une écorce plus acide et de conductivité plus faible que celle des arbres sains. Le processus d'échange protons-cations au niveau du feuillage pourrait donner une explication à cette relation.

Abstract - Forest decline observed in the department of Isère (France) on silver fir trees (Abies alba Mill) and Norway spruce trees (Picea abies (L) Karsten): relationships with the physico-chemical characteristics of the barks. In recent years, the decline of the mountain forests in Europe has been given very serious consideration; this phenomenon has been partly attributed to diffuse atmospheric pollution. As it was impossible for us to study the effects of such pollution on trees directly we concentrated on the relationships that might exist between bark physico-chemical characteristics and criteria of tree decline (thickness of the 3 outermost tree rings, percentage of needle loss, percentage of abnormal colour). In 50% of the trees studied (165 silver firs and 235 Norway spruces), bark samples were taken to measure their pH and conductivity in the laboratory. Before starting this study, methods had to be defined: an experimental procedure had to be drawn up for bark analysis suited to our objectives. The processing of all data was based on statistical methods. The study of the criteria of tree decline, including that of bark physico-chemical characteristics, highlighted the following important points: - the Belledonne massif is more affected by the decline phenomenon and in this massif, silver firs are more affected than Norway spruces (fig 1); - it was noted that Norway spruce barks are on average more acid than silver fir barks, and that they are also thinner (fig 2); - the correlations between bark acidity, conductivity and thickness showed that conductivity is closely related to bark thickness, whereas for acidity, there is no significant difference to be observed (table I); - differences in acidity, conductivity and bark thickness are to be noted between the various massifs; these differences can be explained (figs 3 and 4); - the study of the correlations between the criteria of decline and bark characteristics showed that the seriously damaged silver firs had more acid barks in general than healthy trees. Conversely in the Norway spruces, this aspect was not observed possibly because their barks are already naturally more acid (fig 5); - as far as conductivity is concerned, this appears to be lower in the barks of damaged trees, both silver firs and Norway spruces (fig 5). It is suggested that the differences observed are not directly due to the deposition of polluting agents on the barks, but that the process of 'recretion' in the leaves could provide an explanation: among damaged trees and hence the most defoliated specimens, rainwater is less charged in cations in contact with the leaves, and as the water runs down the bark there is less protoncation exchange, and the bark remains more acid than in the case of a healthy tree.

Key words: forest decline / silver-fir (Abies alba Mill) / Norway spruce (Picea abies (L) Karsten) / Alps / acidity and bark conductivity

Mots clés : dépérissement forestier / sapin (Abies alba Mill) / épicéa (Picea abies (L) Karsten) / Alpes / acidité et conductivité d'écorce