Etudes sur la répartition des essences forestières en France

STUDIES ON THE DISTRIBUTIONOF THE FOREST SPECIES IN FRANCE. INTRODUCTIONby Ph. GUINIER, Directeur de l'École Nationale des Eaux et Forets. At the International Forestry Congress of Paris, in 1913,a report was presented on „The Woody Plants Distribution inFrance" and the author, Ph. Guinier, showed the lack of documents,according to this point of view, and the need to studythis question in detail. The congress proposed that the ForestResearch Institut take charge of this work.The events have delayed the realisation of this proposition.Under favour of the remarkable progress did since in the domainof ecology and phytosociology, the documents which it maybe disposed of are now much more numerous : botanists andforesters are able to give more easily accurate informations.The scheme, appointed in 1913, is carried out by the ForestResearch Institut and a set of studies will be published on thedistribution of the chief forest species in France. CONTRIBUTION TO THE STUDY OF SILVER FIR(Abies Alba Mill.) DISTRIBUTION. Until now, in the botanical or forest litterature, there wasno complete work on the distribution of Silver Fir in France.The author has attempted to meet with this want. He hasdrawn, on a map on the scale of 12.500.000, with the higheststretch of precision according to the scale of the map, the presentarea of Silver Fir in France.Silver Fir is the dominant species in the Vosges and in themiddle high country of the Jura. It is mixed with spruce inthe High Jura and in the Northern Prealps, it is disseminatedin the remainder of the chain of the Alps. In the Pyrenees, itis specially abundant in the Aude High Valley (Country ofSault) and in "Luchonnais"; less abundant elsewhere, it failsin the western end of the chain. It is not scarce in the middlehigh country of Corsica. "Massif Central" has been deforestedvery much and it remains there relatively few Fir Woods wichare found specially in "Beaujolais", "Massif du Pilat", "HighVivarais", "Forez" and "Livradois" : Lastly there is also SilverFir, in a spontaneous state, in the hills of High Normandy.The author asked himself which are the factors settling theexistence of this area. Silver Fir is difficult to grow withoutits natural area. That seems to prove that it requires plainlydefinite and varying only between narrow limits ecologicalconditions.The edaphic factors play an entirely secondary part. Theclimatic ones, on the contrary, hold the first place. Amongstthese factors, the most important seem to be the height ofrainfall and the mean temperature. These ascertainments havebrought the author to examine the relation there is between the distribution of Silver Fir and the aridity index as it has beenPappointed by « de Martonne » : I = P/T+ I0 (I = Index, P = Height of rainfall in millimeter, T = Temperature in centigradedegrees.) y the comparison between a map of the annualaridity index and the map of the Silver Fir distribution settledby the author, it is verified the curve corresponding with thearidity index 50 involves the greater part of the Silver Fir areaand the curve corresponding with the aridity index 45 involvesit entirely. Further, by repeating, on the same map, the annualisotherms of + 8°C and 0°C and by taking into considerationonly the common zone placed within the different curves, theauthor proves that the so obtained figure reproduces, ratherexactly, the Silver Fir area as he has fixed the boundaries of.He gets so the "Graphic Synthesis" of the Silver Fir area.However, the coincidence between the two outlines is not absolute,but this can be explained, in the one hand for the lack ofprecision of the floristic or meteorological informations, in theother hand by the action of the biotic factors and specially bythe action of men. Concerning Silver Fir this action seems tohave been a destructive one. It remains sometimes somewitnesses of that old extension but Silver Fir has completelydisappeared from some regions (Mont Aigoual, Montagne Noire).Now the forest is better protected, the economical conditionshave been altered, Silver Fir tends to win back the lost ground.From the comparison between the two maps, it may be concludedthat, the conditions of temperature being realized (annualmean temperature included between + 8°C and 0°C).1 0 The best zone of Silver Fir (Country of the Wide Fir Woods)corresponds with an aridity index equal or superior to 50;2° When the aridity index is included between 45 and 50 SilverFir can exist but never constitutes the dominant species;3° When the aridity index falls under 45 the existence of SilverFir becomes precarious. (Trad. G. RABOUILLE)