ANOTA LA HORA:

The climate of the M assif Central is predominantly Atlantic. However, the more elevated areas to the east and south are more xeric, with strong winds, low mean temperatures, numerous frost days and a short growing season (Reille et a l , 1992). Climate data for mean minimum January and mean maximum July temperatures, mean number of days per year without defrost (which corresponds to the mean number of consecutive days during when the temperature does not rise above 0°C), mean number of hours of sunshine per year, and mean rain and snow falls per year are presented on six maps of France in Figure 1.4.4 (Kessler and Chambraud, 1986).

Chapter 1.4: The study region 60

Fig. 1.4.3: The volcanic entities and the fault systems in the French M assif C en tral (re-draw n from Truze and Bonifay, 1993).

: : : j Hercynian Crystalline Basement Mesozoic + Tertiary cover

Oligocene Rift sediments Neogene volcanics Major Faults Chaine des Puys M ont-Dore:/: Cezallier : :

Cantal Velay _{O riental}

Deves : Coirons Lyon DD u o> =3 0 g >< 100 km

Chapter 1.4: The study region 61

1.4.2.2. Detailed climatic features

The ombrothermic diagrams drawn from Meteo France weather stations data give a more detailed picture of the climatic conditions existing around some of the lakes studied (figure 1.4.5, from Kessler & Chambraud, 1986). The location of the weather stations is shown in Figure 1.4.6.

• Puy-de-Dôme and Cantal districts

The western part of the Puy-de-Dôme is characterised by high precipitation. The eastern part, the Limagne plain, is sheltered from the wind and rain coming from the west by the volcanic range “Chaîne des Puys” and has very low precipitation. The climate of this district is also characterised by high thermic amplitude and rather cold nights. More or less the same west-east pattern can be drawn for the Cantal district (see figure 1.4.6). In the Puy-de-Dôme district the Besse-en-Chandesse weather station (1050 m) is located close to numerous lakes included in this project (Pavin, Chambon, Estivadoux, Bourdouze, Montcineyre). The ombrothermic diagram for this station shows that precipitation is rather homogeneously spread over the year with a maximum during the Autumn-Winter (November to January) and another during the Spring (May). The high thermic amplitude (-5 to 20°C) over the year and the relative complexity of this diagram is typical of mountainous areas (figure 1.4.5). The Mont-Dore (1050 m) weather station is close to Lake Guéry, but located at lower elevation. The data from this station show a rather more oceanic climate than that of Besse-en-Chandesse, characterised by higher total annual precipitation (174 cm) and lower thermic amplitude (-3 to 19°C). Temperature data are not available for other lakes located in these two districts (Tazenat, Aydat, Cassière, Chau vet, Landie, Laspialades, Crégut, Cousteix, Coindé, Esclauze, Godivelle-d’en-Bas, Godivelle-d’en- Haut, Bordes) but there are data for total annual rainfall (see Figure 1.4.6).

• Haute-Loire and Ardèche districts

In the Haute-Loire district temperatures are generally lower than in the western part of the M assif Central. Days of snow fall are rather numerous (40 days/year at Le Puy en Velay) and snow stands for a long time on the plateau. However, total amount of precipitation is low especially in the Loire and Allier valleys (Kessler & Chambraud, 1986). The weather station of Fay-sur- Lignon gives data relevant for Lake Saint-Front. The corresponding ombrothermic diagram show high thermic amplitude (-4 to 21°C) and a curve for precipitation characterised by a marked minimum in July (figures 1.4.5 & 1.4.6). The Ardèche district has a very contrasted climatic pattern. Although the northern part of this district belongs to the medio-European climatic zone the southem part clearly belongs to the Mediterranean one. The western part, the most mountainous area of the district, has a particularly harsh climate. The plateau around Mount

Chapter 1.4: The study region 62

Gerbier-des-Joncs (the source of the Loire River) has especially cold winters with heavy snow and strong northern winds (“La Burle”). The ombrothermic diagram for Sainte-Eulalie weather station (figure 1.4,5), close to Lake Ferrand, shows high autumnal precipitation (October- November). Issanlas weather station (close to Lac dTssarlès) shows more or less the same ombrothermic diagram than the one for Ste-Eulalie with however markedly lower precipitation. Only total rainfall data are available for the sites of Bouchet and Ribains (87 cm/year), located at close distance to each other (figure 1.4.6).

1.4.2.3. Present vegetation and pedology

In the M assif Central, current arboreal vegetation consists mainly of beech forest, with abundant fir (Abies), and oak forests that are polymorphous in the central part o f the region but dominated by Quercus robur to the north-west and by Q. petraea associated with Castanea to the south­ west. Everywhere abundant Betula pendula and Pinus sylvestris suggest human activity, which has also converted the usual brown acid forest soils with acid mull to brown acid soils with moder or ranker and poor mor or barren brown soils, depending on the geological substratum. Woodlands, however, cover merely half of the region, the rest being occupied by Calluna and Genistae heaths or grassland (Reille et a l , 1992). In the Monts Dore region forests are not even abundant at middle altitudes.

The Devès plateau, with its characteristic pockets of woodland, is heavily exploited for pasture and also for arable farming. The volcanic cones and the poor soils are colonised by heath with

Sarothamnus scoparius and Genista purgans or by Pinus sylvestris woodland, the latter having long been considered by botanists to be a natural formation (Carles, 1957). Holocene pollen sequences have, however, shown that these are secondary, the natural vegetation prior to anthropogenic deforestation being beech woods (Beaulieu et al., 1984).

1.4.3. Previous diatom research in the study region