Différenciation mésoclimatique des terroirs alsaciens et relation avec les paramètres du milieu naturel

The influence of climatic conditions on the development of the vine and on the quality of the wines no longer needs to be demonstrated: at the scale of the vineyard, by the regional climatic characteristics, determining on this scale the viticultural potentialities (Huglin, 1978; Branas, 1946; Riou et al ., 1994); but also on a local scale, at the level of the basic terroir unit (Morlat, 1989), by the landscape differentiation of the natural environment inducing climatic variability within the same vineyard, and partly explaining differences in functioning of the vine, in connection with the processes of maturation and the quality of the wine (Becker, 1977 and 1984; Morlat, 1989 and Lebon, 1993a). According to these authors, the climatic diversity in a wine region constitutes in addition to the edaphic component, an important component of characterization of the Basic Terroir Units (UTB).

Several authors have described spatial climatic variability (Choisnel, 1987; Godart, 1949). Depending on the scale of investigation, they distinguish the macroclimate or regional climate, then the topoclimate resulting from topographic variability and finally the microclimate corresponding to the climate of the plant on the scale of the plot. The concept of mesoclimate, or local climate, is very close to topoclimate. It designates the climate resulting from the spatial differentiation of the regional climate, induced by the variability of the natural environment defining the landscape (Scaeta, 1935 and Godart, 1949).

The influence of topographic parameters; more specifically the declivity and orientation of the slope on solar radiation and on the distribution of air temperatures, have been the subject of numerous studies (Seltzer, 1935; Godart, 1949; Nigond, 1968). More recently, taking into account the type of weather (radiative or overcast) has proven to be important to better analyze and understand the processes of nocturnal thermal differentiation at the mesoclimatic scale (Geiger, 1980; Endlicher, 1980; Paul, 1980). . Erpicum in 1980, thus leads to a descriptive schematization of nocturnal thermal variability in two distinct environments of valley and plateau in Upper Belgium, according to the main types of regional weather.

At this scale of investigation, the advective term is an important parameter to take into account. Ventilation is highly dependent on the quantity and height of the surrounding masks. These can be topographic, vegetal or anthropic (Guyot, 1963). Thus, the analysis of the landscape is necessary during the integrated characterization of the terroirs (Morlat, 1989 and Jacquet et al ., 1995). This work defines simple landscape descriptors such as for example the Landscape Openness Index (LO.P.), making it possible to characterize mesoclimatic differences and lead to a cartographic representation of the landscape (Lebon, 1993b). Based on the spatial variability of global radiation, wind speed and air temperature recorded at the UTB scale of the Alsatian vineyard, the communication proposes a hierarchy of the parameters of the landscape environment generating such differences. climatic.