Stratégies d'occupation de l'espace par les petits ligneux après débroussaillement en région méditerranéenne française. Exemple d'un réseau de pare-feu dans l'Esterel

Résumé - La repousse de la strate arbustive de 6 fruticées typiques de peuplements forestiers du massif de l'Esterel a été suivie pendant 6 ans sur des pare-feu créés il y a plus de 20 ans, et régulièrement entretenus depuis. II s'agit de faciès à cistes (Cistus salviaefolius, C monspeliensis et C albidus), à bruyères (Erica arborea et E scoparia), à myrte (Myrtus communis), à callune (Calluna vulgaris), à calycotome (Calycotome spinosa) et à cytise triflore (Cytisus triflorus). L'analyse de l'accroissement en volume et en poids montre des stratégies d'occupation de l'espace contrastées selon les types de peuplement, avec des faciès à fort encombrement, faible phytomasse et colonisation rapide (cytise, calycotome); des faciès à faible encombrement, forte phytomasse et colonisation rapide (callune); et des faciès à faible encombrement, faible phytomasse et colonisation lente (cistes).

Abstract - Spatial occupation strategy of shrublands after cutting in the French Mediterranean region. Growth rate of shrubs after cutting is the determining factor when programming scrub clearing schedules in fuel-break networks, especially when their maintenance is partly carried out by livestock grazing. As vegetation grows quickly on acid soils, 6 types of shrublands were studied for 6 years in the Esterel hills (southeastern France). These plant communities were growing in 20-year-old fuel-breaks (cut regularly) and were dominated by Calluna vulgaris in the first Calycotome spinosa in the second, Cytisus triflorus in the third, Erica arborea and E scoparia in the fourth, Myrtus communis and Pistacia lentiscus in the fifth and Cistus salviaefolius, C monspeliensis and C albidus in the last. A permanent belt transect 20 m long and 50 cm wide was measured every year in June, just after the spring growth. This method gave brushwood amount, fuel stock and browse biomass. A volumetric space factor was used to estimate brushwood amount and aerial biomass was calculated to evaluate fuel stock. Multiple regression models (table I) classified the shrublands into 3 groups according to biomass or volume growth. An index based on biomass corrected by a coefficient inversely proportional to the initial shrubland cover classified shrublands in relation to their fuel production potential (table II). Volume data are compared with the current volume in the controls (table III) and with the regression models of the dominant species (table V). Changes in the dominant species ratio are also commented on (table IV). The analysis of biomass and volume growth rates gave 2 opposing strategies: Calluna and Cytisus stands sprouted vigorously and then their growth rate decreased steadily, while the other stands, after a slow start increased their growth rate constantly until the fourth year (fig 1). A preliminary growth rate model grouping all the stands with similar dominant species is proposed (fig 2). Altogether, each type of shrubland can be characterized by a particular strategy of spatial occupation: Cytisus or Calycotome stands developed large volume, light biomass and rapidly spreading shrublands; Calluna formed low volume, heavy biomass and rapidly spreading heathlands; while Cistus dominance generated low volume, light biomass and slowly spreading rangelands.