Déterminisme de la phénologie des forêts tempérées françaises : étude sur les peuplements du réseau Renecofor

The spatial and temporal variability of bud burst (838 observations) and yellowing (449) of 103 coniferous and broadleaved stands (10 species) sampled in the Renecofor network were studied over the period 1997- 2006. For oak stands, the growing season starts in mid-April and stops in mid-October. The growing season is shorter in eastern France as a result of later bud burst (roughly 2 days delay per degree of longitude) and earlier yellowing (5 to 10 days). The growing season for beech begins somewhat later towards the end of the third week of April. Yellowing is seen in early October, making for an average growing season duration of 180 days. Bud burst is even later for coniferous stands and mainly depends on altitude (roughly 1.5 days delay per 100 m). Apart from species, modelling processes highlight the influence of topographic and geographic parameters (longitude, altitude, distance from the sea) and of spring and autumn weather conditions (Turc PET and/or mean temperature in March and October depending on the phenological stage). A 10-mm increase in PET in March advances budbreak from 4 to 9 days depending on species (mean: 6.8 days). For temperature, a 1°C increase in March speeds up bud burst by 2 to 5 days. The overall models predict the three phases — leaf unfolding, yellowing and duration of the growing season — within 8, 10 and 12 days respectively. The resulting equations were used to generate phenological timing maps for the whole of France. Results are discussed, in particular in the context of global warming.