Impact of late frost on height growth in young sessile oak regenerations

The damage due to late frost during the 1995 and 1996 growing seasons was a nalysed in sessile oak seedlings in a natural regeneration. The seedlings w ere 5 years old at the beginning of the 1995 growing season. In 1995, frost occurred after the complete elongation of the growth unit (GU) of the stem which was produced during the first period of elongation. In 1996, frost o ccurred during the elongation period of the first GU. Damage intensity was observed visually and ranged from simple necroses of the leaf or the termin al bud of the new GU to the total loss of the GU. Damage intensity appeared to depend on the growing season and the bud phenological stage when frost occurred but not on the initial dimensions (height and collar diameter) of the seedling. Damage was higher when the seedling was at the shoot elongati on stage (1996 growing season) and lower when the seedling was in apparent rest (1995 growing season). The consequences of frost damage on growth were greater in the 1996 growing season. The bud resting period after the first period of elongation and before regrowth was shorter in 1996. Regrowth occ urred from either a bud from the damaged GU or a bud from the GU grown duri ng the previous growing season. In the latter case, the GU was sometimes lo nger than the GU from an axillary bud of the damaged GU. The second elongat ion period was shorter in 1996 than in 1995 and led to a relatively small G U. The intensity of frost damage, which had an effect on the length of the GUs produced during the second period of elongation, had no subsequent effe ct on the length of those produced during the third elongation period. The number of GUs finally contained in the stem and the annual shoot length of the stem were negatively affected by frost since the GU established during the first elongation period did not often belong to this stem. Monitoring g rowth allowed us to count the number of elongation periods and GUs establis hed and to differentiate the GUs produced during different elongation perio ds. ((C) Inra/Elsevier, Paris.).