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 analysed in sessile oak seedlings in a natural regeneration. The seedlings were 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 occurred dur- ing the elongation period of the first GU. Damage intensity was observed visually and ranged from simple necroses of the leaf or the terminal 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 elongation 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 occurred from either a bud from the damaged GU or a bud from the GU grown during the previous growing season. In the latter case, the GU was sometimes longer than the GU from an axillary bud of the damaged GU. The second elongation period was shorter in 1996 than in 1995 and led to a relatively small GU. 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 effect 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 growth allowed us to count the number of elongation periods and GUs estab- lished and to differentiate the GUs produced during different elongation periods. ( © Inra/Elsevier, Paris.) late frost / rhythmic growth / Quercus petraea seedlings Rsum -Impact des geles tardives sur la croissance en hauteur des jeunes rgnrations de chne sessile. Des dgts occa- sionns par des geles tardives, intervenues au cours de deux saisons de croissance 1995 et 1996, sur des plants de Chne sessile d'une rgnration naturelle ont t analyss. Les plants taient gs de 5 ans au dbut de la saison 1995. En 1995, la gele est surve- nue aprs longation complte de l'unit de croissance (UC) de la tige mise en place au cours de la premire vague. En revanche, en 1996, la gele est intervenue en cours d'longation de cette UC. L'intensit des dgts a t observe visuellement, allant de simples ncroses des feuilles ou du bourgeon terminal de l'UC nouvellement mise en place, la destruction totale de cette UC. Cette intensit des dgts est apparue dpendante de l'anne et du stade de dveloppement du plant lorsque la gele est survenue, mais indpendante de leurs dimensions initiales (hauteur et diamtre au collet). Les dgts taient plus importants lorsque le plant se trouvait au stade longation de la pousse (saison 1996) que lorsque le plant tait en repos apparent (saison 1995). Les consquences de ces dgts de gel sur la croissance taient plus importantes au cours de la saison 1996. La dure du repos entre la premire et la deuxime vagues de croissance a t relativement plus courte en 1996. La reprise de croissance a t assure soit partir d'un bourgeon issu de l'UC endommage, soit partir d'un bourgeon issu de l'UC forme la saison prcdente. Dans ce der- nier cas l'UC forme tait parfois plus longue que celle issue d'un bourgeon axillaire de la pousse endommage. La dure de la * Correspondence and reprints