Tammen (Quercus robur L.) kukintabiologia ja -fenologia : kukintahavainnot 1999-2006, Bromarv Framnäs, Parainen Lenholm | Flowering biology and phenology of pedunculate oak (Quercus robur L.)

The phenology and flowering of the pedunculate oak (Quercus robur L.) in Southern Finland was monitored during 1999-2006, in the Framnäs and Lenholm oak forests, which are mixed with hazels, birches, aspens, rowans, lindens, junipers, spruces, pines. Both stands are about 20 ha and located by the northern limit of natural distribution of oaks. The distance between the groves is about 60 km. A long-term pasture of about 150 years, on both islands has reduced the oak stands, however in recent decades selective cuttings have been made to help the oaks. In the south-western coastal areas degree days ranged from 1405,7 to 1737,7 d.d. in 1999-2006, indicating higher cumulating heat sums than on average over the normal period for the years 1981-2000. A total of 20 oaks × 3 branches /each tree sits in both stands, which equals 120 branches that were surveyed. The cork oak flowering model was modified to common oaks. Phases: 0-7 in male flowering were specified: from the dormant period (0) to swelling of the buds, from the onset of flowering to withering of the inflorescences (6). During the female flowering similar phases were found - from the rest period of apex buds (0) to the development of auxiliary buds on same year shoots. Further from the onset of first tiny female flowers to the withering of stigmas on the latest ones (6). The common oak flowering model was accurate enough and easy to apply in the survey. Male flowering began in 1999-2006 between 17th May and 6th June. Anthesis followed a few days later, when most of the staminate inflorescences had ripened. The catkins in the full length turned from green to bright yellow just before pollen release. Ripening was equal to phase 5 in the model of staminate flowering. Onset of the first female flowers began when the phase for stamens was about 3-4. Pollen receptivity of the first glossy reddish stigmas began when male flowering was about 5-5,5. The peak for female flowering took place a few days later than the anthesis, indicating protandry in common oak flowering. The period from pollination to fertilization is still a competition sequence for the pollen tubes. The stigmas, the developing female flower organs in gynoecium and the pollen tube cell tissue interact at the cell level. The self-incompatibility system acts on preventing selfing. Hence the loss of female flowers is huge, in some trees and in some years about 50% of the flowers fall down by the time of fertilization. Only 0-7,5% of the female flowers of the peak blossom period developed into acorns, which was less than in many other investigations. The suitable period for the common oak reproduction is very limited in the northernmost parts of its natural distribution. If the onset of the male flowering was delayed to early June, the number of female flowers was consequently low (years 1999, 2003, 2005). The detected linear regression is a topic for further studies. It remains unclear in this current scrutiny how the regulatory process acts on limiting the seed set and preventing extra ecological costs of any failing reproduction. In 2004-2006 a new potential pest, the black-dotted groundling, Stenolechia gemmella L. attacked the common oaks in many parts of Southern Finland. The moth larvae hollowed out new, same year shoots by length of few centimeters. Slightly afterwards shoots with their leaves and flowers turned brown. In July 2005 hundreds of withering shoots appeared in large oak crowns and of the monitored 1500 shoots in Framnäs 249 were infested by the moth larvae. Larvae and pupae are present in shoots in July, the adults emerge later in August or September.