Foryngelse hos lind Tilia cordata i Sør-Norge

Abstract

The strategy for genetic conservation in Norway aims to manage existing adaptions and thus secure each species" ability to adapt to environmental change and to evolve. The main aims of this report are three-fold: register the regeneration of lime/linden Tilia cordata Miller in three regions of Southern Norway, identify factors influencing regeneration, and suggest means to secure a sustainable management of this species. A total of 27 sites were studied in the eastern, southern, and western regions of South-Norway. Environmental variables were sampled from 217 genets of lime. The topography was categorised, shoots (shorter than 137 cm above grond) counted, and the diameters of all stems (taller than 137 cm) measured. A search was made for seedlings at every site. Of the adult limes, 53% were rooted in ground sloping more than 30° in topography classified as scree or precipice. There was a positive assosiation between clonal regeneration, expressed as number of stems per genet, and steep terrain. The populations of lime in Southern Norway may thus be the survivors after more extensive forests 5-6000 years before present. By looking at frequencies of stems per diameter class, after an approach by Leak (1965), one may evaluate population dynamics based on data collected from one season only. According to this approach the populations for all three regions are evaluated as stable or growing in the sense that the density of stems per genet may be increasing. Only 11 seedlings of lime were found. These were all found in the climatically warmest parts of the country, in southern Vest Agder and inner parts of Oslofjorden. Most of them had established in dense vegetation on relatively level ground. Hence, different factors may govern sexual and clonal regeneration - and steep terrain does not seem to be a part of the regeneration niche of seedlings. A series of studies from Great Britain demonstrate that lack of sexual regeneration in lime is attributable to low temperatures. Fruits collected in northern England and Scotland are unfertilise because the pollen tube does not grow into the stigma in the prevailing climate of that region. Many parts of South-Norway have comparable temperatures during the flowering season of lime. Based on fruit collections from three years in Sogn og Fjordane, in 1994, 1995, and 2004, there is evidence that at least some years fertilisation takes place and some embryos develop. Still, temperature is the most plausible factor that may limit the sexual regeneration of lime in South-Norway - many of the embryos have not ripened properly, indicating a too short growing season. The importance of this factor should be further studied by germination experiments. In addition to germination tests, mapping of genetic variation both at the within and at the between stand level of lime is recommended to ensure a sustainable management can be given to lime in Norway.

Referanse

Mong C.E. 2005. Foryngelse hos lind Tilia cordata i Sør-Norge. Aktuelt fra skogforskningen 4/05: 17 s.