I92I. No. 7. STUDIES ON THE LICHEN FLORA OF NORWAY. 1 5 



Xantlioria parictina and A', lycliiica an quick growers, and the}- can 

 attain fructification in a few years. Stercocaitloii pasclialc can produce full- 

 grown podetia in 5 — 6 years. Certain species of Cetraria- Gyrophora and 

 Parmelia e-g. just Paniiclia pliysodes are also relatively quick growers. 

 But the lichens that co\er the greatest areas in Norway, Cladonia alpestris 

 and Cladonia si/vafica (sensu lat.), grow very slowly. We do not know the 

 time necessary for them to attain full development, but it is hardly possible 

 to mention a lower figure than 25 30 years under favourable circumstances. 



It is, however, not sufficient to know the rate of spreading and the 

 rate of growth. It is also necessary to know the conditions which a lichen 

 spore, soredium or thallus fragment require to start growth, and our know- 

 ledge on this point is next to nothing. 



If a lichen vegetation has been disturbed or driven away from its 

 natural habitat it will in many cases require a long time before it again 

 can cover it. This is best seen after forest fires. I have seen gaps made 

 b\' old forest fire, 50 years or more old, where the range of the fire 

 could be traced on the lichen \-egetation, Cladonia alpestris was more 

 scarce and less developed there than outside the range of the fire. 



Even quite small fire gaps require an extremely long time before they will 

 be covered with the same lichens again. I have seen experimental fields, 

 I m- large in fine Cladonia fields where no trace of Cladonia alpestris was 

 visible 5 — 6 years after the fire, and Lapponian tent fire-places so old that 

 even their surrounding stones were sunk into the earth where the place was 

 recognizable on the vegetation. The original lichen-cover was gone, some- 

 times replaced by mosses or by other lichens, as Peltigera spuria and 

 P. aplitosa, even if the nearest station for these plants was far distant. 



After a fire the composition of the soil is altered, and this will also 

 alter the conditions of life at the cost of the lichens. But even at places 

 where everything is unchanged it takes a long time before the field is 

 covered again by Hchens if the lichen vegetation has been destroyed. It 

 is well known in reindeer districts that intense trampling can destroy a 

 lichen field (fragile Cladonia alpestris) for a long time, and this has been 

 fully confirmed by experiments. 



We may conclude that it takes an extremely long time before Cladonia 

 alpestris can fill the possible stations in a country. 



With the insignificant and uncertain exception of an eventual 'Nunatak'- 

 vegetation our entire flora must have immigrated after the glacial period. 

 The exact statements (de Geer) regarding the time which has elapsed 

 since this period have been contested. But if the time has really been so 

 short as has been supposed, it is a possible hypothesis to see the relative 

 small number of lichen ubiquists in Norway in the light of their slow 

 propagation, to suggest that historical causes may have been a factor which 

 has influenced the distribution of our lichens, — in cooperation with 

 climatic causes and route of immigration. 



