Regeneration of Betula pendula and B. pubescens coll. above and below the natural altitudinal distribution limit of B. pendula in south-east Norway.
During the period 1877 to 1902, a number of Betula pendula saplings were planted in an area above the natural altitudinal distribution limit of this species in SE Norway. In 1989, I compared the regeneration of the trees in this area with regeneration in a naturally established B. pendula stand below its altitudinal distribution limit. The regeneration of Betula pubescens coll., which grows at higher altitudes than B. pendula, was also studied. The age structure showed that few of the trees in the B. pendula plantation were old enough to have been planted, instead most of the trees appear to have become established by natural recruitment of seedlings from their planted ancestors in the 1920-1930's. But no young trees or seedlings were found, and there was almost no production of viable seeds. In contrast, seedlings and saplings occurred in the B. pendula stand below its distribution limit, and the production of viable seeds was relatively high. B. pubescens ooll. showed greater sexual reproduction than B. pendula, especially above the natural altitudinal limit of B. pendula. This suggests that better sexual reproduction in the mountain area may be important for the higher tree-line of B. pubescens coll.
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