REPRODUCTIVE PATTERNS OF BETULA-PENDULA AND B-PUBESCENS COLL ALONG A REGIONAL ALTITUDINAL GRADIENT IN NORTHERN SWEDEN

The Scandinavian birches have different distribution patterns. Betula pendula reaches its upper tree limit at the bottom of the valleys in the Scandes, while B. pubescens ssp. pubescens extends above the B. pendula limit, and merges into B. pubescens ssp. tortuosa at higher elevations. In a 6 yr study, I quantified the spatial and temporal variation in the reproduction of these species along a regional altitudinal gradient. Betula pendula showed a steeper altitudinal decrease of seed germinability and seed germination rate than B. pubescens coll. Seed weight was strongly and positively correlated with altitude for B. pubescens, but not for B. pendula. The proportion of seeds infected by gall midges, Semudobia coll., was much higher in B. pendula than in B. pubescens coll., especially in the mountain area. All birch taxa showed large annual variations in quantity and quality of seeds. The sapling density was higher in B. pubescens ssp. pubescens stands than in B. pendula and B. pubescens ssp. tortuosa stands in the mountains. The vegetative reproduction was about three rimes higher for B. pubescens ssp. tortuosa, than for the other two birches. These results indicate that the altitudinal variation in reproduction among the birch taxa minor their present distribution patterns.