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 times 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 mirror their present distribution patterns