| Summary: | The aim of this thesis was to study patterns of reproduction of Betula pendula and B. pubescens coll. along an altitudinal, coastal-inland, gradient in northern Sweden. The altitudinal variation was related to the distribution of the birch taxa along the gradient. Six years field data showed a steep decrease of seed germinability of B. pendula towards its altitudinal range limit in the Scandes every year. In contrast, B. pubescens ssp. pubescens showed significant positive correlations between seed germinability and altitude in three of the six years. Furthermore, there was a highly significant positive correlation between seed weight and altitude for B. pubescens coll., but not for B. pendula. Production of viable seeds fluctuated strongly between years in most populations, except in marginal B. pendula populations in the mountain area where it was constantly very low. On average 15 - 41 % of the seeds produced in B. pendula populations above 400 m altitude were attacked by gall midges (Semudobia ssp.). Corresponding values for B. pendula populations below 400 m altitude were 4 - 7 %. In B. pubescens populations, the seeds attacked by Semudobia ssp. never exceeded 3 %. The high frequency of Semudobia attackes in high altitude marginal B. pendula populations was suggested to be due to limited resources for defense against the seed predator. A 3-yr study documented large variations in pollination and seed quality between taxa, high and low altitude populations, and between years. Empty seeds (without embryos) dominated among the sampled seeds in most cases. This proportion was decreased by pollen addition, in both B. pendula and B. pubescens, in mountain populations, but not in coastal populations. The high percentage of empty seeds was therefore suggested to be partly caused by pollen-limitation, but failure of pollen tube penetration - fertilisation, or maternal resource supply could also have had an influence. A laboratory experiment showed increased pollen germination and length of the longest pollen tube per ...
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