| Summary: | The colonisation of a forest-forming tree species into an early successional environment, following long-distance dispersal, presents a unique opportunity to study the dynamics of a founder population in a setting that differs from its origin. This thesis takes advantage of one such rare event and seeks to identify factors and processes determining the population’s successes and failures. Around 1990, mountain birch (Betula pubescens ssp. tortuosa) established on Skeiðarársandur outwash plain in southeast Iceland. Nearest populations, and sources of origin, are ~10 km away. At the time of study, the first generation had recently reached reproductive maturity. Seed densities were high but when all losses were accounted for, only 2.7% of the crop remained viable and germinated. Most externally intact seeds did not contain developed embryos, despite being visually indistinguishable from filled seeds. Externally evident losses amounted to ~45%, mostly due to predation by gall midge larvae (Semudobia betulae). Even so, germinable seeds/m2 ranged from 0.5–75.9. Birch establishment is strongly shaped by microsite quality and extreme early mortality is common. On Skeiðarársandur, seeds were most likely to germinate in thin moss and survivorship was exceptionally high in the first 1–2 years, mostly >50%. Despite Skeiðarársandur’s apparent homogeneity, a comparison to 2008 population data revealed that a decade later, most measured variables (e.g. tree size, plant growth form, and seedling density) showed site-specific divergence. The mountain birch is predicted to act as an ecosystem engineer in this sparsely vegetated environment, leaving a transforming but spatially heterogeneous imprint on the ecosystem. Ilmbjörk (Betula pubescens ssp. tortuosa) nam skyndilega land á Skeiðarársandi um 1990, eftir að fræ hafði dreifst um 10 km leið frá Bæjarstaðarskógi og finnst nú á >35 km2 svæði. Markmið verkefnisins var að nýta þetta einstaka tækifæri til að greina afdrif, viðgang og þróun fyrstu kynslóða einangraðs stofns í ...
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