| Summary: | Understanding the drivers of vegetation change, and interactions between them, is fundamental to understanding ecological systems. I report on the results of three studies in which I investigated the role of competition from <i>Calluna vulgaris </i>or <i>Molinia caerulea </i>and browsing by large herbivores in determining the fitness of birch <i>(Betula pubescens) </i>saplings that are established in heath vegetation. The three studies were: (1) an observational field study to measure the effect of competition and browsing on sapling performance <i>in-situ; </i>(2) an <i>ex-situ </i>pot based study to measure the effects and patterns of these drivers on the growth, morphology and internal N cycling of saplings; and (3) a glasshouse based experiment in which the response of birch to a single aspect of competition (heterogeneity of light availability) was measured. There was a strong effect of deer density, on the growth of the saplings <i>in-situ, </i>indicating the importance of this driver. However, when deer densities (and therefore the amount of browsing damage) were low, sapling growth was influenced by the species of competing vegetation. Specifically, those growing with <i>C. vulgaris </i>were smaller than those growing with <i>M. caerulea. </i>The pattern of competition in the <i>ex-situ </i>experiment concurred with this. There was a negative effect of competition, explained by below-ground interactions. However, differences in the competitive effect to the two species were explained by a facilitative effect of <i>M. caerulea </i>shoots on sapling growth. The effect of competition on sapling growth also influenced internal N cycling due to the effect on source-sink interactions. The ability of birch to forage for light was demonstrated in the glasshouse experiment and may explain the lack of negative effects of above-ground interactions.
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