Community- and species-level consequences of competition in an unproductive environment: an experimental approach using boreal forest understory vegetation

In this thesis, I describe three experimental studies that investigate the hotly debated role of competition in structuring communities in unproductive habitats. The studies were done in a boreal forest understory plant community in the southwestern Yukon. The first study was a traditional neighbour removal experiment. Ten of the most common species were transplanted as seedlings into transects with and without neighbours in a factorial design with two levels of water addition and two levels of fertilizer addition. The presence of neighbours increased survival and biomass of 6 species indicating a facilitative effect of neighbouring plants. The second study used the Community Density Series (CDS) methodology. The first of these was a 10-speciesexperimental community established from seed and grown in sandboxes at 6 densities with 2 watering levels and 2 fertilizer levels in a factorial design. At the community level, density dependence was observed at all life stages, but was not consistently competitive or facilitative - both emergence and final per plant shoot mass were density dependent, while survival to the end of the season was inversely density dependent. The effect of water was positive at seed emergence whereas fertilizer negatively affected survival. Species specific responses were also dependent on life stage. The final study was a 4-year CDS in the field using 9 common understory species at 6 densities and 3 fertilizer levels. Density negatively affected the community every year except for the first with competition being important at all densities above x1/8th the average community density. Constant final yield was reached in plots above the naturalx1 density for the last two years of the study. Responses to density were species-specific and 7 species declined with increasing density. No facilitative effects were observed. These studies demonstrate that density dependence is important in structuring this unproductive boreal understory habitat. The CDS approach allows us to quantify both the intensity and importance of plant competition at the community and species levels and to determine whether the importance of these biotic interactions depend on abiotic factors. The results clearly show that species-specific responses to biotic interactions are not necessarily the same as community level responses and if we are to understand community structure, it is necessary to use appropriate methodologies.Science, Faculty ofBotany, Department ofGraduat