The northern side of Mount St. Helens violently erupted in May 1980 providing a unique opportunity to study how foundational species drive plant community change during primary succession. We seek to understand which abiotic and biotic factors imposed by the slowly establishing pioneer conifer, Abies procera, most strongly influences succession. Understanding which competitive and facilitative interactions created by this species will provide insight into mechanisms that drive the transition to a woody forest. To test which facilitative and competitive factors influence plant communities associated with pioneering Abies procera (Noble Fir), forty experimental plots were established across an elevational gradient. Each macro-plot consisted of a control and four equidistant experimental plots; a naturally established Abies procera tree, and three experimental plots that have carbon, nitrogen, or shade additions. For carbon addition plots, Abies procera needles were collected, dried, weighed and dispersed to each plot. For shade addition plots, a shade netting was applied early in the growing season. For nitrogen addition plots, the soil will be treated with a nitrogen supplement based on natural nitrogen levels found close to Abies procera trees. Plant species abundance and coverage was assessed at peak growth.
Results/Conclusions
Species richness, evenness, and community similarity (non-metric multidimensional scaling, NMDS) was completed using R statistical program. Species richness was statistically significantly higher in sites of lower elevation (p<0.05). Across high and low elevation sites, there was no significant difference in species richness between experimental treatment plots. Plots treated with shade proved to have no significant species evenness when compared to the control, despite visual observations that the vegetation appeared less drought stressed. The evenness was higher in shade cloth plots than Abies procera tree plots in the lower elevation, which was statistically significant (p=0.022). NMDS results showed dissimilarity between high and low elevation plots. This was likely driven by a large abundance of Lupinus lepidus in the high elevation plots. NMDS results showed a strong similarity between experimentally manipulated plots and a large dissimilarity between those and the established tree. Understanding how woody pioneer species influence surrounding plant community composition change over time will provide insight into the successional trajectories on Mount St. Helens. These insights of community assembly manipulation may inform restoration efforts of other previously forested communities following disturbance.