Flowering Patterns of Understory Herbs 30 Years after Disturbance of Subalpine Old-Growth Forests by Tephra from Mount St. Helens

Abstract

Premise of research. We sought to determine the role of flowering in recovery of understory herbs from a major disturbance and to determine the effects of plant and environmental factors on flowering patterns.Methodology. We counted flowering and nonflowering shoots in permanent plots eight to 10 times over a 30-year period for all 48 understory herb species in four subalpine old-growth conifer forests that received tephra (aerially transported volcanic ejecta) from the 1980 eruptions of Mount St. Helens, Washington. We defined two measures of flowering and related them to environmental and plant characteristics.Pivotal results. Patterns of flowering varied widely among species both among and within growth forms. Flowering increased with time, especially where it was initially low (for evergreen clonal plants, species that also grow in early seral habitats, and plants in deep tephra and from herb-poor sites). Some significant differences that occurred during the first 20 years disappeared by year 30. Percent of shoots flowering declined as shoot density increased, but the significance of this relationship declined until it became nonsignificant by year 25. There was a significant but weak relationship between the proportion of shoots flowering for a species and its proportional increase in shoot numbers during the 30 years of vegetation redevelopment; some species expanded populations only by flowering, while others became dominant while flowering little.Conclusions. Flowering patterns changed with time; this constitutes an important aspect of successional change. Studies of flowering and other aspects of sexual reproduction are important for understanding mechanisms of succession.