Flowering and seedling production of understory herbs in old-growth forests affected by 1980 tephra from Mount St. Helen

摘要

To determine the variation in flowering among species and the contribution of flowering to the recovery of forest herbs, we counted the flowering shoots, vegetative shoots, and first-year seedlings in old-growth forests affected by tephra from the 1980 eruption of Mount St. Helens. Using permanent 1A m super(2) plots with either undisturbed tephra or with the tephra removed, we obtained 2-9A years of flowering data during 1980-2000 for six sites that received 2-15A cm of tephra. Flowering was infrequent for most species. Most of the 12 commonest species had 2%-4% of their shoots flowering (range 0%-15%). Among growth forms, deciduous nonclonal species flowered most. Flowering percentage increased with plant density; thus microsites favorable for growth were also favorable for flowering. Flowering varied considerably among sampling years. Incidence of flowering was higher on shallow than on deep tephra, and higher on natural tephra than in cleared plots. Seedlings were more common on shallow than on deep tephra. The 20-year increase in shoot density on tephra (ratio of 2000 to 1981 values) was positively related among species to the number of seedlings produced per previous-year flowering shoot. Our results indicate how species vary in the quantity and timing of flowering and also in their reliance on seedling establishment versus clonal growth for population increase following disturbance.