A DECADE OF RECOVERY OF UNDERSTORY VEGETATION BURIED BY VOLCANIC TEPHRA FROM MOUNT ST-HELENS

We examined changes in understory vegetation under an intact forest ca nopy during the first decade following the deposition of tephra (aeria lly transported volcanic ejecta) during the 1980 eruption of Mount St. Helens, Washington State, USA. Objectives were (1) to document vegeta tion response to a major disturbance that has received little attentio n but is widespread and relatively frequent in the northwestern United States, and (2) to analyze vegetation responses in terms of character istics of the disturbance, responses of growth forms as well as those of species, components of vegetation change, and species autecology. W e used permanent plots at four study sites, representing two tephra de pths (approximate to 4.5 and 15 cm), to examine understory vegetation change in old-growth, subalpine conifer forests. The two sites at each tephra depth differed in understory vegetation and amount of snowpack at the time of disturbance. At each site, plant cover and density wer e measured in 100 1-m(2) plots with undisturbed tephra covering the so il surface, in 50 1-m(2) plots from which the tephra was removed in 19 80 (cleared plots), and, at 1 site, in 50 1-m(2) plots from which the tephra was removed in 1982. Values in cleared plots were used to estim ate pre-eruption vegetation composition and to calculate inertia and c omponents of resilience. After a decade, the impact of the tephra was still pronounced: cover was reduced for bryophytes at all sites, for h erbs in deep tephra, and for shrubs where deep tephra had fallen on sn ow. Most cover was contributed by plants that survived the eruption. V ascular species initially absent from the site contributed little to t he vegetation. Seedlings of most herb and shrub species did not surviv e, and many surviving species produced no seedlings. In contrast, at l east two species of weedy, widespread mosses (Ceratodon purpureus and Pohlia annotina) extensively colonized the tephra surface. Although co ver of surviving small trees changed little after the eruption, many s eedlings of the shade-tolerant conifer species that dominated the cano py established and survived well, with the most germinants and highest survival on undisturbed deep tephra. Tsuga spp. constituted a higher proportion of the new seedlings than of the pre-eruption seedling popu lation. A dense layer of conifer saplings appeared to be developing, u nlike any present before the eruption. We calculated inertia (the perc entage of pre-eruption importance remaining after the eruption) and fo ur measures of resilience for each understory growth form. For shrubs, herbs, and bryophytes combined, inertia and one measure of resilience , the importance of the growth form at the end of the decade as a perc entage of pre-eruption importance (called ''c/a''), were highly correl ated; both components increased as tephra depth decreased, as plant si ze increased, and where the snowpack had melted before the eruption, a nd both were higher for species richness and shoot density than for co ver. Inertia for species density decreased with tephra depth and incre ased with plant height. Regressions predicting inertia for single grow th forms included only a single, and different, environmental factor f or each growth form: for shrubs, cover of snowpack; for herbs, tephra depth; and for bryophytes, light intensity. Other measures of resilien ce were less variable than c/a. Although this disturbance did not gene rally favor establishment of new species or totally eliminate species from the system, it did modify greatly the structure, species composit ion, and overall abundance of the forest understory. Recovery to the o riginal state will not occur soon. Even for this example of vegetation recovery in a single stratum, generalities are difficult to state, be cause the relationships of inertia and resilience to growth form and v ariation in the disturbance were so complex. Because the initial damag e played the dominant role in determining understory patterns a decade later, it is critical to focus attention on details of the disturbanc e and mechanisms of survival in order to understand early vegetation r ecovery following tephra deposition.