Early primary succession on the volcano Mount St. Helens

Abstract. Primary succession on Mount St. Helens, Washington State, USA, was studied using long-term observational and experimental methods. Distance from potential colonists is a major factor that impedes early primary succession. Sites near undisturbed vegetation remain low in plant cover, but species richness is comparable to intact vegetation. Sites over 500 m from sources of potential colonists have as many species, but mean species richness is much lower than in undisturbed plots. Cover is barely measurable after 11 growing seasons. Highly vagrant species of Asteraceae and Epilobium dominate isolated sites. Sites contiguous to undisturbed communities are dominated by large-seeded species. For a new surface to offer suitable conditions to invading plants, weathering, erosion and nutrient inputs must first occur. The earliest colonists are usually confined to specific microsites that offer some physical protection and enhanced resources. Primary succession on Mount St. Helens has been very slow because most habitats are isolated and physically stressful. Well-dispersed species lack the ability to establish until physical processes ameliorate the site. Species capable of establishment lack suitable dispersal abilities. Subsequently, facilitation may occur, for example through symbiotic nitrogen fixation, but these effects are thus far of only local importance. Lupinus lepidus usually facilitates colonization of other species only after it dies, leaving behind enriched soil lacking any competitors. Experiments and fine-scale observations suggest that successional sequences on Mount St. Helens are not mechanistically necessary. Rather, they result from local circumstances, landscape effects and chance.     

Early succession on lahars spawned by Mount St. Helens

The effects of isolation on primary succession are poorly documented. I monitored vegetation recovery on two Mount St. Helens lahars (mud flows) with different degrees of isolation using contiguous plots. Seventeen years after the eruption, species richness was stable, but cover continued to increase. That isolation affects community structure was confirmed in several ways. The dominance hierarchies of the lahars differed sharply. Detrended correspondence analysis on Lahar I showed a trend related to distance from an adjacent woodland, whereas vegetation on Lahar II was relatively homogeneous. Spectra of growth forms and dispersal types also differed. Lahar I was dominated by species with modest dispersal ability, while Lahar II was dominated by species with better dispersal. Variation between plots should decline through time, a prediction confirmed on Lahar II. Lahar I remained heterogeneous despite having developed significantly higher cover. Here, the increasing distance from the forest has prevented plots from becoming more homogeneous. At this stage of early primary succession, neither lahar is converging towards the species composition of adjacent vegetation. This study shows that isolation and differential dispersal ability combine to determine initial vegetation structure. Stochastic effects resulting from dispersal limitations may resist the more deterministic effects of competition that could lead to floristic convergence.     

Linking community and ecosystem development on Mount St. Helens

In the two decades following the 1980 eruption of Mount St. Helens in Washington State, the N₂-fixing colonizer Lupinus lepidus is associated with striking heterogeneity in plant community and soil development. We report on differences in nutrient availability and plant tissue chemistry between older, dense patches (core) of L. lepidus and more recently established low density patches (edge). In addition, we conducted a factorial nitrogen and phosphorus fertilization experiment in core patches to examine the degree of N and P limitation in early primary succession. We found that there were no significant differences in N or P availability between core and edge L. lepidus patches during the dry summer months, although nutrient availability is very low across the landscape. In the high density patches we found lower tissue N content and higher fiber content in L. lepidus tissue than in the younger edge patches. The addition of nutrients substantially altered plant community composition, with N addition causing an increase in other forb biomass and a corresponding competition-induced decline in L. lepidus biomass. The majority of the positive biomass response came from Hypochaeris radicata. In the second year of the fertilization experiment, the addition of N significantly increased total community biomass while L. lepidus biomass declined by more than 50%. The response of every species other than L. lepidus to N additions suggests that N may be the macronutrient most limiting plant production on Mount St. Helens but that the gains in productivity were somewhat offset by a decline of the dominant species. By the third year of the experiment, L. lepidus began to increase in abundance with P addition. This result suggests co-limitation of the community by N and P.     

Vegetation development on pumice at Mount St. Helens, USA

This study explores early vegetation development on pumice at Mount St.Helens. We monitored species annually in a grid of 200 contiguous100-m² plots between 1989 and 1999. Of interestwere how vegetation changed and if it became more homogeneous over time.Speciesrichness and cover increased annually. Diversity(H) stabilized by 1996 and began to decline aslong-livedstress-tolerant species such as Agrostis pallens, Carexmertensii and Penstemon cardwellii began todominate. Protected sites had more species and higher cover than did exposedones. Plots next to relict vegetation had more species and cover than diddistant plots. The vegetation initially was dominated by species with gooddispersal, but subsequently those with poor dispersal became dominant. Wecompared species expansion patterns to a model based on random colonization.Theresults implied that populations with poor dispersal derived from a fewcolonists that then produced seeds for local expansion. Detrendedcorrespondenceanalysis showed a pronounced shift in species composition. This analysis alsoshowed that species composition was becoming more homogeneous over time.However, significant heterogeneity remained and some plots are diverging fromothers. As yet, this vegetation is not developing towards a regional vegetationtype. Rather, an unusual community with Agrostis spp.,Carex spp., Penstemon cardwellii, Lupinuslepidus, Anaphalis margaritacea and Salixcommutata has developed. The accumulation phase of primarysuccessionis nearly complete. The next phase, in which vertical structure develops asSalix and conifers mature, has scarcely begun. It shouldbemarked by the invasion of forest understory species and loss of subalpinemeadowspecies. Assembly rules based on biotic interactions may then become evident.     

Early primary succession on Mount St. Helens,Washington, USA

Abstract. The north slope of Mount St. Helens was sampled with 141 circular 100-m² plots to describe vegetation and environmental patterns 13 yr after the 1980 eruption. At least 114 vascular plant taxa were encountered. We recognized four habitat types: Refugia, Pumice barrens, Pyroclastic surfaces and Drainages. A fifth category, Lupine patches, includes samples on primary surfaces that were rapidly colonized. Refugia provided small enclaves where underground portions of several species survived the eruption. They retained an inconsistent array of forest understory species and contained 86 species (mean = 20.8 per plot). Refugia are dominated by woody species such as Penstemon cardwellii, Rubus spp., Ribes spp. and Alnus sinuata, with herbs such as Agrostis diegoensis, Luzula parviflora and Anaphalis margaritacea. Anaphalis represents a suite of species that invaded Refugia after the eruption. Diversity (N₂ and H′) is significantly greater in Refugia than in any other habitat. No plants survived on primary sites, which remain sparsely vegetated and dominated by readily dispersed taxa. Total richness ranges from 36 species (9.9 per plot) on pyroclastic surfaces, through 42 species (11.2 per plot) in drainages, to 66 (11.7 per plot) on Pumice barrens. H′ and N₂ of the three habitats do not differ significantly. Lupine-dominated vegetation occurs sporadically in Pyroclastic and drainage habitats. Lupine patches are characterized by high Lupinus cover and a suite of invaders. These sites have high cover and 52 species (12.6 per plot). H′ and N₂ scores were significantly lower than any other habitat due to strong lupine dominance. Canonical Correspondence Analysis showed that site history and slope contributed most to species composition. Geographic effects accounted for 10 25 % of the explained species-environment relationship. Forest understory species have migrated only short distances and have made negligible contributions to vegetation development. A few species common in Refugia, including Agrostis diegoensis and Carex mertensii, have invaded barren surfaces, but most have not. Refugia also have been invaded by open site species abundant on the Pumice Plain. The heterogeneity of plots within habitat types and small statistical linkage of vegetation to environmental and spatial factors suggests that stochastic events have played a leading role in early primary succession.     

Recovery of forest understories buried by tephra from Mount St. Helens

To determine the effects of tephra (volcanic aerial ejecta) on forest understory plants, six sites were chosen along a tephra depth gradient (23 to 150 mm) northeast of Mount St. Helens, USA. All sites were in old forests beyond the limits of direct blast damage from the volcanic eruption. At each site, 150 one m² plots were permanently marked; all tephra was removed from 50 of these in 1980. Cover and density of plant species were recorded during 1980, 1981, and 1982.Tephra 23 mm deep had almost no effect on cover and density of vascular plants, and reduced bryophyte cover for only two years. Tephra 45 mm deep destroyed almost all bryophytes. Although damaged by 45 mm tephra, deciduous herbs recovered by 1982, but some evergreen species did not. Tephra 75 mm deep reduced herb cover in 1982 to 32% and density to 26% of that in cleared plots. At two sites with an average tephra depth of 150 mm, almost all herbs were eliminated except in microsites where tephra was thin, but shrub abundance was greatly reduced only where snow had been present during tephra deposition. Almost all cover was contributed by plants established previous to the eruption; seedling cover never exceeded 0.2%. Refugia with thin tephra, resulting from erosion, were vital to the survival of many species, especially bryophytes.Nomenclature of vascular plants follows Hitchcock & Cronquist (1973); moss nomenclature follows Lawton (1971).We thank T. Hill, C. Halpern and B. Smith for field assistance. The USDA Forest Service, and especially J. F. Franklin, facilitated entry into the restricted area around Mount St. Helens. This word was supported by the National Science Foundation, USDA Science and Education Administration, and the U.S. Forest Service Pacific Northwest Forest and Range Experiment Station.     

Seedling establishment in different microsites on Mount St. Helens,Washington, USA

On the volcanically devastated Pumice Plain of Mount St. Helens, plant species colonized microsites differentially. Peak colonization did not occur in the same microsites as peak establishment and growth. In addition, observed microsite colonization patterns differed between years. Two studies were conducted. The first assessed seedling establishment and growth from seeds sown at different microsites. The second assessed colonization into four microsite types that were constructed on the Pumice Plain. Hypochaeris radicata was the most common species to survive when the same number of seeds of four species were planted; however, Anaphalis margaritacea was the most common colonizer of microsites. Microsites with the largest biomass plants in the first study generally had the highest colonization in the second study. Sites that do not possess features to trap seeds, such as flats and ridges, are not opportune places for a plant to grow since there is little microclimatic or substrate amelioration. Thus, flat microsites had low biomass in the establishment experiment due to the lack of amelioration and contained few plants in the colonization experiment due to a lack of seed trapping mechanisms. These results show that in the primary successional landscape of Mount St. Helens microsites are critical to revegetation dynamics. Changes in the pattern of microsite colonization between years emphasizes the dynamic nature of the landscape and the important influences of climate, substrate amelioration and seed rain to plant establishment.     

Increasing deterministic control of primary succession on Mount St. Helens,Washington

Question: Does the relative importance of stochastic and deterministic factors change during primary succession? Location: Small depressions (potholes) located on Mount St. Helens, Washington (46°13′51″N, 122°09′10″W, 1290 m). Methods: Pothole vegetation was described in 1993, 1997 and from 2001 to 2008. Explanatory variables included location and elevation (spatial factors), soil factors and Lupinus lepidus cover from prior years (a fertility surrogate). RDA assessed species‐variable relationships. DCA calculated β diversity and within‐year heterogeneity. Flexible sorting classified the vegetation. Species composition, richness, cover, H′ and evenness were also calculated. Results: Vegetation cover increased through 2001, and then fluctuated due to changes in L. lepidus cover. Richness peaked in 2005, after which pioneer species began to decline as persistent evergreen species increased. The six CTs recognized in 2008 were more scattered than were the six different CTs from 2001. DCA demonstrated that woody and rhizomatous species increased as pothole vegetation became less variable. RDA revealed weak spatial relationships in 1993, 1997 and 2001; thereafter, environmental and biological factors became important. The species‐explanatory data relationship increased during this study from 10.2% to 36.0%, leaving 64.0% of the variation unexplained. Conclusions: This is the first temporal study to demonstrate that deterministic control of vegetation development increases during succession. Pothole vegetation has converged somewhat due to deterministic factors, but the initial effects of chance, local disturbances and history remain large and may prevent strong convergence.     

The role of mycorrhizal fungi and microsites in primary succession on Mount St. Helens

This study was designed to examine the role of vesicular-arbuscular mycorrhizae (VAM) and microsites on the growth of pioneer species. Flat, rill, near-rock, and dead lupine microsites were created in plots in barren areas of the Pumice Plain of Mount St. Helens. VAM propagules were added to the soil in half of the plots. Six pioneer species were planted into both VAM and non-VAM inoculated microsites. Plants in dead lupine microsites were greater in biomass than those in flat, rill, and near-rock microsites. Significant effects of VAM on plant biomass did not occur. Microsites continue to be important to plant colonization on the Pumice Plain, but VAM do not yet appear to play an important role. This may be due to limited nutrient availability and the facultatively mycotrophic nature of the colonizing plant species. It is unlikely that VAM play an important role in successional processes in newly emplaced nutrient-poor surfaces.     

Gradients in compositional variation on lahars,Mount St. Helens,Washington, USA

How isolation affects primary succession remains unresolved. Our hypotheses are: 1) the rate of succession is slowed within 50 m from sources and 2) dominance increases more rapidly if the growing season is longer. We sampled lahar vegetation near conifer forests on Mount St. Helens 23 years after the eruption using transects (Muddy) and grids (Butte Camp, BC). Transects were compared to isolated plots of the same age, while grids were compared to vegetation that was 400 years older. Cover declined with distance and with elevation on the Muddy due to denser seed rain near forests and shorter growing seasons at higher elevation. On BC-1, next to a forest, richness measures increased with distance, while cover decreased with elevation. On BC-2, more exposed than BC-1, mean richness and cover declined with elevation, but increased towards seed sources. Equitability increased with distance and with elevation on more stressful transects and on BC-1. Percent similarity (PS), a measure of floristic heterogeneity, declined with elevation on Muddy-SW. BC-1 PS decreased with both distance and elevation. Data from grids collected since 1987 showed that both sites became more homogeneous through time. Directional changes on BC-1 were greater than on BC-2, while annual variation of DCA scores declined only on BC-2. These differences are related to conifer dominance patterns. Succession is influenced by isolation, which controls the seed rain, and by stress, which controls establishment, the rate of biomass accumulation, and the spread of immigrants. Although community development is governed by environmental factors, this study shows that the effects of dispersal that result from distance factors can persist and may be a source of unexplained variation in mature communities.     

Impact of Mount St. Helens Eruption on Bacteriology of Lakes in the Blast Zone

Lakes lying within the blast zone of Mount St. Helens showed dramatic increases in heterotrophic bacterial numbers after the eruption of 18 May 1980. The total microscopic counts of bacteria in some of the most severely affected lakes were more than 10⁷ cells per ml, an order of magnitude above the counts in outlying control lakes. Likewise, the numbers of viable bacteria reached levels of more than 10⁶ cells per ml, compated with fewer than 10⁴ cells per ml in control lakes. The CPS medium used for enumeration provided growth of up to 81.5% of the bacteria during sampling of one of the blast zone lakes. The high numbers of bacteria and the efficacy of the viable enumeration procedure are evidence that the lakes have been transformed rapidly from oligotrophy to eutrophy due to the eruption and its aftermath. Organic material leached from the devastated forest vegetation is thought to be responsible for the enrichment of heterotrophs. Total coliform bacteria were found in all of the blast zone lakes, and some lakes contained fecal coliform bacteria. Klebsiella pneumoniae was the predominant total coliform and was also identified as one of the fecal coliform bacteria, although Escherichia coli was the predominant species in that category. Our data indicate that bacterial populations peaked in the outer blast zone lakes in the summer of 1980 and in most of the inner lakes during the summer of 1981.     

Primary succession trajectories on a barren plain,Mount St. Helens,Washington

Questions: Have predictable relationships between environmental variables and vegetation developed in primary succession following a volcanic eruption? Has the rate of succession changed? Have vegetation trajectories converged or diverged? Location: The Abraham Plain of Mount St. Helens, Washington, USA (46°12′42″N, 122°08′27″W, elevation 1360 m), was sterilized in 1980 by a blast, scoured by lahars and buried by pumice. Method: We monitored 400 100 m² contiguous permanent plots annually (1988–2008), and classified each plot from every year into ten community types (CTs). We characterized the terrain by topography and surface features. Redundancy analysis assessed relationships between vegetation and possible explanatory variables, which included sample location. We used detrended correspondence analysis (DCA) to assess successional rates and trends. Results: Relationships between species composition and explanatory variables were only significant after 1996, when position and presence of rills became significant. By 2006, explained variation remained low (13%) but significant. Species accumulated slowly, restricted by stress and isolation. Changes in mean DCA position slowed. Composition shifted from pioneer to persistent species and vegetation became more stable with time. Species accumulated for two decades and then stabilized, while cover has continued to increase. Diversity increased and then declined slightly as dominance developed and pioneer species became less common. Conclusions: We demonstrate weak but increasingly predictable trends in species composition using environmental variables. The rate of succession slowed and trajectories formed a reticulate network of transitions dominated by divergence. Convergence was not evident because vegetation responded distinctively to minor topographic features that allowed alternative stable communities to develop.     

Trait and phylogenetic patterns reveal deterministic community assembly mechanisms on Mount St. Helens

Plant Ecology - Primary plant succession provides an excellent natural experiment to test ecological questions about community assembly following major disturbances. Temporal phylogenetic and...     

Seedling establishment patterns on the Pumice Plain,Mount St. Helens,Washington

Abstract. We examined the factors that control seedling establishment on barren substrates on the pyroclastic flows from Mount St. Helens. From June to September in 1993, we monitored seedling and microhabitat changes in 240 20 cm × 20 cm quadrats on the Pumice Plain. Seedlings emerged in 104 quadrats (43.3 %). The most abundant species were Anaphalis margaritacea, Hypochaeris radicata, Lupinus lepidus and Epilobium angustifolium. Measured site characteristics included topography, particle size distribution, ground surface movements, soil water content, organic matter, pH, and presence or absence of dead lupines. Quadrats with seedlings had higher cover of dead lupines, higher amount of rock and gravel substrate, and a greater cover of rills. More seedlings emerged where eroded material accumulated. Compared to coarse-textured surfaces, silt surfaces had higher organic matter, held more water, and showed higher pH. However, seedlings became established more frequently on coarse-textured surfaces. In greenhouse experiments, a higher percentage of Hypochaeris seeds germinated on silt than on sand or gravel. The germination of Anaphalis and Epilobium did not differ with soil texture, but was higher at higher moisture levels. Seedling colonization is more dependent on ground surface microtopography, particle size, and ground movement than on the chemical status of these volcanic deposits.     

The role of refugia and dispersal in primary succession on Mount St. Helens,Washington

Abstract. An intense lateral blast devastated Mount St. Helens in 1980, but forest understory species survived in some north‐slope ‘refugia’. We explored the effects of refugia on colonization of barren pumice in 1997 and 1998, 18 yr after the eruption. The seed rain of 23 colonizers came mostly from populations that had previously established in refugia. Parachutists had small, vagile seeds, parasailors had winged seeds, and tumblers were blown along the ground. The latter two groups are heavier and dispersed more slowly, but are more likely to survive. The proportion of the vegetation represented by wind‐dispersed species increased with distance from refugia. Parachutist's density declined with time and proximity to refugia. As vegetation adjacent to refugia developed, populations of parasailors and tumblers expanded, foreshadowing their dominance in more remote pumice. Refugia played a critical role in determining the rate and course of succession by providing fertile islands that permitted pioneers and dry meadow species to establish near barren pumice. Species that survived in refugia played a negligible role in colonization. This study showed that when refugia contrast sharply with new substrates, they accelerate recovery by facilitating the invasion of pioneer species.     

Early primary succession on a barren volcanic plain at Mount St. Helens,Washington

The invasion pattern on a barren plain on the eastern flank of Mount St. Helens, Washington, devastated by the 1980 volcanic eruption was monitored between 1988 and 1992. All vascular plants on a grid consisting of 400 contiguous 100-square-meter quadrats were recorded with a cover score. The substrate was initially homogeneous, but significant heterogeneity had developed by 1988. Vascular plant species richness increased from 24 in 1988 to 41 in 1992. Mean species richness per quadrat increased from 0.44 to 5.71, mean cover increased from 0.04% to 0.51%, and mean diversity index (H') increased from 0.08 to 1.56. A variance/mean test of species richness pattern showed that invasion occurred sporadically since plots tended to have either several or no species. By 1992, mean species richness was more evenly distributed. Most seedlings continue to result from long-distance dispersal, but reproductive colonies of species are developing. Seedling distributions are controlled by microsites. Eleven common species strongly and similarly preferred safe-sites created by small rocks, undulations, or rills. However, many apparent safe-sites are empty, suggesting that seeds are scarce and that even the most favorable microsites are marginal. The niches of these species seem to overlap broadly. The Plains of Abraham is in the earliest stage of primary succession. The detailed invasion pattern permitted us to distinguish species still dependent on immigration from those now locally established. Pronounced microsite preferences emphasize that physical amelioration (e.g., nutrient input and erosion) must occur before further succession can commence. We have observed the early stages of succession where an inhospitable site is gradually and heterogeneously changed into a habitat where safe-sites do not limit succession, but where stochastic factors remain important.     

Low Frankia inoculation potentials in primary successional sites at Mount St. Helens, Washington, USA

The ability of 23 year old volcanic soils from Mount St. Helens, USA, to nodulate actinorhizal Sitka alder (Alnus viridis ssp. sinuata) was studied by estimating Frankia Inoculation Unit density (FIU) in four types of primary successional communities using a host-plant bioassay. Rhizospheric Mycelial Inoculation Unit (MIU) density and the effects of inoculation with alder soil and phosphorus (P) on seedling growth were also examined. FIU and MIU were highest in alder thickets, lower in lupin patch and riparian zone sites, and lowest, with no FIUs, in bare areas. Seedling dry mass was higher in treatments inoculated with alder soil but not with P addition, while root:shoot mass ratios decreased with both inoculation and P. The extremely low densities of suitable symbionts in sparsely vegetated primary successional areas at Mount St. Helens may explain the surprisingly slow rate of spread of alder from nearby seed sources.     

Trophic Interactions during Primary Succession: Herbivores Slow a Plant Reinvasion at Mount St. Helens

Lupines (Lupinus lepidus var. lobbii), the earliest plant colonists of primary successional habitats at Mount St. Helens, were expected to strongly affect successional trajectories through facilitative effects. However, their effects remain localized because initially high rates of reinvasive spread were short lived, despite widespread habitat availability. We experimentally tested whether insect herbivores, by reducing plant growth and fecundity at the edge of the expanding lupine population, could curtail the rate of reinvasion and whether those herbivores had comparable impacts in the older, more successionally advanced core region. We found that removing insect herbivores increased both the areal growth of individual lupine plants and the production of new plants in the edge region, thereby accelerating the lupine's intrinsic rate of increase at the front of the lupine reinvasion. We found no such impacts of herbivory in the core region, where low plant quality or a complex of recently arrived natural enemies may hold herbivores in check. In the context of invasion theory, herbivore-mediated decreases in lupine population growth rate in the edge region translate into decreased rates of lupine spread, which we quantify here using diffusion models. In the Mount St. Helens system, decreased rate of lupine reinvasion will result in reductions in rates of soil formation, nitrogen input, and entrapment of seeds and detritus that are likely to postpone or alter trajectories of primary succession. If the type of spatial subtleties in herbivore effects we found here are common, with herbivory focused on the edge of an expanding plant population and suppressed or ineffective in the larger, denser central region (where the plants might be more readily noticed and studied), then insect herbivores may have stronger impacts on the dynamics of primary succession and plant invasions than previously recognized.     

Colonization genetics of an animal-dispersed plant (Vaccinium membranaceum) at Mount St Helens, Washington

Population founding and spatial spread may profoundly influence later population genetic structure, but their effects are difficult to quantify when population history is unknown. We examined the genetic effects of founder group formation in a recently founded population of the animal-dispersed Vaccinium membranaceum (black huckleberry) on new volcanic deposits at Mount St Helens (Washington, USA) 24 years post-eruption. Using amplified fragment length polymorphisms and assignment tests, we determined sources of the newly founded population and characterized genetic variation within new and source populations. Our analyses indicate that while founders were derived from many sources, about half originated from a small number of plants that survived the 1980 eruption in pockets of remnant soil embedded within primary successional areas. We found no evidence of a strong founder effect in the new population; indeed genetic diversity in the newly founded population tended to be higher than in some of the source regions. Similarly, formation of the new population did not increase among-population genetic variance, and there was no evidence of kin-structured dispersal in the new population. These results indicate that high gene flow among sources and long-distance dispersal were important processes shaping the genetic diversity in this young V. membranaceum population. Other species with similar dispersal abilities may also be able to colonize new habitats without significant reduction in genetic diversity or increase in differentiation among populations.