Jonathan H. Titus

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

. 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.

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.

Proximity, Microsites, and Biotic Interactions During Early Succession

Our studies of succession on mudflows and pumice surfaces at Mount St. Helens support the view that plant succession is determined as much by chance and landscape context as by the characteristics of the site itself. Early primary succession is dominated by the probabilistic assembly of species, not by repeatable deterministic mechanisms. Before most plant immigrants can establish, some physical amelioration in the form of nutrient inputs or the creation of microsites may occur. As vegetation matures, there is a shift from amelioration to inhibition (Wilson 1999), but the magnitude of this shift varies in space and time. Species-establishment order is not preordained as stated by classic succession models (Clements 1916; Eriksson and Eriksson 1998). Life-history traits influence both arrival probability and establishment success, and the best dispersers are usually less adept at establishment. Therefore, interactions between site amelioration and proximity to colonists affect the arrival sequence and initial biodiversity. Unique disturbance events combine with usually low colonization probabilities to produce different species assemblages after each disturbance at a site. Early in primary succession, individuals just accumulate. However, over time, interactions begin that cause species to be replaced. Here we describe how a few struggling colonists slowly developed into pioneer communities (see Tsuyuzaki et al. 1997) and suggest how these communities may develop further.

N-P co-limitation of primary production and response of arthropods to N and P in early primary succession on Mount St. Helens volcano.

Background The effect of low nutrient availability on plant-consumer interactions during early succession is poorly understood. The low productivity and complexity of primary successional communities are expected to limit diversity and abundance of arthropods, but few studies have examined arthropod responses to enhanced nutrient supply in this context. We investigated the effects of nitrogen (N) and phosphorus (P) addition on plant productivity and arthropod abundance on 24-yr-old soils at Mount St. Helens volcano. Methodology/Principal Findings We measured the relative abundance of eight arthropod orders and five families in plots that received N, P, or no nutrients for 3–5 years. We also measured plant % cover, leaf %N, and plant diversity. Vegetation responded rapidly to N addition but showed a lagged response to P that, combined with evidence of increased N fixation, suggested P-limitation to N availability. After 3 yrs of fertilization, orthopterans (primarily Anabrus simplex (Tettigoniidae) and Melanoplus spp (Acrididae)) showed a striking attraction to P addition plots, while no other taxa responded to fertilization. After 5 yrs of fertilization, orthopteran density in the same plots increased 80%–130% with P addition and 40% with N. Using structural equation modeling, we show that in year 3 orthopteran abundance was associated with a P-mediated increase in plant cover (or correlated increases in resource quality), whereas in year 5 orthopteran density was not related to cover, diversity or plant %N, but rather to unmeasured effects of P, such as its influence on other aspects of resource quality. Conclusions/Significance The marked surprising response to P by orthopterans, combined with a previous observation of P-limitation in lepidopteran herbivores at these sites, suggests that P-mediated effects of food quantity or quality are critical to insect herbivores in this N-P co-limited primary successional system. Our results also support a previous suggestion that the availability of N in these soils is P-limited.

Vesicular-arbuscular mycorrhizae influence Mount St. Helens pioneer species in greenhouse experiments

Vesicular-arbuscular mycorrhizae (VAM) are present in most terrestrial ecosystems and play a major role in community structure and function. However, their role in primary succession remains poorly understood. Two greenhouse studies examined the role of VAM in Mount St. Helens pioneer species under three nutrient regimes and four competitive scenarios. Nutrient levels were complete, complete without phosphorus (- P), and tap water (very low nutrient levels). In tap water a negative effect from VAM colonization was observed perhaps due to parasitic action of the VAM fungi. A weak but apparent benefit from VAM occurred in the -P treatment since plants in the -P treatment were usually not less in biomass than those in the complete nutrient treatment and VAM colonization levels were greater in the -P treatment. VAM colonization was more beneficial to plants under the complete nutrient treatment than under the tap water treatment. VAM assisted the facultatively mycotrophic Hypochaeris radicata in competition with the non-mycotrophic Carex mertensii. Lack of VAM improved the competitive ability of Carex mertensii when in competition with facultatively mycotrophic species. However, VAM did not significantly influence competitive outcomes between facultatively mycotrophic species.

Salvage logging and replanting reduce understory cover and richness compared to unsalvaged-unplanted sites at Mount St. Helens, Washington

Abstract The 1980 eruption of Mount St. Helens killed trees in a broad 600-km2 swath north of the crater. Over most of the blast zone, dead trees were salvage logged and Abies procera was planted, except in areas within the Mount St. Helens National Volcanic Monument. We compared salvage-replanted sites and unsalvaged sites in 1 area of the blast zone where the sites were adjacent by using twenty-five 200-m2 plots for each treatment. Salvaged-replanted plots had significantly lower herb and shrub cover, richness, diversity, litter depth, downed woody debris, nitrate, and phosphate. Salvaged-replanted sites also had significantly more stumps, bare area, and moss cover than unsalvaged plots. Soil organic matter and nonnative species cover did not differ. Nonnative species were not important components of any plots. Nitrate, total nitrogen, organic matter, and litter were correlated with the major patterns of species distribution in a canonical correspondence analysis of the salvaged-replanted plots. In the unsalvaged plots, slope, downed woody debris, and elevation were correlated with the major patterns of species distribution.

Microsite effects on forest regeneration in a bottomland swamp in western New York1

Abstract The distribution of woody plant in wetlands may be influenced by topographic microsites that differ in flooding duration, substrate composition, moisture retention, canopy cover, nutrient availability, and seed trapping ability. In order to assess the effects of microsites on woody plant regeneration in Bonita Swamp, a wetland in Western New York, we conducted an assessment of 15 microsite types, a survey of 400 trees, and a woody seedling count using nineteen 100 m2 quadrats in a 4.3 ha study area. Acer × freemanii E.Murr., Fraxinus pennsylvanica Marshall, and Ulmus americana L. dominated the overstory and the often inundated swales occupied 50.3% of the soil surface. Relative to area occupied by each microsite, seedlings were found more often than expected on elevated moss, near elevated moss/soil, near log/root, and swale microsites with 66% of woody seedlings occurring in swales. Acer × freemanii and Fraxinus pennsylvanica occurred more often than expected in the swales. Seedlings appear to prefer moss substrates over soil and wood substrates. Hydrochory may explain the preference of seedlings for “near” microsites onto which wind, water currents and receding spring flood waters deposit floating seeds. Mean percent canopy cover did not vary over the different microsite types, over different species, or between the first and second year (or older) seedlings. Woody seedlings that were two years old or older were found at significantly higher relative elevations and were much less frequent than first year seedlings indicating high mortality and the importance of elevation to survival and establishment. The elevations of microsites are important to woody plant regeneration in this system.

Ski Slope Vegetation of Mount Hood, Oregon, U.S.A.

Ski slope vegetation on Mount Hood, Oregon, U.S.A. was surveyed to assess vegetation that has developed under a constant disturbance regime and the environmental factors that are important in structuring the vegetation. Ski runs extend from 1200 to 2200 m on the south face of Mount Hood. TWINSPAN distinguished 17 plant communities including 4 above treeline and 13 below treeline; 3 of the latter also occur in the forest adjacent to the ski runs. Elevation, which is correlated to temperature, precipitation, depth of snowpack, and timing of snow melt, is the most important variable structuring the vegetation. Soil texture is also important. Distance to the forest boundary influences the vegetation only at lower elevations. Most of the slopes we examined were thickly vegetated, except above treeline where vegetation is typically sparse. Non-native species were detected only at the lowest elevations and were infrequently dominant. Non-native species cover and richness were correlated with percent bare area. R...

SKI SLOPE VEGETATION OF LEE CANYON, NEVADA, USA

Abstract Ski-slope plant communities on Lee Canyon in the Spring Mountains of southern Nevada were surveyed to assess effects of disturbance and environmental factors important in structuring vegetation. Ski runs extended from 2,620 to 2,890 m on the north face of Mount Charleston. Eleven plant communities were identified, including 4 types only on slopes, 1 type only in adjacent forest, and 6 types that occurred on both slopes and in forest. Distance to gravel service roads on the ski runs, soil compaction, ski run width, distance to the forest edge, and gullies were the most important variables in structuring vegetation. Nonnative species were found at all elevations and were frequently dominant. Nonnative species cover and richness were negatively correlated with percent bare area, because the nonnative species vegetated the slopes more densely than the native species. Relative to ski runs in other areas, those on Mount Charleston had high nonnative species invasion and few bare areas; vegetation rarely resembled that of surrounding forests and meadows, except in areas of heavy tree invasion.

Assessing the Reintroduction Potential of the Endangered Huachuca Water Umbel in Southeastern Arizona

The Huachuca water umbel (HWU, Lilaeopsis schaffneriana ssp. recurva) is a federally endangered aquatic perennial plant endemic to southeastern Arizona and northern Sonora, Mexico. The species was listed because of threats posed by the degradation and loss of wetlands throughout its limited range. Although the species is easily grown in a greenhouse, information regarding specific requirements that allow long-term persistence of HWU in natural habitats is lacking, and few efforts to reintroduce this species have been attempted. Using greenhouse-propagated material, we introduced 128 individual HWU plugs into four spring-fed wetland sites near Elgin, Arizona. The sites represent a range of habitat conditions. After two years, overall survival of transplanted plugs was 60% and the area occupied had increased by 845%. This study documented the response of transplanted HWU to periodic drying, disturbance due to scouring and trampling, and sediment deposition. We also examined the number of viable seeds incorporated into a seed bank at the study location in the first season after transplanting. This case study offers a model for watershedwide reintroduction efforts of endangered plants. It also illustrates the importance of low-level disturbance and the necessity of long-term monitoring and maintenance of competing plant species in establishing viable species reintroductions.