SEEDLING ESTABLISHMENT IN FORESTS AFFECTED BY TEPHRA FROM MOUNT ST. HELENS

Seedling density on permanent plots at five sites was monitored for the first four summers following the deposition of 4.5 to 15 cm of tephra from the 1980 eruption of Mount St. Helens. Because the old-growth forests at the sites were not destroyed by the volcanic eruption, the plots were under a normal tree canopy. Almost no seedlings established in 1980. By 1983 tree seedling density exceeded 35 m^−-2 at all sites. Tsuga heterophylla seedlings were most common, with Abies amabilis seedlings next in abundance. A dense layer of small trees may develop on the tephra and long-term forest dynamics could be affected. There was no successful invasion by species not already present in the stands. Seedlings of some forest herbs, especially Tiarella trifoliata, were common, but many other common forest species produced none. It is still problematical if or when these species will return to areas of the forest from which they were extirpated.     

Spatially structured herbivory and primary succession at Mount St Helens: field surveys and experimental growth studies suggest a role for nutrients

Abstract. 1. The 1980 eruption of Mount St Helens (Washington, U.S.A.) created a 60‐km² region of primary successional habitat. Since colonising in 1981, the spatial spread of the legume Lupinus lepidus at Mount St Helens, Washington, U.S.A., has afforded intriguing opportunities to study the effect of trophic dynamics on primary succession. 2. Insect herbivory on this lupine has exhibited striking spatial structure for over a decade, with inverse density‐dependent damage patterns occurring over both small (10–100 m) and large (1–10 km) spatial scales. 3. Hypothesising that lupine nutritional chemistry might underlie the spatial patterns in herbivory, the distribution of elemental macronutrients (nitrogen, phosphorus) across the landscape was characterised. 4. Samples of soil and lupine tissue (roots and leaves) were collected from sites along both local and regional gradients in lupine density. On both large and small spatial scales, lupine leaves from low‐density conditions were significantly more nutrient rich. 5. In addition, in a laboratory growth study native lepidopteran herbivores that specialise on lupines (Gelechiidae: Filatima sp.) performed better when fed leaves from low‐density, high‐nutrient lupines than on diets of low‐nutrient lupine leaves from high‐density areas a few metres away. 6. These data suggest that spatial heterogeneity in lupine nutrient chemistry may underlie the remarkable herbivory gradients witnessed at Mount St Helens.     

Lupine influence on soil carbon,nitrogen and microbial activity in developing ecosystems at Mount St. Helens

Summary Lupine influence on soil C, N, and microbial activity was estimated by comparing root-zone soil (LR) to nonroot-zone soil (NR) collected at Mount St. Helens. Samples were collected from 5 sites forming a gradient of C and N levels as a reflection of different locations and varying volcanic disturbance by the 1980 eruption. In volcanic substrates undergoing primary ecosystem development, C and N levels were low, as would be expected, but higher in LR soil than NR soil. At the least disturbed sites, N was only slightly greater in LR soil whereas significantly less C was observed in LR soil than in surrounding NR soil. Inorganic-N concentrations were small at all sites but comprised a significant proportion of the total amount of soil N in volcanic substrates. In general, LR zone soil contained significantly more NH _inf4 ^sup+ –N. The addition of glucose increased respiration in soils from all sites with the greatest relative response in volcanic soil from the low end of the C and N gradient. Active soil microbial biomass-C and cumulative respiration were correlated with C and N and were significantly greater in LR soil than in NR soil for all sites. These results are consistent with some expected trends in ecosystem development and demonstrate the significance of resource dynamics and lupines in determining patterns of ecosystem response to disturbance at Mount St. Helens. They also suggest that processes leading to soil heterogeneity can be related to either development or to degradation depending on the context of the specific ecosystem or resource under consideration.     

DEATH OF STEPPE CRYPTOGAMS UNDER THE ASH FROM MOUNT ST. HELENS

Terrestrial mosses and lichens in the steppe of Washington were buried under volcanic ash from the 18 May 1980 eruption of Mount St. Helens. Unlike adult vascular plants in these communities, these cryptogams died following the deposition of silt-size ash. Death within as little as four months was demonstrated by anatomical deterioration and these organisms' inability to fix CO₂, to fluoresce in IR radiation, and to absorb vital stains. Although cryptogams in the steppe have probably been destroyed repeatedly by ash falls during the Holocene, this recent destruction is the first since the arrival of alien plants in the nineteenth century. Death of cryptogams may allow the initiation of further colonization by alien plants because a cryptogamic crust forms a barrier to seedling establishment.     

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.     

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.     

Species-specific acclimation to strong shade modifies susceptibility of conifers to photoinhibition

Photoinhibitory processes in the photosynthetic apparatus of the seedlings of Abies alba (Mill.), Picea abies (Karst.), and Pinus mugo (Turra) growing under strong shade (5 % of full solar irradiance) or full irradiance conditions were investigated in winter and spring using chlorophyll a fluorescence techniques. The extent of photoinhibition in needles as indicated by a decrease in maximum quantum yield of PS II photochemistry (F_v/F_m) depended on species, air temperature and acclimation to the light environment. Unexpectedly, shade-tolerant Abies alba was less affected by low-temperature photoinhibition compared to the other species. F_v/F_m recovered with increasing air temperature. During winter, the seedlings of Picea abies growing in shade showed higher F_v/F_m than those from full light. Non-photochemical quenching of fluorescence (NPQ) measured at the same levels of actinic light was higher in needles acclimated to full light except for Abies alba in February. Photosynthetic performance in term of ETR (apparent electron transfer rate) was also higher in full light-acclimated needles. In April, at ambient temperature, recovery of PS II efficiency from the stress induced by illumination with saturating light was faster in the needles of Picea abies than in those of Abies alba. The shade-acclimated needles of Abies alba and Picea abies showed greater down-regulation of PS II induced by high light stress.     

Legionella in aquatic habitats in the Mount Saint Helens blast zone

Illnesses of undiagnosed etiology among researchers exposed to lakes and streams in the Mt. St. Helens blast zone after the 18 May 1980 eruption prompted us to determine the occurrence and potential virulence ofLegionella (Legionnaries' disease bacteria) in aquatic habitats near Mt. St. Helens during the summers of 1981 and 1982. Concentrations ofL. pneumophila, L. micdadei, L. gormanii, L. dumoffii, andL. bozemanii, determined by microscopic counts using direct immunofluorescent staining, ranged from <10⁴ to 10⁵ cells/l in lakes and rivers outside the Mt. St. Helens blast zone while the numbers ofLegionella in aquatic habitats inside the blast zone were from 10⁵ to 10⁷ cells/l.Legionella numbers were consistently highest in North Coldwater and Spirit lakes, which received water from hydrothermal seeps.Legionella pneumophila serogroups 4 and 6 were isolated from North Coldwater Lake in 1981 and from South Coldwater Creek in 1982, indicating that potentially virulent strains ofLegionella persist in aquatic habitats in the blast zone of Mt. St. Helens.Technical paper no. 6923, Oregon Agricultural Experiment Station.     

CONSPICUOUS ALGAL COLONIZATION OF THE ASH FROM MOUNT ST. HELENS1

Conspicuous growth of green and yellow-green algae developed in arid steppe communities in eastern Washington on the silt-sized ash from the 18 May 1980 eruption of Mount St. Helens. In a veneer on the ash, cell numbers per gram dry ash of these aerial immigrants equaled or exceeded the highest reported values for terrestrial chlorophytes (10⁸ cells · g^?1 dry soil). Such pronounced growth was restricted largely to three microsites (under the canopy of Artemisia tridentata, along the runways of the vole, Microtus montanus, and surrounding emergent agarics), apparently in response to water-holding properties of the ash, localized nutrient input and possible reduction of predation by nematodes. Highest algal counts were in Microtus runways where daily water and urea-N input per adult animal may be 8 mL and 23 mg, respectively. Cyanophytes, commonly considered early colonizers of volcanic ash, were extremely rare. The role of blue-green algae in succession on volcanic materials may be more restricted geographically than previously recognized.     

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.     

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.     

SHOOT STRUCTURAL EFFECTS ON NEEDLE TEMPERATURES AND PHOTOSYNTHESIS IN CONIFERS

Greater needle packing (number of needles per unit stem length) among shoots of Picea engelmannii (Parry ex. Engelm.), Abies lasiocarpa ([Hook] Nutt.), and Pinus contorta (Engelm.) collected at sun-exposed locations resulted in leaf temperatures (T₁) that were well above air temperature (T_a) and which were closer to optimum for maximum photosynthesis under field conditions. Maximum photosynthesis in these species occurred at leaf temperatures well above mean maximum T_a during most of the summer growth period. An approximate doubling in needle packing on sun shoots of these species, however, led to an estimated temperature enhancement of daily photosynthetic carbon gain of 21 to 36% compared to estimations assuming T₁ = T_a for the entire day. Elevated T₁ due to needle packing may lead to substantial increases in summer carbon gain for conifers experiencing cold-limited, short growth seasons.     

Increased straying by adult steelhead trout,Salmo gairdneri,following the 1980 eruption of Mount St. Helens

Synopsis This paper describes the magnitude and geographic dispersion of adult steelhead trout, Salmo gairdneri, diverted from Columbia River tributaries which were impacted by the 1980 volcanic eruption of Mount St. Helens. Compelling circumstantial evidence suggests that for 1–3 yr after the eruption, large numbers of adult steelhead migrating toward affected tributaries entered two non-affected tributaries flowing into the Columbia River upstream of natal streams. Streams downstream of affected tributaries did not appear to receive substantial numbers of displaced fish. I estimated that the percentage of non-natal steelhead migrating into the lower portions of the nearest upstream river increased from 16% pre-eruption to 45% post-eruption. Winter-run steelhead strays probably originated from both the Cowlitz and Toutle rivers, whereas summer-run strays originated predominantly from the Toutle River. Increased natural production in tributaries resulting from the infusion of large numbers of strays was of insufficient magnitude to be detected by the methods of this study.     

Plant—herbivore interactions in streams near Mount St Helens

1. In four separate field experiments near Mount St Helens (Washington, U.S.A.) during 1986, the grazing effects of two large benthic herbivores, tadpoles of the tailed frog Ascaphus truei and larvae of the caddisfly Dicosmoecus gilvipes, were investigated using streamside channels and in-stream manipulations. In the experimental channels, abundances of periphyton and small benthic invertebrates declined significantly with increasing density of these larger herbivores. 2. In eleven small, high-gradient streams affected to varying degrees by the May 1980 eruption, in-stream platforms were used to reduce grazing by A, truei tadpoles on tile substrates. Single platforms erected in each tributary and compared to grazed controls revealed only minor grazing effects, and no significant differences among streams varying in disturbance intensity (and, consequently, tadpole density). However, results probably were confounded by high variability among streams in factors other than tadpole abundance. 3. Grazing effects were further examined in two unshaded streams with different tadpole densities, using five platforms per stream. In the stream with five tadpoles m^?2, grazing reduced periphyton biomass by 98% and chlorophyll a by 82%. In the stream lacking tadpoles, no significant grazing effects were revealed. Low algal abundance on both platforms and controls, and high invertebrate density in that stream (c. 30000m^?2) suggests that grazing by small, vagile invertebrates was approximately equivalent to that of tadpoles. 4. The influence of large benthic herbivores on algal and invertebrate communities in streams of Mount St Helens can be important, but reponses vary spatially in relation to stream disturbance history, local environmental factors, and herbivore distributional patterns and abundance.     

Plant succession in areas of scorched and blown-down forest after the 1980 eruption of Mount St. Helens,Washington

Abstract. Patterns of plant succession were studied in areas of scorched and blown-down forest resulting from the 1980 eruption of Mount St. Helens, Washington. Changes in species abundance were observed for 7 years in permanent sample plots representing four post-disturbance habitats, or site types. Total plant cover and species richness increased with time on all site types. In blown-down forests supporting snowpack at the time of eruption, understory recovery was dominated by the vegetative regeneration of species persisting through disturbance. In forests without snowpacks, plant survival was poorer. Increases in cover and diversity were dominated first by introduced grasses, then by colonizing forbs characteristic of early successional sites. Epilo-bium angustifolium and Anaphalis margaritacea showed widespread recruitment and clonal expansion throughout the devastated area. As a result, species composition on previously forested sites converged toward that on formerly clearcut sites, where early serai forbs resprouted vigorously from beneath the tephra. Total plant cover and species diversity were poorly correlated with post-disturbance habitat and general site characteristics (e.g. distance from the crater, elevation, slope, and aspect). However, distributions of several life-forms (e.g. low sub-shrubs and tall shrubs) were strongly correlated with depth of burial by tephra and with cover of tree rootwads. Thus, early community recovery may reflect microsite variation or chance survival and recruitment rather than broad-scale gradients in environment or disturbance. Recovery of pre-disturbance composition and structure will undoubtedly be much slower than after other types of catastrophic disturbance. The rate and direction of community recovery will largely depend on the degree to which original understory species survived the eruption.     

LOTIC ALGAL COMMUNITIES IN THE MT. ST. HELENS REGION SIX YEARS FOLLOWING THE ERUPTION1

A survey of 21 lotic habitats from the Mt. St. Helens region, Washington, was conducted during late summer 1986, six years after the eruption. A total of 152 taxa was observed (94 diatoms, 36 chlorophytes, 19 cyanophytes, 2 xanthophytes, and I rhodophyte). Sampling sites were classified by TWINSPAN (two-way indicator species analysis) and ordinated by DECORANA (detrended correspondence analysis). In general, both techniques corresponded well with each other and grouped sites according to the intensity of disturbance that they experienced. Species richness and diversity values tended to be lowest at the most heavily disturbed sites. Chlorophyll a values ranged over two orders of magnitude and tended to be higher at sites sampled from small, low-discharge tributaries. Loowit Creek, a thermally influenced stream that begins within the volcanic crater, was dominated by thick mats of the cyanophyte Mastigocladus laminosus. A general comparison of the present study with one conducted just after Mt. St. Helens erupted suggests that successional processes are operating in these lotic algal communities.     

Isolation of cuticular membranes from various conifer needles

Summary A method of isolating intact needle cuticles is presented. Cuticles were separated enzymatically from needles of Abies alba Mill., Picea abies (L.) Karst., Picea pungens Engelm., Pinus mugo Turra, and Taxus baccata L. Cuticle separation depended on the enzyme concentration, the developmental stage of the needles and the duration of incubation in the hydrolytic pectinase/cellulase solution. Cuticles could not be removed from needles older than 2 years. Scanning electron micrographs of enzymatically isolated cuticles are presented. The permeance coefficients for water and oxygen transport across the isolated cuticular membranes indicate their functional intactness. But permeance coefficients also show that isolation of cuticular membranes with chromic acid is an unacceptable method, since they are lo longer structurally or functionally intact following isolation by this method.     

How predation can slow, stop or reverse a prey invasion

Observations on Mount St Helens indicate that the spread of recolonizing lupin plants has been slowed due to the presence of insect herbivores and it is possible that the spread of lupins could be reversed in the future by intense insect herbivory [Fagan, W. F. and J. Bishop (2000). Trophic interactions during primary sucession: herbivores slow a plant reinvasion at Mount St. Helens. Amer. Nat. 155, 238–251]. In this paper we investigate mechanisms by which herbivory can contain the spatial spread of recolonizing plants. Our approach is to analyse a series of predator-prey reaction-diffusion models and spatially coupled ordinary differential equation models to derive conditions under which predation pressure can slow, stall or reverse a spatial invasion of prey. We focus on models where prey disperse more slowly than predators. We comment on the types of functional response which give such solutions, and the circumstances under which the models are appropriate.     

Composition and dynamics of wetland seed banks on Mount St. Helens, Washington, USA

The 1980 eruption of Mount St. Helens deposited new substrates upon which wetlands are now developing. We collected soil from five wetlands south of Spirit Lake, Mount St. Helens, Washington, to examine the seed banks. Seedling emergence density in the top 5 cm was highly variable, and ranged from (mean±s.d.) 15,700±15,200 to 38,000±31,500 per m². Seedling emergence from soil at 5 to 10 cm depth varied from 800±600 to 18,000±24,800 per m², and averaged 1/3 as many seeds as the surface. The high proportion of buried seeds may be due to continuing deposition of upland sediments. We identified 17 taxa from the wetland soil samples withEpilobium ciliatum, Juncus bufonius, Agrostis exarata andJuncus ensifolius being the dominant species. Wetland vegetation was dominated byEquisetum arvense, Salix sitchensis, Typha latifolia and the four dominant species in the seed bank. Correlations between vegetation and seed bank were not significant for four of the five wetlands, and the dominance byTypha latifolia in two wetlands contributed to the low correlations. Thus, vegetation of these early successional wetlands generally was not similar to seed bank composition. Canonical correspondence analysis was used to relate soil parameters to vegetation and seed bank data. The vegetation was correlated primarily with soil particle size and secondarily to water temperature, while the seed bank was correlated to pH, texture and temperature.     

Effect of community assembly and primary succession on the species-area relationship in disturbed ecosystems

The species-area relationship (SAR) provides a cornerstone for ecological theory. Implicit in SAR studies is the assumption that SAR properties, especially SAR slopes, remain constant through time, even though the ecosystem characteristics that they encompass–the spatial distribution and abundance of species–change on seasonal to evolutionary time scales. Focusing on disturbed subalpine systems, we evaluated whether SAR properties are a function of stage of succession at Mount St. Helens, WA, and at Gothic, CO. We found that the SAR flattens and shifts upward as these systems mature. The decrease in SAR curvature at Mount St. Helens suggests a transition toward power-law SAR behavior with assembly. Overall, the observed changes in SAR properties raise questions about the appropriateness of applying contemporary SARs to predict future levels of species richness in disturbed or successional systems.