Seed rain,seedling establishment and clonal growth strategies on a glacier foreland

Abstract. Seed production, composition of the seed rain, germination, and seedling mortality, as well as vegetative growth characteristics of common pioneer plant species were studied on the foreland of the retreating Morteratsch glacier in the Swiss Alps. The frequency of diaspores trapped in different successional stages was related to their dispersal mode and was highly skewed towards a few species. Plenty of diaspores well adapted for dispersal by wind are a precondition for the most important pioneer species. Seeds from all pioneer species investigated had a good germination success, provided that the moisture content of the soil was high enough. However, requirement for seedling establishment differed among sites of increasing terrain age and among species. Only specialized pioneers such as Cerastium pedunulatum, Linaria alpina, Oxyria digyna and Saxifraga aizoides tolerate the cold and moist conditions near the glacier. However, these species are restricted to early successional stages. Seedlings of Epilobium fleischeri are affected not only by the cold and moist conditions near the glacier but also by moderately dry conditions on older sites. Availability of safe sites becomes crucial for most species with increasing age of sites and with drier conditions. Most species playing a dominant role during early succession and persisting during later successional stages have a distinctive ability to spread clonally and have a growth form with more or less widely spaced ramets: Achillea moschata. Cerastium pedunculatum. Epilobium fleischeri and Hieraium staticifolium. The growth strategy and demography of the clonal E. fleischeri is presented as an example. The life cycle of this species is characterized in succession by (1) the colonization of safe sites by small seeds adapted for wind dispersal, (2) horizontal spread by clonal growth, and (3) the persistence through phenotypic morphological plasticity in later successional stages. Seedling establishment and clonal growth are thus complementary mechanisms in plant succession on recently deglaciated terrain.     

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.     

Patterns of Primary Succession on the Foreland of Coleman Glacier,Washington, USA

Patterns of community development vary among studied glacier forelands around the world. However, there have been few studies of primary succession on glacial forelands in temperate regions of North America. We described patterns in community composition, vegetation cover, diversity, and vegetation heterogeneity during primary succession on the foreland of Coleman Glacier, in Washington State, USA. Community composition changed rapidly with high turnover between age classes. Cover increased through succession as expected. Species richness and diversity were highest in early succession at small scales and in late succession at larger scales. At small scales, heterogeneity decreased in early succession but increased in mature sites. At larger scales, heterogeneity reached its lowest point earlier in succession. These scale-dependent patterns in diversity and heterogeneity differ from results of other studies of glacier forelands. We hypothesize that these patterns arise due to the development of a dense canopy of the deciduous shrub Alnus viridis followed by a dense canopy of Abies amabilis, Tsuga heterophylla, and Pseudotsuga menziesii.     

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.     

Spatial patterns of microsite colonisation on two young lava flows on Mount Hekla,Iceland

Questions: How does vegetation first establish on newly‐formed lava substrates? Do very small (cm) and meso‐scale (m) variations in the physical environment influence this process and subsequent vegetation development? Location: Mount Hekla, southern Iceland (64°00’ N, 19°40’ W). Methods: Data on vegetation structure and the incidence of ‘safe sites’ suitable for colonisation were collected from high and low points on the surfaces of lava flows emplaced during the 1991 and 2000 A.D. eruptions of Mount Hekla. Effects of flow age and meso‐topographic position on vegetation structure (moss cover, patch density, stem length) were assessed by two‐way analyses of variance. The distributions of colonisation events and available safe sites were analysed using point pattern techniques. Results: Rapid colonisation of the lava surface was observed, despite stressful environmental conditions. The 1991 and 2000 flows differed significantly in vegetation structure, but there were no significant differences in moss cover, patch density and stem length between ‘high’ and ‘low’ sites. Conclusions: Colonisation events are invariably associated with small‐scale irregularities on the surface of the lava. The colonisation process appears to be spatially random. Development of the moss ‘carpet’ proceeds by vertical thickening and lateral growth and coalescence of moss patches that establish in ‘safe sites’. This process is rapid, with close to 100% of available safe sites exploited within 20 years. Topographic position makes no difference to the very early stages of vegetation development and cannot be used to ‘forecast’ the later stages of development.     

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.     

Experimental demography of the rare Gentianella germanica: seed bank formation and microsite effects on seedling establishment

The persistence of populations of short-lived species requires regular reproduction and seedling establishment. A persistent seed bank can buffer populations against extinction in unfavourable years. We experimentally investigated seed fate in Gentianella germanica, an endangered biennial species characteristic for species-rich nutrient-poor calcareous grasslands in central Europe. We studied the effect of experimental gaps on seedling establishment from sown seeds and the fate of seeds buried in bags over two years. In December 1993 experiments were established at seven calcareous grassland sites in the Swiss Jura mountains. In spring 1994 seedlings emerged in all plots where seeds had been sown, including previously unoccupied patches. This suggests that limited dispersal within sites contributes to small population sizes. Significantly more seedlings emerged at sites with current populations of G. germanica than at unoccupied sites (5.95% vs 3.40%). Because this difference was not explained by germinations from the natural seed bank it indicates differences in habitat quality. Clipping of the vegetation and disturbance of the soil reduced vegetation cover in the following spring and enhanced seedling emergence. In undisturbed plots 4.5% of seeds sown produced a seedling in spring 1994, whereas in plots with clipped vegetation 9.9% and in disturbed plots 12.7% produced seedlings (p>0.01). This suggests that management measures which create gaps in the vegetation (e.g. grazing) could positively influence population size and persistence of G. geymanica. On average, we recovered 7.55% viable seeds after one year of burial in bags, and 4.05% after two years, indicating that G. geymanica has a persistent seed bank. The demographic data indicate that the number of viable seeds in the seed bank exceeds the number of established plants in a population at least by a factor of 20. Restoration of extinct populations of the species from the seed bank may thus be possible if appropriate management measures are taken within a few years.     

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.     

Effects of landslides on the mountain vegetation of Flores Island,Azores

Question: What are the consequences of frequently occurring landslides on vegetation dynamics, floristic and structural diversity? Location: 39°27′N; 31°13′W – Morro Alto, Flores Island, Azores, Portugal. Methods: Six comparable landslides were selected. Plots were placed at the top, slope and toe of landslides. Data on floristic composition and biovolume, demography and size structure of the dominant tree species (Juniperus brevifolia) were collected. Hierarchical agglomerative clustering and Principal Component Analysis were used in order to identify succession stages and compare succession pathways and vegetation recovery in different parts of the landslides. Results: Four stages of primary succession on substrates formed by landslides were identified: pioneer (Festuca‐Sphagnum grassland), assembly (Juniperus‐Festuca‐Sphagnum open scrub), building (Juniperus‐Sphagnum scrub) and mature (Juniperus‐Sphagnum woodland). Concerning J. brevifolia populations, the succession pathways are independent of location on the landslide. However, at the floristic level, there are some differences, mainly in the pioneer stage at the toes of landslides. Better abiotic conditions, resulting in a higher succession rate, are probably responsible for a faster vegetation recovery on landslide toes. Conclusion: Landslides trigger succession processes that enable a massive regeneration of the dominant tree species and existence of species not present in mature forests. They are also responsible for the simultaneous occurrence of vegetation of different structures. Overall, landslides increase the floristic and structural diversity of the vegetation, consequently increasing landscape heterogeneity.     

Post‐fire tree establishment patterns at the alpine treeline ecotone: Mount Rainier National Park,Washington, USA

Questions: Does tree establishment: (1) occur at a treeline depressed by fire, (2) cause the forest line to ascend upslope, and/or (3) alter landscape heterogeneity? (4) What abiotic and biotic local site conditions are most important in structuring establishment patterns? (5) Does the abiotic setting become more important with increasing upslope distance from the forest line? Location: Western slopes of Mount Rainier, USA. Methods: We performed classification analysis of 1970 satellite imagery and 2003 aerial photography to delineate establishment. Local site conditions were calculated from a LIDAR‐based DEM, ancillary climate data, and 1970 tree locations in a GIS. We used logistic regression on a spatially weighted landscape matrix to rank variables. Results: Considerable establishment after 1970 caused forest line elevation to increase over 150 m in specific locations. Landscape heterogeneity increased with distance from the 1970 forest line. At a broad spatial context, we found establishment was most common near existing trees (0‐50 m) and at low elevations (1250‐1350 m). Slope aspect (W, NW, N, NE, and E), slope angle (40‐60°), and other abiotic factors emerged as important predictors of establishment with increasing upslope distance from the forest line to restricted spatial extents. Conclusions: Favorable climatic conditions likely triggered widespread tree establishment. Readily available seed probably enhanced establishment rates near sexually mature trees, particularly in the less stressful environment at low elevations. The mass effect of nearly ubiquitous establishment in these areas may have obscured the importance of the abiotic setting to restricted spatial extents. Topographic variability apparently produced favorable sites that facilitated opportunistic establishment with increasing upslope distance from the forest line, thereby enabling additional trees to invade the alpine tundra.     

Dynamics of herbaceous vegetation recovery on Mount St. Helens,Washington, USA,after a volcanic eruption

Recovery of herbaceous vegetation on Mount St. Helens was studied annually after the massive lateral eruption of May 18, 1980. Measures such as species richness, cover, and diversity were combined with detrended correspondence analysis to describe vegetation recovery rates under different combinations of initial impact intensity and degree of isolation from recolonization sources. A major key to recovery is whether any plants survived the devastation. Survival of even a few individuals markedly accelerated recovery. Where no plants survived, the degree of isolation becomes paramount. New, barren substrates, a few meters from undisturbed sites, have begun to develop some vegetation, while more isolated sites have scarcely any subalpine plants present. On any site, plant-mediated processes that improve conditions for growth and the invasion of other species predominate in the early stages, but as vegetation develops, biotic inhibition and establishment of seedlings from adults already in the habitat gain importance. The rate at which this conversion occurs is a function of the size and intensity of the initial impact.Abbreviations DCA Detrended correspondence analysis     

Diversity and Succession of Actinobacteria in the Forelands of the Tianshan Glacier,China

Actinobacteria are typically soil bacteria that have important roles in soil development and biogeochemical cycling. However, little is known about the occurrence or the succession of communities of Actinobacteria in new habitats. In this study, we investigated the diversity and succession of the actinobacterial communities that inhabited the forelands of the Tianshan Glacier (China), which ranged in successional age from 0 to 100 years since the forefield was deglaciated. Actinobacteria was one of the dominant phyla in the glacier foreland and included the orders Acidimicrobiales, Actinomycetales, Rubrobacteriales and Solirubrobacteriales. Actinomycetales was the dominant order, but its relative abundance decreased through the chronosequence. Acidimicrobiales and Solirubrobacteriales were more abundant in the late stages of succession than in the early ones. The abundance of Rubrobacteriales was only high at 74a. The dominant genera Nocardioides and Arthrobacter were widely distributed and were found in each stage of succession. With nonparametric and rarefaction estimated analyses, we found that the phylotype richness of Actinobacteria was significantly correlated with time (r = 0.886, p = 0.019). The succession of actinobacterial communities was divided into 3 stages: the early stage (6a), the intermediate stage (10a and 20a) and the late stage (60a, 74a, and 100a). Based on the canonical correspondence analysis, the actinobacterial communities were affected significantly by soil pH (r = ?0.834, p = 0.039) and somewhat by the C/N ratio (r = 0.783, p = 0.066). The nonmetric multidimensional scaling analysis showed that the effect of geographical isolation on the actinobacterial communities was greater than that of the soils in the development of the chronosequence.     

Effects of slug herbivory on the seedling establishment of two montane Asteraceae species

Seedling establishment as the life stage transition most sensitive to herbivory might be impossible even if the herbivory losses suffered by adult plants of the same species are tolerable. We tested the hypothesis that herbivory impedes seedling establishment of two montane Asteraceae species on their lower altitudinal distribution border.In a submontane meadow in the Harz Mountains, Germany, the montane grassland perennials Arnica montana and Centaurea pseudophrygia (Asteraceae) were sown into artificially created gaps of two different sizes, half of which were protected from slug grazing by molluscicide application in the initial phase of the experiments. The Arnica experiments started in spring and late summer of 1999 and in spring of 2000; whereas Centaurea was only sown in the spring of 2000. Seedling survival was monitored until the autumn of 2000.Ten weeks after sowing, mollusc exclusion resulted in significantly higher Arnica or Centaurea seedling numbers in all experiments; whereas grasses and dicotyledons that emerged from the seedbank were promoted by slug exclosure in only one experiment. Species diversity was not affected by molluscicide application. By the end of the second growing season, the survivorship of Arnica sown in the spring of 1999 was reduced to less than 1% of the number of sown seeds in the gaps with natural slug grazing, compared to 7–15% in the gaps with molluscicide treatment. Plant density of most species was higher in the small than in the large gaps, indicating a microclimatic shelter effect of the surrounding vegetation.The results show that slug herbivory might exert large effects on species composition in grassland; it does strongly influence the seedling establishment of the highly palatable, slow-growing A. montana, while the seedlings of C. pseudophrygia, although of similar palatability, are more able to tolerate slug feeding because of their higher growth rate.     

干扰强度对群落中紫茎泽兰种子萌发、幼苗定居和生长的影响

干扰与外来植物入侵密切相关,种子萌发和幼苗定居是植物生活史中最脆弱、也是外来植物入侵最关键的阶段.为研究干扰在恶性外来入侵植物紫茎泽兰成功入侵过程中的作用,采用人工牧草群落代替自然群落,人为干扰(去除不同面积的牧草)模拟自然干扰的方式,研究了紫茎泽兰入侵初期种子萌发、幼苗定居和生长对不同干扰强度的响应与适应.结果表明:...     

Succession following the catastrophic eruption of Ksudach volcano (Kamchatka, 1907)

Ksudach Volcano, southern Kamchatka Peninsula, erupted in 1907 and impacted over 2000 km² of forests with air-fall pumice deposits. We identified three impact zones. In Zone I, deposits deeper than 100 cm destroyed all vegetation. Two early successional stages occur, a lichen-dominated desert and isolated patches of a pioneer herb stage. Zone II is defined by pumice deposits 30 to 100 cm deep. Deposits of 70 to 100 cm destroyed all vegetation, but left scattered snags. Here primary succession dominates recovery, but its rate varies. Isolated trees survived in deposits of 30 to 70 cm and primary and secondary successional stages form a complex mosaic termed an intermediate succession. In Zone II, the primary stages found in Zone I are joined by a dwarf shrub-herb stage and a secondary birch forest stage. Zone III occurs where thinner deposits permitted some vegetation to survive in all locations. Secondary succession dominates in deposits of 10 to 30 cm. Trees suffered damage, but survived deposits of 20 to 30 cm, while other vegetation layers were eliminated. Deposits of 10 to 20 cm eliminated mosses and lichens and but only reduced the number of dwarf shrubs and herbs. Deposits of less than 10 cm damaged herb, moss and lichen layers but did not eliminate any species. All sampled vegetation remains in a pre-climax state, having yet to recover fully from earlier eruptions. Reconstructed vegetation maps for before 1907 and for ca. 1925 are compared to the map of vegetation in 1994. Based on degree of soil formation, vegetation recovery and colonization rates at different pumice depths, and the current vegetation, we estimate that full recovery of the soil-vegetation system will take more than 2000 years.