Changes in biotic communities developing from freshwater wetland sediments under experimental salinity and water regimes

1. Reduction in diversity of both freshwater aquatic and terrestrial ecosystems has been attributed to salinity increase and such increases are a symptom of changes to land use. Hydrological alteration to ground and surface water are likely to be associated with salinity increase and its influence on biodiversity. However the combined effects of salinity and hydrology on aquatic biodiversity have not been elucidated fully in either field or experimental situations. 2. The effect of salinity and water regime on the biota in sediments from seven wetlands from inland south‐eastern Australia was tested experimentally using germination of aquatic plant seeds (five salinity and two water levels) and emergence of zooplankton eggs (five salinity levels). Salinity levels were <300, 1000, 2000, 3000, 5000 mg L^?1 and water regimes were damp (waterlogged) and submerged. 3. Aquatic plant germination and zooplankton hatching was not consistent for all seven wetland sediments. Four of the wetland sediments, Narran Lakes, Gwydir Wetlands, Macquarie Marshes and Billybung Lagoon showed similar responses to salinity and water regime but the other three wetland sediments from Lake Cowal, Great Cumbung Swamp and Darling Anabranch did not. 4. As salinity increased above 1000 mg L^?1 there was a decrease in the species richness and the abundance of biota germinating or hatching from sediment from four of the wetlands. 5. Salinity had a particularly strong effect in reducing germination from sediments in damp conditions when compared to the flooded conditions. In parallel, salts accumulated in the sediment in damp conditions but did not in flooded conditions. 6. There is potential for increasing salinity in freshwater rivers and wetlands to decrease the species richness of aquatic communities and thus of the wetland community as a whole, resulting in loss of wetland biodiversity. This reduction in diversity varies between wetlands and is at least partly related to hydrology. For aquatic plants the reduction in diversity will be more marked for plants germinating from seed banks at the edges of wetlands where plants are not completely submerged than for the same seed bank germinating in submerged conditions.     

Seed bank spatial pattern in a temperate secondary forest

Abstract. Seed bank spatial pattern was studied in a secondary forest dominated by Fagus sylvatica and Betula celtiberica in the Urkiola Natural Park (N Spain). Soil samples were taken every 2 m in a regular grid (196 points) and divided into two fractions (0–3 cm and 3–10 cm deep). The viable seed bank was studied by monitoring seedling emergence for ten months. The effect of different factors on seed bank composition and patterning was analysed using constrained ordination as a hypothesis testing tool. Furthermore, the existence of spatial autocorrelation was evaluated by geostatistical analysis. Seed density was high, 7057 seed.m^?2, with a few species dominating. Species composition in the various layers were significantly correlated. The seed bank showed significant spatial structure, which was partially explainable by the spatial structure of the canopy and understorey vegetation. Spatial clumping from 0–8 m was observed in seed bank density and composition, mainly due to the pattern of two abundant taxa Juncus effusus and Ericaceae. The Ericaceae seed bank was related to the spatial distribution of dead stumps of Erica arborea. J. effusus was not present in the above‐ground vegetation, which indicates that its seed bank was formed in the past. As expected, the seed bank of this forest reflects its history, which is characterized by complex man‐induced perturbations. The seed bank appears to be structured as a consequence of contrasting driving forces such as canopy structure, understorey composition and structural and microhabitat features.     

Soil seed bank in different habitats of the Eastern Desert of Egypt

The floristic composition and species diversity of the germinable soil seed bank were studied in three different habitats (desert salinized land, desert wadi, and reclaimed land) in the Eastern Desert of Egypt. Moreover, the degree of similarity between the seed bank and the above-ground vegetation was determined. The seed bank was studied in 40 stands representing the three habitats. Ten soil samples (each 25 × 20 cm and 5 cm depth) were randomly taken per stand. The seed bank was investigated by the seedling emergence method. Some 61 species belonging to 21 families and 54 genera were identified in the germinable seed bank. The recorded species include 43 annuals and 18 perennials. Ordination of stands by Detrended Correspondence Analysis (DCA) indicates that the stands of the three habitats are markedly distinguishable and show a clear pattern of segregation on the ordination planes. This indicates variations in the species composition among habitats. The results also demonstrate significant associations between the floristic composition of the seed bank and edaphic factors such as CaCO₃, electrical conductivity, organic carbon and soil texture. The reclaimed land has the highest values of species richness, Shannon-index of diversity and the density of the germinable seed bank followed by the habitats of desert wadi and desert salinized land. Motyka’s similarity index between the seed bank and the above-ground vegetation is significantly higher in reclaimed land (75.1%) compared to desert wadi (38.4%) and desert salinized land (36.5%).     

Understory vegetation indicates historic fire regimes in ponderosa pine‐dominated ecosystems in the Colorado Front Range

Question: Can current understory vegetation composition across an elevation gradient of Pinus ponderosa‐dominated forests be used to identify areas that, prior to 20th century fire suppression, were characterized by different fire frequencies and severities (i.e., historic fire regimes)? Location: P. ponderosa‐dominated forests in the montane zone of the northern Colorado Front Range, Boulder and Larimer Counties, Colorado, USA. Methods: Understory species composition and stand characteristics were sampled at 43 sites with previously determined fire histories. Indicator species analyses and indirect ordination were used to determine: (1) if stands within a particular historic fire regime had similar understory compositions, and (2) if understory vegetation was associated with the same environmental gradients that influence fire regime. Classification and regression tree analysis was used to ascertain which species could predict fire regimes. Results: Indicator species analysis identified 34 understory species as significant indicators of three distinct historic fire regimes along an elevation gradient from low‐ to high‐elevation P. ponderosa forests. A predictive model derived from a classification tree identified five species as reliable predictors of fire regime. Conclusions: P. ponderosa‐dominated forests shaped by three distinct historic fire regimes have significantly different floristic composition, and current understory compositions can be used as reliable indicators of historical differences in past fire frequency and severity. The feasibility demonstrated in the current study using current understory vegetation properties to detect different historic fire regimes, should be examined in other fire‐prone forest ecosystems.     

生态位限制和物种库限制对湖滨湿地植物群落分布格局的影响

研究了物种库限制与生态位限制在湖滨湿地植物分布格局形成过程中的相对重要性。在龙感湖湖滨湿地具有明显水位梯度的湿生植 物区、挺水植物区和沉水植物区采集种子库土样, 采用幼苗萌发法确定了不同水位区种子库的物种成分;并将不同水位区的种子库土样分别置于0、25和50cm3个水位下萌发和生长, 45和90d后比较不同取样区种子库在不同水位处理下所建立的植物群落的异同。结果表明, 不同取样区的种子库物种成分有显著差异, 沿水深梯度呈现明显的带状分布格局。水位处理实验表明, 0cm水位条件下的群落主要由湿生植物和挺水植物组成, 而25和50cm水位下只有沉水植物, 表明不同功能群的物种对水深有不同的耐受力, 生态位限制是决定湿地植物分布格局的关键因子。同时, 挺水植物区的种子库置于沉水条件下, 以及沉水植物区的种子库置于0cm水位下都只能形成极为简单的植物群落, 表明物种库限制对湿地植物群落的形成同样具有显著影响。研究表明, 湿地植物的群落构成与分布格局是由生态位限制和物种库限制共同决定的, 两者的相对重要性可能取决于水体的稳定性。     

How do depth,duration and frequency of flooding influence the establishment of wetland plant communities?

In many temporary wetlands such as those on the Northern Tablelands of New South Wales Australia, the development of plant communities is largely the result of germination and establishment from a long-lived, dormant seed bank, and vegetative propagules that survive drought. In these wetlands the pattern of plant zonation can differ from year to year and season to season, and depth is not always a good indicator of the plant community composition in different zones. In order to determine which aspects of water regime (depth, duration or frequency of flooding) were important in the development of plant communities an experiment using seed bank material from two wetlands was undertaken over a 16 week period in late spring–early summer 1995–1996. Seed bank samples were exposed to 17 different water-level treatments with different depths, durations and frequencies of flooding. Species richness and biomass of the communities that established from the seed bank were assessed at the end of the experiment and the data were examined to determine which aspects of water regime were important in the development of the different communities. It was found that depth, duration and frequency of inundation influenced plant community composition, but depth was least important, and also that the duration of individual flooding events was important in segregating the plant communities. Species were grouped according to their ability to tolerate or respond to fluctuations in flooding and drying. The highest biomass and species richness developed in pots that were never flooded. Least biomass and species richness developed in pots that were continuously flooded. Short frequent floods promoted high species richness and biomass especially of Amphibious fluctuation-tolerator species and Amphibious fluctuation-responder species that have heterophylly. Terrestrial species were able to establish during dry phases between short floods. Depth was important in determining whether Amphibious fluctuation-tolerator or Amphibious fluctuation-responder species had greater biomass. Longer durations of flooding lowered species richness and the biomass of terrestrial species. Experiments of this kind can assist in predicting vegetation response to water-level variation in natural and modified wetlands.     

Variations of the germinable soil seed bank along the altitudinal gradient in the northwestern Red Sea region

The variations in the size, composition and diversity of the germinable soil seed bank were studied along an altitudinal gradient in the northwestern Red Sea region. The standing vegetation and the germinable seed bank were studied in 58 stands distributed along the altitudinal range from sea level to coastal mountain peaks. The classification of the germinable seed bank by the two-way indicator species analysis (TWINSPAN) led to the recognition of five groups representing different altitudinal ranges. Detrended correspondence analysis (DCA) shows that these groups are clearly distinguished by the first two DCA axes. The results demonstrate significant associations between the floristic composition of the seed bank and the edaphic factors such as CaCO₃, electrical conductivity, organic carbon and soil texture. Species richness, Shannon index of diversity and the size of the germinable seed bank show a hump-shaped curve along the altitudinal gradient, whereas evenness shows a weak increase with elevation increasing. Beta diversity of the seed bank declines with altitude increasing. The similarity between the standing vegetation and the seed bank approaches a U-shaped pattern along the elevation gradient. About 34.8% of the species that constitute the standing vegetation are vulnerable to elimination from the standing vegetation because they are not represented in the seed bank. Soil seed bank can be used for restoration of the vegetation at some of the degraded sites.     

Spatial distribution pattern of the riparian vegetation in a basin in the NW Spain

The riparian vegetation of a basin in the NW Spain was studied to establish its spatial variation pattern and to relate floristic and structural differences in the community to environmental factors. Eighty-seven sampling units in 43 sampling stations were used. Samples were classified in 5 groups using Two Way Indicator Species Analysis (TWINSPAN). Three groups represented reaches with riparian wood along their banks: Mediterranean alderwoods and shrubby willow woods. The remaining two corresponded to floodplains with vegetation colonizing moderately eutrophicated deposits of gravel. Structural characteristics of richness and diversity differentiated the alder woods. In these, the shaded environment created by the woody species limited herbaceous vegetation development. This was dominated by Carex acuta subsp. broteriana. Classification and CCA ordination results were compared. The TWINSPAN groups could be recognized in the CCA graph. The ordination was related to a pollution gradient associated with altitude. This pollution gradient involved bank ruderalization, incorporation of nitrophilous species and a decrease in the vegetation quality. The influence of the lithological features on vegetation was also evident.     

Seed Bank Variation Along a Water Depth Gradient in a Subtropical Lakeshore Marsh,Longgan Lake,China

Although zonation patterns of the standing vegetation along a water depth gradient in wetlands have been well described, few studies have explored whether such patterns also occur in the seed bank. This study examined patterns of the seed bank along a water depth gradient in three vegetation types (submerged zone, floating-leaved zone, and emergent zone) of a subtropical lakeshore marsh, Longgan Lake, China. Submerged zone is the deepest water and never exposed its soil to air, and was dominant by submerged species; floating-leaved zone is waterlogged soil even during drawdown and was dominant by Nelumbo nucifera; emergent zone is rarely exceeded 1 m water depth during the wet season (summer and autumn), and the marsh soil was usually exposed to air during drawdown (winter and spring), and is dominant by Zizania latifolia, Polygonum hydropiper and Scirpus yagara. It was found that many species were ubiquitous in the seed bank. Frequency of distribution and densities of the dominant species, however, varied significantly from zone to zone. A total of 22 species was recorded in submerged zone, 20 in floating-leaved zone, and 29 in emergent zone. Germinated seedling density was 1,580, 8,994 and 20,424 seedlings m⁻² in submerged zone, floating-leaved zone, and emergent zone, respectively. Submerged and floating-leaved species were significantly abundant in the submerged zone, while the emergent species were found predominantly in the emergent zone. A fern species, Ceratopsis pterioides, was the most abundant seedling in seed banks from all three zones. A TWINSPAN dendrogram and CCA ordination diagram clearly showed separation of species among sites with the emergent zone being well separated from the submerged zone and floating-leaved zone, thus revealing marked zonal patterns in species distributions in the seed bank. This pattern of zonation reflected the pattern in the standing vegetation.     

Seed banks of grazed and ungrazed Baltic seashore meadows

Abstract. The seed banks of three grazed and three ungrazed seashore meadows were studied on the west coast of Finland. 8486 seedlings (mean 13 669 seedlings/m²) germinated from cold-treated samples (n = 343; depth = 10 cm). Most seedlings and species were monocots and perennials. The seed bank flora included 54 dicots vs. 28 monocots and 66 perennials vs. 16 annuals. The most abundant species were Juncus gerardii, Schoenoplectus tabernaemontani, Eleocharis uniglumis, Agrostis stolonifera, Juncus bufonius and Carex nigra, which made up 73% of the seed bank. Numbers of species and seedlings differed between elevation classes. Species richness was highest in elevation class 50–70 cm. The highest seed density occurred in class 20–50 cm. A model for size and species composition of the seed bank in relation to elevation is presented. The seed bank was larger and richer in species in the ungrazed than in the grazed sites, but not so in the upper elevations and closest to the open sea. Grazing reduced the size of the seed bank of Agrostis stolonifera, A. capillaris, Calamagrostis stricta, Elymus repens, Juncus bufonius, Limosella aquatica and Schoenoplectus tabernaemontani, but increased that of J. gerardii. 32 species germinated only from ungrazed samples and 11 species only from grazed ones. Multivariate classification resulted in nine sample groups. The ordination scatter was best explained by the flooding stress variables, elevation, the distance from the water line and the number of helophyte species in samples. 75 species were found both in the seed bank and in the vegetation, but there was a significant lack of resemblance (in the Mantel test) due to over-representation of some species. Eight species occurring only in the seed bank were mainly annuals or biennials (63%); those occurring only in the established vegetation (86 species) were mainly perennials (86%).     

Temporary floating island formation maintains wetland plant species richness: The role of the seed bank

In wetlands, fluctuating water levels create opportunities for recruitment of new individuals from seed banks, and drawdown periods often favor the establishment of species adapted for life in shallow water. In this study, floating island formation functioned similarly to drawdowns in water level by creating patches of sediment that were less inundated relative to the surrounding deep water marsh. The disturbance of floating island formation (i.e., mats of sediment and vegetation rising vertically in the water column) also resulted in reduced cover of the dominant rooted, floating-leaved macrophyte, thereby creating temporary gaps for the establishment of other species. To assess how floating island formation influences species richness and composition of wetland plant assemblages relative to surrounding deep water marsh, field surveys of plant percent cover on and off of islands were conducted over 2 years, along with a controlled greenhouse seed bank experiment in which levels of inundation were manipulated. Five plant species were present in deep water marsh compared to 22 in surrounding on floating islands. Plant assemblages on floating islands consisted primarily of emergent species, while floating-leaved perennials dominated the deep water marsh. Species richness and assemblage composition in the greenhouse seed bank experiment differed among water level treatments in a manner consistent with differences observed in field surveys. Assemblages germinating under minimal inundation treatments were more species rich (3.5–4.3 species per sample) and contained more emergent species (>450 individuals m⁻²) than did those germinating under flooded conditions (2.8–2.9 species per sample and <405 individuals m⁻²). This study, in addition to being the only reported seed bank study of temporary (i.e., seasonal) floating islands, demonstrates that islands altered levels of inundation favoring the germination of more species-rich, emergent wetland plant assemblages. Because these islands persisted long enough for several species to set seed, their formation may be one mechanism by which the seed bank is replenished and populations of otherwise uncommon species are maintained.     

Composition and seasonal flux of the soil seed bank of species-rich Themeda triandra grasslands in relation to burning history

Abstract. Most species-rich grasslands dominated by Themeda triandra in southeastern Australia have been ungrazed and frequently burnt for decades. The seedling emergence technique was used to determine the size and taxonomic composition of the soil seed bank of five grasslands that had different fire histories (i.e. burnt at 1 yr, 3 yr and > 10 yr intervals) and this was compared to the standing vegetation at each site. A nested sampling design (subplot, plot, site) was used to determine the effect of spatial scale on the patterns observed in both the vegetation and the seed bank. Temporal variation in the seed bank was assessed by repeated soil sampling over a two year period. 61 native and 30 exotic species were recorded in the vegetation. Richness varied more between sites than within sites. Sites were therefore internally homogeneous for species richness. However, no correlation between burning frequency and richness was found. DCA ordination separated the sites into distinct groups, but sites with similar fire history did not necessarily group closely. 60 taxa germinated from the soil seed bank, comprising 32 native and 28 exotic species; 11 species, mostly therophytes, were restricted to the seed bank. The richness of the seed bank was significantly lower than the vegetation at all spatial scales. No correlation between seed bank richness and fire history was found. The seed bank of species-rich grasslands is dominated by a limited number of widespread, highly clumped, annual, native and exotic monocots. Most native hemicryptophytes, and perennials in general, were represented in the soil by a transient seed bank. Only 12 % of study species, all therophytes, were considered to form large, persistent seed banks, the size of which was greater in unburnt grasslands at all times of the year. The distinct floristic patterns observed in the vegetation are less clearly represented in the seed bank. The seed bank represents a floristically distinct (and less variable) component of the vegetation when compared to the standing flora. The size of the long-term soil seed bank suggests that it has little functional importance for many native species and probably contributes little to seedling regeneration processes following disturbance. Altering established fire regimes is likely to only change the composition and small-scale richness of the existing site vegetation and will not re-integrate species previously lost from the vegetation due to past management. It is suggested that the maintenance of vegetation remnants and processes that encompass a range of long-term burning histories will be necessary if the flora is to be conserved in situ. Restoration of degraded grasslands cannot rely on the soil seed bank but rather, will be dependent on the reintroduction of propagules.     

Vegetation and seed-bank relationships on a lakeshore

SUMMARY.

• 1 Possible convergence in species richness and species composition between the (standing) vegetation and soil seed bank was studied along gradients of wave exposure, estimated as soil organic matter content, on five levels of a shore of a river-lake in northern Sweden.
• 2 Species with seeds having little natural ability to float were most common in the vegetation, whereas species with seeds capable of prolonged floating were most abundant in the seed bank. Species richness in the vegetation and seed bank varied with wave exposure in the same way. Species richness decreased with exposure in both communities at the upper three levels but did not vary with exposure at the lower levels.
• 3 Floristic similarity between the vegetation and seed bank was 0.25 (Sørensen's index of similarity) for the entire flora. The variation in floristic similarity between communities did not differ between levels. Floristic similarity per sample plot increased with wave exposure.

     

The role of the permanent soil seed bank in early stages of a post-agricultural succession in the Inland Pampa,Argentina

We studied the soil seed bank composition in four old fields of different ages, after abandonment from agriculture. Complete seed bank composition was assessed by direct seed separation from soil samples and identification to species. Most species found in the seed bank were not important in the present seral communities. Seed of the species that dominated the early succession were generally not found. Additionally, there were very few propagules rather than on the germination of in situ propagules. We suggest that pampean grasses evolved under that the course of post-agricultural succession will depend strongly on the pattern of arrival of exogenous propagules rather than the germination of in situ propagules. We suggest that pampean grasses evolved under disturbances of low intensity and/or a disturbance regime dominated by small gaps, in which open areas could be rapidly colonized from the edges and/or by remnant vegetative propagules. The changes produced by the introduction of agriculture triggered the invasion by exotic species adapted to the new disturbance regime.     

Photosynthetic use of inorganic carbon among primary and secondary water plants in streams

1. We analysed photosynthetic rates and inorganic carbon use of thirty-five vascular macrophyte species collected submerged in eight nutrient- and CO₂-rich Danish lowland streams. The species were classified in four groups as mainly terrestrial, homophyllous and heterophyllous amphibious and truly submerged. These groups represent plant species differently adapted to life in water. 2. Photosynthetic rates measured in water increased in the gradual transition from mainly terrestrial, through amphibious to truly submerged species. Species normally in contact with air adapted to submergence by increasing the photosynthetic rate at limiting CO₂. Photosynthetic rates of submerged parts of heterophyllous amphibious species were close to those of submerged species. Submerged species with thin or finely dissected leaves had the highest photosynthetic rates, probably because of low diffusional resistance to uptake of nutrients and gases. 3. In contrast to submerged species, terrestrial and amphibious species were unable to use HCO₃^?. Extensive oversaturation with CO₂ in the streams allows, however, many amphibious species to photosynthesize well under water, based on CO₂-use alone. Amphibious CO₂-users, with very few structural adaptations to life under water, can therefore be as dominant in the submerged vegetation of lowland streams as HCO₃^?-using water plants. Moreover, the streams provide open space for colonization from the dense vegetation ashore. 4. Among the 1265 Danish herbaceous species no less than seventy-five terrestrial species occasionally grow submerged, forty-five species are amphibious, and fifty-one are true water plants. These numbers suggest that adaptation to permanent or temporary submergence is an ongoing process involving many species and that the land-water interface does not represent as difficult a barrier as often believed.     

Persistent soil seed banks and floristic diversity in Fagus orientalis forest communities in the Hyrcanian vegetation region of Iran

We assessed the size of seed bank, species diversity and similarity between seed bank and standing vegetation in four oriental beech (Fagus orientalis Lipsky) community types of the central Hyrcanian forests of northern Iran. For this purpose a total of 52 relevés was established in two associations and two subassociations of the beech forests, and six soil samples (20 × 20 cm square and to a depth of 10 cm) were collected in each relevé in mid-spring, after the germination season had ended. Soil seed bank was investigated using the seedling emergence method. A total of 63 species, 57 genera and 36 families was represented in the persistent soil seed bank of the forest communities. The seed bank contained 28 species not found as adult plants in the vegetation, but these were mostly early successional species. Size of the seed bank ranged from 3740 to 4676 individuals m⁻² in the Rusco hyrcani-Fagetum orientalis and Danae racemosae-Fagetum orientalis associations, respectively. Species composition of seed banks and aboveground vegetation had low similarity with an average of 24.3% in the four plant communities, because only 38% of the species were the same in the vegetation and the seed banks. Most seeds in the seed bank were from early successional species, and the only tree with a large persistent seed bank was the fast-growing pioneer Alnus subcordata. DCA ordination also demonstrated low similarity between soil seed bank and vegetation. The soil seed banks of the four beech communities did not differ significantly in size, composition, diversity and uniformity. Although above ground vegetation in the four community types is floristically distinct, there is considerable overlap among the soil seed banks because they contain in a similar way early successional species. Further, the absence of typical forest species in the soil seed bank indicates that restoration of forest tree species cannot rely on the soil seed bank.     

天山中部天山云杉林土壤种子库年际变化

通过定点采样,采用萌发法对天山中部天山云杉(Picea schrenkiana Fisch.et Mey.)近熟林(101-120a)和成熟林(121-160a) 2004-2011年(8a)土壤种子库物种组成、种子密度的年际变化和不同间隔年限土壤种子库物种组成的相似性进行了分析.结果表明:(1)土壤种子库中共萌发鉴定出种子植物87种,隶属29科70属,其中乔木种子植物2种,灌木种子植物2种,草本种子植物83种,土壤种子库中草本植物种子密度远远大于木本植物种子密度;8个采样年份土壤种子库恒有种仅有6种;(2)土壤种子库种子密度及其中天山云杉种子密度存在巨大的年际变动,且不具有同步性;土壤种子库种子密度最大(2009年)值为(953.75±66.12)粒/m2,最小(2008年)值为(186.50±20.37)粒/m2,其中天山云杉种子密度最高(2006年)达到(584.50±53.58)粒/m2,最低(2005年)仅有(0.25±0.26)粒/m2;(3)天山云杉林土壤种子库年际间物种组成的相似性不高,Czekanowski相似系数均值仅为0.344,并随间隔年限的增加呈现减小—增大—减小的变化趋势.天山云杉林土壤种子库物种组成和种子密度稳定性差,年际间相差悬殊,物种组成的相似性不高,种子库中天山云杉种子密度主要受其种子库采样前一年天山云杉结实丰歉的影响,属间断型.土壤种子库年际变化特征可为天山森林的更新恢复和可持续经营提供科学依据.     

Regeneration of monsoon rain forest in northern Australia: the dormant seed bank

Abstract. As part of a wider study examining regeneration pathways in monsoon rain forest vegetation in northern Australia, the dormant component of the soil seed bank was assessed by storing soil samples for over six dry season months, before watering in shade-house trials. Six soil samples were collected from each of 34 sites broadly representative of the range of regional monsoon rain forest vegetation. Four floristic seed bank groups were derived through TWINSPAN classification. Mean group densities of germinants ranged from 25–144/m². Dormant seed banks were least dense, and most sparsely distributed, in sandy soils. Seed bank samples were dominated by woody pioneer monsoon rain forest species, especially figs; exotic weeds and savanna taxa (e.g. Poaceae) were relatively more common at seasonally dry sites. Dormant seed banks comprised species mostly present in the standing vegetation, although a small number of germinants represented species not growing at half the sites. Regeneration of woody pioneers from dormant seed banks is least likely to be of importance on infertile, seasonally dry sites.     

Disturbance Effects on the Seed Bank of Mexican Cloud Forest Fragments1

The density and floristic composition of the soil seed bank was assessed in six cloud forest fragments with different levels of human disturbance in central Veracruz, Mexico. A total of 8416 seeds germinated in 60 soil samples, at 5‐cm depth, corresponding to 107 species, 85 genera, and 48 families. Significant differences were found among study sites in seed densities with values ranging from 873 to 3632/m². Tree species contributed 20 percent of the total soil seed bank in four sites and herbs accounted for the majority of the species in each site. Among tree species, Trema micrantha displayed the highest seed density, accounting for 84 percent of the germinated seeds. In general, the tree species composition of the soil seed bank did not closely reflect the composition of the tree community. Results suggest that disturbance produced by human activities (trail use, selective cutting of trees, livestock) may influence the size and composition of the soil seed bank in forest fragments. Sites where human activity has been reduced showed the highest proportion of dormant seeds.     

A STUDY OF THE SEED BANK OF A SALT MARSH IN NORTHERN SAN FRANCISCO BAY

Abundance, species composition, and distribution of buried seeds in a San Francisco Bay salt marsh were studied by collecting soil samples in October and February and observing seedling emergence in the greenhouse. Results were compared with existing vegetation patterns and field germination. Average numbers of buried viable seed down to a 5-cm depth were 380/ m² in October and 700/m² in February, with field germination averaging 118/m². Salicornia virginica dominated the seed bank in the greenhouse and in field germination. Most other marsh species were present in the seed bank but numbers of seeds were low. A significant correlation was found between highest species diversity and proximity to channels. The nature of this low-diversity, low-density seed bank reflects dominance of long-lived perennial species, seed dispersal patterns and selective environmental pressures.