内蒙古草甸草原地下芽库对极端干旱的响应

全球变化引发的极端气候事件严重影响草地生态系统结构与功能。然而,作为多年生草地重要繁殖体库,半干旱草甸草原地下芽库如何响应极端干旱尚未明确。本研究以内蒙古呼伦贝尔草甸草原为对象,利用模拟极端干旱样地平台,探究地下芽库及其与地上植被关系对模拟生长季极端干旱的响应。结果表明,环境降水水平与干旱处理下总芽密度分别为1443和1128芽·m^-2;分株总密度分别为1791和1346株·m^-2;群落整体分生组织制约系数分别为0.84和0.83。极端干旱对草甸草原地下芽库、地上分株密度以及二者之间关联均无显著影响。就优势植物功能群根茎型禾草而言,极端干旱对其地下芽库同样无显著影响。因此,地下芽库表现出对极端干旱一定程度的抵抗力,可作为草甸草原植被应对极端气候事件的保险策略,有效促进草甸草原在极端气候胁迫下的种群更新与植被恢复,有助于草甸草原植被稳定性与生态系统功能维持。     

Soil seed bank in different vegetation types in the Loess Plateau region and its role in vegetation restoration

In the Loess Plateau region, soil erosion is a serious problem. Vegetation restoration is an effective approach to control soil erosion and improve ecosystems. The soil seed bank generally plays an important role in vegetation restoration after disturbance. Thus, we reviewed soil seed bank studies to reveal the soil seed bank characteristics and its role in vegetation restoration in three vegetation types (forest, forest‐steppe, and steppe). We selected 38 seed bank studies and analyzed several seed bank characteristics, such as seed density, species composition, and the relationship between seed size and seed bank. We also assessed the role of the soil seed bank in vegetation restoration. The soil seed bank density ranged from 2,331 ± 1,993 to 6,985 ± 4,047 seeds/m² among the different vegetation types. In the soil seed bank, perennial herbs and grasses accounted for 51.5% of the total species. Native species that were dominant or common in the standing vegetation usually had relatively high seed bank densities. Moreover, species with smaller seeds generally had higher soil seed bank densities. The present study indicates that the soil seed bank plays a significant role in spontaneous vegetation restoration, especially during the early successional stages in abandoned slope farmlands and grazing‐excluded grasslands. However, species with large seeds or transient soil seed banks should be reintroduced through seeding to accelerate target species restoration. More studies on soil seed banks need to be conducted to comprehensively reveal their characteristics.     

Resilience of Mediterranean shrubland to a severe drought episode: the role of seed bank and seedling emergence

Extreme climate events, such as severe drought episodes, may induce changes in vegetation if they induce species‐specific adult mortality and changes in the seedling recruitment pattern. In 2005 a severe drought occurred in Doñana National Park (south Spain) causing extensive shrubland mortality. Over the following years we monitored the soil seed bank and seedling emergence via a gradient of canopy dieback induced by the drought episode. The canopy dieback corresponded to an increase in emergence of seedlings of woody species in 2007, probably because of the reduced competition induced by canopy loss. The soil seed bank of woody species sampled in 2008 was less abundant on plots with a higher proportion of dead vegetation, probably because of depletion of the seed bank as a result of the increased germination in the previous year and also as a result of a reduction in seed supply in these sites. Accordingly, in 2009 we detected reduced emergence of woody species on plots that had suffered the greatest shrub mortality. We failed to find any significant changes in patterns of the soil seed bank and seedling emergence of short‐lived herbaceous species, indicating greater resilience in these types of species. This study highlights the resilience of Mediterranean shrublands to climate fluctuations at one extreme of the variability characteristic of these ecosystems. An increase in the frequency of severe drought episodes – increasingly probable under the new climate conditions – does have the potential, however, to induce changes in vegetation, especially in woody communities that need more time to replenish their seed banks.     

Responses to Riparian Restoration in the Spring Creek Watershed, Central Pennsylvania

Riparian treatments, consisting of 3‐ to 4‐m buffer strips, stream bank stabilization, and rock‐lined stream crossings, were installed in two streams with livestock grazing to reduce sediment loading and stream bank erosion. Cedar Run and Slab Cabin Run, the treatment streams, and Spring Creek, an adjacent reference stream without riparian grazing, were monitored prior to (1991–1992) and 3–5 years after (2001–2003) riparian buffer installation to assess channel morphology, stream substrate composition, suspended sediments, and macroinvertebrate communities. Few changes were found in channel widths and depths, but channel‐structuring flow events were rare in the drought period after restoration. Stream bank vegetation increased from 50% or less to 100% in nearly all formerly grazed riparian buffers. The proportion of fine sediments in stream substrates decreased in Cedar Run but not in Slab Cabin Run. After riparian treatments, suspended sediments during base flow and storm flow decreased 47–87% in both streams. Macroinvertebrate diversity did not improve after restoration in either treated stream. Relative to Spring Creek, macroinvertebrate densities increased in both treated streams by the end of the posttreatment sampling period. Despite drought conditions that may have altered physical and biological effects of riparian treatments, goals of the riparian restoration to minimize erosion and sedimentation were met. A relatively narrow grass buffer along 2.4 km of each stream was effective in improving water quality, stream substrates, and some biological metrics.     

Postfire seed bank dynamics in semiarid grasslands

We evaluated the combined effects of fire after drought on the seed bank composition and its role in the postfire recovery of NW Patagonia grasslands. During three years, we monitored the seed bank and the aboveground vegetation. Species were arranged in functional groups and Detrended Correspondence Analysis was used to separate sites according to species and functional groups. Similarity between aboveground vegetation and seed bank was calculated with SØrensen Index. In the first year, the seed density was similar in the control and burned sites and was lower than following years in all the sites. The species that survived the high temperatures were all annuals with the exception of the perennial species Fabiana imbricata and Rumex acetosella. In the second postfire year, the diversity and seed density increased due to the contribution of fugitive species (rare in the community) and exotic annual species. Seed bank of perennial species was the most affected by fire and just recovered in the third year. Drought did not affect the similarity between the seed bank and vegetation. Fire had low impact on the total seed bank, probably due to the heat buffering nature of the soil, whereas drought reduced significantly seed bank size and richness. Seed bank contributes to grassland richness maintenance.     

Vegetational changes on two eroding banks of a short-term regulated river reservoir in northern Sweden

This study deals with the vegetational changes and the course of erosion on two types of river banks bordering a river reservoir, in northern Sweden. The water-level has been short-term regulated since 1961, with an amplitude of about 3 m in the uppermost reaches of the reservoir and about 1 m furthest downstream. The pattern of regulation includes both a weekly and a daily rhythm. The banks of the river reservoir have been cut in fine-grained sediments. The two sites selected for study were (1) a formerly littoral area (2) a formerly dry-land area. Percentage cover and number of shoots were determined annually within belt transects subdivided into 10 times 10 cm quadrats, during the period 1977–1981.
The course of bank erosion was initiated and mainly continued by water movements. In the formerly littoral area, erosion proceeded only slowly, whereas in the formerly dry-land area erosion was rapid and the bank was continuously being undercut, leading to the formation of an overhanging mat of vegetation. As erosion progressed, this overhanging mat split up and slumped down into the reservoir. The vegetational changes on the overhang were considerable, while the terrace maintained a more stable vegetation cover. In the long-term, the composition of the vegetation on the former littoral river-bank remained more stable than on that cut into the former dry-land area. The break-up of the vegetation cover on the overhang also initiated secondary vegetational successions on the exposed soil. Considerable differences in the vegetation cover were also recorded from one year to the next. These differences were considered to be mainly due to differences in the rate and local course of erosion. Annual differences in the rhythm and/or amplitude of the fluctuations in water-level were considered to be less important in this respect.     

红壤侵蚀区植被恢复过程中土壤种子库变化特征

土壤侵蚀作为一种自然营力和自然干扰形式对土壤种子库的次分布与物种组成具有一定的影响。本研究对典型红壤侵蚀区裸地(Ⅰ号)、马尾松林地(Ⅱ、Ⅲ、Ⅳ号)和次生林(Ⅴ号)等5处不同植被恢复区的土壤种子库物种组成、储量及分布格局进行研究,探究在植被恢复过程中土壤侵蚀对土壤种子库的影响。结果表明: 研究区土壤种子库共统计到21种物种,物种丰富度低,以草本种类为主。各样地土壤种子库密度为56.7～793.3粒·m^-2,样地间差异显著,土壤种子库密度随着土壤侵蚀强度加重而明显下降。各样地浅层0～2 cm土壤种子库密度随着上坡-中坡-下坡的变化均呈增加趋势;剧烈侵蚀地和强烈侵蚀地土壤种子库主要分布在5～10 cm深层土壤内,且中上坡0～2 cm土层几乎没有种子。土壤侵蚀使得土壤种子库在土层中的分布呈现深层化,植被恢复后深层种子库积累仍需要较长的时间。     

沙丘生态系统种子库研究现状、趋势与挑战

风蚀和沙埋赋予沙丘生态系统独特的自然物理和生物过程。沙丘生态系统种子库研究对于沙区植物多样性保护和生态恢复至关重要。但是人们对沙丘生态系统种子库的时空格局、形成机制和生态功能的认识还很肤浅。近些年, 沙丘生态系统种子库开始受到关注。该文对20年来沙丘生态系统种子库研究进行了回顾总结, 论述了沙丘生态系统种子库研究现状, 剖析了沙丘生态系统种子库现有结论, 阐述了沙丘生态系统种子库研究趋势与面临的挑战。沙丘生态系统种子库研究已获得以下认识: 1)沙丘生态系统类型、部位和深度决定种子库大小与组成; 2)沙丘生态系统种子库季节、年际变化受风沙活动和降水调控; 3)沙丘生态系统种子库受各种干扰的综合影响; 4)生境对沙丘生态系统种子库具有重要影响; 5)沙丘生态系统种子库结构和组成与植物繁殖对策密切关联; 6)种子库对沙丘生态系统植被恢复贡献率因沙丘类型而异。沙丘生态系统种子库研究应强化如下方面: 1)种子库区域分异规律研究; 2)种子库与植物生活史阶段的联系的研究; 3)多种干扰方式综合影响下的种子库研究; 4)种子库对沙丘生态系统植被恢复贡献的研究; 5)沙丘生态系统自然物理过程与种子库耦联关系的研究。     

Bank erosion hazard index as an indicator of near-bank aquatic habitat and community structure in a southeastern Piedmont stream

Streams in the southeastern Piedmont region display varying degrees of bank erosion, which in turn can influence multiple physical and biological processes. In efforts to assess and predict bank erosion, the multi-metric tool Bank Erosion Hazard Index (BEHI) is often calculated. As designed, the BEHI takes into consideration many physical phenomena and scores stream banks categorically from very low to extreme bank erosion hazard; however the power of this index to directly predict in-stream biological conditions is unknown. The predictive ability of BEHI was evaluated on near-bank aquatic biota and organic matter (OM) retention in an AL Piedmont stream with a range of bank conditions. Fifteen banks were characterized using the BEHI and subsequently, macroinvertebrates, crayfishes, fishes and organic matter associated with each bank were quantified. Results suggest increased macroinvertebrate richness and OM retention with low BEHI score. Multivariate analyses suggest functional similarity in macroinvertebrate assemblages associated with low BEHI banks and greater variation in assemblages as BEHI increased. Crayfish were more abundant at banks with high BEHI, a pattern attributed to increases in a generalist species in more-degraded habitats. Fish response showed no discernible pattern in relationship to BEHI score. These results suggest that BEHI may have applications as an informative ecological evaluation tool integrating physical and biological conditions in streams.     

Testing a Passive Revegetation Approach for Restoring Coastal Plain Depression Wetlands

Restoration of coastal plain depressions, a biologically significant and threatened wetland type of the southeastern United States, has received little systematic research. Within the context of an experimental project designed to evaluate several restoration approaches, we tested whether successful revegetation can be achieved by passive methods (recruitment from seed banks or seed dispersal) that allow for wetland "self-design" in response to hydrologic recovery. For 16 forested depressions that historically had been drained and altered, drainage ditches were plugged to reestablish natural ponding regimes, and the successional forest was harvested to open the sites and promote establishment of emergent wetland vegetation. We sampled seed bank and vegetation composition 1 year before restoration and monitored vegetation response for 3 years after. Following forest removal and ditch plugging, the restored wetlands quickly developed a dense cover of herbaceous plant species, of which roughly half were wetland species. Seed banks were a major source of wetland species for early revegetation. However, hydrologic recovery was slowed by a prolonged drought, which allowed nonwetland plant species to establish from seed banks and dispersal or to regrow after site harvest. Some nonwetland species were later suppressed by ponded conditions in the third year, but resprouting woody plants persisted and could alter the future trajectory of revegetation. Some characteristic wetland species were largely absent in the restored sites, indicating that passive methods may not fully replicate the composition of reference systems. Passive revegetation was partially successful, but regional droughts present inherent challenges to restoring depressional wetlands whose hydrologic regimes are strongly controlled by rainfall variability.     

Response of fishes and aquatic habitats to sand-bed stream restoration using large woody debris

Effects of habitat rehabilitation of Little Topashaw Creek, a sinuous, sand-bed stream draining 37 km² in northwest Mississippi are described. The rehabilitation project consisted of placing 72 large woody debris structures along eroding concave banks and planting 4000 willow cuttings in sandbars. Response was measured by monitoring flow, channel geometry, physical aquatic habitat, and fish populations. Initially, debris structures reduced high flow velocities at concave bank toes, preventing further erosion and inducing deposition. Physical response during the first year following construction included creation of sand berms along eroding banks and slight increases in base flow water width and depth. Fish collections showed assemblages typical of incising streams within the region, but minor initial responses to debris addition were evident. Progressive failure of the structures and renewed erosion were observed during the second year after construction.     

Interannual dynamics of a rare vegetation on emerged river gravels with special attention to the critically endangered species <Emphasis Type="Italic">Corrigiola litoralis</Emphasis> L

Emerged gravelly bottoms of large European rivers are very dynamic ecosystems endangered by modifications of their hydrological regime and water pollution. In the present study we investigated the vegetation on an emerged gravelly bottom of an incised segment of the river Elbe in the northwestern part of the Czech Republic. Our knowledge about the vegetation response to great seasonal fluctuations of the water table in incised rivers has so far been limited. We asked the following questions: (1) How are the vegetation composition and (2) occurrence of the critically endangered annual species Corrigiola litoralis affected by their position on gravel deposit, and (3) how are these factors affected by inter-annual variability in water table dynamics? We established nine permanent transects, three at each of three existing extensive gravelly sites, stretching perpendicularly from the river bank and observed annually from 2008 to 2015. At the time when the vegetation was best developed, in the period of the lowest river flow, we recorded all species present in 755 quadrats (1 m² in size) located along the transects. The results show that the year of observation, the distance from the bank and the time for which the sites were exposed significantly influenced the vegetation. Whereas perennial plants were concentrated close to the bank in places least affected by water table fluctuations, annual plants dominated in parts of the gravel deposit more prone to inundation. The frequency of C. litoralis was low; it occurred in only 2.1% of the quadrats. The minimum period of the bottom’s emergence for the establishment of the species was 50 days. The greatest frequency was recorded in 2015, which was the driest year during the study period. The continuation of the present river geomorphology and water table dynamics seems to be sufficient to preserve the continued existence of the vegetation under study, including the populations of C. litoralis.     

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.     

Effects of livestock exclusion on in‐stream habitat and benthic invertebrate assemblages in montane streams

1. Stream and riparian ecosystems in arid montane areas, like the interior western United States, are often just narrow mesic strands, but support diverse and productive habitats. Meadows along many such streams have long been used for rangeland grazing, and, while impacts to riparian areas are relatively well known, the effect of livestock grazing on aquatic life in streams has received less attention. 2. Attempts to link grazing impacts to disturbance have been hindered by the lack of spatial and temporal replication. In this study, we compared channel features and benthic macroinvertebrate communities (i) between 16 stream reaches on two grazed allotments and between 22 reaches on two allotments where livestock had been completely removed for 4 years, (ii) before and after the 4‐year grazing respite at a subset of eight sites and (iii) inside and outside of small‐scale fenced grazing exclosures (eight pairings; 10+ year exclosures) in the meadows of the Golden Trout Wilderness, California (U.S.A.). 3. We evaluated grazing disturbance at the reach scale in terms of the effects of livestock trampling on per cent bank erosion and found that macroinvertebrate richness metrics were negatively correlated with bank erosion, while the percentage of tolerant taxa increased. 4. All macroinvertebrate richness metrics were significantly lower in grazed areas. Bank angle, temperature, fine sediment cover and erosion were higher in grazed areas, while riparian cover was lower. Regression models identified riparian cover, in‐stream substratum, bank conditions and bankfull width‐to‐depth ratios as the most important for explaining variability in macroinvertebrate richness metrics. 5. Small‐scale grazing exclosures showed no improvements for in‐stream communities and only moderate positive effects on riparian vegetation. In contrast, metrics of macroinvertebrate richness increased significantly after a 4‐year period of no grazing. 6. The success of grazing removal reported here suggests that short‐term removal of livestock at the larger, allotment meadow spatial scale is more effective than long‐term, but small‐scale, local riparian area fencing, and yields promising results in achieving stream channel, riparian and aquatic biological recovery.     

Fire effects on the soil seed bank and post‐fire resilience of a semi‐arid shrubland in central Argentina

Soil seed bank is an important source of resilience of plant communities who suffered disturbances. We analysed the effect of an intense fire in the soil seed bank of a semi‐arid shrubland of Córdoba Argentina. We asked if the fire affected seed abundance, floristic and functional composition of the soil seed bank at two different layers (0–5 cm and 5–10 cm), and if fire could compromise the role of the soil seed bank as a source of resilience for the vegetation. We collected soil samples from a burned site and from a control site that had not burned. Samples were installed in a greenhouse under controlled conditions. During 12 months, we recorded all germinated seedlings. We compare soil seed bank with pre‐fire vegetation in terms of floristic and functional composition. The high‐intensity fire deeply affected the abundance of seeds in the soil, but it did not affect its floristic or functional composition. Floristic and functional composition of soil seed banks – at burned and unburned sites‐ differed markedly from that of the pre‐fire vegetation, although a previous study at the same site indicated high resilience after fire of this plant community. Our results indicate that resilience of this system is not strongly dependent on direct germination from seeds buried in the soil. Other sources of resilience, like colonization from neighbouring vegetation patches and resprouting from underground organs appear to gain relevance after an intense fire.     

Hidden biodiversity in the Arctic – a study of soil seed banks at Disko Island,Qeqertarsuaq, West Greenland

Seed dispersal is a key process in plant community dynamics, and soil seed banks represent seed dispersal in time rather than in space. Despite their potential importance, seed bank dynamics in the Arctic are poorly understood. We investigated soil seed banks and corresponding plant community composition in three contrasting vegetation types in West Greenland, viz. dwarf shrub heaths, herb slopes and fell‐fields. Through germination testing, 31 species were documented in soil seed banks. All of these were herbaceous, while no dwarf‐shrub species, which represents the dominating growth form in the above‐ground vegetation, were emerging from the seed bank. Consequently, across vegetation types, the lowest similarity between seed bank and above‐ground vegetation was found in dwarf shrub heath. Nine plant species were exclusively found in seed bank, all of which were non‐clonal forbs. Seed bank size (total number of seeds) and species richness seemed to increase with the level of natural disturbance. Additionally, we examined the effect of different experimental light and temperature conditions on the quantity and diversity of germinating seeds. The difference in diversity in vegetation and seed bank at the species level will impact population dynamics, regeneration of vegetation after disturbances and its potential to respond to climate change.     

森林草原过渡带持久土壤种子库沿降水梯度的格局

为了解全球气候变化背景下森林草原过渡带持久土壤种子库对未来降水减少的响应,本研究以呼伦贝尔森林草原过渡带为研究区域,沿降水梯度采集0～10 cm土层的持久土壤种子库样本,研究种子库密度、物种组成、多样性及其与地上植被的关系,并利用结构方程模型研究年降水量对持久土壤种子库的直接影响及其通过地上植被和土壤有效氮、有效磷、土壤pH值产生的间接影响。结果表明: 随着降水量的降低,种子库密度和物种丰富度有增加趋势,森林草原过渡带草地土壤种子库物种多样性高于森林。土壤种子库与地上植被相似性整体较低。结构方程模型结果显示,年降水量对种子库的密度和物种丰富度的总效应为负效应,标准路径系数为-0.051和-0.122。年降水量对种子库的密度和物种丰富度的直接效应为正效应,降水量通过土壤全氮对种子库密度和物种丰富度产生显著的间接正效应,通过土壤pH和土壤有效磷对种子库物种丰富度产生显著的间接负效应,通过土壤pH对种子库密度产生显著的间接负效应。气候变化下降水减少会改变植物应对风险的策略,森林草原过渡带的持久土壤种子库对应对未来可能发生的降水减少具有一定的缓冲作用。     

The role of the seed bank,seed rain and the timing of disturbance in gap regeneration

Abstract. Question: Does the degree and timing of disturbance contribute significantly to the pattern and process of regeneration in plant communities as a consequence of the availability and number of species of propagules present? Location: Acid grassland at 230 m a.s.l., eastern Scotland, UK. Methods : Plots were surface disturbed or had their soil profile inverted at monthly intervals at 12 dates during a year. Seed bank and seed rain were assessed at each treatment time. The effect of disturbance intensity and timing on the regenerating vegetation was assessed. Results: Removing the seed bank significantly slowed regeneration, as it contributed 43 % of developing cover after one year where it was present. At an individual seed level, seed in the seed rain had a much higher likelihood of contributing to the regenerating vegetation than a seed in the seed bank. Some species showed a reliance on the seed bank for regeneration, and hence there was a significant difference in the vegetation that developed between plots with the seed bank intact and those where it was removed. Winter disturbed plots (little seed rain) had slower rates of re‐vegetation than summer disturbed plots. Timing had little effect on species composition, though a significantly higher cover of perennial forb species developed on the winter disturbed plots. Conclusion: Removing the contribution of the seed bank had a greater effect on the composition of regenerating vegetation than the effect of seasonal variation on the seed rain.     

Seed Bank and Vegetation Development of Sandy Grasslands After Goose Breeding

Four hypotheses were tested using long-term observations of vegetation development (12 years) and present-day seed bank data in a sandy grassland area overgrazed by domestic geese: i) Gap regeneration is crucial in maintaining species richness; thus, closed vegetation of the lower sites prevents continuous establishment of short-lived species. ii) Short-lived, early successional species comprise most of the seed banks and late successional perennials have at most sparse seed banks. iii) Composition of seed banks is more similar to pioneer vegetation than to later successional stages. iv) The similarity is higher between vegetation and seed banks in the upper-positioned plots than in the closed, lower-positioned ones. Two sites, located in the upper part of dune slopes, and another two, positioned on the lower part, were studied. In each site five 2?×?2 m permanent plots were surveyed between 1991 and 2002. Percentage cover was estimated three times a year. In the last study year, soil seed banks were sampled. Two vertical segments (0–5, 5–10 cm) were separately analyzed. The seedling emergence method was applied on concentrated samples. We found that the vegetation developed from open, annual dominated weedy assemblages to grasslands dominated by perennial graminoids. In the lower-positioned sites perennial clonal grasses (Cynodon dactylon, Poa angustifolia and P. pratensis) formed more closed vegetation, which was accompanied by lower species richness compared to the upper-positioned sites. Seed density varied between 10,300 and 40,900 seeds/m². Significantly higher seed densities were found in upper sites than in the lower ones. Annuals and short-lived perennial dicots comprised most of the seed bank. The dominant perennial graminoids also built up dense seed banks. We found a low to medium similarity between vegetation and the seed bank; similarity was the highest with the vegetation of the 1994–1998 period. In the upper sites the similarity between seed bank and the vegetation of the last studied years was also high. The vertical position had a significant effect on regeneration after overgrazing. The large cover of grasses in lower sites decreased species richness and it also decreased the seed density preventing the seed bank formation of annuals and short-lived perennials. Here, further management practices are needed to increase the species richness.     

Hydrological Variation and Fish Assemblage Structure in the Middle Wabash River

We assessed the effect of a severe drought in 1999 upon stream morphology and brook trout (Salvelinus fontinalis) populations in seven headwater streams in the Greenbrier and Potomac River watersheds, West Virginia. During the drought, stream discharge was 96% lower than in years of normal precipitation. As a result, habitat availability and quality over all study streams was significantly lower. Riffle area was greatly reduced (?54%) relative to available pool area (?2%). Fine sediment levels (<0.063 mm) significantly increased within spawning substrate (p=0.01). Water temperature and dissolved oxygen were adequate (mean 15.8?°C, >6.0 mg l^?1, respectively) for brook trout survival in all streams during the drought. Brook trout populations were significantly reduced (adult 60%, Young-of-the-year 67%), and individual fish had significantly lower body condition during the drought relative to the post-drought period. Reductions in brook trout density and population condition during, and in the-post drought period, were related to spatially-limited food resources and/or increased fine sediment levels, but not to degraded water quality. Fisheries managers should consider the effect of periodic drought on brook trout populations and consider short-term harvest restrictions to abet recovery after such stochastic events.