Revegetation following soil disturbance in a California meadow: the role of propagule supply

Revegetation following a disturbance event initially should be constrained by the abundance and types of propagules available at the disturbed site. I tested this idea by conducting two experiments in which I created artificial soil disturbances by excavating or burying pre-existing grassland vegetation. In the first experiment, I varied disturbance intensity (depth), to investigate the consequences for revegetation when numbers of surviving propagules (dormant seeds and bulbs) were altered. In the second experiment, I varied the timing of disturbance, to investigate the consequences when disturbed sites experienced differing exposures to seasonal patterns of clonal growth and seed dispersal. I sampled these experiments from 1991 to 1993, and have interpreted their results using measurements of the seed bank, the bulb bank, and the seed rain. In the first (depth) experiment, bulbs declined in abundance with burial depth and were scarcer in deeper excavations. In contrast, numbers of annual graminoids initially showed no trends with respect to disturbance depth. These results reflect the depth distributions of the seed and bulb banks. Since bulbs occur deeply in the soil, progressively deeper disturbances left fewer survivors. Similarly, perennial graminoids could grow through the shallowest burials. In contrast, since the annual-graminoid-dominated seed bank is concentrated near the soil surface, disturbance depth mattered less to these species: any disturbance removing the surface layer was equally destructive. In the second (timing) experiment, more annual graminoids initially occurred in older plots. This result reflects seasonal patterns of seed production: plots exposed to more of the annual-graminoid-dominated seed rain supported higher densities of annual graminoids as a result. In subsequent years, the vegetation of most plots in both experiments was increasingly dominated by annual graminoids, again as a consequence of their great abundance in the seed rain. These results indicate that interactions between soil disturbances and sources of propagules play an important role in controlling early stages of succession in newly created gaps. They also suggest that disturbance may play different roles in communities characterized by species with different reproductive strategies. Understanding sources of colonists will improve our ability to predict the effects of disturbance.     

Seed supply of native and cultivated grasses in pine forests of the southwestern United States and the potential for vegetation recovery following wildfire

In this study we examined seed supply (seed banks and seed rain) and vegetation of seeded cultivated grasses and naturally occurring native grasses following a wildfire in northern New Mexico, USA. We specifically examined density of native and cultivated grass seeds and plants in areas of high, moderate, and low fire severity. We also examined the similarity between density of native and cultivated grass seeds to density of above ground plants. Density of native grass seed per square meter was higher in areas that burned under low fire severity (85.18 ± 44.83) compared to areas of moderate (18.52 ± 11.26) and high (7.41 ± 4.90) fire severity; however, differences were not statistically significant due, in part, to the high error associated with estimates. Density of cultivated grass seed per square meter was higher than that of native grass seed in areas of high (439.60 ± 117.98) and moderate (437.02 ± 146.50) fire severity, areas that were seeded with cultivated grasses after the wildfire for erosion control. Density of seeded grass plants per square meter was also higher than that of native grass plants in areas of high (18.78 ± 4.59 versus 0.33 ± 0.24) and moderate (8.22 ± 1.76 versus 0.22 ± 0.15) fire severity. There was a higher correspondence between the density of cultivated grass seeds and plants (highest value 0.32 ± 0.11) compared to density of native grass seeds and plants (highest value 0.05 ± 0.04). The high density of seeds, plants, and correspondence indicated that seeds from cultivated grasses are more likely to establish as post-fire vegetation than seeds from native grasses. Seeding with cultivated grasses following a wildfire may slow or inhibit recovery of native grasses in the short term. Longer-term implications for site occupancy deserve further study.     

Plants with longer-lived seeds have lower local extinction rates in grassland remnants 1950–1985

We investigated whether plant species with longer-lived seeds in the soil had lower rates of local extinction between 1950 and 1985 than species with shorter-lived seeds in 26 intact remnants of extensively used calcareous grasslands. In a previous study we had found higher rates of local extinction for smaller populations, for species with shorter life cycles, and for species with higher habitat specificity. We compiled information on seed longevity from recent literature. Seed longevity had a highly significant effect on the rate of population extinction. Rates of local extinction were lower for species with seed longevity >5 years (33.8%; 31 species) compared with species with shorter-lived seeds (1–5 years: 60.7%, 46 species; <1 year: 58.5%, 81 species). The integration of seed longevity into our analysis did not change our previous conclusions on the dependence of local extinction rates on habitat specificity and on life form. Species in our data set with higher habitat specificity tended to have shorter-lived seeds than species with lower habitat specificity. Thus, most characteristic species of calcareous grassland do not persist in the seed bank and can not rely on this mechanism as a buffer against local extinction. Received: 23 December 1998 / Accepted: 29 May 1999     

Vertical structure of seed banks and the impact of depth of burial on recruitment in two temporary marshes

The structure of the seed bank (including Chara oospores), in relation to depth within the sediment and disturbance, was studied in two Rhône delta temporary marshes for two years. The seeds of all species were concentrated in the top 2 cm of sediment with very low numbers beeing found below 4 cm. When an exclosure eliminated disturbances of the sediment by animals, the vertical repartition of seeds at site 2 was more pronounced than outside the exclosure.In experiment 1, the emergence capacity of seeds from different depths and buried under layers of sterile equivalent to those in the field was measured. Depending of the species, 22 to 98% of the seeds germinated from unburied seeds in the top 2 cm. Only 1% of the oospores of Chara (from site 2) at 2 to 4 cm depth in the sediment emerged.In experiment 2, surface seed bank samples were placed under 0, 2 or 4 cm sterile sediment depth. The samples contained numerous recent seeds and the emergence percentage reached 41% (for Ruppia maritima). Only the seeds of Zannichellia spp failed to germinate from a depth of 2 cm or more. The emergence percentage from 2 cm depth or more was always lower than at the surface. These experiments showed that both burial and ageing of seeds decrease germination capacity.The majority of the active seeds located at the surface germinate when the marsh is flooded. Seeds located between 2 and 4 cm can be brought back to the surface by disturbances and play the role of a reserve involved in maintenance of populations that go without seed production for one or some years.     

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.     

Impact of flooding on potential and realised grassland species richness

In order to reduce flood risk, river management policies advise floodplain restoration and the recreation of water retention areas. These measures may also offer opportunities for the restoration of species-rich floodplain habitats through rewetting and the restoration of flood dynamics. The potential to enhance biodiversity in such flood restoration areas is, however, still subject to debate. In this paper we investigate whether flooding along a small altered lowland river can contribute to the potential and realised species richness of semi-natural meadows. We compare the seed bank and vegetation composition of flooded and non-flooded semi-natural meadows and test the hypothesis that flooding contributes to an input of diaspores into the meadow seed banks, thereby promoting seed density and potential species richness. Furthermore we hypothesise that, where habitat conditions are suitable, flooding leads to a higher realised species richness. Results showed that seed densities in flooded meadows were significantly higher than in non-flooded meadows. The seed banks of flooded meadows also contained a higher proportion of exclusively hydrochorous species. However, the seed bank species richness, as well as the species richness realised in the vegetation did not differ significantly between flooded and non-flooded meadows. Finally, the seed bank and standing vegetation of flooded sites showed larger differences in species composition and Ellenberg nitrogen distribution than non-flooded sites. From these results we conclude that, although flooding does contribute to the density and composition of the seed bank, most imported seeds belong to only a few species. Therefore, it is unlikely that flooding substantially enhances the potential species richness. Furthermore, even if new species are imported as seeds into the seed bank, it seems unlikely that they would be able to establish in the standing vegetation. However, it is unclear which factors impede the establishment of imported species in the vegetation. The implications of our findings for flood meadow restoration are discussed.     

Response of two oligohaline marsh communities to lethal and nonlethal disturbance

Severity is recognized as an important attribute of disturbance in many plant communities. However, the effects of disturbances of different severity on patterns of regeneration in oligohaline marsh vegetation have not been experimentally examined. In these communities, a critical difference in the effects of disturbance severity may be whether the vegetation dies as a result of the disturbance or is merely damaged and hence capable of resprouting. We described the regeneration of vegetation in two Louisiana marsh community types, one dominated by Sagittaria lancifolia L. and the other by Spartina patens (Ait.) Muhl., following three levels of disturbance: no disturbance, a nonlethal disturbance, and a lethal disturbance. In the nonlethal disturbance, aboveground vegetation was clipped to simulate common disturbances such as fire and herbivory that remove aboveground vegetation but leave rhizomes intact. In the lethal disturbance vegetation was killed using herbicide to simulate disturbances causing plant mortality such as wrack deposition, sedimentation, scouring, and flooding following fire or herbivory. Regeneration was assessed over a 2-year period by measuring plant species richness, relative abundance, relative dominance, cover, and final biomass. To elucidate mechanisms for observed responses of vegetation, the species composition of the seed bank, light penetration, water level, salinity, and soil redox potential were evaluated. Despite differences in the structure of undisturbed vegetation in the two community types, they exhibited the same overall pattern of regeneration. Following nonlethal disturbance, the dominant species resprouted and quickly reestablished the structure of the vegetation. In contrast, recolonization following lethal disturbance occurred primarily via seedling recruitment, which resulted in marked shifts in community structure that persisted throughout the study. While the two communities responded similarly overall to disturbance, the response of individual species was not uniform; abundance, dominance, biomass, or cover increased for some species but decreased for others in response to disturbance. Seed bank species occurred in the vegetation following lethal disturbance in the Spartina community and in both disturbed and undisturbed plots in the Sagittaria community, indicating that the seed bank is a source of propagules for regeneration and maintenance of oligohaline marshes. Of the environmental variables measured, light level was most closely related to the effect of disturbance severity on community structure. Our results suggest that lethal and nonlethal disturbances have differential effects on regeneration of vegetation that can create pattern in oligohaline marshes communities. Received: 29 September 1997 / Accepted: 12 May 1998     

林窗研究进展（综述）

林窗研究已为当前森林生态学关注的热点之一。它是森林循环更新的一个重要阶段,也是维持森林生物多样性的一个重要环境。本文从林窗的基础理论、概念、特征、林窗的影响作用、林窗模型以及林窗与森林经营等方面,简要介绍了当前国内外有关林窗的研究状况、热点及展望。     

Establishment and reproduction of Aeschynomene virginica (L.) (Fabaceae) a rare,annual, wetland species in relation to vegetation removal and water level

Aeschynomene virginica is a rare annual plant found in freshwater tidal wetlands of the eastern United States. We hypothesized that standing vegetation and water inundation were two important environmental factors in its population dynamics. To test these hypotheses, we sowed seeds into plots with undisturbed vegetation or plots with all aboveground vegetation removed in 1998 and 1999. Presence/absence of seedlings was noted and seedling survival to reproduction, final size, and seed set were measured throughout both growing seasons. Seedling establishment from germination to the first true leaf stage increased with decreasing water depth. Vegetation removal plots had greater seedling establishment, higher seedling survival, and higher seed set per plant than non-removal plots. In a greenhouse study designed to test the effects of water level on seed germination and seedling establishment, no seedlings established in submerged soils, and seed germination and seedling establishment were lower in waterlogged soil than in wet soil. Physical stress associated with deeper water likely limits the distribution of A. virginica to higher elevations, where seeds that colonize patches with low vegetative cover are more likely to produce reproductive adults that produce more seeds relative to patches with established vegetation. A. virginica appears to be a fugitive species specializing on open habitat patches in tidal wetlands. This species may be dependent on disturbances for population establishment and maintenance.     

鼢鼠土丘植物群落演替生态位动态及草地质量指数

采用\"空间代替时间\"的方法,以不同演替系列的鼢鼠土丘植物群落为对象,通过种群生态位动态和功能群变化探讨其演替机制.选取20个主要群落组分种,采用Shannon生态位宽度和Pianka生态位重叠指数进行定量分析.结果表明:较大的生态位宽度常伴随着较大的生态位重叠,但一些具较小生态位宽度的物种之间也存在着较大的生态位重叠;物种之间的生态位重叠普遍较大.功能群变化分析表明:随着演替的进行,1、2年生杂草先迅速减小后缓慢增大,但总体减小;多年生杂草先增大后减小;多年生禾草类线性增加.反映了多年生禾草类最终在演替顶极群落中占据主导地位.采用草地质量指数分析了鼢鼠土丘植被状况,表明随着演替的进行,草地质量指数显著增加.     

植物生活史繁殖对策与干扰关系的研究

植物生活史繁殖对策研究是涉及植物的适应或进化、生态系统退化与恢复过程、生物多样性保护等多方面理论生态学和应用生态学研究内容的生态学研究领域。按Grime的植物生活史繁殖对策分类、植物营养繁殖与干扰适应、种子形态学与干扰适应、土壤种子库与干扰适应、植物繁殖体传播和萌发与干扰适应论述了当今极受关注的植物生活史繁殖对策与干扰关系,简述了我国干旱区干扰与植物生活史繁殖对策关系研究。     

Role of soil seed bank along a disturbance gradient in an alpine meadow on the Tibet plateau

We examined the role of the soil seed bank along a grazing disturbance gradient and its relationship with the vegetation of alpine meadows on the Tibet plateau, and discussed the implications for restoration. The seed bank had a high potential for restoration of species-rich vegetation; 62 species were identified in the vegetation and 87 in the seed bank, 39 species being common to both. Mean seed density was 3069–6105 viable seeds m⁻². The density of buried seeds increased significantly with increasing disturbance, indicating that restoration of disturbed areas is not seed limited. Seed density and species richness decreased with depth. The proportion of perennial species decreased with decrease in disturbance both in seed bank and in vegetation. A large portion of species with persistent seeds in the disturbed areas indicate that this seed type can be regarded a strategy of adaptation to current disturbances. Detrended correspondence analysis (DCA) showed significant differences of species composition between seed bank and vegetation, except for the seriously disturbed site. Our results suggest that the establishment of new species in severely disturbed areas is more dependent on the seed bank. By contrast, the restoration in less-disturbed and mature meadows does not rely on seed banks, and the establishment of the vegetation in these communities is more likely to rely on seed dispersal from the standing vegetation and on species with vegetative reproduction.     

Soil seed bank composition and diversity in a managed temperate deciduous forest

Little is known about the influence of forest management on the interaction between seed bank and aboveground vegetation. We surveyed seed banks and vegetation in 10 forest stands under similar abiotic conditions but submitted either to a coppice-with-standards treatment (n=5) or to a selective-cutting system (n=5). We analyzed species composition and diversity, community ecological profile, and distribution of taxa among different life forms, strategy, morphology and functional type categories. A total of 2085 seedlings (8296 seedsm^–2) germinated-corresponding to 28 species, among which Juncus effusus was the most abundant. Fifty-seven percent of the species were also recorded in the aboveground vegetation, the dominant species being Rubus fruticosus agg., but only 28% of the aboveground species were present in the seed bank. Our results suggest that (1) vernal geophytes and shade-tolerant perennials, which group most true forest species, are not incorporated in the seed bank, (2) parent plants of most seeds were present either in the stand in an earlier dynamic stage or apart from the stand and long-distance dispersed, (3) as expected, early-successional species are well represented in the seed bank, (4) forestry vehicles seem to be a major means of dispersion for stress-tolerant species normally found in forest lanes and wheel tracks. We conclude that seed banks contain species that have a potentially negative impact on the true forest flora and, thus, forest management should minimize soil disturbance and retain remnants of old-coppice woods to conserve disturbance-sensitive true forest species.     

沙坡头人工固沙区的种子库动态

沙坡头人工固沙区的土壤种子库中共有４６种植物的种子,属于１７个科。其中菊科植物最多,达１１种;其次为藜科,７种;禾本科７种;豆科４种。既是现有植被组分种,其种子又存在于种子库中的,占总种数的３３．３％ 。油蒿（Ａｒｔｅｍ ｉｓｉａ ｏｒｄｏｓｉｃａ）、花棒（Ｈｅｄｙｓａｒｕｍ ｓｃｏｐａｒｉｕｍ ）和柠条（Ｃａｒａｇａｎａ ｋｏｒｓｈｉｎｓｋｉｉ）的种子产量为５４５±６７６．７３粒／ｍ ２、６．６７±１３．７４粒／ｍ ２、０．０３２±２．４３×１０－ ３粒／ｍ ２。油蒿在植株上成熟时不受虫害,而花棒、柠条的种子受害比例分别为３１．９２％ 和８３．２９％ 。在最适埋藏深度下,油蒿、柠条、花棒种子的累积发芽率分别可达７８．３％ 、８５．０％ 和６６．７％ ;萌发速度：油蒿＞ 柠条＞ 花棒。根据种子库动态分析和植被调查结果可初步预测：沙坡头人工固沙植被的演替趋向将是灌木以油蒿为主,草本以雾冰藜（Ｂａｓｓｉａ ｄａｓｙｐｈｙｌｌａ）、小画眉草（Ｅｒａｇｒｏｓｔｉｓｐｏａｅｏｉｄｅｓ）为主的群落