Seed dynamics during forest succession in Costa Rica

Soil seed banks and current seed inputs each play a role in tropical succession. We compared the abundance and floristic composition of seeds from these two sources at a Costa Rican site by germinating seeds from the soil, measuring seed inputs for 3 yr, and monitoring the earliest colonists in a forest clearing.There were an estimated 6800 viable seeds/m² in the soil of 3.3-yr-old vegetation, 9500 seeds/m² in 11-yr-old vegetation, and 7000 seeds/m² in a 75-yr-old forest. An estimated 10100 seeds/m² fell on the soil surface of the young successional vegetation during 3 yr and 3700 seeds/m² fell during that same time in the forest.Locally produced seeds accounted for about 75% of the seed input to the soil surface early in succession. Seeds dispersed out of young successional vegetation increased the quantity and species richness of the seed input and storage in an adjacent forest. Much of the species richness of the young successional vegetation resulted from seeds dispersed there from other communities by animals.Deforestation stimulated germination of most seeds in the surface soil of the old forest, including seeds of the dominant canopy tree. The recruitment of seedlings from the soil seed bank numerically overwhelmed that from post-disturbance seed rain and sprouts.We evaluated patterns of soil seed storage during succession and predicted the ability of vegetation of differing ages to respond to disturbance. Immediately after disturbance the number of seeds in the soil plummeted due to mortality, low inputs, and germination. As the vegetation regrew, the soil seed bank increased to a peak after 4 to 7 yr, then gradually decreased to its pre-disturbance size. High-frequency pulses of disturbance should result in reduced species richness, dominance by species with long-lived seeds, and fast recovery by seedling recruitment from the soil seed bank.Journal series number 6459 from the Institute of Food and Agricultural Sciences, University of Florida, Gainesville, Florida 32611, USA.Reprint requests to J. J. E. at Florida.     

Germination and seedling survival in fens undergoing succession

We studied mechanisms of vegetation change in fens subject to succession from open water to floating mats and finally herbaceous rich-fens. Earlier research showed that these systems are characterized by transient seed banks. Our main question was whether seedlings of later successional fen stages are already present in earlier stages, remaining subordinate in the vegetation until conditions become suitable for them. If, however, conditions during succession change in a way that only a limited set of species can survive as seedlings during each of the successional stages, no seedling bank will exist. The transient character of the seeds would then imply that seeds will not germinate and will subsequently die and that seeds that have germinated in the “wrong” stage will not become established. We hypothesized that: (1) germination and seedling survival of fen species are significantly better in the successional fen stage for which these species are characteristic, (2) as a consequence no seedling banks occur in these fens. In a field experiment, seeds of five characteristic fen species in the standing vegetation of three successional fen stages i.e. raft fen, quaking fen and rich fen were sown in each of these stages in a turf pond in the Tienhoven area, The Netherlands. Germination and seedling survival were measured over two growing seasons together with environmental variables. Germination was higher in the “own” stage for all species groups as was survival for quaking fen species and rich fen species. For both these stages, percentage of germination and survival of four out of five characteristic species were significantly higher in the “own” stage. Germination and survival can be considered stage-dependent and it was concluded that seedling banks do not exist in these fens. Site-specific environmental variables act as a sieve and differentiate on species presence already during early life history stages. We found clues that the environmental sieve acts at the level of nutrient availability, tolerance for high sulphide concentration and light climate. Because of the transient seed bank and absence of a seedling bank in these fen wetlands, successful establishment of species necessitates a continuous dispersal of characteristic species until the environmental conditions permit establishment. This also implies that species of the whole successional sere should be present within dispersal distance.     

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.     

西双版纳白背桐次生林土壤种子库、种子雨研究

 本文讨论了西双版纳常见的次生林白背桐(Mallotus paniculatus )林的土壤种子库和种子雨的组成及动态。结果表明该林地的种子雨年输入量为745粒/m2,土壤种子库储量在雨季末期为3345±438粒/m2,旱季末期为4555±554粒/m2,种子雨的输入高峰出现在旱季,导致旱季末期土壤种子库储量大于雨季末期。森林破碎化导致的隔离效应阻碍了种子的传播途径,使土壤种子库储量下降,种子雨和土壤种子库中鸟类传播的种子比例增大。传播成熟林树种的大型动物到达不了该样地,植被的演替受到了阻碍。     

Salt stress limitation of seedling recruitment in a salt marsh plant community

Summary Seedling recruitment in salt marsh plant communities is generally precluded in dense vegetation by competition from adults, but is also relatively rare in disturbance-generated bare space. We examined the constraints on seedling recruitment in New England salt marsh bare patches. Under typical bare patch conditions seed germination is severely limited by high substrate salinities. We examined the germination requirements of common high marsh plants and found that except for one notably patch-dependent fugitive species, the germination of high marsh plants is strongly inhibited by the high soil salinities routinely encountered in natural bare patches. Watering high marsh soil in the greenhouse to alleviate salt stress resulted in the emergence of up to 600 seedlings/225 cm². The vast majority of this seed bank consisted of Juncus gerardi, the only common high marsh plant with high seed set. We tested the hypothesis that salt stress limits seedling contributions to marsh patch secondary succession in the field. Watering bare patches with fresh water partially alleviated patch soil salinities and dramatically increased both the emergence and survival of seedlings. Our results show that seedling recruitment by high marsh perennial turfs is limited by high soil salinities and that consequently their population dynamics are determined primarily by clonal growth processes. In contrast, populations of patch-dependent fugitive marsh plants which cannot colonize vegetatively are likely governed by spatially and temporally unpredictable windows of low salinities in bare patches.     

The role of seed dispersal in the natural regeneration of rain forest after strip-cutting in the Peruvian Amazon

Seed dispersal and forest regeneration were studied on a 30×150 m strip cleared by strip-cutting, a system of forest management designed for sustained yield (Hartshorn 1989), in high terrace rain forest in the Department of Loreto, Peru. After one year the strip was dominated by seedlings of a few bat- and bird-dispersed pioneer tree species (Cecropia spp., Melastomataceae, and Alchornea triplinervia); stump sprouts from cut trees and saplings that survived the clearing were less numerous. The density of saplings (>2 m in height) surviving the clearing was 903 per hectare; 94% of these survived the subsequent 18 months. About 30% of 417 stumps (>7.5 cm diameter at breast height) resprouted within 3 months, with an additional 10% sprouting in the subsequent 10 months. Sprouting frequency was greater for small stumps than large and varied greatly among plant families. Seed deposition over this year was much lower in the interior of the strip, both in species richness and numbers of seeds, than within the forest; strip edges were intermediate in richness and number. The decline in seed input from forest to edge to strip, both in species and in numbers of seeds, was most pronounced for bird-dispersed taxa (primarily Melastomataceae); bat- and wind- dispersed taxa were more evenly distributed. The similarity in bat species composition between the strip and nearby primary forest was higher than the similarity in bird species composition between these habitats, reflecting a failure of many forest bird species to venture into the strip. The predominance of Cecropia spp. and other pioneers of minimal commercial value in the regeneration question the sustainability of strip-cutting. Subsequent succession and future tree species composition on the cleared strip will depend not only on the survivorship and growth of sprouts, survivors, and seedlings, but also on responses of different seed-dispersing animal taxa to changes in the species composition and structure of the vegetation in the strip.     

Plant traits: Their role in the regeneration of alpine plant communities in sub‐arctic Finland

Abstract. Question: How do the relative frequencies of plant traits (clonality, growth form, seed weight, diaspore morphology) vary during the life cycle and how does this affect regeneration? Location: Alpine meadow and heath communities at Kilpisjärvi, sub‐Arctic Finland. Methods: Control plots and three treatments were used to measure relative species abundances for five life cycle stages: standing vegetation, seed rain, seed bank and seedlings emerging in gaps and in closed vegetation. Results: The relative frequencies of plant traits varied between the life cycle stages. The meadows were dominated by weakly clonal herbs, small or intermediate seeds and unappendaged diaspores, while the heaths were dominated by clonal dwarf shrubs, small seeds and fleshy fruits. In the meadows, species with small seeds dominated during the seed rain and in the seedling stage in gaps, while species with intermediate seeds dominated the seed bank and the seedling stage in closed vegetation. Species with unappendaged diaspores dominated throughout the life cycle. In the heaths, seed bank and seedling stage were practically absent. Conclusions: The observed differences in plant trait spectra between life cycle stages indicate that important environmental factors differ among the stages. Small seeds are advantageous for dispersal, whereas intermediate seeds have a greater probability of germinating and establishing in closed vegetation. Appendages facilitate dispersal, whereas unappendaged diaspores favour seed burial. Although the plant growth form spectrum largely reflects environmental constraints during the regeneration cycle, information on seed weight and diaspore morphology improves our knowledge of the relative importance of morphological adaptations of sexual structures in different stages during the life cycle.     

Development of vegetation patterns in early primary succession

Question: We investigated colonisation filters in early plant community development on a glacial outwash plain. We asked if these were related to seed limitation or to a lack of safe sites, if topographical heterogeneity affected species patchiness and how species life cycles influence successional trajectories. Location: An outwash plain (Skeiðarársandur) in southeast Iceland. Methods: We identified surface heterogeneity at two different scales, ca. 10–15 cm (larger stones and established plants) and ca. 50 m (shallow depressions representing dry river beds) at two study sites. We quantified species cover, flowering plant density, seed production, seed rain, seed bank density, seedling emergence and seedling survival from June 2005 to June 2007 for the whole plant community, and measured seed production for five species. Results: Mean vegetation cover was <2.5% at the sites. Low emergence rates and high seedling mortality were the two main recruitment filters. Only 1.4% of seedlings emerging in 2005 survived into the 2007 growing season. Topographical heterogeneity had little effect on plant colonisation. High annual variation was recorded, and the two study sites (ca. 2 km apart) differed in their colonisation success. Of the five species, establishment of Cerastium alpinum and Silene uniflora was most limited by lack of seeds, whereas establishment of Luzula spicata, Poa glauca and Rumex acetosella was most limited by safe sites. Conclusions: We conclude that colonisation processes and patterns in early primary succession on Skeiðarársandur were largely influenced by stochastic factors.     

Rainfall partitioning and related hydrochemical fluxes in a diverse and in a mono specific (Phenakospermum guyannense) secondary vegetation stand in eastern Amazonia

Rainfall partitioning into throughfall and stemflow was studied in a diverse and in a mono specific stand of secondary vegetation in Eastern Amazonia. The nutrient concentrations in the water were analysed in order to quantify the related hydrochemical fluxes. Secondary vegetation forms the fallow in the local shifting cultivation system and is usually dominated by shrubs and trees. Phenakospermum guyannense (Strelitziaceae), a banana-like herb, is one of the predominant non-woody species. The study was conducted during an 18-month period in a 2.5-year-old relatively species-rich stand and a 10-year-old stand dominated by P. guyannense. In a year with 1956 mm of rainfall 65% (1281 mm) of this quantity reached the soil as throughfall in the diverse stand and 38% (743 mm) in the mono specific stand. Stemflow was estimated to be 23% and 41% respectively. P. guyannense and Banara guianensis (Flacourtiaceae), a tree species, were causing these high funnelling effects. In the young diverse stand B. guianensis had a stemflow of more than 200 l year⁻¹ and P. guyannense had a median flux of 77 l year⁻¹ per pseudostem. In the older stand the taller plants of P.␣guyannense collected 644 l year⁻¹ per pseudostem on the median. The reason for these high values could be the banana-like growth form of P. guyannense and the crown morphology of B. guianensis, which has inclined branches. The low proportion of throughfall and the high stemflow values differ from all previous studies in Amazonian primary forests. The proximity to the Atlantic Ocean strongly influenced the nutrient fluxes via rainfall at our study site. This becomes obvious from the high Na and Cl fluxes with rainfall (19.7 kg Na ha⁻¹ year⁻¹, 37.2 kg Cl ha⁻¹ year⁻¹) which were approximately equal to the Na and Cl fluxes with the sum of throughfall and stemflow for both stands. K fluxes in throughfall and stemflow in both stands were higher than in rainfall by a factor of 8. The high K enrichment during the crown passage is assumed to be caused by a␣high K concentration in the leaf tissue resulting in enhanced leaching from the leaves. In months with low␣rainfall the concentrations of Ca, Mg, S and Cl in throughfall of the diverse stand were significantly higher than in months with high rainfall. This was mainly due to vegetation burns in the dry period, which resulted in ash deposition on the canopy and subsequent wash-off and solution of ash particles. Received: 11 May 1997 / Accepted: 2 November 1997     

Soil carbon dynamics in regrowing forest of eastern Amazonia

The future flora of Amazonia will include significant areas of secondary forest as degraded pastures are abandoned and secondary succession proceeds. The rate at which secondary forests regain carbon (C) stocks and re-establish biogeochemical cycles that resemble those of primary forests will influence the biogeochemistry of the region. Most studies have focused on the effects of deforestation on biogeochemical cycles. In this study, we present data on the recuperation of carbon stocks and carbon fluxes within a secondary forest of the eastern Amazon, and we compare these measurements to those for primary forest, degraded pasture, and productive pasture. Along a transect from a 23-y-old degraded pasture, through a 7-y-old secondary forest, through a 16-year-old secondary forest, and to a primary forest, the δ¹³C values of soil organic matter (SOM) in the top 10 cm of soil were – 21.0, – 26.5, – 27.4, and – 27.9‰, respectively, indicating that the isotopic signature of SOM from C3 forest plants was rapidly re-established. The degraded pasture also had significant inputs of C from C3 plants. Radiocarbon data indicated that most of the C in the top 10 cm of soil had been fixed by plants during the last 30 years. Differences in soil C inventory among land use types were small compared to uncertainties in their measurement. Root inputs were nearly identical in primary and secondary forests, and litterfall in the secondary forest was 88% of the litterfall rate of the primary forest. In contrast, the secondary forest had only 17% of the above ground biomass. Because of rapid cycling rates of soil C and rapid recovery of C fluxes to and from the soil, the below ground C cycle in this secondary forest was nearly identical with those of the unaltered primary forest.     

Small-scale patterning in the recruitment of forest species during succession in tropical dry forest,Mozambique

Within a few years of field abandonment on the coastal dunes, a considerable number of species of trees and woody lianes of the climax evergreen forest enter the succession. At a relatively large-scale of analysis (a 100 m² plot), there is no evidence of soil development during the early succession, with no accumulation of soil organic matter nor nutrients being detected. At this scale there is, therefore, little support for the facilitation model of succession. However, when smaller scales of analysis are employed (e.g. a plot equivalent to the canopy area of a sapling) a very different picture emerges; most recruitment of forest woody species occurs beneath saplings found in early succession. Nutrient concentrations were higher in soils under sapling canopies than in soils beyond sapling canopies. With increasing sapling size, an increasing number of seedlings of forest woody species were found and there is progressive soil amelioration. Thus, during the early succession, there is considerable small-scale patterning in soil development and recruitment. This pattern is largely established through bird dispersal, with the intensity of recruitment being dependent on the distance to seed sources.     

Regeneration processes of biological soil crusts,macro-cryptogams and vascular plant species after fine-scale disturbance in a temperate region: Recolonization or successional replacement?

Open base-rich sandy habitats are often characterised by an extensive cover of biological soil crusts (BSCs). We studied the impact of mechanical disturbances on cyanobacteria/algae-dominated BSCs, macro-cryptogams (bryophytes, lichens), phanerogams and their regeneration processes in a 3-year field experiment in a temperate region. We compared controls with two fine-scale disturbance regimes, each based on 50 plots (20 cm×20 cm): 1. BSCs were raked (weak disturbance), 2. BSCs were completely removed (strong disturbance). We used a Sileno conicae-Cerastion semidecandri habitat as a model area for the field study and ask if the crusts are able to regenerate or will be replaced by macro-cryptogams and/or phanerogams.     

Altitudinal gradients in seed dynamics of Calluna vulgaris in eastern Scotland

Abstract. Relationships between seed deposition, size of soil seed banks and some of the environmental factors affecting them were assessed for Calluna vulgaris throughout its altitudinal range (150–960 m a.s.l.) in eastern Scotland. Seed rain was assessed using pitfall traps, collected every 5–10 wk for 3 yr; germinability was determined by laboratory incubation. Seed bank size was estimated, once only, by counting seedlings emerging from soil cores kept for 50 wk in a glasshouse. Seed deposition varied annually, was related to parent plant cover but always declined with altitude, falling sharply above 600 m a.s.l. Seed bank size was more closely correlated with the proportion of organic matter in the soil than with the amount of seed rain. Seed bank sizes declined gradually with altitude but did not differ significantly between four altitudinal zones. The mean density of buried seeds was less than half the mean annual seed rain at sites below 300 m a.s.l. but was over 200 × greater than annual seed rain above 800 m, suggesting that seeds buried at high altitudes remain viable for much longer than those at lower altitudes.     

Foraging activity of an Amazonian leaf-cutting ant: responses to changes in the availability of woody plants and to previous plant damage

This study investigates some factors affecting the foraging activity of the leaf-cutting ant Atta laevigata. The study was conducted on an abandoned farm near Manaus, in Brazilian Amazonia, where forest was beginning to regenerate. I determined how temporal changes in the structure of the woody plant community (linked to the regeneration process) affected the leaf harvesting activity of this Neotropical ant. A 0.39-ha plot was established, and woody plants existing and emerging in this plot were identified, tagged, measured and mapped. At intervals of approximately 14 days all marked plants were checked to determine which had been attacked by A. laevigata during that period. Woody plant density doubled over the 18-months of the study. However, the number of plants attacked by A. laevigata, controlling for seasonal differences in ant activity, was independent of the number available for attack. The number of plants attacked was also independent of the number of new plants emerged in the previous 2 months. Apart from changes in total plant density, the relative abundance of most species also changed with time. Data on the composition of the plant community and on the composition of the ants' diet at different time intervals were subject to ordination analysis. The analyses revealed that successional changes in the composition of the woody plant community did significantly affect the composition of the leaf diet of A. laevigata. Variations in the ants' diet correlated most strongly with variations in the abundance of Bellucia imperialis. As this increased in abundance its relative contribution to the ants' diet increased. In addition, I observed a small decline in the diversity of plant resources exploited by the ants. B. imperialis was one of the species most preferred by A. laevigata, suggesting that variations in the diversity of its diet were related to the abundance of preferred species. The chances of an individual plant being attacked was independent of whether or not the plant had been attacked during the previous 5 months. This suggests that plant resistance against ant herbivory is not affected by previous damage. Two other lines of evidence also pointed to this. First, there was no significant difference in the leaf area cut (as measured during assays of leaf preference with A. laevigata) between leaves of plants that had been subject to artificial defoliation 3 mo earlier and those of control plants. Second, the number of attacks by A. laevigata on individuals of a given species usually followed a random distribution, even though a uniform or clumped distribution would have been expected under the hypothesis that damage by leaf-cutting ants affects the plant's resistance to further damage. Received: 3 March 1997 / Accepted: 23 June 1997     

Recovery of conservation values in Central African rain forest after logging and shifting cultivation

Secondary forests in Central Africa are increasing in importance for biodiversity conservation as old growth forests outside the few protected areas are disappearing rapidly. We examined vegetation recovery in a lowland rain forest area in Cameroon based on a detailed botanical survey of old growth forest and different-aged logging gaps (5–27 years) and shifting cultivation fields (10–60 years). Our analysis focuses on the long-term recovery of botanical conservation values by analysing trends in vegetation structure, species composition, species diversity and levels of endemism and rarity. In the total survey (4.25 ha), we recorded 834 species of which 23% were endemic to the Lower Guinea forest region. The proportion of endemic species was high in shrubs and low in herbs. Geographic range and (local) rarity were not significantly associated. The proportion of rare species (relative frequency <10%) was high in woody climbers and low in trees. In logging gaps, recovery of all vegetation characteristics was relatively quick (5–14 years). Recovery in shifting cultivation sites took longer (30–60 years). Endemic species were found to be highly sensitive to shifting cultivation practices and even after 50–60 years the level of endemism was still significantly lower compared to old growth forest. The proportion of rare species was not significantly different between disturbed sites and old growth forest. We conclude that secondary forests can contribute to biodiversity conservation, e.g. as buffer zones around protected areas. However, this contribution should be assessed differently between land use types and widespread versus endemic species.     

Seed bank versus seed rain in the regeneration of a tropical pioneer tree

Summary We used the tropical pioneer tree, Cecropia obtusifolia to evaluate the relative importance of different sources of seeds in the regeneration of species that depend on ephemeral sites. We studied seed production in a population established in a 5 ha plot, and dispersal, dormancy and seed predation in two recent treefall gaps (<1 year-old), two building or successional forest patches (10–15 since disturbed), and two mature forest patches (>35 years since disturbed) for a one year period at Los Tuxtlas (Mexico). Flowers and fruits were counted at monthly intervals. Annual fecundity per tree ranged from 1.4×10⁴ to 1.4×10⁷ seeds. Seeds were continuously available on the trees and on the ground. Average annual seed rain per m² (as measured by 0.5×0.5 m seed traps) varied from 184 to 1925 among the six sites. Distance to nearest seed source and patch type explained more than 60% of the seed rain variation among sites. Soil seed density, estimated by counting seeds from ten samples (78.5 cm²×10 cm deep) collected from each site in October and January, ranged among the six sites from 269 to 4485 seeds per m² in January and from 204 to 5073 in October. Soil seed viabilities were much lower (17.1% in October and 5.1% in January) than those of rain seeds (48.26%). Annual survivorships of 2.2% were estimated for seeds artificially sown on the soil surface of a gap and a mature patch, and 3.75% in a building patch. In two other experiments seed removal rates ranged from 27% to 98% in 4 days. Removal rates were significantly higher in gap and mature patches than in building patches. Ants (Paratrechina vividula) and grasshopper nymphs (Hygronemobius. sp.) were the main predators. We draw three main conclusions from our data: (1) Pathogens and predators determine low survivorship of C. obtusifolia's seeds in the soil and a rapid turnover rate (1.07 to 1.02 years) of its seed bank; (2) a continuous and copious seed production and an abundant and extensive seed rain replenish the soil seed pool in patches with different disturbance ages at least up to 86 m from nearest source; (3) more than 90% of the seeds contributing to C. obtusifolia seedling recruitment in gaps are less than one year-old. We discuss our results in the context of previous similar studies for tropical forests.     

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.     

The role of ecological knowledge in explaining biogeography and biodiversity in Amazonia

Biogeographical studies in Amazonia have commonly taken a historical, rather than an ecological approach. General patterns have been sought in the distribution maps of different species, and these have been explained in terms of past or present distribution barriers, especially past climates and large rivers. Implicitly, and often also explicitly, it is assumed that Amazonia is ecologically so uniform that present-day ecological conditions are rather insignificant in determining species distribution patterns and speciation. However, this assumption is more based on the lack of relevant data than on actual observations of environmental uniformity or ecological unspecialization of the species. Recent studies have indeed documented ecological heterogeneity and floristic differences among sites that were previously thought similar. In the absence of direct knowledge of the past, more complete ecological and environmental understanding of the present-day Amazonia are needed for evaluating the relative roles of historical and ecological factors in Amazonian biogeography and biodiversity.     

科尔沁沙地植被恢复演替进程中群落土壤种子库研究

研究了科尔沁沙地植被恢复演替进程中各群落土壤种子库种类组成、密度以及物种多样性等特征。主要结论是:①科尔沁沙地植被退化后的恢复演替是一种次生的中途恢复演替。演替各阶段土壤种子库密度为流动沙丘<半流动沙丘<固定沙丘,从流动到半流动沙丘阶段,土壤种子库密度平均增加了709%;从半流动到固定沙丘阶段,土壤种子库密度平均增加了393%。从流动到半流动沙丘阶段是土壤种子库密度的快速增长期。②植被恢复演替过程中,土壤种子库组成均以1年生草本植物为主(优势度为60.40%~91.83%),到演替中后期阶段,多年生草本植物的种类有所增加,但其所占比例仍很小。③演替各阶段群落种子库物种多样性指数分别为0.6616、0.7736、0.7281、1.0939、1.0648和0.9682,可见种子库物种多样性最高的群落并非是演替历史最大的群落。④恢复演替系列各阶段土壤种子库间的相似性系数都较大,在0.368~1.000范围之内,任一群落总是与其下一阶段最邻近的群落具有最高的相似性系数。