Plant-soil feedback in spruce (Picea abies) and mixed spruce-beech (Fagus sylvatica) stands as indicated by dendrochemistry

Ten pairs of secondary pure spruce (Picea abies) and adjacent mixed spruce-beech (Fagus sylvatica) stands on comparable sites were selected on two different bedrocks for soil formation (Flysch: nutrient rich and high soil pH; Molasse: poor nutrient supply and acidic) to study how an admixture of beech to spruce stands affects nutrient cycling and consequently soil chemistry. Soil analyses indicated accumulation of Ca under the mixed stands while the top soil under pure spruce was acidified. It was hypothesized that changes of soil chemical properties due to species composition over the last six decades are reflected in the stem wood of spruce. Three healthy dominant spruce trees per plot were selected for coring. Cores were crossdated and half-decadal samples were analyzed for Ca, Mg, Mn and Al. Calcium and Mg concentrations in stem wood of spruce were significantly higher for the pure spruce than for the mixed stands in spite of lower Ca and Mg stores in the soil. We assume that acidification caused by pure spruce mobilized these cations temporarily, increasing soil solution contents and consequently stem wood concentrations. It was possible to reconstruct soil pH from the element ratios Ca/Al (pure stands) and Ca/Mg (mixed stands), since these ratios in the stem wood of the last half-decade did correlate with soil pH for selected soil depths. Reconstructed soil pH showed a decline over the last 60 years under both species compositions due to accumulation of base cations in the increasing biomass. Comparisons of reconstructed soil pH in 0–5 and 10–20 cm soil depth indicated more pronounced top soil acidification (lower soil pH in 0–5 cm) by spruce on the nutrient rich soil (Flysch) than on the acidic soil (Molasse). However, admixture of beech caused higher pH values in 0–5 cm than in 10–20 cm soil depth on Flysch due to the observed Ca-pump effect of beech (uptake of Ca from deeper soil horizons).     

Factors determining species richness of soil seed banks in lowland ancient woodlands

The demise of coppicing in UK ancient woodlands, combined with the planting of non-native, fast-growing conifers in the twentieth century, heightens the potential recharge value of ground flora seed banks. Soil cores from adjoining semi-natural and conifer-containing stands in four lowland ancient woods in central England were removed to establish seed bank species richness. During a fourteen-month germination trial soil from two depths yielded 6554 seedlings from 81 species, ten of which showed a strong affinity for ancient woodland conditions. Juncus effusus accounted for 80% of emergent seeds whilst 23 other species, including Lysimachia nummularia and Potentilla sterilis, were represented by only one individual. Species richness is described by a model that explains 40% of observed variance (P < 0.00001). The model has three significant variables: species richness increases as soil pH rises, and decreases with both depth and increasing time since the most recent planting/disturbance event. No difference was found in the density of seeds from species common to paired semi-natural and conifer-containing stands that were separated only by a woodland ride, suggesting prior management and environmental conditions have a greater influence on seed banks than current stand type. Sørensen similarity index values revealed poor congruence between above-ground vegetation and species in the seed bank. Taking pH measurements in conifer stands identified as younger in terms of planting/disturbance may help locate areas where greater numbers of species (including woodland specialists) are located. Caution is required, however, as these seed banks may also contain non-target, competitive species that may swamp the regeneration of woodland specialists.     

The dynamics of three shrub species in a fire-prone temperate savanna: the interplay between the seed bank, seed rain and fire regime

A model was developed to assess how the seed rain and fire regime affect seed bank dynamics and seedling establishment of three native shrub species (Acanthostyles buniifolius, Baccharis pingraea and Baccharis dracunculifolia) with different regeneration strategies, in temperate South American savanna. Seed bank and seed rain were quantified for each species under different fire regimes, and their relative roles in regeneration were evaluated. All species had short-term persistent seed banks and high annual variability in seed production. A high proportion of seeds deposited in the seed rain produced seedlings after fire; few entered the soil seed bank. Fire killed a high proportion of the seeds in the soil seed bank. Seedlings derived from the seed rain had a higher probability of surviving for 2 years than seedlings emerging from the soil seed bank. In the absence of fire, establishment depended on germination both from the seed rain and the soil seed bank, whereas with annual fire, establishment was primarily dependent on germination of seeds arriving in the annual seed rain, regardless of species’ regeneration strategies. These results help to explain changes in the vegetation of South American temperate savannas as a result of changes in fire regime and grazing management during the last 50 years. By revealing the crucial roles of the soil seed bank and seed rain in regeneration, this study provides vital information for the development of appropriate management practices to control populations of shrub species with different regeneration strategies in South American temperate savannas.     

What we don’t seed: the role of long-lived seed banks as hidden legacies of invasive plants

Acacia dealbata is an invasive south-eastern Australian tree that produces a persistent soil seed bank. In order to characterize the seed bank in plots invaded by A. dealbata and to understand its implications for management and re-invasion risk, density, germinability, and viability of the seed bank were evaluated in five sites in central Portugal. Soil samples were collected in A. dealbata invaded plots and adjacent areas and screened for seeds, which were then quantified and germinated at 25 °C. A subset of seeds was first exposed to 60 °C to assess the effect of high soil temperature on dormancy breaking. Variables influencing differences between sites were explored with generalized linear mixed models with a Poisson distribution. Inside A. dealbata invaded plots the seed bank averaged 4608 seeds/m², reaching up to 62,747 seeds/m²; in adjacent areas, up to 14 m from the plots, only 9 seeds/m² were found. Seed bank densities were mostly influenced by stoniness, number of fires in the last 10 years, and density of trees and roots. Almost 90% of seeds were viable, but only 8.6% germinated without treatment. Nearly 70% of seeds exposed to 60 °C germinated without any physical stimulation, suggesting that high soil temperatures can effectively break seed dormancy. The high density and viability of A. dealbata’s seed bank and its ability to disperse seeds far from the parent plants contribute to the species’ invasive success. These features combined with heat events that can overcome seed dormancy need to be considered in the management of this species.     

川西米亚罗亚高山云杉林种子雨和土壤种子库研究

 对川西米亚罗亚高山20、30、60年人工云杉(Picea asperata)林及天然林的种子雨和土壤种子库进行了研究。结果表明：该区云杉种子雨一般从每年的10月初开始下落,一直到翌年的1月底结束,但不同林龄的云杉种子雨强度不同,60年人工林种子雨量最大,然后依次为30年人工林、天然林、20年人工林,其大小分别为(1 088.16±52.34)粒•m－2、(973.45±63.12)粒•m－2、(579.99±28.93)粒•m－2、(66.73±5.71)粒•m－2。云杉降雨历程与其林龄大小也有一定的关系,表现在种子雨发生时间和降雨高峰期的不同。30、60年人工云杉林及天然林下土壤种子库存在显著差异,两次取样（2003年3月26日和2003年8月21日）土壤种子库大小分别为(507.2±40.32)粒•m－2和(267.6±25.14)粒•m－2、(1 065.6±88.06)粒•m－2和(872.8±77.12)粒•m－2、(472.8±20.82)粒•m－2和(185.5±22.48)粒•m－2。土壤种子库中云杉种子主要都集中在枯枝落叶层,占66%左右,0～2 cm层占24%左右,2～5 cm层占10%左右。到8月21日,土壤中所有云杉种子都失去活力,腐烂死亡和动物取食是种子库损耗的主要因素。云杉种子库属于Thompson和Grime定义的第Ⅱ类。该区云杉幼苗一般在6月初开始出现,6月中旬左右达到出苗高峰,3种云杉林下幼苗密度差异很大,60年人工林下幼苗最多,其次为30年人工林和天然林。由于生境适合度低,死亡的幼苗数量较多,特别是自然死亡数。综合种子雨、种子库和转换的幼苗数量几个方面来看,在一定龄级范围内,人工林结实能力和更新潜力随林龄增加而增加,且人工林在更新潜力方面比天然林（约150年）要好。虽然该区云杉林下有大量种子下落,但由于种子大量损耗和幼苗死亡,种子萌发为幼苗再通过环境筛作用而最终补充到云杉种群的个体数量非常有限。     

Effects of changes in tree species composition on water flow dynamics – Model applications and their limitations

Water-plant relations play a key role in the water cycling in terrestrial ecosystems. Consequently, changes in tree species composition may have distinct effects on the water retention capacity as well as on the pattern of streamflow generation. Such changes may result from modified interception properties and transpiration related to differences in canopy properties and root distribution. In order to evaluate the potential hydrological effects of the current silvicultural conversion from monocultural conifer stands into mixed or pure deciduous stands the hydrological model BROOK90 was applied to two forested upland catchments in Germany. The Rotherdbach catchment (9.4 ha, 93 yr-old Norway spruce) is situated in the Eastern Ore Mountains. The Schluchsee catchment (11 ha, 55-yr-old Norway spruce) is located in the higher altitudes of the Black Forest. The calibrated model is capable to describe rather well the temporal variation of streamflow but also the portions of the individual flow components. Data for a beech scenario were adapted for each site using a standard parameter set for deciduous trees provided by BROOK90. The annual discharge in the fictional beech stand at Rotherdbach is 30 to 50% higher compared to spruce with an increase of soil moisture and especially the slow streamflow components. This mainly results from low interception rates during winter time. In contrast, the spruce stand has a permanently higher interception rate. Effects of tree species conversion are moderate at Schluchsee. The annual discharge of a fictional beech stand at Schluchsee is 7 to 14% higher compared to spruce. There in contrast to Rotherdbach, effects of tree species conversion on soil moisture dynamics are small since vertical percolation in the highly permeable soil dominates and precipitation is abundant. Practical forestry will favorably establish mixed beech–spruce rather than pure beech stands. However, it is critical to simulate mixed stands with BROOK90. Therefore, a simple summation of model results from spruce and beech according to their respective area in a fictional mixed stand can only be a first approximation. Advanced hydrological simulation of mixed stand conditions should regard interactions of tree species and spatial parameter distribution. However, this is not yet feasible due to a distinct lack of information. As a consequence, there is a strong need to collect relevant hydrological and ecophysiological data in mixed stands in the future.     

Role of trichomes and pericarp in the seed biology of the desert annual Lachnoloma lehmannii (Brassicaceae)

In many angiosperms, the fruit rather than the seed is the dispersal/germination unit, and this is the case with Lachnoloma lehmannii, a desert annual ephemeral in central southwestern Asia with indehiscent nonmucilaginuous silicles covered with trichomes. The primary aim of this study was to assess the role of trichomes and pericarp in dispersal, anchorage of diaspores, and seed germination of this species. Mature silicles are dispersed by wind and gravity, and trichomes not only significantly increased their dispersal distance, adherence to sandy soil particles, mass of water imbibed and moisture content, but also decreased the rate of water loss and moisture content of seeds. A significantly higher percentage of seeds within silicles than of isolated seeds retained viability after exposure to 60 °C for 24 h. Seed dormancy is due to the pericarp and to nondeep physiological dormancy, as shown by the increase in germination percentage of isolated seeds following dry storage and treatment with GA₃. Removal of pericarp increased germination of 6-month-old seeds from 0 to 80–90 %, and leachate from both pericarp and trichomes significantly inhibited germination of isolated seeds. Ninety-five percent of seeds within silicles buried in soil for 2 years were viable, but only 28 % of them germinated in light at 15/2 °C; thus L. lehmannii forms a persistent soil seed bank. The pericarp and its trichomes may maximize plant fitness by determining the settlement location of silicles, thus helping to ensure that seeds germinate during the cool season for seedling survival in the desert environment.     

The effectiveness of active and passive restoration on recovery of indigenous vegetation in riparian zones in the Western Cape,South Africa: A preliminary assessment

Riparian ecosystems in South Africa's fynbos biome are heavily invaded by alien woody plants. Although large-scale clearing of these species is underway, the assumption that native vegetation will self-repair after clearing has not been thoroughly tested. Understanding the processes that mediate the recruitment of native species following clearing of invasive species is crucial for optimising restoration techniques.This study aimed to determine native species recovery patterns following implementation of different management interventions. We tested the influence of two clearing treatments (“fell & remove” and “fell & stack burn”) on the outcomes of passive restoration (natural recovery of native riparian species) and active restoration (seed sowing and planting of cuttings) along the Berg River in the Western Cape. Under greenhouse conditions we investigated seed viability and germination pre-treatments of selected native species.There was no recruitment of native species in sites that were not seeded (passive restoration sites), possibly because of the dominance of alien herbaceous species and graminoids or the lack of native species in the soil-stored seed bank. Germination of our targeted native species in the field was low in both “fell & remove” and “fell & stack burn” treatments. However, “fell & stack burn” gave better germination for the species Searsia angustifolia, Leonotis leonurus and Melianthus major. Seedling survival in the field was significantly reduced in summer, with drought stress being the main cause for seedling mortality. Germination rates in the greenhouse were high, an indication that harvested seeds were viable. Most seeds germinated without germination pre-treatments.We conclude that failure of native seeds to germinate under field conditions, secondary invasion of alien herbs and graminoids, the lack of native species in the soil-stored seed bank, and dry summer conditions hamper seedling establishment and recovery on sites cleared of dense stands of alien trees. For active restoration to achieve its goals, effective recruitment and propagation strategies need to be established.     

The dynamics of the soil seed bank after a fire event in a woody savanna in central Brazil

The seed-bank dynamics of cerrado, a savanna-like vegetation type in central Brazil, was monitored for a year after a fire event in the mid-dry season. Fifty paired soil and litter samples were collected 1 day before and 1 day after the fire to record the immediate effects on the seed bank, and thereafter at monthly intervals to investigate the post-fire seed bank dynamics. The samples were hand-sorted and the intact seeds were classified as monocot or dicot and counted. All seeds underwent germination trials in a germination chamber for 1 month. Seeds that did not germinate were checked for the presence and viability of the embryo. The sorted soil samples were placed in a greenhouse for 6 months, and the count of emerging seedlings was added to the number of germinated and dormant seeds from the germination trials to estimate the total number of viable seeds per sample. The fire did not affect the total seed-bank density: 63 ± 8 seeds m^?2 before the fire, and 83 ± 20 seeds m^?2 (mean ± se) immediately after it. Although monocots represented 65 % of the pre-fire seed bank, 1 year after the fire, the monocot seed density did not reach the pre-fire value, whereas the density of dicot seeds increased threefold. After the fire, the viable seed density and species richness, decreased with the onset of the rainy season coinciding with germination in the field. Therefore, post-fire recruitment increases genetic variability and contributes to the persistence of plant populations in cerrado communities.     

Seed rain and soil seed bank of Picea asperata Mast. in subalpine coniferous forest of western Sichuan,China

Abstract

Dragon spruce (Picea asperata Mast.) is widely planted on clear-cuts from natural subalpine coniferous forests in western Sichuan. To assess the natural regeneration potential of this species in spruce plantations of different ages, field studies on the seed rain, seed bank, and seedling recruitment were conducted in 20-year, 30-year, and 60-year plantations, and in a retained natural forest ca. 150 years old for comparison. Moreover, a series of temperature and light regimes were also designed in March 2003 to test germination/dormancy responses of the P. asperata seeds to different conditions. In the plantations considered, both the densities of seed rain and soil seed bank increased with increasing stand age, whereas they were both low beneath the natural forest partially due to low adult density of P. asperata adult trees. P. asperata has a transient seed bank, and ca. 60% of seeds were found in the litter layer. Seed decay and seed predation were the two most important factors affecting soil seed bank dynamics, which caused a number of seed losses from the soil. Only a small fraction germinated and produced seedlings, and the indices of the losses from the seed bank via germination in the 30-year plantation, 60-year plantation and natural forest were 4.75, 5.10, and 2.80%, respectively. High seedling mortality was observed after seedling emergence, and most of the germinated seedlings died out within one growing season. The P. asperata seeds showed a high percent germination and no significant differences under most light and temperature regimes except for full sunlight or temperature below 5°C. In conclusion, despite a substantial number of seed produced, the high depletion of soil seed, the low seedling output, and high seedling mortality may obscure the natural regeneration potential of this tree species.     

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.     

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.     

Germination biology and seedling growth of Clusia hilariana Schltdl., a dominant CAM-tree of drought-prone sandy coastal plains

The spatial and temporal fluctuations of water availability can be an obstacle for recruitment of many species in the restinga and might restrict seed germination and seedling growth in specific regeneration safe-sites. Clusia hilariana is one of the most dominant species of Restinga de Jurubatiba. This species has a high proportion of seedling establishment occurring inside the tanks of soil bromeliads underneath vegetation patches. Given the thin seed coats, the fast germination time and seed dispersal of C. hilariana during the dry season, we hypothesized that their major regeneration niche (the tanks of soil bromeliads) is related to susceptibility of seed germination and also seedling growth to low water availability. To test this hypothesis, seeds were germinated under decreasing water potentials using PEG 6000 solutions and seedlings were grown under varying water regimes. The percentage of seed germination progressively decreased at lower water potentials. After 38 days in ?1.0 MPa no seeds germinated. However, approximately 90% of seeds germinated when transferred to Ψ = 0 MPa. The relative growth rates of seedlings of C. hilariana did not differ between water treatments. Thus, the major regeneration niche of C. hilariana is not a consequence of a high sensitivity of seeds and seedlings to water shortage. Nonetheless, C. hilariana showed an array of seed and seedling traits that may help to overcome establishment constraints of the harsh environment of restingas.     

Comparison of spatial patterns of soil seed banks between larch plantations and adjacent secondary forests in Northeast China: implication for spatial distribution of larch plantations

In response to increasing timber demands, the natural secondary forest (NSF) is largely converted to larch plantation (LP) in Northeast China. However, it causes a series of problems such as mono-species composition, declined soil fertility and unsustainable productivity. On the other hand, it brings a great challenge for the forest managers to answer how to keep the LP with higher natural regeneration capability and how to change the pure plantations into mixed forests with more complex stand structures. Two typical spatial distribution patterns of LP within the NSF ecosystem, i.e., the Contour Type (CT, NSF and LP locating at the same slope position and aspect) and the Up-and-Down Type (UDT, LP locating at the down slope of adjacent NSF in the same aspect) were selected. The objectives of this study were to find out the optimal spatial distribution type of the LP to improve the regeneration potential (i.e., seeds in the soil stock) and the possibility inducing them into larch-broadleaved mixed forests. The spatial patterns of seed bank along the transects with the NSF-boundary-LP gradient were compared between CT and UDT. We found that in the CT stands, seed density of all species varied as a quadratic curve from NSF to LP and the largest seed density occurred at the boundary; for the dominant tree species in NSF, seed density of Fraxinus rhynchophylla tended to decrease logarithmically from NSF to LP and the seeds could only appear at 4 m away from the boundary in LP; litter depth was closely correlated with seed density and species richness in seed bank. In the UDT stands, seed density of all species tended to increase linearly from NSF to LP, and seed density in LP was 1.95 times higher than that in NSF significantly; seed densities of F. rhynchophylla and Acer mono increased as a quadratic curve from NSF to LP; the canopy openness was linearly correlated with seed density. Our results indicated that the establishment of the Up (NSF)-and-Down (LP) Type of stands could be more feasible for seed invasion. Comparative studies on the patterns of seed banks across the two spatial distribution types of larch plantations within the secondary forest ecosystem may provide some new insights into the tending measures on the LP management.     

Role of indehiscent pericarp in formation of soil seed bank in five cold desert Brassicaceae species

The dispersal and germination unit of some Brassicaceae species is the fruit, and we hypothesized that it could affect germination phenology and promote formation of a soil seed bank. We determined the effects of the indehiscent pericarp on germination and longevity of buried seeds of five Brassicaceae species native to cold deserts of central Asia. Germination phenology (seedling emergence) was monitored for intact dispersal units and isolated seeds of Chorispora sibirica, Goldbachia laevigata, Spirorrhynchus sabulosus, Tauscheria lasiocarpa (annuals), and Sterigmostemum fuhaiense (perennial) at natural temperatures in watered and non-watered (natural precipitation) soil. Intact dispersal units and isolated seeds were buried under natural conditions and exhumed at regular intervals for 35 months to monitor germination, viability and moisture content of isolated seeds, seeds in dispersal units, and seeds removed from dispersal units after burial. Isolated seeds of Goldbachia, Spirorrhynchus, and Tauscheria germinated only the first autumn and those of Chorispora and Sterigmostemum the first autumn and first spring, with higher germination percentages in all species in watered than in non-watered soil. A high percentage of seeds in buried dispersal units of Chorispora, Goldbachia, and Sterigmostemum was viable after 35 months, and seeds exhibited a 6-month dormancy cycle, being non-dormant only in autumn and spring. Seeds in buried dispersal units of Spirorrhynchus and Tauscheria germinated when exhumed in the first spring, but all non-germinated seeds were dead after 1 year. Thus, the presence of the pericarp allows Chorispora, Goldbachia, and Sterigmostemum to form a persistent seed bank but not Spirorrhynchus and Tauscheria.     

Soil seed bank dynamics of fire-prone wooded grassland, woodland and dry forest ecosystems in Ethiopia

In order to reveal the role of soil seed banks in vegetation recovery after fire in savanna, the spatial distribution and temporal changes in the soil seed banks of regularly burning savanna in Gambella, western Ethiopia, was studied. The seedling emergence technique was employed to determine the species composition and density of the soil seed bank of six sites ranging in fire severity from wooded grassland with frequent fires over woodland with intermediate fire frequency to forest with absence of fires. Species composition and density of seeds in the soil were compared between seasons, depths and sites with different types of standing vegetation. Fourteen plant species were recorded in the soil seed bank from the grassland and woodland sites and 6 from the dry forests; 60 % of the taxa in the soil seed bank were annuals and 40 % were perennials. The soil seed banks were largely dominated by graminoids and 48–97 % of the soil seed bank in the grasslands and woodlands was of a single grass species, Hyparrhenia confinis , which was absent from the dry forests. The soil seed pools ranged from less than 100 to 4700 seeds per m² depending upon the season. The soil seed bank of graminoids was nearly empty after the onset of the rainy season whereas seeds of broadleaved herbs and woody species able to germinate were still found after this time. Floristic composition, representation of life forms and density of seeds in soil did not correspond closely with that of the standing vegetation, but within graminoids there was a strong similarity between the soil seed bank and the standing vegetation. The current fire regime of Ethiopian savanna woodlands appears to maintain the dominance of graminoids over broadleaved herbs and woody plants both as seeds in the soil and in the standing vegetation.     

The invasive Leptoglossus seed bug,a threat for commercial seed crops,but for conifer diversity?

Among the recent introductions of alien insects in Europe, the polyphagous western conifer seed bug, Leptoglossus occidentalis Heidemann (Heteroptera; Coreidae) can seriously be regarded as a major threat for all the European conifer forests. In the current study combining laboratory and field experimentations, we characterized first bug damage by developing specific damage categories on seeds of different conifer species by the use of X-ray. Secondly, we investigated the impact of the invasive bug on key conifer species used for afforestation in Western and Central Europe. For this purpose, we performed germination tests on predated seeds which revealed that even light damage (consumption of <1/3 of the whole seed content) strongly reduced the germination capability of the seed. We also compared the impact of feeding on the proportion of filled seeds. Second year cones of Pinus sylvestris and Pinus nigra have been enclosed and offered to different life stages (nymphs and adults) and the results showed a significant reduction of filled seeds whatever the life stage. In field, we annually surveyed the bug seed damage for six different conifer species planted in southwestern French seed orchards. Taking into account the economic value of improved seeds in seed orchards, economic impact of bug damage was important although never exceeded 25 %. Two natural or semi-natural alpine pine stands were also surveyed and appeared to be highly affected by the bug (up to 70 % of damaged seeds). Therefore, bug damage could also be considered as a serious threat for seed production in natural stands.     

植冠与土壤种子库储存种子的萌发特性及策略

为明确黄土丘陵沟壑区植物种子库如何调控种子萌发来提高个体适合度,选择研究区7种具有种子库的主要物种为研究对象,以刚成熟和室内储存种子为对照,比较植冠宿存(5个宿存期)和土壤埋藏(5a埋藏期)对植物种子萌发特性的影响,探讨植冠种子库与土壤种子库储存下的种子萌发策略。结果表明:7种植物种子经过不同种子库储存后萌发特性表现出明显的种间差异,黄刺玫(Rosa xanthina)和水栒子(Cotoneaster multiflorus)种子萌发力表现为植冠宿存不变型、土壤储存增强型,土壤储存明显提高水栒子种子萌发速率;达乌里胡枝子(Lespedeza davurica)和狼牙刺(Sophora davidii)种子萌发力表现为植冠宿存增强型、土壤储存减弱型,种子萌发历时表现为植冠宿存延长型,土壤种子库储存还可加快达乌里胡枝子萌发速率、缩短萌发历时;茭蒿(Artemisia giralaii)和铁杆蒿(Artemisia gmelinii),种子萌发率随植冠宿存时间先升高后降低,随土壤储存时间先降低后升高,土壤储存可推迟其萌发,铁杆蒿种子萌发速率在植冠与土壤储存后均加快;紫丁香(Syringa oblata)种子萌发率随植冠宿存先升高后降低,土壤储存明显加快其种子启动萌发与速率。在黄土丘陵沟壑区,植物种子经过植冠或土壤种子库储存,或增加、加快、提早萌发充分利用有利条件提高占据性,或减少、减缓、推迟萌发分摊不利条件的风险;而且该区植物植冠与土壤储存后种子萌发特性间的关系,体现各自适应环境与应对干扰的分工与协作策略,主要表现为:单一主导型和相辅相成型。     

Impact of management on vegetation dynamics and seed bank formation of inland dune grassland in Hungary

Seed bank formation and 7 years of vegetation dynamics were studied on permanent plots of a dry sandy pasture, Cynodonti-Festucetum pseudovinae. A stand overgrazed by domestic geese and a reference stand void of overgrazing were compared. Apart from this both stands were accidentally grazed by cattle. Vegetation of the overgrazed stand was significantly more species-rich, especially in summer annuals. The reference was dominated with perennials and winter annuals. Composition and dominance changed considerably at both stands but only composition became more alike. Declining species richness and increasing dominance of perennials was found in the overgrazed stand. The reference stand became dominated with the dwarf-shrub Thymus degenianus. Species richness of the overgrazed stand showed greater seasonal and year-to-year variation than that of the reference. No temporal change of nutrient availability was found and neither was a difference detected between the stands.Greenhouse germination revealed more dense and more species-rich seed bank in the overgrazed stand. Its established vegetation and soil seed bank were also more alike. Higher species richness of the overgrazed stand can be associated with intensive propagule dispersal of geese, as indicated by dense seed banks of zoochorous hygrophytes delivered from neighbouring wetlands. Relatively high representation of persistent seed bank records suggests that, except for some sensitive perennials, the studied community is adapted to recurrent disturbances. Community regeneration seems to be limited by slow spread of perennial graminoids.     

Greater accumulation of litter in spruce (Picea abies) compared to beech (Fagus sylvatica) stands is not a consequence of the inherent recalcitrance of needles

Background and aims

Replacement of beech by spruce is associated with changes in soil acidity, soil structure and humus form, which are commonly ascribed to the recalcitrance of spruce needles. It is of practical relevance to know how much beech must be admixed to pure spruce stands in order to increase litter decomposition and associated nutrient cycling. We addressed the impact of tree species mixture within forest stands and within litter on mass loss and nutritional release from litter.

Methods

Litter decomposition was measured in three adjacent stands of pure spruce (Picea abies), mixed beech-spruce and pure beech (Fagus sylvatica) on three nutrient-rich sites and three nutrient-poor sites over a three-year period using the litterbag method (single species and mixed species bags).

Results

Mass loss of beech litter was not higher than mass loss of spruce litter. Mass loss and nutrient release were not affected by litter mixing. Litter decay indicated non-additive patterns, since similar remaining masses under pure beech (47%) and mixed beech-spruce (48%) were significantly lower than under pure spruce stands (67%). Release of the main components of the organic substance (C_org, N_tot, P, S, lignin) and associated K were related to mass loss, while release of other nutrients was not related to mass loss.

Conclusions

In contradiction to the widely held assumption of slow decomposition of spruce needles, we conclude that accumulation of litter in spruce stands is not caused by recalcitrance of spruce needles to decay; rather adverse environmental conditions in spruce stands retard decomposition. Mixed beech-spruce stands appear to be as effective as pure beech stands in counteracting these adverse conditions.