古尔班通古特沙漠植物种子大小变异的空间格局

在古尔班通古特沙漠70个固定样地中采集了87种植物种子,通过测量种子百粒重,分析了种子大小在不同分类群中的变异规律,并调查了样地中不同类型种子总数目的空间格局。结果发现:1)古尔班通古特沙漠植物种子大小可分为:A(0.1—1 mg)、B(1—10 mg)、C(10—100 mg)、D(100—1000 mg)、E(1000—5000 mg)5种类型。拥有不同类型物种数量差异较大,其中D型种子物种最多,含41种占总物种数目的 47.1%,其次C型种子30种占总物种数目的 34.5%,E型种子占总物种数的13.8%,B型种子总物种数的3.5%,A型种子占总物种数的1.1%。沙漠植物中中等种子大小物种占多数,与以色列特拉维夫以北的Poleg自然保护区地中海沿岸沙漠地区相似。由于本地属、种为地中海区、西亚至中亚分布,所以认为本沙漠种子大小主要受系统演化影响。2)科间种子大小存在极显著差异,百粒重最大科是蓼科(29315 mg)、最小的是列当科(0.3 mg)。藜科,菊科,紫草科的属间种子大小差异显著。3)生活型间种子大小差异显著,种子大小从大到小排序为:灌木类短命植物多年生草本短命植物半灌木1年生草本。4)种子大小与物种相对分布频度相关性不显著,但将相对分布频度小于1.4%的环境特化物种去除后,种子大小与物种相对分布频度呈指数型极显著负相关。5)B型、C型、D型三类种子在样地中的总数目随海拔、经度和降水的增加而极显著增加,随纬度的增加而极显著降低,E型种子则与之相反。表明古尔班通古特沙漠未来随着降水的增加,B型、C型、D型种子的植物优势性逐渐增加,而E型种子的灌木类等物种优势性逐渐减少,注意加以保护。     

A continental-scale study of seed lifespan in experimental storage examining seed,plant, and environmental traits associated with longevity

Management of seed banks conserving the biodiversity of phylogenetically diverse species requires insight into seed longevity. This study determined the seed longevity of 172 species sourced from across the mega-diverse flora of the Australia continent. Seeds were aged via a controlled ageing experiment through storage at 45 °C and 60 % RH, or 60 °C and 60 % RH, and regularly tested for germination. Relative seed longevity between species was determined by comparing the time to 50 % viability loss (p ₅₀), calculated via probit analysis of seed survival curves. Seed, plant, and environmental traits were examined for associations with longevity. The p ₅₀ values varied between species from 3.0 to 588.6 days. Serotinous species, and woody trees and shrubs, had significantly longer-lived seeds than geosporous species, and species of herbaceous habit. Seeds that possess physical dormancy, and seeds with large embryos with little endosperm, were also long-lived. There was a weak, but significant, positive correlation between seed mass and longevity. Seeds sourced from regions of higher mean annual temperature and rainfall were significantly longer-lived than seeds from cooler and drier regions, although both environmental factors were weakly associated with longevity. Compared with species from other regions of the world, prolonged longevity is a feature of many Australian species. Nevertheless, seed life-spans vary substantially between species and close consideration of seed traits along with biotic and abiotic components of the plants and their environment can assist to differentiate between potentially long- and short-lived seeds.     

Attributes of the seed bank after a century of primary succession on islands in Lake Hjälmaren, Sweden

1 A large number of islands was created when the water table of Lake Hjälmaren, south central Sweden, was lowered between 1882 and 1886. We have complete lists of vascular plant species for 40 of these islands from 1886, 1892, 1903–04, 1927–28 and 1984–85.
2 We have investigated the seed bank on nine of these islands and compared species composition at different soil depths with the species lists from the islands in 1886–1985, and with the present vegetation in the area of seed bank sampling. We have also investigated the distribution in the soil of seeds from species with different ecological attributes, including seed longevity, successional status, seed weight, seed form and species longevity.
3 Seeds in soil samples were allowed to germinate over the course of two summers with an intermediate cold storage. We found 1944 seeds representing 65 taxa. The mean seed density was 84 seeds dm ^–2 .
4 The similarity between the surface soil (0–3 cm) seed bank and the vegetation at the different vegetation analyses increased from 1886 to 1993. The similarity between the present vegetation and the seed bank decreased with increasing soil depth, and the soil at 12–15 cm had no species in common with the present vegetation. Several species now absent from the vegetation were found in the seed bank.
5 Deeply buried seeds came from early successional, annual species with long-term persistent and low-weight seeds, as expected from seed bank theories, but were slightly elongated, which was in contrast to theories. Spherical seeds were associated with the surface soil, as were short-lived and high-weight seeds from late successional, perennial species.     

Potential of endozoochorous seed dispersal by sheep in calcareous grasslands: correlations with seed traits

Questions: What is the potential of sheep to serve as seed dispersers via ingestion and defecation in calcareous grasslands? Is the presence of viable seeds from dung correlated with specific seed traits? Location: Calcareous grasslands, South Limburg, the Netherlands/Belgium. Methods: Dung samples (n=24) from sheep were collected between September 2006 and November 2007 from five sites with Mesobromion plant communities, and communities of Nardo‐Galion saxatilis. Germinability and identity of seeds in the dung samples were ascertained from germination of seedlings under glasshouse conditions. Seed traits of species with viable seeds in dung were compared with those present in the local species pool. Results: Seventy‐two plant species from 23 plant families had viable seeds in sheep dung. The plant families encountered most frequently were Gramineae and Compositae. The most abundant and frequently recorded plant species in dung samples was Urtica dioica, accounting for >80% of the total number of seeds. Mean seed density in sheep dung was 0.8 seeds g^?1 dry matter. Seeds with low seed mass and a high seed longevity index were over‐represented in dung. Viable seeds >2.5 mg were infrequent in the dung samples. Conclusions: We conclude that sheep are potentially important dispersers of plant species in Dutch calcareous grasslands. Although smaller seeds were relatively abundant in sheep dung, it cannot be excluded that this was mainly caused by differences in seed abundance.     

Effects of seed size of wind-dispersed pines (Pinus) on secondary seed dispersal and the caching behavior of rodents

Seeds and nuts dispersed by scatter-hoarding animals are relatively large compared to propagules dispersed by other means. Possible selective forces in the evolution of large seed size include the selectivity of foraging animals and the ways that food-storing animals treat seeds and nuts after harvest. Treatment by rodents, primarily yellow pine chipmunks ( Tamias amoenus ), of four species of pine seeds that vary in size was studied in the Carson Range of western Nevada. The pines, lodgepole pine ( Pinus contorta , 8.7 mg), ponderosa pine ( P. ponderosa , 55 mg), Jeffrey pine ( P. jeffreyi , 157 mg), and sugar pine ( P. lambertiana , 213 mg), produce winged seeds that are initially wind-dispersed but are gathered by rodents and cached in the soil. Radioactive scandium-46 was used to follow the fates of seeds of all fours species placed around three source trees during autumn 1998 to 2000. Rodents gathered the seeds of all four species, but they took fewer of the lodgepole pine seeds and only six lodgepole seed caches (n=2106 total caches) were found during the three years. Among the other three species, number of seeds per cache decreased with increasing seed mass. However, the product of number of seeds per cache and seed mass was similar for all species. Sugar pine seeds were cached slightly deeper than ponderosa and Jeffrey pine seeds. For the species examined, seed size appeared to have had little effect on several other attributes, including mean dispersal distance, substrate choice, and microhabitat choice. Large size decreases wind dispersibility of pine seeds, but secondary dispersal by scatter-hoarding rodents compensates for poor wind dispersal so that total dispersibility of large-seeded pines is not compromised.     

Size and taxonomic constraints determine the seed preferences of Carabidae (Coleoptera)

Several species of carabid beetles are important postdispersal predators of the seeds of herbaceous plants. The preferences of carabids for particular seeds differ, but the factors that determine their choice are little studied. We tested the hypothesis that preferences are determined by taxonomic constraints (carabid species affiliation), and carabid and seed size. The preferences were determined for adults of 30 species of central European field carabids mainly belonging to the tribes Zabrini (17 species) and Harpalini (10 species) (body mass 1–36 mg). In a cafeteria experiment the beetles were offered an excess of seeds from 28 species of dicotyledoneous herbaceous plants (mass 0.1–8.7 mg). The number of seeds eaten during a 5-day experiment was used as an estimate of preference. Mass of the preferred species of seed eaten was positively related to carabid body mass in both tribes. Multivariate analysis indicated three groups of carabid species with marked preferences for particular species of seeds: (i) species of Harpalini favoured mainly the seed of Cirsium arvense and Viola arvensis, (ii) some species of Zabrini the seeds of Asteraceae (Taraxacum officinale, Tripleurospermum inodorum and Crepis biennis) and (iii) other species of Zabrini the small seeds of Brassicaceae and Caryophyllaceae. The species of Harpalini were more generalist and accepted a greater proportion of seed species than Zabrini of the same size. Preferences of carabid seed predators were thus determined by taxonomic and size constraints, as in other groups of predators.     

湖北三峡大老岭自然保护区光叶水青冈群落种子雨10年观测: 种子雨密度、物种构成及其与群落的关系

种子雨是植物种子扩散的起点, 对群落更新及种群动态起着关键作用。该文以三峡大老岭自然保护区内一片面积为1.3 hm ²的光叶水青冈(Fagus lucida)群落固定样地为研究对象, 运用分层随机设计, 在10个不同的地形部位放置了100个种子雨收集框, 自2001年起进行种子雨观测, 对该群落种子雨的数量与物种多样性的年际动态、种子雨和群落物种构成的关系等进行了统计分析。结果表明: 1)过去10年间, 共收集到来自48种木本植物的60 926粒种子, 种子雨的多年平均密度为(82.9 ± 61.5) seeds·m^-2·a^-1(mean ± SD), 平均物种丰富度为(16.7 ± 5.5) species·a^-1(mean ± SD)。2)种子生产的种间差异极为显著, 种子量排名前三的植物贡献了累计种子雨总量的70%。3)群落种子雨的密度和物种丰富度在10年中基本同步, 均呈现显著的周期性波动, 并出现了3个大年。乔木和灌木种子雨密度的年际波动无显著相关性, 但物种数变化显著正相关; 4)种子雨与样地群落共有种为23种, 分别占种子雨和群落中木本植物种数的47.92%和54.76%, 但这些共有种贡献了种子总量的96.22%, 表明扩散限制在研究群落中十分显著。与国内其他森林群落种子雨研究结果相比, 该研究群落的种子雨密度明显较低。     

Seed size and seedling emergence in 16 temperate forest herbs and one dwarf-shrub

The impact of seed size (seed mass) on seedling emergence beneath a leaf litter layer and post-dispersal seed predation was investigated in two field experiments including 16 forest herbs and one dwarf-shrub in southeastern Sweden. In the first experiment, I studied the relationship between seed mass of eight forest herbs (0.3–16.7 mg) and seedling emergence after removal of litter and reduction of seed predation (rodents and insects). Removal of litter and reduction of seed predation did not affect seedling emergence. However, regardless of treatment, species with large seeds (> 3 mg) had a higher seedling emergence than those with small seeds (< 1 mg). In the second experiment, I investigated the relationship between seed mass of 12 species (0.007–18.4 mg) and seedling emergence after removal of litter, reduction of seed predation (insects) and seedling herbivory (molluscs). Total emergence over three years was significantly higher in species with large seeds (> 3 mg) than in those with small seeds (< 2 mg). Removal of litter increased total seedling emergence, while application of insecticide and molluscicide had no effect. Similar results were obtained from both a deciduous and a mixed coniferous forest, but seedling emergence was in general higher in the mixed coniferous forest. Seedling emergence in temperate forest herbs and dwarf-shrubs seems to be higher in species with large than in those with small seeds, and it is often enhanced by disturbance.     

Persistence of seed bank under thick volcanic deposits twenty years after eruptions of Mount Usu, Hokkaido Island, Japan

The topsoil that contained the seed bank became buried under thick tephra after the eruptions of Mount Usu during 1977 and 1978. To determine the seed bank potential of the topsoil 20 yr after the eruptions, i.e., in 1998, 408 100-cm(3) samples were excavated under 115-185 cm of volcanic deposits. The topsoil was collected at 10-cm intervals along the horizontal scale and was divided into a 0-5 cm deep upper layer and a 5-10 cm deep lower layer. The seed bank was estimated by both the germination (GM) and flotation (FM) methods. In total, 23 species with an average seed density of 1317 seeds/m(2) were identified by GM, and 30 species with a density of 2986 seeds/m(2) were extracted by FM. The dominant species was Rumex obtusifolius, and perennial herbs, such as Carex oxyandra, Viola grypoceras, and Poa pratensis, were common. For nine species this study provided the first records for field seed longevity >20 yr. The seed density in the upper layer was double that in the lower layer, and the horizontal distribution was heterogeneous even at 10-cm intervals. We concluded that the seed bank has retained the original structure of the seed bank under the tephra and will persist longer with soil water content between 20 and 40%, no light, and low temperature fluctuations (±0.17°C of standard deviation in a day).     

Chemical Defense and the Persistence of Pioneer Plant Seeds in the Soil of a Tropical Cloud Forest

We present evidence that differences in soil seedbank persistence among pioneer plants in the cloud forest of Monteverde, Costa Rica, are influenced by differences in seed chemical defense. We used extracted seed chemicals from Bocconia frutescens (Papaveraceae), Guettarda poasana (Rubiaceae) , Phytolacca rivinoides (Phytolaccaceae) , Urera elata (Urticaceae) , Cecropia polyphlebia (Cecropiaceae), and Witheringia meiantha (Solanaceae) to assess seed chemical defense in two ways: (1) a plant pathogen inhibition assay using Pythium irregulare ; and (2) a brine shrimp toxicity assay using Artemia salina . The combined performance of each species in the two assays positively correlated with seedbank persistence. In the pathogen assay, mycelium growth was reduced when Pythium was cultured on media containing seed extracts from the three species with the greatest seed longevity in the soil ( i.e., Bocconia , Guettarda, and Phytolacca ). Bocconia , the most persistent species, was the only species that contained chemicals toxic to brine shrimp , an indication of defense against arthropods. We focused on Bocconia defense by isolating the chemicals toxic to brine shrimp and identified them as dihydrosanguinarine, dihydrochelirubine, and dihydrochelerthrine. We found these alkaloids in Bocconia seeds at much higher concentrations (∼50 mg/g seed material) than in leaves. These chemicals are likely responsible for the exceptional longevity of Bocconia seeds in the soil. Phytolacca and Guettarda seeds also remain viable in the soil for long periods probably due to antipathogen chemicals detected in our analyses. In contrast, the species that do not persist ( i.e., Urera , Cecropia , and Witheringia ) lacked seed defensive chemicals in our assays.
Abstract in Spanish is available at http://www.blackwell-synergy.com/loi/btp .     

种子重量的生态学研究进展

作为植物生活史中的一个关键性特征,种子重量与其它许多植物性状和生态因子有关,种子重量的分异与其它一些植物性状及环境的变化关系在进化生物学上已经成为一个非常有意义的研究内容,且具有一定的实践意义。种子重量被发现与下列的一些植物学和群落学性状有关:植物的生活型、种子的散布能力、种子的散布方式、植物的高度、植物的冠幅、植物的比叶面积、植物的寿命、动物的捕食、植被中植物的数量或多度、土壤中种子的数量或多度、种子的休眠、种子在土壤中的持久性和植物的净初级生产力等,另外生态因子如降雨、温度、坡向、海拔、经度、纬度、光强和干扰等都影响种子的重量。种子的重量被认为是在大量小种子和少量大种子之间的进化折衷,在一定的能量限度内,较大重量的种子一般具有较少的数量,而较小重量的种子一般数量较多,这是种子重量和数量方面具有的一种反向关系。与其它性状相比,很多研究都表明种子重量和植物的生活型的关系密切。没有散布结构或风散布的种子比以动物和水作为散布媒介的种子重量要小。种子重量与捕食的关系现发现有3种格局。种子重量和形状与种子在土壤中的持久性的关系有4种格局。在干旱和阴暗的环境条件下,种子有变大的趋势。大重量种子比小种子赋予幼苗较优势的竞争地位,其原理尚有争论,尚不清楚是否是幼苗阶段的竞争决定了世界上大部分植被类型的物种组成。未来的研究方向主要有以下几个方面:1) 种子重量与植物系统学相结合,探索种子重量的变化规律;2)调查群落三向(纬度、经度和海拔)性的种子重量谱变化规律;3) 群落演替与群落种子重量谱的变化;4) 种子重量与群落中植物个体和种子的数量的关系及机理研究;5) 微生境、微地形如坡向、坡位和林间隙等对种子重量的影响;6) 全球气候变化和种子重量变化的关系。     

Seed size, shape and vertical distribution in the soil: indicators of seed longevity

1. We investigated the vertical distribution of seeds in the soil, using data from nine studies in five European countries. We discovered significant correlations between seed shape and distribution in the soil.
2. The classification of the longevity of seeds of plant species has been improved by the introduction of a 'longevity index', expressing on a continuous scale the most recent information on seed longevity represented as the proportion of non-transient seed bank records in the database of Thompson et al. (1997 ). Remarkably, no difference in seed longevity was found if the index was based on direct observations only when compared with the index based on the complete data set where indirect, 'depth-derived' observations were included.
3. Seed longevity was best estimated using a multiple regression model with an integrated measure of seed size and shape and depth distribution of seeds.
4. The shape of seeds, known to be a consistent character of species, was shown to be constant within species, whereas depth distribution of seeds was highly variable among sites. This is consistent with the variability of seed longevity found in published seed bank data.     

Does pre-dispersal seed predation limit reproduction and population growth in the alpine clonal plant Geum reptans?

We studied the impact of the seed damaging gall midge larva Geomyia alpina on its perennial alpine host plant Geum reptans. We analysed the effect of seed predation on reproduction by seeds, i.e. seed number, seed mass, and seed viability and on growth and clonal propagation of non-protected plants in comparison to plants protected from predation by an insecticide. Additionally, we assessed the consequences of seed predation for population growth using matrix projection modelling. Seed predation resulted in a decrease in total seed mass per flower head by 23.8% in non-protected plants (P < 0.05). Individual seed mass decreased with increasing infestation intensity (P < 0.05). Seed number remained unaffected because the sucking feeding behaviour by gall midge larvae does not evoke seed abortion. Percent germination of seeds from non-protected plants was reduced by 97.9% compared to seeds from protected plants. According to reduced seed viability, modelling revealed a decrease in population growth rate from λ = 1.055 to λ = 1.041. Predation did neither influence total plant biomass nor biomass fractions. But stolon dry-weight of non-protected plants increased by 24.1% (P < 0.05), which may indicate a trade-off between sexual reproduction and clonal propagation. Our results demonstrate that despite substantial reduction of viable seeds, predation by gall midge larvae only slightly affected population growth of G. reptans suggesting that in this alpine species, persistence by longevity and clonal propagation can balance potential seed losses by predation, at least for local population growth.     

Seed size in relation to phylogeny,growth form and longevity in a subalpine meadow on the east of the Tibetan plateau

This study examined seed size distribution, and seed size in relation to phylogeny, growth form and longevity, where seed size is expected to be approximately of a log-normal frequency distribution and correlate dwith phylogeny, growth form and longevity. We made use of a data set of 229 species from alpine meadow on the east of the Tibetan plateau. Species spanned 10⁴ range of seed size, and the frequency of seed mass classes on a logarithmic scale produced an approximately normal distribution, but largely shifted towards smaller-sized seeds compared to those of the temperate zone and the tropics. It was evident that seed size was strongly related to phylogeny, where order, family, genus and species accounted for 2%, 32.9%, 48.9% and 16.2% of total variation in log seed mass. However, both growth form and longevity did not account for variation in log seed mass, which was opposite to the previously reported patterns of trait correlation. The implications of our results are discussed.     

Composition of harvested seeds and seed selection by the invasive tropical fire ant, <Emphasis Type="Italic">Solenopsis geminata</Emphasis> (Hymenoptera: Formicidae) in Taiwan

Solenopsis geminata (F.) is an invasive ant that is widely distributed in weedy areas and agricultural fields in Taiwan. Previous studies have found that S. geminata harvests the seeds of numerous plants. In the present study, we further investigated the composition of harvested seeds in ant nests and seed selection by workers. The seed caches in S. geminata nests sampled in eight areas in Taiwan suggest that the seeds harvested by workers were diverse and belonged to 52 plant species in 17 plant families. Twenty-three species (44%) belong to the family Gramineae, and most of the seeds weighed from 0.02 to 2.29 mg, which might suggest that these are the main seeds harvested by S. geminata from their habitat. Ten common species with similar seed weights were used to compare the seed preferences of workers from two experimental sites. The results suggest that the seed preference was different between the two experimental sites. The seeds of Casuarina equisetifolia showed the most obvious difference in seed removal speed, which might suggest that S. geminata potentially prefers the encountered seed species in the habitat. The various plant species in the ant nests and seed preference suggest that fire ants easily accept newly encountered plant species. As more than half of the plant species (52%) and the total seed number (63%) belonged to exotic species, the role of S. geminata might be negative because it tends to harvest exotic seeds and has a high opportunity to improve the establishment of exotic seedlings.     

Effects of burial and storage on germination and seed reserves of 18 tree species in a tropical deciduous forest in Mexico

The changes in germination and seed reserve composition that occur while seeds are stored in the laboratory or buried in the soil are important for understanding the potential and ecological longevity of seeds as well as seed-bank dynamics. Both germination and seed-bank dynamics depend on water availability. We studied 18 tree species, including those with permeable or impermeable seeds, from a tropical deciduous forest in Mexico. We measured seed germination in a growth chamber after (1) dispersal, (2) laboratory storage, (3) seed burial at two field sites and directly in the field, and (4) two rainy seasons. Lipids, nitrogen, and nonstructural carbohydrates were quantified after dispersal and after laboratory or field storage. Sixteen species were viable after three periods of laboratory storage (~3 years). Eleven species were viable after two burial periods in the field (~2 years). Nitrogen concentration decreased after storage and burial in 11 species. Species lipid concentration had a negative relationship with species water content at dispersal and after one burial period, whereas nonstructural carbohydrates showed the opposite trend. Potential and ecological longevities were similar in impermeable seeds. Most of the species studied can form persistent seed banks consisting mainly of species with impermeable seeds that can remain in the soil without degrading their viability. Germination in the field is staggered following natural precipitation pulses as a strategy to stagger seedling recruitment, which may insure against unfavorable conditions.     

Ecological correlates of ex situ seed longevity: a comparative study on 195 species

Background and Aims

Extended seed longevity in the dry state is the basis for the ex situ conservation of ‘orthodox’ seeds. However, even under identical storage conditions there is wide variation in seed life-span between species. Here, the effects of seed traits and environmental conditions at the site of collection on seed longevity is explored for195 wild species from 71 families from environments ranging from cold deserts to tropical forests.

Methods

Seeds were rapidly aged at elevated temperature and relative humidity (either 45°C and 60% RH or 60°C and 60% RH) and regularly sampled for germination. The time taken in storage for viability to fall to 50% (p₅₀) was determined using Probit analysis and used as a measure of relative seed longevity between species.

Key Results

Across species, p₅₀ at 45°C and 60% RH varied from 0·1 d to 771 d. Endospermic seeds were, in general, shorter lived than non-endospermic seeds and seeds from hot, dry environments were longer lived than those from cool, wet conditions. These relationships remained significant when controlling for the effects of phylogenetic relatedness using phylogenetically independent contrasts. Seed mass and oil content were not correlated with p₅₀.

Conclusions

The data suggest that the endospermic seeds of early angiosperms which evolved in forest understorey habitats are short-lived. Extended longevity presumably evolved as a response to climatic change or the invasion of drier areas. The apparent short-lived nature of endospermic seeds from cool wet environments may have implications for re-collection and re-testing strategies in ex situ conservation.Key words: Gene banks, seed ageing, seed longevity, taxonomic trends, climate, seed structure     

Fourteen months of seed rain in three Australian semi‐arid communities

The abundance and composition of seed rain was measured over 14 months (February 2004 to March 2005) in Currawinya National Park, western Queensland. The experimental design included four measurement periods, three vegetation communities and two grazing regimes. A total of 12 586 seeds from 104 species were captured. There were significantly more seeds and species captured during the measurement period with the least rainfall, although no significant correlation was found between the amount of rainfall and the number of seeds or species captured. More seeds and species were captured where native and feral grazing pressure was removed, but this was only significant for the number of species. The above‐ground vegetation showed no significant difference between grazing treatments over the study period and exhibited far fewer species than the seed rain. However, the majority of species found in the above‐ground vegetation were represented in the seed rain. Hypotheses are explored as an attempt to understand the apparent lack of a relationship between seed rain and rainfall. The effects of grazing and seed movement and storage are also discussed.     

Herbaceous plant strategies in disturbed habitats

A systematic theoretical evaluation has been made of three important plant life history traits: adult longevity, seed longevity and seed mass, where seed mass is interpreted as being indicative of dispersal distance and seedling vigour. This model study examined the role of these three traits in relation to environmental disturbance. We chose temperate grasslands, widespread in north Western Europe and northern and eastern America, as our reference system for our simulations. Eight plant strategies were defined by allowing two levels in each of the three and combining them in all eight possible ways. A simple, spatially explicit model was developed to simulate competition among individuals with these eight trait combinations at different levels of disturbance.
Simulation results were compared with the actual occurrence over a disturbance gradient of species with similar plant trait combinations in a large database from the Sheffield area (UK). This showed that with increasing disturbance level, non-dormant perennials, dormant perennials, non-dormant annuals and dormant annuals, respectively, became dominant but only if small-seeded, indicating the relative viability of these particular strategies with respect to disturbance.
A new prediction from the model was that stable coexistence occurs between plant strategies with dormant and with non-dormant seeds over a range of levels of disturbance. Plant strategies with large seeds were inferior to small-seeded ones if competitive ability of seedlings is proportional to seed weight. This difference was highest at low seed densities and low germination probabilities, indicating that large-seeded species secure no advantage from being dormant (i.e. having a low germination probability). Finally, the results indicated that dormancy is superior to dispersal as a method of coping with disturbance.     

Soil seed banks in dry Afromontane forests of Ethiopia

Abstract. The soil seed bank was investigated in four dry Afromontane forests of Ethiopia. At least 167 plant species were identified in the 0–9 cm soil layer with total densities ranging between 12 300 and 24 000 seeds/m². Herbs were represented with the largest numbers of species and seeds in the seed bank, while the contribution of tree species was generally low. The overall vertical distribution of seeds was similar at all sites with the highest densities occurring in the upper three cm of soil and gradually decreasing densities with increasing depth. Relatively high densities also occurred in the litter layer. There were large differences in depth distribution between species, suggesting differences in seed longevity. A large number of species in dry Afromontane forests evidently store quantities of seeds in the soil and this is in contrast to the situation in most tropical rain forests, dry lowland forests and savannas, where both the number of seeds and the number of species are relatively small. It is possible that the strongly seasonal and unpredictable climate of this region may have selected for high levels of dormancy, and that herb regeneration is associated with small scale disturbance. The fact that most of the dominant tree species do not accumulate seeds in the soil suggests that their regeneration from seed would be unlikely if mature individuals disappeared. Most tree species have relatively large seeds and poor long-distance dispersal; this implies that restoration of Afromontane forests after destruction would be difficult. Since there is a diverse seed bank of the ground flora, this component of the vegetation would have a better chance of reestablishing. However, because most cleared forest land is used for agricultural crop production, it is probable that the seed bank will be depleted in only a few years. Therefore, the future of the Afromontane forest flora seems to depend on the successful conservation of the few fragments of remaining natural forest.