Rodent seed predation and seedling recruitment in mesic grassland

Seedling recruitment of two grasses (Arrhenatherum elatius and Festuca rubra) and two herbs (Centaurea nigra and Rumex acetosa) was measured in areas with and without rodents to which seeds of each species were sown at three seed densities (1000, 10,000 and 50,000 seeds m⁻²) in two seasons (spring and autumn 1995). Seed removal was measured for 10-day periods and the fate of seedlings was followed for 15 months after sowing. The proportion of seed removed ranged from 6 to 85% and increased with increasing seed density for each species. Rodents had no effect on seedling emergence or survival in the spring sowing. In the autumn sowing, rodents reduced seedling emergence of all four species sown at 1000 and 10,000 seeds m⁻² but had no impact at 50,000 seeds m⁻², presumably because of microsite limitation. We suggest the difference between spring and autumn arose because emergence was seed limited in autumn but microsite limited in spring; microsite availability was higher in autumn because a summer drought killed plants, reduced plant biomass and opened up the sward. Fifteen months after the autumn sowing, fewer A. elatius and C. nigra seedlings survived on plots exposed to rodents. This result reflected not only the reduced seedling emergence but also increased seedling mortality (seedling herbivory) in sites exposed to rodents. In contrast, F. rubra and R.acteosa showed density-dependent seedling survival which compensated for initial differences in seedling emergence, so that no effect of rodents remained after 15 months. The results suggest that rodent seed predation and seedling herbivory exert strong effects on seedling recruitment of A.elatius and C. nigra when recruitment conditions are favourable (conditions that lead to high microsite availability) and may contribute to both species being maintained at low densities in the grassland. The results also demonstrate that highly significant impacts of rodent seed predation at the seedling emergence stage can disappear by the time of plant maturation. Received: 2 March 1998 / Accepted: 28 September 1998     

可获得种子量和种子出苗对草地裸沙斑植被恢复的相对制约

以科尔沁沙化草地的裸斑为研究对象,布设种子捕捉器监测生长季传播到裸斑的种子数量、物种组成,并人工播种监测裸斑中出苗情况,研究可获得种子数量和出苗对裸斑植被恢复的制约.结果表明:沙生植物3种,种子数为71 ～623粒·m-2,非沙生植物9种,种子数为73 ～410粒·m-2;裸斑中沙生植物的出苗数(9.3 ～22.3株·m-2)高于非沙生植物(0 ～3.4株·m-2);沙生植物在裸斑的出苗率最高为沙蓬(12.2％ ～34.1％),其次是乌丹蒿(4.5％ ～10.3％),小叶锦鸡儿(0～0.5％),非沙生植物狗尾草出苗率为0～6％,蒲公英为0 ～0.4％,其余植物种出苗率为0;沙生植物裸斑出苗数(9.3 ～22.3株·m-2)高于植被区(7.7株·m-2),而非沙生植物在裸斑的出苗数(0～3.4株·m-2)低于植被区(4.5株·m-2).可见,裸斑植被恢复中,沙生植物的可获得种子量制约高于出苗制约;非沙生植物的可获得种子量制约与出苗制约相当.     

A comparison between reclamation stockpile and boreal forest seed banks and plant communities

Soil stockpiles are essential to the reclamation of large‐ and small‐scale mining and other industrial sites. However, stockpiling soils can lead to the degradation of seed banks. This study examines the diversity, composition, depth of seed storage, and relationships between the aboveground and seed bank plant communities in stockpiles and compares them to the nearby boreal forest. The seed bank and aboveground vegetation sampled at eight stockpiles and six mature forest sites were near Fort McMurray (57.337°N, 111.755°W) and Cold Lake (54.695°N, 110.730°W), Alberta, Canada. Seed bank samples were taken from the forest floor (LFH) and depths of 0–5, 5–10, 10–20, 20–30, >50 cm. Aboveground vegetation cover was also estimated at these locations. The seed bank composition was determined using the seedling emergence method in a greenhouse. Stockpile seed banks had higher seedling abundance and species richness than nearby forested sites but were dominated by grasses and non‐native forbs. Most seeds germinated from the surface layer, with 92% of seeds germinating from the LFH layers in the forested sites, and 68% from the 0 to 5 cm layer in the stockpiles. Mature forest sites had more similar aboveground and seed bank communities than the stockpiles. Overall, integrating information on seed bank and aboveground plant communities would improve reclamation decisions, rather than relying on aboveground vegetation alone.     

浑善达克沙地冰草种子萌发、出苗和幼苗生长对土壤水分的反应

研究了浑善达克沙地 4～ 10月份土壤含水量变动情况和冰草种子萌发、出苗和幼苗生长对土壤含水量的响应。结果表明 ,4月下旬至 5月上中旬的土壤含水量对冰草种子萌发、出苗和定居极为关键。控制条件下 ,冰草种子萌发和出苗的最适土壤含水量范围是 12 %～ 2 0 % ,幼苗生长的最适土壤含水量是 12 %。当土壤含水量低于 3% ,冰草种子不能萌发 ,土壤含水量低于 6 %时 ,幼苗不能出土并定居。当土壤含水量达到 16 %时 ,冰草幼苗生物量有所下降。在 6 %～ 8%的土壤含水量条件下 ,植株将更多的生物量投资于根的生长     

Wind-dispersed seed deposition patterns and seedling recruitment of Artemisia halodendron in a moving sandy land

* BACKGROUND AND AIMS: Artemisia halodendron is a native sub-shrub that occurs mainly in moving and semi-fixed sandy lands in Inner Mongolia, China. Information on the spatial patterns of wind-dispersed seed deposition and seedling recruitment of A. halodendron inhabiting moving sandy lands is very limited. The aim of this study was to examine wind-dispersed seed deposition patterns and post-dispersal recruitment of A. halodendron seedlings. * METHODS: The spatial patterns of wind-dispersed seed deposition and seedling recruitment of A. halodendron were examined by investigating the numbers of deposited seeds, emerged and surviving seedlings using sampling points at a range of distances from the parent plant in eight compass directions for two consecutive growing seasons. * KEY RESULTS: Wind-dispersed seed deposition showed considerable variation between directions and years. Wind transported A. halodendron seeds only a few meters away from the parent plant in all eight directions. Seedling emergence and establishment also showed between-direction and between-year variability, but the spatial pattern of seedling distribution differed from that of seed deposition. Only a very small fraction (<1 %) of the deposited seeds emerged in the field and survived for long enough to be included in our seedling censuses at the end of the growing season. * CONCLUSIONS: The spatial variation in wind speed and frequency strongly affects the pattern of seed deposition, although the variation in seed deposition does not determine the spatial pattern of seedling recruitment. Seeds of A. halodendron are not dispersed very well by wind. The low probability of recruitment success for A. halodendron seedlings suggests that this species does not rely on seedling recruitment for its persistence and maintenance of population.     

生物土壤结皮对种子萌发和幼苗建植的影响研究进展

生物土壤结皮（BSC）在陆地生态系统中具有重要的生态地位,尤其是旱地生态系统中,BSC占据了种子植物之间的广阔地面。因此,BSC的发展必然影响种子植物更新过程与植被空间格局;但其作用方式、影响程度等因相关研究涉及多气候要素、土壤类型、BSC组成物种和种子植物物种的差异及其不同组合,导致目前的研究结论存在广泛争议。研究综合论述了BSC改变地表微形态对种子传播过程的影响;BSC改变土壤特性（物理、化学、生物学）对种子萌发和幼苗存活与建植等关键环节的影响;并结合种子形态特征及种子萌发、幼苗建植的性状等,综合分析了BSC对种子传播、种子萌发与幼苗建植等更新过程的潜在影响机理;探讨了目前研究矛盾性结论产生的原因。总体来说,深入研究并全面揭示BSC对种子植物更新过程的影响,应加强学科交叉,将分子生物学、植物生理学、生物化学等微观研究,与遥感、野外生态因子过程监测、控制实验等宏观、中观研究结合,从机理到过程方面动态研究BSC对种子植物更新过程的影响,并引入水文模型、气候模型、种群动态模型等模型预测方法,研究气候变化、各类干扰频发的情景下,BSC对种子传播、萌发及幼苗建植过程的潜在影响,以期促进对BSC与种子植物间相互关系的研究,加深对干旱脆弱生态系统植被发展规律的认识。     

Quantitative genetic analysis of seed vigour and pre-emergence seedling growth traits in Brassica oleracea

The ability of seeds to germinate and establish seedlings in a predictable manner under a range of conditions has a direct contribution to the economic success of commercial crops, and should therefore be considered in crop improvement. We measured traits associated with seed vigour and pre-emergence seedling growth in a segregating population of 105 doubled haploid Brassica oleracea lines. The germination traits measured were: mean germination times for unstressed germination; germination under water stress or germination after a heat treatment; and conductivity of seed leachate. The seedling growth traits measured were: seed weight; seedling growth rate; and seedling size at the end of the exponential growth phase. There were some correlations, notably among germination traits, and between seed weight and pre-emergence seedling growth. Heritability of the various traits was typically in the 10–15% range, with heritability of conductivity and mean germination time under water stress 25 and 24% respectively. Collectively the results indicate that germination and pre-emergence seedling growth are under separate genetic control. Quantitative trait loci analyses were carried out on all measurements and revealed significant loci on linkage groups O1, O3, O6, O7 and O9. We suggest that genes at these loci are important in determining predictable seed germination and seedling establishment in practice.     

Episodic recruitment of the seedling banks in balsam fir and white spruce

• Premise of the study: In ecosystems where seed production is low and masting years are sporadic, or with species that have short-lived seeds, regeneration is assured by seedling banks rather than seed banks. Seedling establishment and survival play a critical role in determining the composition of these plant communities by supplying new individuals for their maintenance. Seedling emergence and mortality were investigated to test the hypothesis that recruitment into the seedling bank is periodic. • Materials and methods: Seed production and seedling emergence and survival was monitored during 1994-2007 in balsam fir (Abies balsamea) and white spruce (Picea glauca) in four pristine stands of the boreal forest of Quebec, Canada. Measurements were collected twice per month by sampling one permanent plot of 20 × 20 m per stand. • Key results: Seed-rain abundance reached 9 × 10³ seeds m⁻² year⁻¹, and was characterized by synchronous sequences of low and high seed production. New seedlings appeared only during the year following a seed production of at least 1 × 10³ and 1.5 × 10² seeds m⁻² year⁻¹ for balsam fir and white spruce, respectively. Seedlings emerged in July and survived 34-52 d on average, with balsam fir showing a longer lifespan and lower mortality, although 85–99% of seedlings died before completing one year of life. • Conclusions: The emergence of young seedlings was coupled with massive seed rains, which allowed synchronous replenishment of the seedling banks among stands and species, and generated different cohorts, yielding a discontinuous age structure.     

沙埋和种子大小对柠条锦鸡儿种子萌发、出苗和幼苗生长的影响

研究了沙埋深度和种子大小对内蒙古毛乌素沙地植被群落中占优势的柠条锦鸡儿(Caragana korshinskii Kom.)种子萌发、出苗、幼苗存活和生长的影响.结果表明,沙埋深度显著影响柠条锦鸡儿的种子萌发率、休眠率、出苗率、幼苗存活率及生物量.在0.5-2cm的浅层沙埋下,种子萌发率、出苗率、幼苗存活率及生物量最高,休眠率最低;沙埋深度≥4 cm时,柠条锦鸡儿的种子萌发率、出苗率、幼苗存活率及生物量随着沙埋深度增加显著降低,而休眠率却显著升高;沙埋深度≥12 cm时,柠条锦鸡儿种子不能够出苗,幼苗也不能够存活.种子大小对柠条锦鸡儿种子萌发率没有显著影响,但对出苗率、幼苗存活率及生物量影响显著.在各个沙埋深度下,不同大小的柠条锦鸡儿种子间的萌发率没有显著差异.当沙埋深度≤6 cm时,不同大小的柠条锦鸡儿种子在同一沙埋深度下的出苗率间没有显著差异;但当沙埋深度≥8 cm时,在同一沙埋深度下,大种子的出苗率显著高于中种子和小种子的出苗率,而中种子和小种子出苗率间没有显著差异.0.5-10 cm的沙埋深度中,除6 cm和8 cm深度下中种子和小种子萌发幼苗的生物量间没有显著差异外,其余深度下,大种子萌发的幼苗的存活率及生物量显著高于同一沙埋深度下中种子萌发的幼苗的存活率及生物量,后者又显著高于小种子萌发的幼苗的存活率及生物量.可能正是种子萌发对沙埋环境的忍耐或响应能力以及种子的多态性提高了柠条锦鸡儿在毛乌素沙地的适合度,为其在流动或半流动沙丘环境中成功定居并形成优势群落奠定了基础.     

The effect of heat and smoke on the soil seed banks of heathlands on permanent freshwater swamps

In South Australia, Swamps of the Fleurieu Peninsula are critically endangered due to past vegetation clearance and changes in hydrology, but still contain a high diversity of threatened plant species. This vegetation community provides habitat for 82 threatened ground-stratum plant species, including the nationally critically endangered subshrub Hibbertia tenuis which is endemic to these swamps. With infrequent burning, native ferns and taller shrubs outcompete these species. We conducted glasshouse trials to determine the potential of fire to regenerate threatened and other swamp plant species. Soil samples from eight swamps were used in germination trials with half of each sample treated with heat plus smoke, and half left untreated. Samples were spread onto trays (n = 188), and seedling emergence was recorded for twelve months. Emergence was dominated by native species (2649 seedlings m⁻²) compared to exotics (675 seedlings m⁻²). In total, 48 native and 25 introduced species germinated, with 21% of germinated native species absent from the above-ground vegetation. The dominant native fern Gleichenia microphylla did not emerge from any soil samples, indicating that recovery likely lags behind that of ground-strata species, giving them time to establish and set seed before being outcompeted. Thirty-four rare or threatened species germinated, including five species absent from above-ground vegetation. Of all native species that emerged, only five were confined to heat plus smoke treatments, suggesting most species will regenerate without fire if overstorey competition is reduced. However, seedling emergence of native shrubs/subshrubs more than doubled with the fire treatment. Of particular importance, H. tenuis showed an 18-fold increase in germination when treated with heat plus smoke. This study supports the utility of ecological burns as a management tool to regenerate threatened plant species in long-undisturbed peaty heathlands on permanent swamps.     

Small vertebrate granivores reduce seedling emergence in native tallgrass prairie restoration

High seed cost and low rates of establishment make tallgrass prairie restorations challenging and expensive endeavors. Typical seedling emergence rates in prairie restorations are approximately 10% and the causes of seed mortality are poorly understood. In this study, we examined the impact of small vertebrate granivores on prairie restoration by comparison of seedling emergence in open (sham) versus closed exclosures at three newly restored sites. To assess other causes of seed loss, we also tracked seed fates at one prairie restoration site. We coated seeds of four prairie species with fluorescent dye, placed them under closed exclosures, and monitored their fate (emerging seedling, partially germinated, nongerminated/viable, and nongerminated/nonviable) over a 5‐month period. On average, 9.6 more seedlings/m² emerged in the closed than the opened exclosures, suggesting that small vertebrate granivores reduce seedling emergence in prairie restoration. Granivores influenced the composition of the emerging community but did not preferentially consume large‐seeded species. In the seed‐tracking experiment, we found that greater than 70% of seeds were lost within 30 days of sowing, that seed recovery and viability both decreased with time in soil, and that seed fates differed between species. Collectively, our results indicate that small vertebrate granivores are an important cause of seed loss in prairie restoration, but unidentified belowground (e.g. fungal decomposition, invertebrate predation) and environmental (wind, rain) factors account for a greater proportion of total seed loss. Until these causes of seed loss are better understood, high seed costs will persist and continue to impede prairie restoration.     

A comparative study of seed number, seed size, seedling size and recruitment in grassland plants

In this study we analyse relationships between seed number, seed size, seedling size and recruitment success in grassland plants. The often hypothesised trade-off between seed size and seed number was supported by a cross-species analysis and by an analysis of 35 phylogenetically independent contrasts, derived from a data-set of 72 species. Apart from among-species relatedness, we also controlled for possible confounding effect of plant size that may influence both seed size and seed number. A sowing experiment with 50 species was performed in the field. The seeds were sown in a grassland and subjected to two treatments, disturbance and undisturbed sward. Evidence for seed-limited recruitment was obtained for 45 of the species. Disturbance had a significant, or nearly significant, positive effect on recruitment for 16 of the 45 species. The relative recruitment in undisturbed sward increased with increased seed size, and both recruitment success and seedling size were positively related to seed size. We suggest that a trade-off between competitive ability and number of recruitment opportunities follows from the trade-off between seed size and seed number, through a causal chain from seed size via seedling size to recruitment success. The relationships between seed size, seed number and recruitment may be an important underlying mechanism for abundance and dynamics of plant species in grassland vegetation. This is an example of a direct link between evolutionary life-history theory, and theory of plant community structure.     

The roles of seed mass and persistent seed banks in gap colonisation in grassland

Gaps in grassland created by animals are often sites for species regeneration. The persistent seed banks of ant-hills and surrounding soils in a calcareous grassland in southern Britain were compared and analysed in terms of seed mass and longevity. The relative abundance of species on ant-hills compared to the pasture was highly correlated with seed abundance in ant-hill soil. The abundance of plant species on ant-hills compared to the pasture was significantly negatively correlated with seed mass, and positively correlated with seed longevity. These results suggest that germination from the seed bank in ant-hill soils, as well as limited seed dispersal and vegetative invasion, contributes to the distinctive ant-hill vegetation. The same may apply to other gaps created by animals in grasslands.