A census of seedling establishment in sugar-beet crops

Analysis of seedling establishment in 254 experiments from 1970 to 1977 showed that the target of 70%, needed for an adequate plant population and maximum yield, was achieved in only 19% of fields. However, because the plants were only counted once, when they had about six leaves, the cause and timing of the losses could not be identified. This, therefore, gave no sound basis for seeking improvements. Eight studies of all phases of establishment, from sowing to the 6-leaf stage, were then made at Broom's Barn from 1978 to 1981. The studies were on sandy loam or clay loam soils with seedbeds prepared by standard, recommended methods and sown either early or late in spring. These standard treatments were supplemented by plots which tested irrigation or protection from rain, fine or coarse tilths, and the use of a soil applied insecticide or a sterilant. Despite the widely differing weather experienced by these crops, 70% establishment was achieved in seven studies; in the eighth, seedlings emerged at 75% of seed stations, but 31% were then killed, mainly by birds. On average, the drill failed to place seeds at 2·8% of target positions, dead seed was sown at 6%, seedlings died after germination but before emergence at 12·4% and plants died thereafter at 6·5%. Almost every viable seed sown in the field germinated eventually; waterlogging seemed to be the cause of failure in a few small areas where this was not so. In most studies the first 50% of seeds germinated as expected on the basis of the thermal time required in the laboratory test. The remainder sometimes took longer, mainly because they experienced periods when the water supply was inadequate. Seedlings failed to emerge for several reasons; of which the principal ones were dehydration, restriction under a soil crust or stone, and pest damage. It seemed that waterlogging and herbicide damage were important causes in a few cases. After emergence bird grazing was the major cause of death, preferentially of the seedlings which emerged first and had the greatest yield potential. Because losses occurred at every stage and from many causes the solution to the problem of poor establishment requires a multidisciplinary approach.     

长久性紫茎泽兰土壤种子库

具有长久性土壤种子库的植物在适应多变的生境和不良的生长条件方面具有优越性。于紫茎泽兰（Eupatorium adenophorum）的主要萌发时段后（7月到次年的4月间）在云南的5个地点采集了共１９个不同植被覆盖下的土壤种子库样本,萌发实验结果表明,紫茎泽兰具有长久性的土壤种子库,其在云南的不同生境的土壤中广泛分布,所有19个样地中都有长久性的紫茎泽兰种子。 0～10 cm土层的种子密度变动于47～13 806 ind.·m-2,平均为2 202 ind.·m-2。种子密度与样地内地表的紫茎泽兰间没有直接的联系,但与植被的覆盖状况有关,种子库密度由滑坡堆积物（47 ind.·m-2）到草地（801 ind.·m-2）到灌丛（2 349 ind.·m-2）到森林（3 255 ind.·m-2）间逐渐增加。种子在各种类型土壤的采样点间出现的频度为60%～100%。在土壤的垂直方向上,0～2 cm土层分布有较多的种子,2～5 cm土层次之,5～10 cm土层最少,其各层占总数的比例的平均值分别为56.1%、25.2%和18.6%。但值得注意的是,虽然紫茎泽兰的种子在5～10 cm 深的土层内的存在量占总量的比例相对较少,但如果折合成密度值,其量仍高达270 ind.·m-2,仍有形成危害的潜在可能。广泛分布且数量巨大的具有长久性特性的紫茎泽兰土壤种子库对各种防治措施的制订意义重大,它要求我们长远地、大尺度地考虑防治措施。     

The chemical nature of P accumulation in agricultural soils��implications for fertiliser management and design: an Australian perspective

Sand burial, persistent seed bank and soil water content (SWC) are three factors that potentially can affect regeneration in sand dune plant populations. To evaluate the effects of these three factors on population regeneration of Eremosparton songoricum, a rare and endangered legume, we investigated seed germination, seedling emergence and seedling survival in greenhouse and controlled field experiments in different sand dunes microsites. Freshly matured seeds are physically dormant, and the highest germination was only 9.3?±?5.8% at 25/10°C. Seed germination occurred at burial depths from 0 to 10 cm, but the maximum depth from which seedlings emerged was 6 cm; from 1 to 6 cm, the deeper the burial, the lower the percentage of seedling emergence. Only 36.2% of the total soil seed banks occurred at depths of 0?C6 cm. For artificially sown seeds at different dune microsites, mean seedling emergence percentage was 6.8%. Of 150 seedlings that emerged in the field investigation at the study site, only those germinating in flat sandy areas survived, and mean survival percentage was only 2.0%. Thus, the proportion of non-dormant seeds in soil seed banks that developed into seedlings and survived to the end of the growing season was only 0.2%. Minimum SWC for seed germination, seedling emergence and seedling survival was 2.0%. During monitoring of emergent seedlings in the field, low seedling recruitment was at least partly due to the rate of root extension (1.6?±?0.3 cm day^?1) into the sandy soil, which was slower than that of the downward movement of plant-available moisture (2.8?±?0.6 cm day^?1). Thus, population regeneration under natural conditions rarely occurred via sexual reproduction, owing to the limited water resources available for seedling establishment. Rational field seeding practices, including manually scarified or dry stored seeds before sowing, sowing the seeds at right time and suitable place, are suggested for ecological restoration of endangered E. songoricum populations.     

Seed Dormancy,Seedling Establishment and Dynamics of the Soil Seed Bank of Stipa bungeana (Poaceae) on the Loess Plateau of Northwestern China

Studying seed dormancy and its consequent effect can provide important information for vegetation restoration and management. The present study investigated seed dormancy, seedling emergence and seed survival in the soil seed bank of Stipa bungeana, a grass species used in restoration of degraded land on the Loess Plateau in northwest China. Dormancy of fresh seeds was determined by incubation of seeds over a range of temperatures in both light and dark. Seed germination was evaluated after mechanical removal of palea and lemma (hulls), chemical scarification and dry storage. Fresh and one-year-stored seeds were sown in the field, and seedling emergence was monitored weekly for 8 weeks. Furthermore, seeds were buried at different soil depths, and then retrieved every 1 or 2 months to determine seed dormancy and seed viability in the laboratory. Fresh seeds (caryopses enclosed by palea and lemma) had non-deep physiological dormancy. Removal of palea and lemma, chemical scarification, dry storage (afterripening), gibberellin (GA₃) and potassium nitrate (KNO₃) significantly improved germination. Dormancy was completely released by removal of the hulls, but seeds on which hulls were put back to their original position germinated to only 46%. Pretreatment of seeds with a 30% NaOH solution for 60 min increased germination from 25% to 82%. Speed of seedling emergence from fresh seeds was significantly lower than that of seeds stored for 1 year. However, final percentage of seedling emergence did not differ significantly for seeds sown at depths of 0 and 1 cm. Most fresh seeds of S. bungeana buried in the field in early July either had germinated or lost viability by September. All seeds buried at a depth of 5 cm had lost viability after 5 months, whereas 12% and 4% seeds of those sown on the soil surface were viable after 5 and 12 months, respectively.     

矮沙冬青种子特性和萌发影响因素的研究

 对矮沙冬青(Ammopiptanthus nanus)种子的特性和萌发影响因素进行了初步研究,结果表明：种子不易传播;虫蛀率高,室温贮藏60 d的种子虫害率为38%;易形成硬实,含水量为7.68%的种子在30 ℃温水中浸泡90 h,只有33.33%能吸水膨胀。种子萌发时不需光,在15～30 ℃和室温（18～32 ℃）条件下,经9 d的萌发,发芽率均可达80%以上,30℃时萌发最快;在1～2 cm深的沙壤中,种子出苗率可达75%以上,超过3 cm显著降低,超过6 cm则低于20%;种子在沙壤中萌发时,沙壤的适宜湿度为19.35%～28.75%,高于32.43%或低于3.85%,很少有种子萌发;含水量分别为19.36%、10.64% 和7.68%的种子发芽率无显著差异,在-10 ℃和5 ℃下贮藏7个月,发芽率也无显著降低,但在室温和35 ℃下贮藏7个月则显著下降,发芽率下降的速度与种子本身的含水量和贮藏温度正相关;在湿度分别为7.41%、13.79%和28.57%的沙壤中播种育苗,幼苗死亡率高达77.49%、81.25%和89.49%,即使用三唑酮拌种,死亡率亦高达50.27%、69.53%和76.03%,幼苗死亡率与沙壤湿度正相关。     

The effects of soil conditions, soil sterilisation and seed treatmen on the emergence of cabbage in controlled temperature cabinets and the field

Previous cropping history of the soil and the changes that occurred in soil moisture content and temperature were the major factors determining the emergence of cabbage seedlings from pathogen-free seeds. Seedling losses caused by soil-borne pathogens could be high but these losses were prevented by seed treatment with metalaxyl or soil sterilisation with methyl bromide prior to sowing. The beneficial effects of metalaxyl and methyl bromide treatments were confirmed in field experiments involving repeated sowings of cabbage seeds. In experiments with sterilised soil and cabbage seed infected with Alternaria brassicicola emergence at – 0·2 bars soil moisture potential was reduced by 7·8–23.5% at 5, 10, 20, and 25°C. In the field, losses due to A. brassicicola were greatest when the soil became capped after sowing. The results show that recently developed fungicides may facilitate emergence under conditions extremely favourable for seed- and soil-borne pathogens. For maximum benefit, selection of seed treatments should be based on knowledge of the range of pathogens likely to reduce emergence.     

茂兰喀斯特森林中华蚊母树群落土壤种子库动态初探

 茂兰喀斯特森林中华蚊母树群落900m2样地内共有种子植物59种,其中19种主要植物已繁殖产生种子。群落每年以种子雨形式向种子库输入种子351.1粒·m-2,其中对群落更新有用的成熟有效种子150.8粒·m-2。土壤种子库中活力种子丰富,共有41种2510.5～2646.8粒·m-2。种子库中活力种子的种类和数量均随时间的变化而变化,不同时间种子库的差异主要在于现存植物活力种子的变化。土壤种子库每年输出活力种子171.9粒·m-2,其中萌发输出21.9粒·m-2,动物采食输出133.5粒·m-2,病菌危害引起的输出为3.2粒·m-2,种子自然衰老引起的输出为13.3粒·m-2。每年输出超过输入21.1粒·m-2。演替前期植物种子没有输入,只有缓慢输出,但没有萌发输出。     

Seed Stores for Restoration of Species-Rich Shrubland Vegetation Following Mining in Western Australia

The extreme species richness of native shrubland vegetation (kwongan) near Eneabba, Western Australia, presents a major problem in the restoration of sites following mineral sand mining. Seed sources available for post-mining restoration and those present in the native kwongan vegetation were quantified and compared. Canopy-borne seeds held in persistent woody fruits were the largest seed source of perennial species in the undisturbed native vegetation and also provided the most seeds for restoration. In undisturbed vegetation, the germinable soil seed store (140–174 seeds · m^?2) was only slightly less than the canopy-borne seed store (234–494 seeds · m^?2), but stockpiled topsoil provided only 9% of the germinable seeds applied to the post-mining habitat. The age of stockpiled soil was also important. In the three-year-old stockpiled topsoil, the seed bank was only 10.5 seeds · m^?2 in the surface 2.5 cm, compared to 56.1 to 127.6 seeds · m^?2 in fresh topsoil from undisturbed vegetation sites. In the stockpiled topsoil, most seeds were of annual species and 15–40% of the seeds were of non-native species. In the topsoil from undisturbed vegetation, over 80% of the seeds were of perennial species, and non-native species comprised only 2.7% of the seed bank. Additional seeds of native species were broadcast on restoration areas, and although this represented only 1% of the seed resources applied, the broadcast seed mix was an important resource for increasing post-mining species richness. Knowledge of the life-history characteristics of plant species may relate to seed germination patterns and assist in more accurate restoration where information on germination percentages of all species is not available.     

Morphological dormancy in seeds of the autumn-germinating shrub Lonicera caerulea var. emphyllocalyx (Caprifoliaceae)

To better understand the germination ecophysiology of the genus Lonicera , the dormancy class, temperature requirements for embryo growth and radicle emergence and phenology of seedling emergence were determined for Lonicera caerulea var. emphyllocalyx . At maturity, seeds have an underdeveloped embryo (approximately 28% of the length of full-grown embryos). Embryos in fresh seeds grew to full length at 15, 20, 20/10 and 25/15°C within 3 weeks, but failed to grow at ≤ 10°C and at 30°C. Radicles emerged from 86–100% of freshly matured seeds in light at 15, 20, 20/10 and 25/15°C within 28 days, but failed to emerge at 10°C. Radicles emerged equally well in a 12 h photoperiod and in continuous darkness at 25/15°C. Rapid embryo growth and germination over a range of conditions indicate that seeds of this taxon have morphological dormancy (MD); this is the first report of MD in a species of Lonicera . Seeds are dispersed in summer, at which time high temperatures promote embryo growth. Embryos grow to the critical length for germination in approximately 1 month; the peak of seedling emergence occurs in early autumn. Radicles emerged within 2 months from 98% of seeds buried at soil depths of 2 cm and 10 cm in the field in August in Sapporo, Japan; thus, seeds have no potential to form a persistent soil seed bank. However, seeds sown too late in autumn for embryos to grow remained viable and germinated the following summer when temperatures were high enough to promote embryo growth.     

Seed and Seedling Susceptibility to Phased Moisture Stress in Soil

Seeds of calabrese and carrot treated with fungicide were sownin soil at three levels of moisture stress causing a range ofeffects on seed germination or seedling emergence. After periodsof up to 3 weeks the soil was either rehydrated directly, orrehydrated after a short period of severe dehydration. Seedand seedling losses appeared to be almost entirely due to seedlingdeath after germination in the conditions of severe dehydration,but even then some germinated seeds survived these conditions.Seed deterioration or death was implicated only in carrot whenseeds were dehydrated after being held for from 1 to 3 weeksin soil at —10 ? 10⁵ Pa. A model for seed moisture effectson deterioration and survival is proposed.     

Population dynamics of the shrub Acacia suaveolens (Sm.) Willd.: Fire and the transition to seedlings

Fire, through soil heating effects, causes flushes of seed germination in Acacia suaveolens. Optimal temperatures for germination are between 60 and 80°C for any duration, or up to 100°C for durations less than 1 h. Exposure to temperatures less than 60°C leaves seeds dormant and viable, whilst seed death occurs in increasing proportions with increasing exposure to temperatures greater than 80°C. A field study of temperatures in the soil under simulated burns showed that the innate seed dormancy in A. suaveolens would only be broken for seeds up to a depth of 1 cm in ‘cool’ or 4 cm in ‘hot’ burns. In the hot burns some of the seeds in the top 1 cm of the soil were killed by excessive heating. These simulated burns most resemble cool and moderate/high intensity wildfires, respectively. Seeds can emerge from depths up to 8 cm and, for any seeds buried deeper than this, the probability of emergence is progressively reduced down to nil at 14 cm. Seeds buried between 5 and 10 cm will be heated sufficiently to break their dormancy only in a very high intensity wildfire. Seeds buried between 5 and 10 cm deep mostly occur in nests of an ant, Pheidole sp. Field observations of emergent seedlings confirm that post-fire emergence is concentrated over a small range of soil depths directly related to the intensity and duration of heating that occurs, whilst occasional seedlings may appear from greater or lesser depths largely dependent upon the spatial heterogeneity of soil heating in natural fires.     

Germination and emergence of Ambrosia artemisiifolia L. under changing environmental conditions in China

Laboratory and greenhouse studies were conducted to determine the effects of key environmental factors on germination and seedling emergence of the invasive weed Ambrosia artemisiifolia L. (common ragweed) collected from Mudanjiang (temperate climate), Nanjing (temperate–subtropical) and Nanchang (subtropical) in China. Germination of seeds occurred at temperatures ranging from 5 to 40°C, under both a 12‐h photoperiod and continuous darkness. Germination success exceeded 48% in solutions with pH values between 4 and 12, with maximum rates occurring in distilled water at pH 5.57. Germination was greatly reduced in solutions with osmotic potentials below ?0.8 MPa. Accordingly, the final germination ratio exceeded 69% at <200 mmol/L NaCl, but only reached 8% at 400 mmol/L NaCl. Emergence was greater than 75% at burial seed depths of 1–4 cm; no seedlings emerged from a soil depth of 8 cm. Seeds collected from Mudanjiang, Nanjing and Nanchang had very similar germination traits, with the main differences occurring in relation to temperature. The great germination success of common ragweed over highly variable conditions throughout its Chinese distribution range explains its successful large‐scale invasion.     

Canola seed germination and seedling emergence from pre-hydrated and re-dried seeds subjected to salt and water stresses at low temperatures

Low soil temperatures and low water potentials reduce and delay the seed germination of canola (Brassica rapa L., B. napus L.) in western Canada. Germination is also very sensitive to the salinity effects of nitrogen fertiliser placed with the seed, especially when the seed bed is relatively dry. The effects of pre-hydration and re-drying treatment on canola (Brassica rapa L. cv. Tobin) seed germination and seedling emergence at 10°C subjected to either a water or salt stress were determined. Low water potentials, induced by polyethylene glycol (PEG 8000), low soil moisture, or high concentrations of salts, reduced both germination and seedling emergence, and increased the time to 50% germination and emergence of seeds at 10°C. At equal osmotic potentials, Na₂SO₄ was less inhibitory on low temperature germination than either NaCl or PEG, suggesting that the sulphate ion partially alleviated the inhibitory effects of low water potential. Solutions of NaCI produced more abnormal seedlings compared to Na₂SO₄, suggesting that NaCl was more toxic than Na₂SO₄ during seedling development. Pre-hydration and re-drying partially overcame the inhibitory effects of both low water potential and salts on seed germination and seedling emergence at 10°C. The seed treatment increased the germination rate in Petri dishes and seedling emergence from a sandy loam soil. Water potentials or soil water contents required to inhibit 50% germination or emergence at 10°C were lower for treated seeds compared to control seeds. Salt concentrations inhibiting 50% emergence were higher for treated seeds than control seeds. Neither treated nor control seeds produced seedlings which emerged if the soil water content was lower than 9% or when the soil was continuously irrigated with salt solutions of 100 mmol kg^-1 of NaCl or 50 mmol kg^-1 of Na₂SO₄. These results suggest that the pre-hydration and re-drying treatment did not lower the base water potentials at which seedling emergence could occur. Abnormal seedlings were observed in both treated and control seeds, particularly if the soil was watered with NaCl solutions; however, the seed treatment reduced the number of abnormal seedlings.     

Soil seed densities and emergence patterns in pastures in the seasonally dry tropics of northeastern Australia

Germinable seed densities in the surface (0–10 cm) soil of pasture communities growing at Lansdown, near Townsville, were measured during the late dry season before the first germinating rain and again during the following wet season after germination but before the input of new seed. Seedlings emerging in the field were counted at approximately weekly intervals during this period to determine emergence patterns. Twelve communities were sampled in 1980–81 and six were re-sampled in 1981–82. During the late dry season germinable seed densities ranged from 5000 to 40 000 seeds m^-2. Seeds of the introduced legume, Stylosanthes hamata, were present in all pastures. There were many seeds of annual grasses (Digitaria ciliaris and Brachiaria miliiformis) and sedges (Cyperus and Fimbristylis spp.) but only few seeds of perennial grasses (both native and introduced). Soil seed densities were much lower during the wet season than during the preceding dry season, particularly for the grasses. Emergence commenced and approximately 70% of all seedlings emerged on the first major rainfall of the wet season. The subsequent emergence pattern varied between years. In 1980–81 there was a gradual and continuous increase in seedling numbers under the continuously moist conditions which prevailed. In 1981–82 further emergence occurred in discrete events related to rainfall and intervening dry periods. Maximum seedling densities exceeded 34 000 seedlings m^-2 including 29 000 grass seedlings (mainly annual species). The implications of these results for species survival and pasture composition are discussed.     

Seed germination ecology of Portulaca oleracea L.: an important weed of rice and upland crops

Portulaca oleracea , a C₄ species, is reported to be a serious weed in 45 crops in 81 countries. Experiments were conducted in the laboratory, the screenhouse and the field to determine the influence of environmental factors on seed germination and seedling emergence of P. oleracea . In the laboratory, germination in the dark was low and was not influenced by the tested temperatures (35/25°C, 30/20°C and 25/15°C alternating day/night temperatures). In the light/dark regime, however, germination was lower at 25/15°C and 35/25°C than at 30/20°C (70%, 75% and 81% germination, respectively). In conditions of 106 mM sodium chloride or −0.34 MPa osmotic potential, seeds germinated to only 50% of maximum germination of the control. Germination was not influenced by buffered pH solutions ranging from 5 to 9. In the screenhouse, germination was greatest for seeds placed on the soil surface, but emergence declined with increasing seed burial depth in soil; no seedlings emerged from the depth of 2 cm. Seedling emergence and seedling dry matter were markedly reduced by the addition of rice residue to the soil surface at rates equivalent to 4 to 6 t ha⁻¹. In the field, seedling emergence of P. oleracea was greater under zero till (ZT) (17–20%) than under minimum tillage (6–10%), a likely reflection of low seed burial and exposure of seeds to light with a ZT system. This study identifies some of the factors enabling P. oleracea to be a widespread weed in the humid tropics, and the information could contribute to improved control strategies.     

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

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

Ecological mechanisms involved in dormancy breakage in Ulex parviflorus seeds

Dormancy in the hard seed coats of Mediterranean species is considered a strategy that enables persistent seed banks to be formed in the soil. An important factor related to seed coat fracture and dormancy breakage in Mediterranean ecosystems is heat. Nevertheless, the effect of factors other than heat on dormancy breakage in these species has hardly been studied. To investigate the different ecological factors involved in germination, in the laboratory we applied several scarification treatments to seeds with chromatic polymorphism. We evaluated the effect of soil seed depth during experimental burns by sowing seeds at −1, −3 and −5 cm in the soil profile, and we also studied the effect of seed origin on the posterior germination of seeds from 4 and 10 year-old shrubs as well as from the soil seed bank. U. parviflorus shows clear chromatic polymorphism: its brown seeds present higher dormancy levels than its yellow seeds. The different techniques of dormancy breakage result in different degrees of germination; the highest degree of germination is generated by the mechanical treatment, followed by the acid and the heat treatments, in that order. The depth of the seeds in the soil determines the temperature thresholds and the residence times of these temperatures and whether they stimulate a massive germination at the −1 cm soil profile or only a slight germination at the −5 cm depth. Seeds recently produced by the plant show higher dormancy levels than seeds extracted from soil seed banks. Dormancy levels also depend on the shrubland age used for extracting the soil samples (3>9 years old). In effect, from the point of view of dormancy, the germination behaviour of U. parviflorus seeds seems to follow a multiresponse strategy based on different seed populations and involving both biological and abiotic processes to break dormancy.     

Seasonal and spatial variance of seed bank species composition in an oligotrophic wet meadow

Seasonal and small-scale spatial variation of the seed bank in an oligotrophic wet meadow (Molinion) was assessed by two methods: (1) direct determination of seeds extracted from soil samples (6 cm in diameter, 10 cm depth) and (2) germination of seeds according to Ter Heerdt et al. [1996. An improved method for seed bank analysis: seedling emergence after removing the soil by sieving. Funct. Ecol. 10, 144–151]. Samples were taken three times during the year (July and November 2001, March 2002) from three distinct microhabitats (moss layer, gaps, Molinia caerulea tussocks). Using the extraction method, more seeds were found in the upper 0–3 cm soil layer than in the deeper 3–7 cm layer, significant differences in the number of seeds were found among microhabitats (with the lowest seed bank densities in Molinia tussocks). The highest number of seeds was found in the spring census, but the test was not significant (P<0.10). However, seasonal variation was found when damaged seeds were included in the analysis. Changes in damaged/undamaged and light/heavy seed ratios, and in numbers of grass/herbs/sedges seeds are also discussed. The spatial distribution of seeds is probably influenced to a great degree by abiotic factors, such as microtopography (round and heavy seeds of sedges prevail in gaps and in deeper soil layers). The species composition of the seed bank varied among microhabitats, depth, and throughout the year. No differences were found by the seed germination method.     

Seed Ecology of the Invasive Tropical Tree <Emphasis Type="Italic">Parkinsonia aculeata</Emphasis>

Parkinsonia aculeata is an invasive tree native to tropical America, but introduced to Australia. Propagation and stand regeneration is mainly by seed. To gain baseline knowledge for management decisions, seed bank dynamics were monitored for two months during the fruit dispersal period at a coastal wetland in Costa Rica (native habitat), and at a coastal wetland and two semi-arid rangeland sites in Northern Queensland, Australia (introduced habitats). Seed bank densities underneath dense, uniform Parkinsonia stands were found to be lowest in the Australian wetland but highest in the Costa Rican wetland. Post-dispersal seed losses were highest in the Australian wetland, primarily due to seed germination and/or death. At the other sites, seed losses were minor during the study period, and predation was the most important cause of losses. At the two rangeland sites bruchid beetles accounted for more than 95% of the seed losses by predation. Total predation was lowest in the Costa Rican wetland. In order to test for intrinsic differences of seed characteristics, germination trials were conducted using both canopy seeds and seeds from the soil seed bank. Dormancy release and germination rate were studied under four temperature treatments. In all populations, dormancy release increased with increasing temperature, but averaged responses were significantly different between Costa Rican and Australian seed populations, and between seeds collected from the soil and from trees. Germination rate of scarified seeds was fastest at 35 °C in all tested seed populations. While high seed germination levels seem to explain low seed bank densities in the Australian wetland, the large seed banks at the rangeland sites reflect the lower incidence of favourable conditions for germination. In the Australian wetland biocontrol with bruchids is unlikely to be successful, while control by conventional methods, such as killing stands by basal bark spraying, seems feasible, due to a lower long-term risk of re-infestation from the soil seed bank. At the rangeland sites conventional control will be difficult and costly. Parkinsonia stands may be better left to their own, while bruchid populations are monitored and management efforts are concentrated on preventing further invasion.     

Changes in sensitivity of Amaranthus retroflexus L seeds to ethylene during preincubation. II. Effects of alternating temperature and burial in soil

Abstract. The effects of diurnally alternating temperatures and of prolonged burial in the soil on germination response of redroot pigweed ( Amaranthus retroflexus L.) seeds to ethylene were investigated. Percentage germination in a 12 h/12 h, 23° C/35° C temperature regime roughly equalled that observed at constant 35° C, and greatly exceeded that observed at 30°C. Preincubation for 61 d in alternating temperatures, which were gradually increased to simulate soil warming in spring, caused little germination in the absence of ethylene, but considerably enhanced sensitivity to ethylene. Seeds kept in soil in the same temperature regime failed to show the response to ethylene, and the soil itself removed ethylene from the soil atmosphere.
After burial in a field plot either over winter or during the summer, seeds had a very low ethylene response threshold (0.01−0.05 cm³ m⁻³) and strong response to ethylene (70–95% germination at 51 cm³ m⁻³ compared to 1–20% without ethylene). Germinability of seeds buried overwinter declined between 10 May (85%) and 24 May (7%), and 90% of those recovered on or after 24 May had a visible rupture in the seed coat. Apparently, germination had begun during burial, but was arrested by unknown causes in an early phase and was followed by seed deterioration.
Although the role of ethylene in germination of buried seeds remains uncertain, the greatly enhanced sensitivity to ethylene observed in pigweed seeds after burial deserves further investigation.