Showing papers on "Seedling published in 1999"

Seedling survival of Mediterranean shrubland species in relation to root:shoot ratio, seed size and water and nitrogen use

Abstract: 1. We hypothesized that in Mediterranean plant communities seedling survival of different species during the drought period would be related to their ability to use below-ground resources, particularly water and nitrogen. 2. For 5 years we studied under field conditions the summer seedling survival of 11 dominant species of a Spanish Mediterranean shrubland. We related seedling survival of the different species to their seed size and root allocation estimated as the slope of the function root biomass vs log shoot biomass. We used δ13C for the estimation of water-use efficiency and δ15N and nitrogen concentration to determine the sources of nitrogen utilized. We correlated these variables with root allocation. 3. Seedling survival of the different species was positively correlated with root allocation and seed size. Root allocation was also positively correlated with seed size. δ15N and nitrogen concentration were also positively correlated with root allocation, but δ13C was not. 4. Under the relatively moist conditions occurring during summer 1996, higher root allocation was associated with the use of nitrogen from more nitrogen saturated microsites.

Impact of El Nino and logging on canopy tree recruitment in borneo

Abstract: Dipterocarpaceae, the dominant family of Bornean canopy trees, display the unusual reproductive strategy of strict interspecific mast-fruiting. During 1986–99, more than 50 dipterocarp species dispersed seed only within a 1- to 2-month period every 3 to 4 years during El Nino–Southern Oscillation events. Synchronous seed production occurred across extensive areas and was essential for satiating seed predators. Logging of dipterocarps reduced the extent and intensity of these reproductive episodes and exacerbated local El Nino conditions. Viable seed and seedling establishment have declined as a result of climate, logging, and predators. Since 1991, dipterocarps have experienced recruitment failure within a national park, now surrounded by logged forest.

The Arabidopsis det3 mutant reveals a central role for the vacuolar H+–ATPase in plant growth and development

Abstract: During the development of multicellular organisms, an intricate coordination of cell division and cell enlargement is necessary to achieve both morphogenesis and growth. In contrast to our rapidly growing knowledge of pattern formation and morphogenesis in a variety of model organisms, relatively little is known about the mechanisms that control cell and organ growth and integrate it with morphogenesis. Because plants are sessile, such mechanisms are of pivotal importance as their postembryonic development takes place under a multitude of environmental constraints, including the quality and quantity of light and the availability of water and nutrients. To compensate for their lack of mobility, plants have achieved a unique plasticity of development, which allows them to adapt to their environment. Both the initiation of organs by the apical meristems, and their subsequent growth through further cell divisions and cell expansion, continue throughout the plant life cycle. Therefore, growth and morphogenesis are not only coordinated with each other, but must provide the flexibility for adaptation to suboptimal environmental conditions. One of the most striking examples for developmental plasticity in response to an environmental cue is found during early seedling development. When dicotyledenous seedlings germinate in the absence of light, morphogenesis is inhibited and growth is achieved mostly by organ-specific cell expansion. Hypocotyl cells elongate ≥100-fold of their embryonic length to position the shoot apical meristem into an environment providing light necessary to establish photoautotrophic growth. The closed cotyledons and the formation of the apical hook protect the largely inactive shoot apical meristem. Once this so-called etiolated seedling reaches the light, however, it switches to the photomorphogenetic program in which new organs develop and growth is achieved by both cell division and cell expansion in these newly initiating organs (for review, see Kendrick and Kronenberg 1994). In the deetiolating seedling, the rate of hypocotyl elongation is inhibited while cotyledons unfold and expand and primary leaves are initiated by the shoot apical meristem. Moreover, genes necessary for photoautotrophic growth are expressed and the photosynthetic machinery, absent from etiolated seedlings, is installed. Light triggers this developmental switch; however, it is well known that in particular the hypocotyl growth response is mediated by the action of plant hormones. Physiological studies have shown that gibberellins, auxin, and brassinosteroids (BRs) have a stimulatory function, whereas ethylene, abscisic acid, and cytokinins have inhibitory effects on hypocotyl elongation (Davies 1995). How light might interact with these hormone signal transduction pathways is not understood. Because of the dramatic differences in the body plan of light- and dark-grown seedlings, early seedling development is a striking example for developmental plasticity that is also particularly amenable to genetic dissection of the underlying regulatory mechanisms. In Arabidopsis, genetic screens based on the described differences in seedling morphology have identified >40 mutants, which fall into two phenotypic classes. Light-insensitive mutants (∼20 loci), identified based on their inability to restrict hypocotyl cell expansion in response to light of different wavelengths, comprise the first phenotypic class. The second class of mutations in an additional 20 genes affects the entire morphogenetic program, resulting in initiation of deetiolation in the absence of light. When grown in darkness, these mutants show several features of light-grown seedlings, such as a short hypocotyl, expanded cotyledons, developing leaves, expression of light-regulated genes, and chloroplast development. In one subclass consisting of 10 genes (the COP/DET1/FUS genes), mutations result in seedling lethality, suggesting that these gene products play an essential role in both light and dark development of Arabidopsis (Deng and Quail 1999). Their exclusively recessive nature identifies them as negative regulators and the molecular analysis reveals that they are nuclear proteins, although their precise mechanism of action is not known (for review, see Fankhauser and Chory 1997; Deng and Quail 1999). The second subclass of deetiolated mutants has revealed that BRs play a key role in the control of photomorphogenesis. Mutants affected in either the BR biosynthesis (Li et al. 1996; Szekeres et al. 1996) or response pathways (Clouse et al. 1996; Kauschmann et al. 1996; Li and Chory 1997b) show a deetiolated phenotype when grown in the dark and are characteristic dark-green dwarfs with reduced male fertility, reduced apical dominance, and delayed senescence when grown in the light. The det3 mutant (Cabrera y Poch et al. 1993) is unique among the deetiolated mutants as it uncouples the morphological and molecular aspects of deetiolation and combines features of both subclasses. After prolonged growth in the dark, det3 seedlings do not only have a short hypocotyl, expanded cotyledons, and numerous leaves, they even undergo the transition to the reproductive phase and form flower buds (Fig. ​(Fig.1).1). In contrast to other deetiolated mutants, the morphological changes are not accompanied by a derepression of light-specific genes or signs of chloroplast development. When grown in the light, an organ-specific reduction of cell elongation leads to adult det3 plants with reduced stature and apical dominance. Moreover, it has been reported that det3, again unlike most other members of the deetiolated class of mutants, does not show hypocotyl elongation in response to BRs (Szekeres et al. 1996), indicating a possible role for DET3 either as a component of a branch of BR signal transduction controlling deetiolation or as a downstream target of BR signaling. Here we show that the phenotype of the det3 mutant is caused by a weak mutation in the gene for subunit C of the vacuolar H+–ATPase (V-ATPase) and provide evidence that this ubiquitous eukaryotic enzyme complex plays an important role in the control of growth and morphogenesis of Arabidopsis seedlings. Figure 1 Phenotype of the det3 mutant. Col-0 (left) and homozygous det3 mutant plants (right) were grown for 5 weeks in the dark in the presence of 1% sucrose (A) or in the light on soil (B).

Water potential and ionic effects on germination and seedling growth of two cold desert shrubs.

Abstract: We tested expectations that two desert shrubs would differ in germination and seedling relative growth rate (RGR) responses to Na and Cs stress. The study species, Chrysothamnus nauseosusssp. consimilis and Sarcobatus vermiculatus (hereafter referred to by genus), differ in their distribution along salinity gradients, with Chrysothamnusinhabiting only less saline areas. In growth chamber studies, declining Cs (20.82 to 22.71 MPa) inhibited germination of both species, and Chrysothamnus was less tolerant of Cs stress than Sarcobatus. Germination fell below 10% for Chrysothamnus at 21.64 MPa (NaCl and PEG), and for Sarcobatus at 22.4 MPa PEG. Neither species exhibited ion toxicity. There was substantial ion enhancement for Sarcobatus in lower Cs, allowing for 40% germination in 22.71 MPa NaCl. For seedling RGR, species were not different at 20.29 or 20.82 MPa (0 and 100 mmol/L NaCl, respectively), but Chrysothamnus RGR declined substantially at 21.3 MPa (200 mmol/L NaCl). The greater stress tolerance of Sarcobatus was not associated with a lower RGR under nonsaline conditions. Species differences in seed and seedling Cs stress tolerance probably contribute to the restricted distribution of Chrysothamnus to less saline areas. The Na uptake of Sarcobatus seedlings enhances its ability to deal with declining Cs and establish in more saline areas.

Circadian dysfunction causes aberrant hypocotyl elongation patterns in Arabidopsis.

Abstract: Many endogenous and environmental signals control seedling growth, including several phototransduction pathways. We demonstrate that the circadian clock controls the elongation of the Arabidopsis hypocotyl immediately upon germination. The pattern of hypocotyl elongation in constant light includes a daily growth arrest spanning subjective dawn and an interval of rapid growth at subjective dusk. Maximal hypocotyl growth coincides with the phase during which the cotyledons are raised, in the previously described rhythm of cotyledon movement. The rhythm of hypocotyl elongation was entrained by light-dark cycles applied to the imbibed seed and its period was shortened in the toc1-1 mutant, indicating that it is controlled by a similar circadian system to other rhythmic markers. The daily groth arrest is abolished by the early flowering 3 (elf3) mutation, suggesting that this defect may cause its long-hypocotyl phenotype. Mutations that affect the circadian system can therefore cause gross morphological phenotypes, not because the wild-type gene functions pleiotropically in several signalling pathways, but rather because the circadian clock exerts wide-spread control over plant physiology.

Interspecific and intraspecific variation in tree seedling survival: effects of allocation to roots versus carbohydrate reserves.

Abstract: We examined interspecific and intraspecific variation in tree seedling survival as a function of allocation to carbohydrate reserves and structural root biomass. We predicted that allocation to carbohydrate reserves would vary as a function of the phenology of shoot growth, because of a hypothesized tradeoff between aboveground growth and carbohydrate storage. Intraspecific variation in levels of carbohydrate reserves was induced through experimental defoliation of naturally occurring, 2-year-old seedlings of four northeastern tree species -Acer rubrum, A. saccharum, Quercus rubra, and Prunus serotina- with shoot growth strategies that ranged from highly determinate to indeterminate. Allocation to root structural biomass varied among species and as a function of light, but did not respond to the defoliation treatments. Allocation to carbohydrate reserves varied among species, and the two species with the most determinate shoot growth patterns had the highest total mass of carbohydrate reserves, but not the highest concentrations. Both the total mass and concentrations of carbohydrate reserves were significantly reduced by defoliation. Seedling survival during the year following the defoliation treatments did not vary among species, but did vary dramatically in response to defoliation. In general, there was an approximately linear relationship between carbohydrate reserves and subsequent survival, but no clear relationship between allocation to root structural biomass and subsequent survival. Because of the disproportionate amounts of reserves stored in roots, we would have erroneously concluded that allocation to roots was significantly and positively related to seedling survival if we had failed to distinguish between reserves and structural biomass in roots.

Survival, growth, and photosynthesis of tree seedlings competing with herbaceous vegetation along a water-light-nitrogen gradient

Abstract: In herbaceous dominated patches and ecosystems, tree establishment is influenced partly by the ability of woody seedlings to survive and grow in direct competition with herbaceous vegetation. We studied the importance of season long wet and dry spells on the competitive interactions between herbaceous vegetation and oak seedlings along a light and nitrogen gradient in an infertile secondary successional grassland in central North America. We conducted a field experiment in which seedlings of bur oak (Quercus macrocarpa) and northern pin oak (Q. ellipsoidalis) were exposed to two levels of light (full sun and 80% shade), three levels of nitrogen input (0, 5, 15 g m−1 yr−1), and three levels of water input (low, medium and high). In addition, seedlings were grown with and without the presence of surrounding herbaceous vegetation under both light and all three water levels. Seedling survival, growth, and rate of photosynthesis were significantly affected by competition with herbaceous vegetation and these effects varied along the multiple resource gradient. Overall, seedling survival of both species was significantly greater in wetter and shaded plots and when surrounding herbaceous vegetation was removed and was lower in nitrogen enriched plots. We found that soil water was significantly affected by varying inputs of water, light, and the presence or absence of herbaceous vegetation, and that seedling survival and rate of photosynthesis were highly correlated with available soil water. Our findings show that the impact of season long wet and dry spells on tree seedling success in grasslands can be affected by light and soil nitrogen availability.

Seed size and seedling emergence: an allometric relationship and some ecological implications.

Abstract: We develop a geometric model predicting that maximum seedling emergence depth should scale as the cube root of seed weight. We tested the prediction by planting seeds from 17 species ranging in weight from 0.1 to 100 mg at a variety of depths in a sand medium. The species were spread across 16 genera and 13 families, all occurring in fire-prone fynbos shrublands of South Africa. Maximum emergence depth was found to scale allometrically with seed weight with an exponent of 0.334, close to the predicted value. We used the allometry to predict recruitment response to experimentally simulated variation in fire intensity. Five species with small ( 10 mg) seeds were planted at ≤20-mm and 40-mm depths and exposed to low and high heat treatments and a control. The allometric equation predicted that species with large seeds would be able to emerge from a depth of 40 mm but those with small seeds would not. Only 1% of 481 seedlings from small-seeded species emerged from the 40-mm planting compared with 40% of 626 seedlings from the large-seeded group. The simulated fire treatments killed seeds in shallow, but not deeper, soil layers. At simulated high fire intensities, seedling emergence was poor in small-seeded species but good in large-seeded species, with most seedlings emerging from the 40-mm planting depth. Seed size could be a useful general predictor of recruitment success under different fire intensities in this system. We suggest that allometric relationships in plants deserve wider attention as predictive tools.

Root formation in ethylene-insensitive plants.

Abstract: Experiments with ethylene-insensitive tomato ( Lycopersicon esculentum ) and petunia ( Petunia × hybrida ) plants were conducted to determine if normal or adventitious root formation is affected by ethylene insensitivity. Ethylene-insensitive Never ripe (NR) tomato plants produced more belowground root mass but fewer aboveground adventitious roots than wild-type Pearson plants. Applied auxin (indole-3-butyric acid) increased adventitious root formation on vegetative stem cuttings of wild-type plants but had little or no effect on rooting of NR plants. Reduced adventitious root formation was also observed in ethylene-insensitive transgenic petunia plants. Applied 1-aminocyclopropane-1-carboxylic acid increased adventitious root formation on vegetative stem cuttings from NR and wild-type plants, but NR cuttings produced fewer adventitious roots than wild-type cuttings. These data suggest that the promotive effect of auxin on adventitious rooting is influenced by ethylene responsiveness. Seedling root growth of tomato in response to mechanical impedance was also influenced by ethylene sensitivity. Ninety-six percent of wild-type seedlings germinated and grown on sand for 7 d grew normal roots into the medium, whereas 47% of NR seedlings displayed elongated taproots, shortened hypocotyls, and did not penetrate the medium. These data indicate that ethylene has a critical role in various responses of roots to environmental stimuli.

Seedling density dependence promotes coexistence of bornean rain forest trees

Abstract: Density-dependent processes may occur at various spatial scales and may be generated by, and influence, various life stages in a population. When evaluating evidence for density dependence relevant to species coexistence, the nature of forest communities leads logically to a focus on particular scales and life stages. We assessed evidence for the dependence of seedling survival on the abundance of conspecific trees and seedlings over both wide (150-ha) and local (0.16-ha and 1-m 2 ) scales, in a species-rich Bornean rain forest. At the 150-ha scale, we identified a community-level compensatory trend in seedling survival (i.e., more abundant species had higher seedling mortality). There were parallel trends at the population level and on a local scale; seedling survival was inversely related to conspecific seedling density (in 1-m 2 quadrats) and tree basal area (in the surrounding 0.16 ha), for five out of 15 abundant species and for all species combined. As expected, seedlings of species with the highest adult abundance in 150 ha experienced the highest local densities of both conspecific seedlings and trees. Local density dependence in single- species analyses confirmed a dynamic interpretation of the community compensatory trend, i.e., that wide-scale seedling survival tends to decrease as a species' adult abundance increases. Taken together, our findings constitute strong evidence that density-dependent processes contribute to coexistence in this diverse rain forest community.

The fern understory as an ecological filter : Growth and survival of canopy-tree seedlings

Abstract: We investigated the role of the fern understory of closed-canopy forests as an ecological filter shaping the density, species composition, size structure, and spatial distribution of the seedling bank In New England deciduous forests we tested the hypothesis that the understory stratum is a selective filter that differentially influences growth and survival of tree-seedling species by comparing performance of Acer rubrum, Betula al- leghaniensis, and Quercus rubra seedlings in plots where the fern understory was undis- turbed, partially removed, or completely removed We related seedling growth and survival to microenvironmental characteristics of experimental plots in order to further explore mechanisms responsible for the filtering capacity of the fern understory The presence of a fern understory reduced growth and survival of all seedling species, but the magnitude of the effect differed among species Mortality resulting from resource limitation in Quercus below the fern understory was balanced by mortality resulting from insect herbivory in fern-free areas Relative biomass growth rates of all species were neg- atively influenced by the presence of fern cover, whereas relative height growth rates of Acer and Quercus were uniformly low and insensitive to the presence of fern cover Growth and survival rates indicate that only Quercus seedlings can emerge from the fern stratum in the absence of understory or overstory canopy disturbance A trade-off between persis- tence in low light and maximum growth in understory light levels was observed among species The relative growth rate of Betula in terms of biomass and height was more responsive to light levels than were relative growth rates of Acer or Quercus, and the growth rate of Betula was higher than that of Acer and Quercus in all light levels However, survival of Betula below the fern stratum was lower than survival of Acer and Quercus The fern understory has the capacity to selectively filter tree seedlings as they grow up through it because seedling species respond differentially to the presence of fern cover The selective filtering of tree seedlings by the fern understory results in a seedling spatial structure that reflects the spatial heterogeneity of the fern stratum The seedling pool below the fern stratum has a lower seedling density and different species composition and size structure than the seedling pool in fern-free areas

Herbivores, seed banks and seedling recruitment in mesic grassland.

Abstract: 1 An experiment was carried out in a species-poor acid grassland to determine the effect of insect, mollusc and rabbit herbivory on the size and composition of the seed bank and on seedling recruitment from the seed bank and seed rain. From 1991 to 1997, insects and molluscs were excluded with pesticides, and rabbits with fences. Seedling recruitment was monitored over 22 months in gaps established in the vegetation in summer 1995. 2 The most common species recorded from the seed bank in early summer 1995 were dicots (17 species), but perennial grasses (five species) were numerically the most abundant (65% of total). There was no relationship between the species composition of the seed bank and the established vegetation. 3 The size of the seed bank of eight species was greater on fenced plots, a result that reflected increased seed rain where rabbits were excluded. Insects and molluscs had no effect on the size of the seed bank of any species. The number of species in the seed bank was not affected by any of the herbivore exclusions. 4 A comparison of seedling emergence in gaps formed over the original soil with gaps where the soil had been sterilized indicated that only Galium saxatile and Cytisus scoparius recruited from the seed bank. Seedling recruitment was almost entirely derived from the recent seed rain, was dominated by the most abundant perennial grasses in the vegetation (Festuca rubra and Holcus lanatus), and had a species composition that resembled the established vegetation. Results highlight that the potential for seedling establishment in gaps to bring about vegetation change in this grassland is low. 5 Six species had higher seedling densities on rabbit-fenced plots, but the significant effect of fencing disappeared by plant maturity for most species. Survival of seedlings was lower on fenced plots where non-grazed biomass accumulated, so that after 22 months Agrostis capillaris was the only species with more plants present where rabbits were excluded. Rumex acetosa and Stellaria graminea showed higher seedling emergence where molluscs were excluded. More seedlings of Rumex acetosa were also found where insects were excluded. These invertebrate effects were still evident at plant maturity.

Seed size variation and its effect on germination and seedling performance in the clonal herb Convallaria majalis

Abstract: Variation in seed size is common both within and among plant species. This study examined within-species variation in seed weight, and its implications for some components of fitness in the clonal herb Convallaria majalis . This species produces berries containing 4.1 seeds on average. The average seed weight was 16.5 mg, with a coefficient of variation of 32.7%. Seed packaging in fruits was on average 12.5%, and showed a slight tendency to increase with fruit weight. A trade-off was found between seed weight and seed number both within fruits and within ramets. The probability and timing of germination was not influenced by seed size. A field experiment and indirect evidence suggested that post-dispersal seed predation was not related to seed size. Increasing seed weight conferred an advantage to developing seedlings. This advantage was enhanced if a seedling was growing in the close vicinity of a seedling of another species. It is suggested that seed size variation in C. majalis primarily is the result of resource variation during fruit development. A conflict between parents and offspring may however contribute to increase seed size variation.

Comparison of seedling and adult palatability in annual and perennial plants

Abstract: 1. The relative palatabilities of the seedlings and adults of a range of British herbaceous plant species (14 annuals and 15 perennials) were compared using a generalist native herbivore (the slug Deroceras reticulatum) in a food-choice experiment. A palatability index (0 to 1) was devised. 2. A general linear model analysis showed that, overall, there was a highly significant difference between seedling and adult palatability. 3. In the majority of cases, seedlings were more palatable than adults. However, in species with highly palatable adults the opposite tended to be the case. 4. No consistent difference between annual and perennial species was seen with respect to the relative palatabilies of adults vs seedlings. 5. No evidence of a trade-off between seedling palatability and relative growth rate was found. 6. In view of the role that selective seedling predation may play in determining species composition in the field, the results of this experiment indicate the need for caution in making ecological inferences from the exclusive use of adult material in palatability tests.

The Prolamins of Maize

Abstract: Prolamins are the principal seed storage proteins of cereals These proteins are soluble in alcohol and function in storage of nitrogen and amino acids for the germinating seedling (Osborne, 1908; Osborne and Mendel, 1914) Maize prolamins are called zeins, and they form accretions or protein bodies within the lumen of the rough endoplasmic reticulum (RER) of endosperm cells Zeins are structurally distinct from the prolamins of the more distantly related cereals, wheat, barley, oats and rice (see chapters 3, 4 and 5), but they are very similar to the prolamins of the other panacoid cereals, eg sorghum, millet and Coix (see chapter 7)

Effect of light on the germination of forest trees in Ghana

Abstract: Summary 1 Seed germination in light and dark, and responses to irradiance and light quality, were tested in shadehouse experiments for 19 West African tropical forest tree species representing a wide range of ecological types. Germination in forest gaps of different size was tested for 11 species. 2 Percentage germination was reduced in the dark only for three small-seeded species that are common in forest soil seed banks: Musanga cecropioides, Nauclea diderrichii and Milicia excelsa. Percentage germination of the other 16 species, including four widely regarded as ‘pioneers’, was unaffected. 3 Effects of different irradiances in shadehouses, where the seeds were watered, were significant for some species, but there was no consistent pattern. Irradiance effects in forest gaps, where the seeds received only natural wet season rainfall, were more widespread and substantial, and were most commonly shown as a depression of percentage germination at high irradiance. 4 Effects of light quality (neutral vs. green shade; red : far-red = 0.43) were insignificant at 5% irradiance in shadehouses for all species except Nauclea diderrichii. In growth chamber experiments, the low energy response was only evident at 1.0 µmol m–2 s–1 (< 1% of unshaded forest irradiance) in Musanga and Nauclea. 5 The speed of germination was affected by irradiance in many species, but the effect was small compared with differences between species, in which time to complete germination varied between 3 weeks and over 6 months. 6 Seeds of Ceiba pentandra and Pericopsis elata planted in deep forest shade (2% irradiance) and in a small gap (30% irradiance) germinated well in both sites, showed exponential biomass growth in the gap but a linear decline in mean seedling biomass and subsequent death in deep shade. 7 Light-mediated germination is relatively rare among these forest trees, even among pioneers, so that the working definition of a pioneer should be seen to depend more on a species’ ability to survive in forest shade. The effects of canopy opening on seed germination are small except in the largest openings, which severely depress germination in a number of species, including some species with strongly light-demanding seedlings.

Variability in post-dispersal seed predation in deciduous woodland: relative importance of location, seed species, burial and density

Abstract: The considerable variability found in post-dispersal seed predation and the absence of consistent directional trends (e.g., with reference to seed size) has made it difficult to predict accurately the impact of seed predators on plant communities. We examined the variation attributable to location, seed density and seed burial on the removal of seeds of three tree species: Fraxinus excelsior, Taxus baccata and Ulmus glabra. Experiments were undertaken in five deciduous woodlands in Durham, U.K., and the relative importance of vertebrate and invertebrate seed predators was assessed using selective exclosures. In all five woodlands, seed removal was greatest from treatments to which vertebrates had access, and losses attributable to invertebrates were negligible. Rodents, in particular Apodemus sylvaticus (Muridae) and Clethrionomys glareolus (Cricetidae), were the principal seed consumers in these woodlands. Unidentified vertebrate seed predators (probably birds, rabbits and/or squirrels) appeared to be significant seed removers in three of the five woodlands. Rates of removal differed among the three tree species, increasing in the following order Fraxinus < Taxus < Ulmus but were not related to seed mass. The major effect influencing rates of seed removal was seed burial, which halved rates of seed removal overall. The effect of seed burial was a function of seed size. The larger seeds of Taxus realising little benefit from seed burial whereas encounter of the smaller Ulmus seeds fell by almost two-thirds. Removal was density-dependent for all three species. However, the relative increase in seed encounter through an increase in seed density was a negative function of seed size. This suggests that, for large seeds, the opportunity to escape seed predation via burial or reduced seed density is limited. These results reveal a number of parallels with other studies of post-dispersal predation and identify several generalities regarding the interaction between plants and post-dispersal seed predators. Comparison of the seed predation results with actual seedling distributions suggests that seed predators may influence regeneration of Ulmus glabra but probably play a lesser role in the dynamics of Taxus baccata and Fraxinus excelsior.

Comparative ecology of seedling recruitment in an oligotrophic wet meadow

Abstract: . For the regeneration niche to contribute to the maintenance of species diversity interspecific differences in sensitivity of seedling recruitment to environmental conditions is assumed. We experimentally tested differences between meadow species for the response of seed germination to chilling, and sensitivity of seedling recruitment to microscale heterogeneity. We also compared the dynamics of seedling recruitment in gaps. Seed germination was tested in standard laboratory germination tests, comparing control seeds with seeds chilled at +4 °C, and at –14°C for one month. Species responses varied from significant increases in germinability after chilling (e.g. Cirsium palustre, Betonica officinalis, Angelica sylvestris) to significant decreases (e.g. Hieracium umbellatum, Succisa pratensis, Selinum carvifolia). In some species, chilling at + 4 °C has a similar effect to chilling at –14 °C, in others the effect of chilling at + 4°C was intermediate, and in some, there was no effect of chilling at + 4°C, but an effect of chilling at –14°C. Different chilling temperatures also affect timing and speed of seed germination under greenhouse conditions. The dynamics of seed germination under field conditions was studied by sowing seeds into artificially created gaps and following their germination, both where the seedlings were removed after emergence and where they were not removed. Species differ in their germination dynamics: they all start late April, but then differentiate from an abrupt maximum and early finish of germination (in the second half of May), to prolonged germination without a marked maximum. Seedling removal increased the total number of germinated seeds, with a marked density dependence at this stage. Seeds were also sown into plots with treatments (1) gaps, sod stripped, above-ground vegetation removed; (2) mown, moss layer removed; (3) mown; (4) untouched control. Seedling emergence was monitored for 3 yr. Seedling recruitment decreased from treatments (1) to (4), but sensitivity differed between species. With increasing seed weight, the difference between gaps and other treatments decreased. The results show that there are considerable differences in seedling recruitment sensitivity between species.

Facilitation of conspecific seedling recruitment and shifts in temperate savanna ecotones

Abstract: In contrast to documented increases in woody plant dominance of savannas and grasslands of North America, oak (Quercus L.) savannas that form lower tree lines in the southwestern United States and northwestern Mexico have been stable over the last several centuries. We sought to identify potential biotic and abiotic constraints on seedling recruitment of Quercus emoryi within the context of potential shifts in lower tree line. We used field surveys to describe seedling distribution at and below lower tree line, and to determine the potential for acorn dispersal from lower tree line into adjacent grassland. Field and greenhouse experiments were used to test explanatory hypotheses generated by descriptive surveys. Q. emoryi seedlings were located almost exclusively beneath mature, conspecific tree canopies within the woodland and savanna and were absent from adjacent semidesert grassland in 1993 and 1995. Seed bank surveys indicated that acorns were concentrated beneath tree canopies and were dispersed into adjacent grassland in low numbers. Although soil N, C, and P were about two times greater beneath trees than in adjacent grassland, experimental nutrient amendments to subcanopy and grassland soils indicated that soil nutrients did not limit Q. emoryi growth. Reciprocal transfers of subcanopy and grassland soil to subcanopy and grassland microsites indicated that microsite was more important than soil source for seedling growth. Overstory shade was important at all stages of seedling development investigated: the provision of artificial or natural shade increased rates of seedling emergence and subsequent survival as much as 19-fold and increased recruitment rates between 30- and 60-fold. We conclude that rates of Q. emoryi recruitment within grasslands below tree line are relatively low and are constrained by low rates of seed dispersal coupled with a low probability of seedling emergence. In contrast, large numbers of acorns are dispersed directly beneath Q. emoryi trees, where they have a higher probability of emergence than in adjacent grassland. Survival rates of emerged seedlings were low, regardless of landscape position. Thus, observed patterns of seedling distribution on the landscape resulted from interactions between seed dispersal and habitat-specific response of seedlings to environmental variation. Results of this and complementary research suggest that the lower tree line in southern Arizona is stabilized by self-enhancing feedback mechanisms of overstory shade, seed dispersal, and seedling establishment, coupled with strong abiotic constraints beyond the current ecotone. These processes stabilize the woodland–grassland ecotone both spatially and temporally, consistent with Wilson and Agnew's one-sided positive feedback switch. Although this switch would not produce an indefinitely stable vegetation mosaic, upslope or downslope shifts in lower tree line are apparently resistant to decadal or even century-scale climatic perturbation. The observed shift in tree line in the last millennium was less likely the result of slow, spatial progression of autogenic safe sites than the result of episodic and infrequent allogenic processes that simulated or negated the importance of conspecific, biogenic safe sites.

Soil-based phytotoxicity of 2,4,6-trinitrotoluene (TNT) to terrestrial higher plants

Abstract: Seed germination and early stage seedling growth tests were conducted to determine the ecotoxicological threshold of 2,4, 6-trinitrotoluene (TNT) in two soils of different properties. Soils were amended up to 1,600 mg TNT kg-1 soil and four representative species of higher plants, two dicotyledons (Lepidium sativum L., common name: cress; and Brassica rapa Metzg., turnip) and two monocotyledons (Acena sativa L., oat; and Triticum aestivum L., wheat), were assessed. Cumulative seed germination and fresh shoot biomass were measured as evaluation endpoints. Phytotoxicity of TNT was observed to be affected by soil properties and varied between plant species. Cress and turnip showed higher sensitivity to TNT than did oat and wheat. The lowest observable adverse effect concentration (LOAEC) of TNT derived from this study was 50 mg kg-1 soil. In contrast to high TNT concentrations, low levels of TNT, i.e. , 5-25 mg kg-1 soil for cress and turnip and 25-50 mg kg-1 for oat and wheat, stimulated seedling growth. Oat was capable of tolerating as much as 1,600 mg TNT kg-1 and demonstrated a potential ability of TNT detoxification in one of the soils tested, suggesting that this plant might be useful in the bioremediation of TNT contaminated soils.

Seed mass versus seedling performance in Scots pine: a maternally dependent trait

Abstract: It is generally accepted that larger seeds give rise to seedlings with better performance. On the other hand, the size that a seed reaches is genetically determined by at least two different traits ; the genetic variability of the developing embryo and the genetic variability of the maternal plant. Thus, the relative contributions of these two traits affect seedling performance by influencing seed size. In this paper, I investigate the effect of seed size on seedling performance in the Scots pine (Pinus sylvestris). From eight maternal plants, 50 seeds were planted in each of two soil types (800 seeds in total), and seedling performance was monitored for 1 yr. Seed mass proved to be highly constant within maternal plants. Soil type influenced emergence and survival; however, the effect of soil type differed depending on maternal origin. Seed mass was positively correlated with seedling emergence, although this relationship was not found for seedling survival or date of emergence. The initial growth of the shoot was also positively correlated with seed mass. However, after one growing season, seed mass had no effect on seedling performance, which depended exclusively on maternal origin. Nevertheless, the mean mass of seeds produced by plants was positively correlated with mean values of growth parameters. Thus, first-year seedling performance seems to be a maternal trait indirectly associated with seed size.

Effects of sand burial depth on seed germination and seedling emergence of Cirsium pitcheri

Abstract: A greenhouse study was conducted to determine the effects of sand burial on seed germination and seedling emergence of Cirsium pitcheri, a threatened species along Lake Huron sand dunes. In October 1996, seeds of C. pitcheri were sorted into three groups (small, medium and large) and artificially buried at 2, 4, 6, 8, 10 and 12 cm depths in plastic pots filled with unsterilized sand from Lake Huron sand dunes. These pots were placed outside in sand boxes for natural stratification. In early February, the pots were brought into the greenhouse for germination of seeds. Data showed that percent seed germination and emergence of seedlings were not related to seed size. However, both variables were negatively correlated with depth. Seedling emergence occurred from a maximum depth of 6 cm with most seedlings emerging from 2 cm depth. Only one seedling emerged from 8 cm depth. Seedlings from large seeds produced longer roots than those of small seeds. Larger root system would probably enhance seedling establishment.

Effect of fire on hard-coated Cistaceae seed banks and its influence on techniques for quantifying seed banks.

Abstract: The impact of fire on hard-coated Cistaceae ( Halimium ocymoides, Cistus ladanifer ,a ndC. salvifolius) soil seed banks in a Mediterranean ‘maquis’ shrubland, and its effect on seed germinability were studied. The study also contrasts the effectiveness of two widely used techniques for quantifying seed banks, the seedling emergence and the physical separation methods, in relation to fire. The null hypothesis that a massive enhancement of physicallydormant Cistaceae seed germination by fire would make use of the time-consuming physical separation technique unnecessary was tested. Fire reduced Cistaceae seed banks in the 0‐2 cm deep soil layer by both seed fireconsumption and lethal temperatures, revealed by the significant decreasing of the seed bank density and by the increase of apparently-intact but soft-unviable seeds, respectively. In contrast, no damage was recorded in the 2‐ 5 cm soil layer. A dramatic seed bank depletion (> 90%) in both soil layers was recorded one year after fire in the burnt area, coinciding with a significant increase of seedling density confined to the first post-fire year. The ecological consequences of this massive post-fire seed bank input are discussed. A germinability test revealed that germination of surviving Cistaceae seeds was significantly enhanced in all cases except for the C. salvifolious seed bank in the deeper soil layer. However, final germination levels (60‐75%) did not correspond to the magnitude of seed bank depletion, especially for C. salvifolious, which suggests that other environmental factors not exclusively associated with fire may also be important in softening Cistaceae seeds. Germination enhancement by fire soilheating was not high enough to reject the physical separation technique, at least in the deeper soil layer. The simultaneous use of both seedling emergence and physical separation is recommended for reliable seed bank estimates when a physically-dormant hard-seeded component can be expected in the soil, as in many Mediterranean ecosystems, regardless of fire occurrence. Nomenclature:Tutin et al. (1964‐1980).

Rodent seed predation and seedling recruitment in mesic grassland.

Abstract: 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.

The restoration of sedge meadows: seed viability, seed germination requirements, and seedling growth ofCarex species

Abstract: To better understand how to establishCarex species from seed in created and restored wetlands, a series of experimental studies was conducted onCarex seed and seedlings. These studies included (1) the effect of seed age (1 to 18 months after collection) on viability and germination, (2) storage conditions (+4,-4 or +4/-4°C; wet or dry) on seed germination, (3) soil moisture on seed germination, and (4), soil amendments (fertilizer, topsoil, and compost) on seedling recruitment and growth. Seeds ofCarex species of several provenances would not germinate to any appreciable extent once they were more than six months old. ForCarex aquatilis, germination could be increased by storing its seed at either 4°C or—4°C. ForCarex lacustris andCarex stricta, seed germination decreased significantly, by about 50 to 100% in all 8 storage treatments. Germination ofCarex stipata seed was highest in 1 cm of standing water, and its seed did not germinate in the driest soil moisture treatment.Carex stricta seed germination was not affected very much by soil moistures. NoCarex spp. were recruited from seed in any of the field soil-amendment treatment plots. In both the greenhouse and field, the addition of compost, topsoil, and fertilizer increased the growth ofCarex stricta. In a greenhouse study, above ground, below ground, and total dry mass increased linearly with the percent compost added. Our results suggest that the probability of establishingCarex spp. from seed in created and restored wetlands in the Upper Midwest would be maximized by using fresh seed, preferably seed produced earlier in the same growing season; by keeping soil moisture levels as high as possible; and by raising the soil’s organic matter content, if need by, through the use of suitable soil amendments to levels found in natural sedge meadows.

Inhibition of seedling survival under Rhododendron maximum (Ericaceae): could allelopathy be a cause?

Abstract: In the southern Appalachian mountains a subcanopy species, Rhododendron maximum, inhibits the establishment and survival of canopy tree seedlings. One of the mechanisms by which seedlings could be inhibited is an allelopathic effect of decomposing litter or leachate from the canopy of R. maximum (R.m.) on seed germination, root elongation, or mycorrhizal colonization. The potential for allelopathy by R.m. was tested with two bioassay species (lettuce and cress), with seeds from four native tree species, and with three ectomycorrhizal fungi. Inhibitory influences of throughfall, fresh litter, and decomposed litter (organic layer) from forest with R.m. (+R.m. sites) were compared to similar extractions made from forest without R.m. (-R.m. sites). Throughfall and leachates of the organic layer from both +R.m. and -R.m. sites stimulated germination of the bioassay species above that of the distilled water control, to a similar extent. There was an inhibitory effect of leachates of litter from +R.m. sites on seed germination and root elongation rate of both bioassay species compared with that of litter from -R.m. sites. Native tree seed stratified in forest floor material from both forest types had a slightly higher seed germination rate compared with the control. A 2-yr study of seed germination and seedling mortality of two tree species, Quercus rubra and Prunus serotina, in field plots showed no significant influence of litter or organic layer from either forest type. Incorporating R.m. leaf material into the growth medium in vitro depressed growth of one ectomycorrhizal species but did not affect two other species. Leaf material from other deciduous tree species depressed ectomycorrhizal growth to a similar or greater extent as leaf material from R.m. In conclusion, R.m. litter can have an allelopathic effect on seed germination and root elongation of bioassay species as well as some ectomycorrhizal species. However, this allelopathic affect is not manifest in field sites and is not likely to be an important cause for the inhibition of seedling survival within thickets of R.m.

Seed dynamics and seedling establishment in the invading perennial Solidago altissima under different experimental treatments

Abstract: 1 Population increases in Solidago altissima, an aggressive invader in central Europe, can be due to clonal growth by rhizomes or sexual reproduction by seeds. We studied transitions between different reproductive stages, to assess the potential of S. altissima to colonize new habitats by seed, and the influence of annual mowing on the dynamics of seeds and juvenile plants. 2 The dispersal of the numerous achenes, their subsequent fate on and in the soil, and the growth and survival of seedlings and juvenile plants were followed from 1988 to 1990 in an old-field population. 3 The seed rain was 49 000 seeds m−2 per annum in unmown and 1800 seeds m−2 per annum in adjacent mown plots. Survivorship and germinability of seeds in the soil decreased rapidly in the first 3 months after dispersal. Thereafter the declines were much slower. Germination within the established vegetation was very rare: only 12 out of 150 000 experimentally sown seeds (0.008%) became seedlings. 4 The survival and growth of seedlings transplanted into closed vegetation increased with age and developmental stage. Nevertheless, no transplants flowered during the 24 months over which their fate was followed. 5 Mowing affected neither seed survival nor germinability, nor did it have any effect on the growth, survival and reproduction of seedlings or juvenile plants. 6 The creation of 1-m2 openings in the vegetation strongly increased the number of emerging seedlings. Production of achenes was high (c. 20 000 per sexual shoot) and seed rain data suggested that 80–90% of all seeds may have been dispersed outside the study area. The colonization of nearby open sites is therefore quite probable. 7 Thus, mowing in late summer to control the growth of S. altissima does not provide opportunities for further seedling establishment in existing populations and may prevent the colonization of new sites by reducing seed supply from the source population.