Showing papers on "Seedling published in 1987"

Role of Fire in Seed Germination of Woody Taxa in California Chaparral

Abstract: Germination behavior of 45 tree, shrub, subshrub, and liana taxa from fire- prone coastal sage scrub and chaparral was investigated. Seeds were sown on filter paper or potting soil, in the light or dark, with or without powdered charred wood (charate), and after heating treatments of 1 h at 700, 5 min at 1000, 5 min at 1 20'C, or no heat. Germination medium had a significant effect on >70% of the species, although the pattern of response to other treatments remained the same on both media. Germination of over half of the species was inhibited in the dark, but two species were light inhibited. Charate significantly enhanced the germination of approximately a quarter of the species. A similar proportion of species exhibited heat-stimulated germination. In only a few species did both heat and charate play a role in stimulating germination. Nearly a third of the species had seeds that germinated readily upon wetting, and germination was not further stimulated by any fire- related cue. These different germination syndromes are correlated with different life history syndromes. Most coastal sage subshrubs germinate readily in the absence of fire-related stimuli and thus are capable of colonizing other forms of disturbance. For many of these species, germination was inhibited in the dark. This may result in a portion of the seed pool remaining dormant until fire since, in the case of several species, dark inhibition is overcome by charate. Chaparral shrubs and trees that germinate readily upon wetting seldom establish seedlings after fire. Their persistence in these fire-prone environments is due to resprouting after fire. Seedling establishment and population expansion for such species is dependent upon extended fire-free periods. In contrast, woody species that fail to germinate without some fire-related cue have seedling establishment and potential pop- ulation expansion restricted to postfire conditions. The present mixture in species com- position may be maintained by variable burning regimes.

Role of fire in seed germination of woody taxa

Abstract: Germination behavior of 45 tree, shrub, subshrub, and liana taxa from fire- prone coastal sage scrub and chaparral was investigated. Seeds were sown on filter paper or potting soil, in the light or dark, with or without powdered charred wood (charate), and after heating treatments of 1 h at 700, 5 min at 1000, 5 min at 1 20'C, or no heat. Germination medium had a significant effect on >70% of the species, although the pattern of response to other treatments remained the same on both media. Germination of over half of the species was inhibited in the dark, but two species were light inhibited. Charate significantly enhanced the germination of approximately a quarter of the species. A similar proportion of species exhibited heat-stimulated germination. In only a few species did both heat and charate play a role in stimulating germination. Nearly a third of the species had seeds that germinated readily upon wetting, and germination was not further stimulated by any fire- related cue. These different germination syndromes are correlated with different life history syndromes. Most coastal sage subshrubs germinate readily in the absence of fire-related stimuli and thus are capable of colonizing other forms of disturbance. For many of these species, germination was inhibited in the dark. This may result in a portion of the seed pool remaining dormant until fire since, in the case of several species, dark inhibition is overcome by charate. Chaparral shrubs and trees that germinate readily upon wetting seldom establish seedlings after fire. Their persistence in these fire-prone environments is due to resprouting after fire. Seedling establishment and population expansion for such species is dependent upon extended fire-free periods. In contrast, woody species that fail to germinate without some fire-related cue have seedling establishment and potential pop- ulation expansion restricted to postfire conditions. The present mixture in species com- position may be maintained by variable burning regimes.

The Role of Hormones During Seed Development

Abstract: The formation of a seed in the life cycle of higher plants is a unique adaptation. It incorporates embryo development with various physiological processes that insure the survival of the plant in the next generation. These adaptations include the accumulation of nutritive reserves, an arrest of tissue growth and development, and the ability to withstand desiccation, all of which are of considerable agronomic importance (e.g., nutritive value, yield, germination). The extent of these adaptations are quite spectacular. For example, the embryo must acquire the ability to withstand a reduction in water content from about 85% to 10%; in other plant tissues, such a severe desiccation is lethal. To survive long periods of time in this dry state until environmental conditions are favorable to resume development into a seedling, numerous plants have acquired different mechanisms of seed dormancy. The term “dormancy” is not entirely appropriate for many higher plants; this term can be defined as the absence of germination during environmental conditions which otherwise promote germination. Typically some external stimulus such as light or chilling (stratification) is required. However, many angiosperms undergo the developmental program of maturation, developmental arrest, and desiccation without true dormancy.

Effects of fungal endophytes on the seed and seedling biology of Lolium perenne and Festuca arundinacea

Abstract: Many grasses are infected by endophytic fungi that grow intercellularly in leaves, stems, and flowers and are transmitted maternally by hyphal growth into ovules and seeds. The seed biology and seedling growth of endophyte-infected and uninfected perennial ryegrass (Lolium perenne) and tall fescue (Festuca arundinacea) were investigated under controlled environmental conditions. The percentage of filled seeds produced by infected tall fescue was over twice of uninfected tall fescue; infected and uninfected perennial reegrass had similar percentages. Weights of seeds from infected and uninfected plants were similar in both species. Seeds from infected plants of both species exhibited a higher rate of germination than seeds from uninfected plants. Shoot growth in the greenhouse was compared by making three sequential harvests of above-ground plant parts from infected and uninfected plants of both species. Infected perennial ryegrass plants produced significantly more biomass and tillers than uninfected plants after 6 and 10 weeks of growth and significantly more biomass after 14 weeks of growth. Infected tall fescue plants produced significantly more biomass and tillers than uninfected plants after 10 and 14 weeks of growth. The physiological mechanism of enhancement of growth is not known. The results of this study suggest that infected plants may have a selective advantage in populations with uninfected members.

Mechanisms of Early Primary Succession in Subalpine Habitats on Mount St. Helens

Abstract: In the 6 yr following the 1980 eruption of Mount St. Helens in Washington state, vascular plant invasion of barren substrates in subalpine habitats has been limited despite the proximity of seed sources from resprouted vegetation. From 1983 to 1985 we counted Ist-yr seedlings and estimated percent cover of adults in grids of permanent plots located across the ecotone between residual vegetation and barren substrate created in 1980. We found that (1) seedling recruitment declined from 1983 to 1985, apparently in response to drought; (2) most seedlings occurred within 3 m of a conspecific adult; and (3) plots with an intermediate vegetation cover (5-72%) contained a significantly higher fraction of seedlings than did unvegetated plots. Thus, dispersal is limited in many species and nurse plants may play a key role in trapping seeds and promoting seedling establishment. We sowed 16 000 viable seeds from 22 subalpine species into 264 plots in barren sub- strates at two sites on the volcano. The design was a complete factorial, with species, site, and fertilizer as treatments. Survivorship through 1985 varied from 0 to 12%, with Sitanion jubatum, Stipa occidentalis, Polygonum newberryi, Eriogonum pyrolifolium, and Spraguea umbellata attaining the highest values. Survivorship was correlated with seed mass, and was higher at the site where the pre-eruption surface was exposed. Fertilizer increased the size of most seedlings but had only a marginal effect on survivorship. Species with high environmental tolerance generally dispersed short distances, whereas species that dispersed farther generally had low tolerances and apparently require site amelioration prior to establishment. The path of early succession depends upon the spatial position and dispersal abilities of species in the seed pool, and may not reflect environmental gradients.

Effects of Predation and Light on Seedling Establishment in Gustavia Superba

Abstract: Large-seeded trees whose seeds are dispersed by mammals often have high levels of seed dispersal but high seed losses due to predation. Consequently, seedling establishment is influenced both by the impact of mammalian seed predators as well as by the ability of the seedling to survive where the seed is dispersed. I examined the relative roles of seed/seedling predation by mammals and of the light environment in seedling establishment in three populations of the tropical tree Gustavia superba. One population was located in semideciduous forest on Gigante Peninsula and two were on Barro Colorado Island (BCI), Panama. On Gigante Peninsula, mammals that feed on Gustavia fruit and seeds were much less abundant than on BCI. My objectives were: (1) to quantify seedling densities in each population, (2) to measure the intensity of seed predation, and (3) to determine the relative importance of seed/seedling predation and light in seedling survival and growth. Seedling density was significantly greater on the Gigante Peninsula than on BCI. This pattern was influenced by postdispersal seed predation that was much lower on the Gigante Peninsula than on BCI. Seedling transplant experiments, which measured the effects of site, light condition (gap, edge, understory), and protection from mammals (tall cage, short cage, no cage), showed that protection from mammals significantly increased all three measures of seedling performance: seedling survival, seedling height, and overall seedling success (survival times growth). In addition, as expected from differences in mammal abundance among sites, the three sites differed in the extent to which each measure of

Seed dynamics during forest succession in Costa Rica

Abstract: Soil seed banks and current seed inputs each play a role in tropical succession We compared the abundance and floristic composition of seeds from these two sources at a Costa Rican site by germinating seeds from the soil, measuring seed inputs for 3 yr, and monitoring the earliest colonists in a forest clearing There were an estimated 6800 viable seeds/m2 in the soil of 33-yr-old vegetation, 9500 seeds/m2 in 11-yr-old vegetation, and 7000 seeds/m2 in a 75-yr-old forest An estimated 10100 seeds/m2 fell on the soil surface of the young successional vegetation during 3 yr and 3700 seeds/m2 fell during that same time in the forest Locally produced seeds accounted for about 75% of the seed input to the soil surface early in succession Seeds dispersed out of young successional vegetation increased the quantity and species richness of the seed input and storage in an adjacent forest Much of the species richness of the young successional vegetation resulted from seeds dispersed there from other communities by animals Deforestation stimulated germination of most seeds in the surface soil of the old forest, including seeds of the dominant canopy tree The recruitment of seedlings from the soil seed bank numerically overwhelmed that from post-disturbance seed rain and sprouts We evaluated patterns of soil seed storage during succession and predicted the ability of vegetation of differing ages to respond to disturbance Immediately after disturbance the number of seeds in the soil plummeted due to mortality, low inputs, and germination As the vegetation regrew, the soil seed bank increased to a peak after 4 to 7 yr, then gradually decreased to its pre-disturbance size High-frequency pulses of disturbance should result in reduced species richness, dominance by species with long-lived seeds, and fast recovery by seedling recruitment from the soil seed bank

Effect of soil surface topography and litter cover on the germination, survival, and growth of musk thistle (Carduus nutans)

Abstract: In natural populations of musk thistle (Carduus nutans L.) germination occurs over several months in the fall and spring. To better understand the physical environmental mechanisms that influence this pattern of germination, a greenhouse experiment was established consisting of ten treatments in which soil surface topography and the amount and consistency of litter cover was varied. Significant differences occurred among the treatments for germination percentage, levels of mortality and individual growth rates. Optimum levels ofgermination, survival and growth occurred in treatments that provided microhabitats with reduced evaporation. Treatments with cracks or moderately irregular surface topographies provided the best conditions. A light covering of litter benefited establishment and growth but thick layers of litter prevented seeds from reaching the soil surface and reduced germination and establishment. These results were consistent with what is known about the germination ecology of musk thistle in natural populations. IN MANY PLANT species the highest levels of mortality occur between the seed dispersal and the seedling establishment phases of the life cycle (Harper and White, 1974; Harper, 1977). Unfortunately, this is often the most poorly understood portion of the life cycle. Usual monitoring procedures are capable of quantifying seed production and seedling establishment but are generally incapable of describing interactions between seeds and the soil surface environment. Yet many biologists (e.g., Grubb, 1977; Grime, 1979) feel that these interactions are critical to the reproduction of populations and the composition of plant communities. Successful germination involves complex interactions between the seed's physical position, its physiological state, and its microenvironment. Some species have genetically determined, innate dormancy requirements (Harper, 1977) that prevent germination until after specific environmental keys have been experienced (e.g., Betula pubescens, Black and I Received for publication 13 February 1986; revision accepted 19 August 1986. We wish to express our gratitude to Karen J. Hamrick and Jenna R. Hamrick for their technical assistance. Steve Stewart provided valuable assistance during the data analyses. The comments of two anonymous reviewers greatly improved this paper. This work was supported by funds supplied by University of Kansas General Research Fund Grant #3675 and Environmental Protection Agency Grant R-805455030. 2 Current address: Departments of Botany and Genetics, University of Georgia, Athens, GA 30602. 3Current address: Department of Botany and Plant Sciences, University of Califomia, Riverside, CA 92521. Wareing, 1955; Avenafatua, Marshall and Jain, 1970; Naylor and Jana 1976; Ambrosia artemisifolia, Willemsen 1975a, b; Avena barbata, Marshall and Jain, 1970). Other species have less rigid dormancy (enforced, Harper, 1977) and can germinate whenever minimum environmental conditions are met (usually heat, light, moisture, and proper aeration). Adverse conditions such as drought, cold, or deep burial in soil or litter prevent or retard germination in these species (Werner 1975; Harper, 1977). Several investigators (Harper and Sager, 1953; Harper, Williams and Sager, 1965; Sheldon, 1974) have demonstrated that subtle features of the soil surface topography and the position of the seed in relation to this topography can have dramatic effects on germination and seedling establishment. Studies of the population ecology of musk thistle (Carduus nutans L.) indicate that germination occurs over periods of several months in both fall and spring (Lee and Hamrick, 1983). Subsequent survival, growth and fecundity of monitored individuals was influenced by the their time of germination. Generally, individuals germinating in the early fall had lower mortality, attained larger vegetative sizes and produced more seeds than those that germinated later in the fall or in the spring. Later studies (J. L. Hamrick, unpubl. data) indicate, however, that germination early in the fall can be followed by a period of high mortality if a period of drought immediately follows germination. It is clear, therefore, that the dynamics of musk thistle populations may be

Increases in mycorrhizal colonization and seedling growth in Pinusechinata and Quercusalba in an enriched CO2 atmosphere

Abstract: Forest tree biomass is hypothesized to increase in a CO2-enriched atmosphere if mechanisms exist to ensure acquisition of limiting nutrients in forest soils. Investment of additional photosynthate produced at elevated CO2 into mycorrhizal proliferation and root growth may provide one such mechanism. To test this hypothesis, mycorrhizal density and seedling biomass were measured in shortleaf pine (Pinusechinata Mill.) and white oak (Quercusalba L.) grown in unfertilized forest soil in controlled-environment chambers at 360 μL L−1 and 700 μL L−1 CO2. Mycorrhizal density was greater at elevated CO2 in both species after 6 weeks of exposure; in white oak, the increased density persisted for 24 weeks. Root dry weight was increased 76% in P. echinata and 91% in Q. alba at 700 μL L−1 CO2; total seedling dry weight was increased by 66 and 56%, respectively. It is hypothesized that increased photosynthesis at elevated CO2 offsets the carbon requirement for mycorrhizal establishment on shortleaf pine. Greater mycor...

Elevated atmospheric CO2 effects on seedling growth, nutrient uptake, and rhizosphere bacterial populations ofLiriodendron tulipifera L.

Abstract: Yellow-poplar (Liriodendron tulipifera L.) seedlings were planted in unfertilized forest soil in boxes with a removable side panel and grown in atmospheres containing either ambient (367 μl l−1) or elevated (692 μl l−1) CO2. Numbers of total bacteria, nitrifiers, and phosphate-dissolving bacteria in the rhizosphere and in nonrhizosphere soil were measured every 6 weeks for 24 weeks. Seedling growth and nutrient content were measured at a final whole-plant harvest. Root, leaf, and total dry weights were significantly greater, and specific leaf area was significantly less, in 692 ml l−1 than in ambient CO2. Uptake per gram plant dry weight of N, S, and B was lower at elevated CO2, whereas uptake of P, K, Cu, Al, and Fe was proportional to growth in both CO2 treatments. Total uptake and uptake per g plant dry weight of Ca, Mg, Sr, Ba, Zn, and Mn were not affected by CO2 treatment. Bacterial populations differed due to CO2 only at the final harvest, where there were significantly fewer nitrite-oxidizers and phosphate-dissolving bacteria in the rhizosphere of seedlings grown at 692 μl l−1 CO2.

Population ecology of halophyte seeds

Abstract: Some aspects of the population biology of halophytes are considered in this review. Persistent seed banks have been reported for a number of inland- and coastal-salt marsh plant communities. Seeds of perennial grasses are often under-represented, while annuals and some perennial forbs may be over-represented in the seed bank. The persistent seed bank of annual halophytes appears adaptive, and provides multiple seed germination opportunities which may prevent local extinction when environmental stress increases. Somatic seed polymorphism provides a mechanism by which parent plants can respond to changing environments by partitioning their resources into reproductive units which have distinct germination responses. Parental effects may influence either seed morphology and/or physiological requirements of seeds when they are exposed to environmental stress. A prolonged germination period can provide plant populations with numerous opportunities to establish seedling cohorts. Early cohorts will have a selective advantage under moderate conditions because mortality will be low and plants will survive until maturity. However, fluctuations in salinity levels and tidal activity can cause high mortality in early cohorts in salt marsh habitats, providing later cohorts with an opportunity for establishment. Resource allocation to reproductive structures is related to plant size, which itself can be affected by both abiotic and biotic factors. Larger plants were found to produce more seeds than smaller plants in a population, but the mean seed weight was greater in small plants.

Effect of soil transfer on ectomycorrhiza formation and the survival and growth of conifer seedlings on old, nonreforested clear-cuts

Abstract: Small amounts (150 mL) of soil from established conifer plantations and mature forest were transferred to planting holes on three clear-cuts in southwest Oregon and northern California to enhance mycorrihiza formation. The clear-cuts, 8–27 years old and unsuccessfully reforested, included a range of environmental conditions. At Cedar Camp, a high-elevation (1720 m) southerly slope with sandy soil, transfer of plantation soils increased 1st-year Douglas-fir (Pseudotsugamenziesii (Mirb.) Franco) seedling survival by 50%. Notably, soil from a plantation on a previously burned clear-cut doubled mycorrhiza formation and tripled seedling basal area growth. Soil from mature forest did not improve survival and growth. Less dramatic effects owing to soil transfer were evident on other sites, which were lower in elevation and had clayey soils with greater water-holding capacity, and where woody shrubs had apparently preserved mycorrhizal fungi. At Crazy Peak (1005 m), seedling survival was uniformly good, and soil ...

The seedling regeneration niche: habitat structure of tree seedlings in an oak-pine forest

Abstract: The physical environment to which a seedling is subjected affects its probability of survival and recruitment into a population. Aspects of the physical and biotic environment form components of the plant habitat and regeneration niches defined by Grubb. The importance of different variables may change during the life cycle of a long-lived plant. We measured nine variables that characterize the habitat of six species of one year old tree seedlings in an oak-pine forest in the New Jersey Pine Barrens. Variables were measured for 25 randomly located individuals of each species as well as at 25 random points in the same stand as the seedlings and in two other stands. Principal components analysis (PCA) of seedling plus random point data produced two habitat gradients: Axis I was a gradient from canopy cover to moss and lichen cover and higher light intensity. Axis II was a gradient from high total cover and shallow litter to habitats with less cover and deeper litter layers. Random points were concentrated in areas with deep litter and low light whereas most seedlings grew in areas with more light and less litter. Discriminant functions analysis indicated that seedling habitat breadth was large but that the habitat of Pinus echinata could be distinguished from that of Quercus coccinea and Sassafras albidum. Seedling density differed among the three stands. Analysis by PCA of the random points from the three stands produced similar habitat gradients as in the PCA derived from tree seedling habitats. In the stand where seedling density was lowest, litter layers were significantly deeper, shrub density was greater and light was lower than in the other stands. These trends were confirmed by discriminant functions analysis. The multivariate analysis of seedling habitat and regeneration niches can be used to explain, in part, seedling density in the ground layer of different forest stands.

Seasonal variation in root hardiness of container-grown Scots pine, Norway spruce, and lodgepole pine seedlings

Abstract: Roots of containerized seedlings of Scots pine (PinussylvestrisL.), Norway spruce (Piceaabies (L.) Karst.), and lodgepole pine (Pinuscontorta Dougl.) grown for one season were experimentally frozen during autumn, winter, and spring. After freezing, root growth capacity and seedling survival were determined by growing the seedlings for 3 weeks in a growth chamber. Root vitality was also tested with the triphenyl tetrazolium chloride method. The seedlings showed a progressive increase in root hardiness towards midwinter and a dehardening as spring approached. Scots pine and lodgepole pine roots were more sensitive to low temperatures than Norway spruce roots. Northern provenances of Scots pine and Norway spruce had the hardiest roots during the autumn. During winter and spring, provenance differences became less clear. Northern Scots pine and lodgepole pine seedlings sown in July tended to be more sensitive to low root temperatures than older seedlings sown in May. No age effects were found in southern Scot...

Fire and acacia seeds: a hypothesis of colonization success

Abstract: SUMMARY (1) The effects of natural and simulated fire on insect-infested seed germination, seedling emergence and on the viability of seed pools in the soil of Acacia sieberiana were investigated in a savanna in Queen Elizabeth National Park, Uganda. (2) There was a strong potential for the population of A. sieberiana plants to increase from the viable seed pool of about 800 seeds m-2. Seedling emergence was 167 seedlings m-2 after one fire. Seeds collected from the soil surface of burned areas produced significantly higher total germination rates (650%,) than those from unburned areas (10%/,) when tested in the laboratory. As expected, seeds located at shallow depths were stimulated to germinate more readily by fire then seeds located deeper in the soil. (3) The germination was significantly reduced by the high bruchid larvae (Bruchidius spp.) infestation. Bruchid larvae are more sensitive to fire than the seed embryo and, thus, the heat treatment indirectly leads to higher germination. (4) Regression equations were developed for predicting the mortality of seed and bruchid larvae under different fire intensities. (5) A conceptual diagram of germination success of A. sieberiana was developed.

Seasonal changes in the free amino acids of oat and barley phloem sap in relation to plant growth stage and growth of Rhopalosiphum padi

Abstract: SUMMARY The concentrations and composition of free amino acids in phloem sap from two cultivars of oats and barley, both susceptible to the aphid Rhopalosiphum padi, were determined by means of high performance liquid chromatography Sap was collected from excised aphid stylets at three developmental stages (seedlings, tillering plants and plants undergoing stem elongation) from plants given or not given fertiliser and grown outdoors In connection, the growth of individual R padi nymphs was estimated at the same phenological stages on plants grown in the greenhouse The content of free amino acids was consistently higher in seedlings than in plants at the early tillering stage Only in seedlings did the addition of fertiliser increase amino acid levels Barley phloem sap contained more free amino acids than that of oats when fertiliser was added and at later developmental stages Phloem sap of oats and barley showed similar patterns in their composition of free amino acids at the seedling stage, but as the plants grew older the patterns became increasingly different Plants given fertiliser had higher amounts of dicarboxylic amino acids (glutamic and aspartic acid) than unfertilised plants The concentrations of γ-amino butyric acid, glycine, histidine, and methionine were very low in all treatments The relative growth rates of R padi nymphs were low when amino acid content was low and vice versa The results are discussed in relation to host plant suitability and plant resistance mechanisms

Competitive effects of various grasses and forbs on ponderosa pine seedlings

Abstract: Competition between ponderosa pine seedlings and various grasses and forbs was studied on :I site in northern Arizona burned in 1982 by a wildfire Two-yearold pine seedlings were planted in 305 x 305 m plots in April 1983, followed by the sowing of grass and forb seeds on the same plots in July 1983 after summer rains had begun Predawn xylem water potential of the pine seedlings was measured biweekly throughout the 1983 and 1984 growing seasons Extractable soil nitrogen was measured at the beginning and end of both growing seasons After the wildfire, nitrate and ammonium levels were significantly higher in the burned area than in an adjacent unburned area Nitrate and ammonium concentrations in various grass treatment plots were significantly (P < 005) different at both the beginning and end of the growing season Pine seedling xylem water potential differed among treatments, with potentials being lowest on plots sown with Agrtpyron deserrorrr/n The differences in seedling xylem water potential and available soil nitrogen were reflected in differences in pine seedling growth The most effective competitors were Agropym d~~scworwn and Lkwtylk g/o177~~1~71~ Results indicated that competition occurred for both moisture and available nitrogen FOR SCI 33(2):356-366 ADUITIONAI

The allelopathic potential of alfalfa root medicagenic acid glycosides and their fate in soil environments

Abstract: The allelopathic effect of alfalfa (Medicago media Pers.) and red clover (Trifolium pratense L.) root saponins on winter wheat seedling growth and the fate of these chemicals in soil environments were studied. Seed germination, seedling and test fungus growth were suppressed by water and by alcohol extracts of alfalfa roots, and by crude saponins of alfalfa roots, indicating that medicagenic acid glycosides are the inhibitor. Powdered alfalfa roots inhibited wheat seedling growth when added to sand. At concentrations as low as 0.25% (w/w) the root system was completely destroyed whereas seedling shoots suffered little damage. Red clover roots caused some wheat growth inhibition when incorporated to sand, but their effect was much lower than in the alfalfa root treatment.

Effects of Seed Predation by Ants on Seedling Densities at a Woodland Site in SE Australia

Abstract: Ants are by far the most important post-dispersal seed predators at a woodland site in SE Australia, and they rapidly remove many seeds from the ground. This paper uses experimental manipulations of seed densities and ant populations to examine the likely impact of seed predation by ants on seedling recruitment in Eucalyptus baxteri and Casuarina pusilla. When seedfall was light, as usually occurs in the absence of fire, the elimination of ants resulted in a 15-fold increase in seedling densities for E. baxteri, whose seeds are removed at particularly high rates. Seeds of C. pusilla are removed less rapidly by ants, and increases in seedling densities following ant elimination were less marked for this species. Experimental dumpings of seeds at high densities, simulating the massive release of seeds from woody fruits which occurs following fire, resulted in predator satiation and in comparison to controls produced high numbers of seedlings. These results suggest that seed predation by ants limits seedling recruitment in the absence of fire, and that predator satiation plays an important role in successful recruitment following fire.

Seedling Recruitment Strategies in Obligate-Seeding and Resprouting Banksia Shrubs.

Abstract: Banksia ericifolia is killed by fire and so depends upon seeds for persistence. In contrast, B. oblongifolia can persist after fire by resprouting from protected buds, as well as by recruiting seedlings. These serotinous species coexist in the fire-prone scrublands of the Sydney Basin. Here we report experiments that compared seed release from cones retained in the canopy, seed germination, and seedling establishment in these two species. B. oblongifolia seeds were released more quickly than B. ericifolia seeds after a fire, and B. oblongifolia follicles opened and shed seeds after exposure to lower temperatures than those of B. ericifolia. B. oblongifolia seeds germinated more quickly than those of B. ericifolia. The relative growth rates and leaf mass ratios (mean ratio of leaf dry mass to plant dry mass over sequential harvests) of the two species did not differ. Root : shoot ratios were significantly large and biomass allocation to roots was faster in B. oblongifolia seedlings. B. ericifolia seedlings survived low and intermediate levels of droughting better than B. oblongifolia under greenhouse conditions. In a field experiment in which water was not limiting, large B. oblongifolia seedlings had the highest survival rate. These results suggest that selection has acted on B. ericifolia to minimize the chance of zero seedling survival by distributing the risks over more than one germination episode. There was no consistent evidence to support the hypothesis that selection on this species has increased its probability of seedling survival during any one germination opportunity relative to its congener.

Analysis of seedling banks of a climax beech forest: ecological importance of seedling sprouts

Abstract: Most of the common and dominant tree species in gaps of a climax Japanese beech (Fagus crenata) forest have large seedling banks on the forest floor. These seedlings were classified into “true seedlings” and “seedling sprouts”, each of which was further classified into two and three subtypes, respectively. Each species had a characteristic seedling form spectrum. The seedling bank of beech was composed almost solely of true seedlings, whereas, seedling sprouts predominated in the seedling banks of lower tree species such as Prunus grayana Maxim. and some species of Acer. The age structures suggested that seedling sprouts had longer life spans and had been recruited more constantly than true seedlings. The habit of sprouting and creeping seemed to enable seedlings of these species to persist for long periods, thus increasing their chance for eventual success in canopy gaps.

Growth and nutrition of containerized Pinusresinosa at exponentially increasing nutrient additions

Abstract: Containerized red pine (Pinusresinosa Ait.) seedlings were reared on different fertility regimes that compared conventional applications of a recommended 39 mg N per seedling as a complete nutrient...

Natural Re-Seeding and Trifolium repens Demography in Grazed Hill Pastures. II. Seedling Appearance and Survival

Abstract: (1) The appearance and survival of Trifolium repens seedlings from naturally deposited seed were monitored for 3 years under different sheep grazing managements and phosphate fertilizer inputs, and in four slope/aspect zones, in summer moist New Zealand hill country. (2) A mean of 5 5 seedlings m-2 per year appeared, representing 4 2% of the seeds deposited each summer. (3) Only 44% of seedlings survived to form established, stolon-bearing plants, representing a mean recruitment rate of one seedling per 5 5 m2 per year. Survival was highest on steep north-west sites (10%), and nil on flat south-west sites, reflecting likely competitive stresses from surrounding vegetation. (4) Such low recruitment rates clearly indicate seedling regeneration plays only a minor role in T. repens persistence in this environment. Vegetative stolon densities of 3000-4000 m-2 and high rates of stolon initiation and death confirm that the species behaves essentially as a true perennial. (5) A useful ecological role for low seedling recruitment rates in maintaining genetic diversity within the T. repens population must be recognized, however, especially in highly variable environments.

Soybean growth responses to enhanced levels of ultraviolet-b radiation under greenhouse conditions

Abstract: Soybean (Glycine max [L.] Merr. cv. Essex) was grown in an unshaded greenhouse under three levels of biologically effective ultraviolet-B (UV-BBE) radiation (effective daily dose: 0, 11.5 and 13.6 kJ m-2) for 91 days. Plants were harvested at regular intervals beginning 10 days after germination until reproductive maturity. Mathematical growth analysis revealed that the effects of UV-B radiation varied with plant growth stage. The transition period between vegetative and reproductive growth was the most sensitive to UV-B radiation. Intermediate levels of UV-B had deleterious effects on plant height, leaf area, and total plant dry weight at late vegetative and reproductive stages of development. Specific leaf weight increased during vegetative growth but was unaffected by UV-B during reproductive growth stages. Relative growth, net assimilation, and stem elongation rates were decreased by UV-B radiation during vegetative and early reproductive growth stages. Variation in plant responses may be due in part to changes in microclimate within the plant canopy or to differences in repair or protection mechanisms at differing developmental stages. ENHANCED LEVELS of solar ultraviolet-B radiation (UV-B, 280-320 nm) can substantially restrict plant growth and development (Caldwell, 1971; Van, Garrard and West, 1976; Vu, Allen and Garrard, 1978; Biggs, Kossuth and Teramura, 1981; Tevini, Iwanzik and Thoma, 1981; Teramura, 1983). UV-B sensitivity is influenced by existing microclimatic conditions including ambient levels of visible radiation (Teramura, 1983; Warner and Caldwell, 1983; Mirecki and Teramura, 1984), water regimes (Teramura et al., 1984; Teramura, Forseth and Lydon, 1984; Murali and Teramura, 1986a), and nutrient levels (Murali and Teramura, 1985). Sensitivity to UV-B radiation may also vary depending upon the phenological stage of development. In a recent field study, net photosynthesis in soybean was adversely affected by UV-B, with late vegetative and early reproductive stages being more senI Received for publication 8 July 1986; revision accepted 11 November 1986. The authors would like to thank Drs. Gregory A. Carter and Donald T. Krizek for comments and reviewing this manuscript. This work was supported by the United States Environmental Protection Agency's Environmental Research laboratory in Corvallis, Oregon (CR 812-152-020). Scientific Article No. A-4464, Contribution No. 7456 of the Maryland Agricultural Experiment Station. Although the work described herein was funded in part by the United States Environmental Protection Agency, it has not been subjected to the Agency's required peer and policy review and therefore does not necessarily reflect the views of the Agency and no official endorsement should be inferred. sitive than later reproductive stages (Murali and Teramura, 1986b). In that study, measurements were arbitrarily made only during three growth stages. Some investigators have found that seedlings and young vegetative plants are unaffected by enhanced levels of UV radiation following germination in the field (Becwar, Moore and Burke, 1982; Biggs et al., 1984), while Krizek (1975) reported some abnormal seedling growth after exposure to increased levels of UV-B. These effects included reduced root growth, bronzing of the cotyledons, increased pigmentation, and abnormal shoot curvature. To date, we lack detailed information on the relative sensitivity of various growth stages to enhanced levels of UV-B radiation. Since final crop yield reflects a summation of effects through various stages of growth, an understanding of developmental changes in sensitivity could help in breeding plants more tolerant to UV-B radiation. The purpose of this study was to determine the effects of UV-B radiation on the allometrics of soybean at various phenological stages. MATERIALS AND METHODS-Seeds of soybean (Glycine max [L.] Merr. cv. Essex) were sown in 1.2-liter pots of Metromix 300 potting mixture (composted pine bark, peat moss, vermiculite, perlite, granite, and sand). Plants were grown from May to July 1984 in an unshaded greenhouse at the University of Maryland, College Park, MD. The natural photoperiod ranged from 14 to 16 hr. Midday peak photosynthetic