Showing papers on "Seedling published in 1991"

Seed germination and life history syndromes in the California chaparral

Abstract: Syndromes are life history responses that are correlated to environmental regimes and are shared by a group of species (Stebbins, 1974). In the California chaparral there are two syndromes contrasted by the timing of seedling recruitment relative to wildfires. One syndrome, here called the fire-recruiter or refractory seed syndrome, includes species (both resprouting and non-resprouting) which share the feature that the timing of seedling establishment is specialized to the first rainy season after fire. Included are woody, suffrutescent and annual life forms but no geophytes have this syndrome. These species are linked by the characteristic that their seeds have a dormancy which is readily broken by environmental stimuli such as intense heat shock or chemicals leached from charred wood. Such seeds are referred to as “refractory” and dormancy, in some cases, is due to seed coat impermeability (such seeds are commonly called hardseeded), but in other cases the mechanism is unknown. Seeds of some may require cold stratification and/or light in addition to fire related stimuli. In the absence of fire related cues, a portion or all of a species’ seed pool remains dormant. Most have locally dispersed seeds that persist in the soil seed bank until the site burns. Dispersal of propagules is largely during spring and summer which facilitates the avoidance of flowering and fruiting during the summer and fall drought. Within a life form (e.g., shrub, suffrutescent, etc.), the seeds of these species have less mass than those of species with non-refractory seeds and this possibly reflects the environmental favorableness of the postfire environment for seedling establishment. Regardless of when fire occurs, germination is normally delayed until late winter or early spring. In the absence of fire, or other disturbance, opportunities for population expansion are largely lacking for species with this syndrome.

Canopy seed storage in woody plants

Abstract: The retention of seeds in the plant canopy for one to 30 years or more is termed serotiny. It is well represented floristically and physiognomically in fire-prone, nutrient-poor and seasonally-dry sclerophyll vegetation in Australia, and to a lesser extent, South Africa followed by North America. While the seed-storing structures vary greatly, all will release their propagules following exposure to the heat of a fire (pyriscence). This phenomenon can be contrasted with seed release at maturity (non-storage) and soil storage of seeds. Although the evolutionary requirements for serotiny are clear, its adaptive advantages over other seed storage syndromes are largely the subject of conjecture in the absence of comparative experiments. Nine hypotheses were assessed here. Canopy storage maximises the quantity of seeds available for the next post-fire generation (unlike non-storage). Synchronized post-fire release satiates post-dispersal granivores (unlike non-storage and soil storage) and ensures arrival on a seed bed conducive to seedling recruitment (unlike non-storage). Canopy stored seeds are better insulated from the heat of a fire than non-stored, and probably soil-stored, seeds. Fluctuating annual seed crops, the opportunity for post-fire wind-dispersal, the possible advantages of dense stands of adults, short lifespan of the dispersed seeds and their optimal location in the soil for germination have only a limited role in explaining the advantages of serotiny. It is concluded that canopy seed storage is favoured in regions where seed production is restricted and inter-fire establishment and maturation are unlikely. In addition, these regions have a reliable seasonal rainfall and are subjected to intense fires at intervals occurring within the reproductive lifespan of the species.

Penetration of very strong soils by seedling roots of different plant species

Abstract: The abilities of seedling roots of twenty-two plant species to penetrate a strong growth medium were compared under controlled conditions. Seedlings were grown for 10 days in compression chambers filled with siliceous sandy soil at 0.2 kg kg−1 water content and mean penetrometer resistance of 4.2 MPa. Root elongation and thickening were measured after growth. The results show that soil strength reduced the elongation of roots of all plant species by over 90% and caused the diameters of the roots to increase compared with control plants grown in vermiculite (0 MPa resistance).

Flavonoid and isoflavonoid distribution in developing soybean seedling tissues and in seed and root exudates.

Abstract: The distribution of flavonoids, isoflavonoids, and their conjugates in developing soybean (Glycine max L.) seedling organs and in root and seed exudates has been examined. Conjugates of the isoflavones daidzein and genistein are major metabolites in all embryonic organs within the dry seed and in seedling roots, hypocotyl, and cotyledon tissues at all times after germination. Primary leaf tissues undergo a programmed shift from isoflavonoid to flavonoid metabolism 3 days after germination and become largely predominated by glycosides of the flavonols kampferol, quercetin, and isorhamnetin by 5 days. Cotyledons contain relatively constant and very high levels of conjugates of both daidzein and genistein. Hypocotyl tissues contain a third unidentified compound, P19.3, also present in multiple conjugated forms. Conjugates of daidzein, genistein, and P19.3 are at their highest levels in the hypocotyl hook and fall off progressively down the hypocotyl. These isoflavones also undergo a programmed and dramatic decrease between 2 and 4 days in the hypocotyl hook. All root sections are predominated by daidzein and its conjugates, particularly in the root tip, where they reach the highest levels in the seedling. Light has a pronounced effect on the distribution of the isoflavones; in the dark, isoflavone levels in the root tips are greatly reduced, while those in the cotyledons are higher. Finally, the conjugates of daidzein and genistein and several unidentified aromatic metabolites are selectively excreted into root and seed exudates. Analysis of seed exudates suggests that this is a continuous, but saturable event.

Tree and shrub seedling colonization of old fields in central new york

Abstract: The main objective of this study was to determine the important mechanisms regulating woody plant establishment in recently abandoned agricultural fields in central New York. The study focused on the early life stages, from the dispersal of seeds to the fate of seedlings at the end of the second growing season, of common old-field invaders, primarily Acer rubrum, Pinus strobus, Cornus racemosa, and Rhamnus cathartica. We investigated patterns of survivorship and causes of mortality during each of these life stages in microsites dominated by previously established herbs and in experimental openings in the herb canopy. Establishment of a tree or shrub seedling was much less likely from seeds landing among herbs than in openings. During the fall, seed predation by mice, especially under the cover of herbs (median of averages across all species = 6% in open, 85% under herbs), was an important process regulating the relative abundance and spatial distribution of seeds after dispersal. Seedling emergence rates were generally quite low, but for most species tended to be greater in the presence of perennial herbs (12%) and were lower under annuals and biennials (7%) and in bare soil (6%). For cohorts of seedlings followed for two growing seasons (Experiment 6A), some mortality occurred due to frost heaving (17% in open, 4% under herbs) and apparent heat stress or desiccation during the summer (5% in open, 0% under herbs); however, the largest source of mortality, especially under the herb canopy, was predation by rodents, especially meadow voles (6% in the open, 62% under herbs). Competition with herbs, especially aboveground competition, significantly reduced growth of woody seedlings. In spite of growth reduction in the presence of herbs, there was no significant relationship between seedling size and survivorship within any treatment, in- dicating that mortality could not be directly attributed to competition. For different species, and at different life stages, support was found for all three of the succession models of Connell and Slatyer (1977). The tolerance model was supported in some cases of seed predation (Experiment 2), seedling emergence (Experiments 4 and 5, in part), and seedling predation (Experiment 7). The facilitation model was supported in some cases by increased seedling emergence (Experiments 4 and 5, in part) and increased seedling survivorship in the presence of herbs during periods of environmental stress (Experiments 6A and 8). The inhibition model was also supported in some cases of de- creased growth (Experiment 6B) and decreased survival of seeds and seedlings (Experiments 1, 3, 6A, and 8) in the presence of herbs. The lower survivorship under herbs was largely the result of seed and seedling predation, which were the two most important processes regulating tree and shrub colonization of old fields in this study. Reduced growth of later colonists due to competition with earlier colonists is considered direct inhibition, while increased mortality due to predation on later colonists caused by animals whose activity is necessarily associated with the presence of earlier colonists is considered indirect inhi- bition. Seed and seedling predation are potentially important mechanisms regulating suc- cession and should be explicitly included in studies of succession.

Experiments on Mechanisms of Tree Establishment in Old-Field Succession: Seedling Survival and Growth

Abstract: Mechanisms influencing the relative establishment success of early succes- sional tree species in North Carolina Piedmont old fields were investigated in multi-factor field experiments. Six tree species (Acer rubrum, Fraxinus americana, Liquidambar styr- aciflua, Liriodendron tulipifera, Pinus taeda, and Ulmus alata) were sown into old fields at equivalent densities. In this paper I evaluate relative seedling performance (survival and growth) with respect to two principal factors: (1) competition from old-field vegetation (weeded vs. vegetated plots) and (2) browsing by vertebrate herbivores (exclosures vs. open plots). Manipulation of the first factor tests Connell and Slatyer's (1977) alternative "mod- els" of succession. Seedlings were monitored over a 3-yr period in each of two experiments, one initiated in a wet spring (1984) and the other in a dry spring (1985). Most seedling mortality occurred in the first growing season. In the 1984 experiment, old-field vegetation reduced survival of all species; however, competitive reductions were not large except in Liriodendron, which appeared unable to survive in herbaceous cover, and in Acer, which survived poorly in the absence of competition as well. In the other four species, survival in old-field vegetation was similar; after high mortality in the first year, seedlings exhibited continued slow mor- tality with time. The dry spring of 1985 resulted in greater overall mortality in all species and greater variation in species responses to vegetation cover. In contrast to survival, height growth was strongly reduced by herbaceous vegetation in all hardwood species, but not in Pinus, which grew fairly well in competition with the herbaceous community. Character- ization of these responses as tolerance vs. inhibition is complex and requires consideration of long-term demographic behavior. Three species (Fraxinus, Liquidambar, and Ulmus) were damaged by vertebrate browsers, but browsing intensity varied according to site and was most severe in Ulmus, with the result that its survival and growth were selectively reduced. The expected relative prevalence of these hardwood species, based on their emergence success and seedling performance in the experiments, does not fully correspond to their apparent relative occurrence in young Piedmont hardwood communities. A significant factor that may account for the discrepancies is differential seed rain, which can numerically offset low rates of seedling survival. Patterns of seed production by the hardwood species appear consistent with this hypothesis.

Experiments on mechanisms of tree establishment in old-field succession: seedling emergence'

Abstract: Loblolly pine (Pinus taeda) is the initial successful woody invader of aban- doned old fields in the Piedmont region of North Carolina, but hardwood tree species also invade old fields and develop an understory community beneath young pine stands. The composition of this early hardwood community will be regulated by factors affecting the relative establishment success of these species in old-field habitats. Mechanisms influencing differential seedling emergence of such early successional tree species were studied in multi- factor field experiments. Seeds of six wind-dispersed species (Acer rubrum, Fraxinus amer- icana, Liquidambar styraciflua, Liriodendron tulipifera, Ulmus alata, and Pinus taeda) were sown into old fields at equal densities, and their relative germination (emergence) in two successive years was evaluated with respect to: (1) presence of old-field vegetation (tested with a weeding treatment), (2) vertebrate seed predation (tested with exclosures), (3) vari- ation in life history traits (seed size and germination phenology), and (4) variation in physical factors (spring drought). Emergence of one species (Liriodendron) was limited by intrinsically low seed viability. In 1984, a wet spring, emergence of Pinus exceeded that of all hardwood species, and the smaller-seeded Liquidambar and Ulmus had higher emergence than the larger-seeded Acer and Fraxinus. Seed predators reduced emergence proportionately more in larger-seeded species than in smaller-seeded species. Herbaceous cover facilitated emergence of larger- seeded species by providing a moister germination microenvironment, and perhaps also by reducing visibility of seeds to predators. In 1985 an early spring drought reduced emergence of fall-dispersed species (Pinus, Liquidambar, and Fraxinus), whose earlier germination phenology coincided with the drought. Later-germinating, spring-dispersed species (Ulmus and Acer) avoided this drought period and displayed similar germination responses in both years. The prominence of drought effects in 1985 reduced the apparent impact of other experimental factors upon emergence rates. The emergence stage is an important filter on relative establishment success of tree species colonizing old-field hab- itats, and multiple "mechanisms" interact in complex ways to influence emergence rates.

Germination responses of 64 wetland species in relation to seed size, minimum time to reproduction and seedling relative growth rate.

Abstract: Three germination attributes (lag time, maximum germination rate, and final germination proportion) were measured for 64 species of herbaceous wetland plants. The environmental conditions approximated the drawdown environment known to stimulate germination in wetland plants: a period of cold stratification followed by position of the seed on the surface of wet, but not inundated, substrate in the presence of light and with a 20/300C daily temperature cycle. Correlations were sought between the three germination attributes and average individual seed weight, seedling relative growth rate and a categorical variable indicating miminum time to reproduction (annuals, facultative annuals and obligate perennials). Average seed weight was not correlated with any of the three germination attributes. Seedling relative growth rate was negatively correlated with time to initiation of germination. Species capable of setting seed their first year (annuals and facultative annuals) initiated germination sooner, a larger proportion germinated per day once germination began, and a larger proportion of seeds had germinated by the end of the experiment in comparison with species which require more than 1 year to set seed (obligate perennials). A discriminant analysis showed that the time to initiation of germination could accurately classify 89% of the perennial species as being either facultative annuals or obligate perennials. Key-words: Annuals, drawdown, facultative annuals, germination, perennials, plant strategies, regeneration, seed size, wetlands

Fire Intensity Effects on Germination of Shrubs and Herbs in Southern California Chaparral

Abstract: The study reported here investigates the effects of fire intensity and com- bustion products (ash and char) on shrub and herbaceous seedling emergence in the field in a mixed southern California chaparral, which was dominated by a facultative seeder, Adenostoma fasciculatum, and by an obligate seeder, Ceanothus greggii. Fire effects were studied in two winter bums, one of which was manipulated by adding brush to experimental plots to increase fire intensity. Adenostoma was very sensitive to increasing fire intensity, with decreasing seedling production as fire intensity increased. This was detected within the normal range of fire intensities of a winter bum, and to a greater extent when intensities were experimentally modified. Ceanothus germination was either stimulated by, or very resistant to, increasing fire intensity. Increasing fire intensity also promoted earlier germination of Ceanothus. Herb species were also differentially affected by increasing fire intensities. Among the abundant herbs, fire annuals like Phacelia brachyloba were more resistant to increasing fire intensity than opportunistic annuals like Cryptantha muricata. Seedling production of one opportunistic annual, Lotus strigosus, was, however, stimulated by all levels of increased fire intensity. In most cases, ash and char did not affect field germination. Fire intensity can profoundly modify seedling production of shrubs and herbs after a fire. By substantially altering post-fire seedling populations, fire intensity may represent a major factor in controlling species composition and diversity in chaparral.

Consequences of emergence phenology for reproductive success in ranunculus adoneus (ranunculaceae)

Abstract: This study explores the effects of emergence time and reproductive phenology on seed number, seed size, and seedling survival in a population of the alpine buttercup, Ranunculus adoneus. Phenology in this snow bowl population is structured by snow depth. Plants in late melting interior portions of the bowl emerged and flowered 3 to 4 wk after those in early melting zones at the bowl perimeter during the summers of 1988 and 1989. Flowering time differences of buttercups across the bowl were consistent from one year to the next. In 1988, late flowering plants tended to set fewer seeds than early flowering ones; in 1989 no decrease in seed number accompanied flowering date. Path analysis showed that equal fecundity in early and late emerging portions of the bowl population during 1989 resulted from balancing spatial and temporal constraints on seed production. Spatial aspects of habitat quality improved toward the interior of the bowl, but temporal regimes deteriorated in these late melting sites. In both 1988 and 1989 seed size declined with delays in flowering. Path analysis of 1989 data showed that because of reduced time for seed growth, plants in late melting portions of the bowl set smaller seeds than those in earlier melting zones. Differences in seed size due to parental phenology are likely to influence fitness in snow buttercups. Under natural conditions, seedlings from large seeds (?0.65 mg) have sixfold higher survival than do those from smaller seeds (<0.65 mg). We conclude that seedling recruitment may be infrequent in late-melting portions of the snow bowl due to delayed parental phenology.

Influence of stress during soybean seed fill on seed weight, germination, and seedling growth rate

Abstract: Water and high air-temperature (AT) stresses that occur during soybean (Glycine max (L.) Merr.) seed fill greatly reduce seed yield, but their effects on seed germination and vigor are less clear. ...

Differential seedling responses to litter after Hurricane Hugo in the Luquillo Experimental Forest, Puerto Rico

Abstract: We studied the dynamics of the seedling community in the Luquillo Experimental Forest during one year following Hurricane Hugo, to look at the effect of three litter treatments (continuous litter removal, unaltered litter quantity [controll and continuous litter addition), on seedling emergence, growth, density, and mortality. Total seedling densities (for both newly emerged and established seedlings) were highest in the litter removal treatment, suggesting that litter is a major constraint to recruitment of seedlings. However, species differed in their responses to the three treatments: species characteristic of early succession (Chionanthus domingensis and Cecropia schreberiana ex. C. peltata) were densest in the litter removal treatment, while densities of species characteristic of late succession either did not increase (Sloanea berteriana) or declined (Dacryodes excelsa) in the litter removal treatment. Height growth was lowest and mortality generally highest for seedlings in the litter removal treatment. Variability in species responses to litter after the hurricane may lead to changes in the species composition of the forest.

The effect of understory palms and cyclanths on the growth and survival of Inga seedlings.

Abstract: Seedlings of Inga fagifolia (L.) Wild. and I. pezizifera Benth. were planted into the understory of a Costa Rican rain forest to test whether the proximity of large-leaved dwarf palms and cyclanths affects seedling growth and survival and if so, whether that effect is primarily due to above or belowground processes. In two sites, seedlings were planted either near palms (or palm-like Cyclanthaceae) or nearby in sites with no overarching vegetation under 1.5 m. Half of the seedlings in each treatment were trenched to a depth of 30 cm by cutting all roots 25 cm from the seedling. The proximity of palms or cyclanths significantly affected growth and survival of both species. However, most of this effect could be attributed to loss of stem length due to herbivore or pathogen damage rather than to differences in growth rates. There was no significant effect of trenching on seedling growth. Vegetation data from ten widely distributed sites in the forest showed that seedling density was inversely correlated with the abundance of understory palms and cyclanths. Our data suggest that understory vegetation, especially broad-leaved palms and palm-like cyclanths, acts as a filter affecting the distribution and abundance of establishing seedlings.

Seedling Quality of Southern Pines

Abstract: Seedling quality is related to a seedling’s ability to survive prolonged environmental stresses and produce vigorous growth following outplanting This complex concept includes both a seedling’s “physiological readiness” to grow and suitable morphological features that will allow the seedling to overcome site limitations Seedling ontogeny (genetically coded course of development) and phenology (response to environment) alter seedling physiology (processes), and fluctuations in physiology are, in turn, manifested in seedling morphology (structure) Morphological criteria for assessing seedling quality — shoot height, root-collar diameter, and root and shoot architecture — can be manipulated by nursery cultural practices However, to implement these practices, nursery personnel must understand seedling growth dynamics and allometry (interrelationship of shoot and root growth over time) Physiological criteria for assessing quality include water relations, nutrition, carbohydrates, and dormancy and cold hardiness Water and nutritional status, responsive to environmental changes, also can be influenced by nursery practices and, in turn, affect carbohydrate production and storage and, in part, dormancy and cold hardiness Inappropriate or ill-timed cultural practices can produce seedlings of inferior morphological grade that perform poorly in the field Moreover, careless handling during lifting, packaging, storing, and planting can degrade high-quality seedlings such that field survival and growth suffer To predict quality before planting, nursery managers and foresters can subject seedlings to morphological, physiological, and/or performance tests The better morphological tests are based on diameter, alone or in combination with height Physiological tests include mitotic index, carbohydrate concentration, and cold hardiness, and performance tests include root-growth potential and budbreak with or without stressing Future tests may rely on infrared thermography and spectroradio-metry, chlorophyll a fluorescence, volatile-compound emissions, and biochemical markers

Photosynthesis, water relations and mortality in Abies lasiocarpa seedlings during natural establishment

Abstract: Few studies have investigated the ecophysiology of natural seedling establishment in forest trees not associated with anthropogenic disturbance. Photosynthesis and water relations measurements were made on one- through four-year-old seedlings of Abies lasiocarpa Nutt. (subalpine fir) establishing naturally in an understory environment. First (current)-year seedlings generally had only cotyledons, whereas most second-year seedlings had both cotyledons and primary leaves. Mortality was high (> 60%) in first-year seedlings with the greatest mortality (> 90%) measured at the more open, sun-exposed sites within the understory. Seedling mortality was negligible after the first year of growth at shaded microsites and after the second year of growth at sunny microsites. Photosynthetic CO(2) uptake at light saturation was considerably lower in first-year than in fourth-year seedlings (0.6 micromol m(-2) s(-1) versus 1.7 micromol m(-2) s(-1)) and occurred at lower solar irradiance (240 micromol m(-2) s(-1) versus 600 micromol m(-2) s(-1) of photosynthetically active radiation). Differences in photosynthetic capacity were due to differences in both stomatal and non-stomatal limitations to CO(2) uptake. Carbon dioxide assimilation in first- and second-year seedlings was 28 and 29%, respectively, of the mean value measured for fourth-year seedlings. Although first-year seedlings had low transpiration rates, their water use efficiency (photosynthesis/transpiration) was less than half that of fourth-year seedlings and their water potentials were lower than those of all other age classes (0.46 mol CO(2) mmol(-1) H(2)O and -3.0 MPa, respectively). The stomatal limitation to CO(2) uptake was approximately 21% in first- and second-year seedlings, and increased to 39% in fourth-year seedlings. Intercellular CO(2) concentrations were greater in first- and second-year seedlings (255 and 250 microl l(-1), respectively) than in third- and fourth-year seedlings (203 and 186 microl l(-1), respectively). Thus, abrupt increases in water status and photosynthetic capacity after the first or second year of growth appear crucial for survival to maturity. Moreover, differences in temperature and water relations according to microsite may be major factors determining seedling establishment and, thus, the distributional and successional patterns observed for adult trees of Abies lasiocarpa.

Growth and competition between seedlings of Protea repens (L.) L. and the alien invasive, Acacia saligna (Labill.) Wendl. in relation to nutrient availability

Abstract: Seedlings of Acacia saligna (Labill.) Wendl., an alien invasive shrub of coastal fynbos, and Protea repens (L.) L., an indigenous shrub, were grown in pots, as single seedlings and as a mixture. These were amended with one of a range of treatments representing a broad gradient in nutrient availability. When grown in isolation, both species displayed increases in dry mass, and leaf nitrogen and phosphorus contents in response to increasing levels of nutrient availability. A. saligna however, exhibited a greater response than P. repens, particularly at high levels of nutrient availability. No significant differences in root/ shoot ratio were found in response to variation in nutrient availability, but A. saligna consistently allocated a greater proportion of its total dry mass to below-ground growth. When grown as a mixture, P. repens exhibited reduced growth at high levels of nutrient availability in response to competition with A. saligna, which totally overtopped the indigenous species. In root observation chambers, the depth penetration of the soil by the tap root of seedlings of A. saligna after germination was approximately twice as rapid as that of P. repens. Seed nitrogen and phosphorus contents were significantly linearly related to seed size, except the phosphorus content of A. saligna. The relatively high absolute growth rate of A. saligna seedlings, at practically all levels of nutrient availability found in the fynbos, appears to be one of the key factors which enables them to compete successfully with P. repens seedlings. Key-words: Alien invasion, competitive interaction, phenotypic plasticity, root penetration, root/shoot ratio, seed quality, seed size * Present address: Dr E.T.F. Witkowski, School of Biology, Curtin University of Technology, GPO Box U 1987, Perth 6001, Western Australia. Introduction Coastal fynbos of the south-western Cape has been extensively invaded by Acacia saligna (Labill.) Wendl., a shrub or small trees, native to southwestern Australia, which was introduced during the mid-nineteenth century to stabilize shifting sand-dunes (Shaughnessy, 1980). It has invaded both disturbed and pristine fynbos and allied shrublands, on a wide variety of substrata, and often forms dense monospecific stands (Milton & Hall, 1981). The success of A. saligna has been largely attributed to the large number of viable seeds it produces (Milton & Hall, 1981; Macdonald, 1984; Holmes, Macdonald & Juritz, 1987), although little is know of its resource requirements, particularly in terms of nutrients and soil moisture. However, A. saligna grows to a height of from 3 to lOm and overtops the indigenous vegetation which generally attains a height of less than 2 m. The soils of the south-western Cape, like those of south-western Australia, have a low nutrient status (Witkowski & Mitchell, 1987). The availability of phosphorus and nitrogen in coastal fynbos is raised after a fire for a short period of time (Brown & Mitchell, 1986; Stock & Lewis, 1986), and may influence the establishment of seedlings. Seed germination of both indigenous and alien species in the fire-prone fynbos vegetation occurs largely after fire, during the moist winter period of this mediterranean-climate region. Fire frequencies in fynbos vegetation usually occur at intervals between 6 and 40 years (Kruger & Bigalke, 1984). Seedling establishment may critically depend on the ability of these plants to produce a rooting system which penetrates to the deeper layers of the soil profile, where sufficient supplies of moisture for survival and growth during the period of summer drought are available. In addition, the ability to produce a deeply penetrating root system may in part require seeds with high contents of limiting nutrients. In this study, the effects of variation in nutrient This content downloaded from 157.55.39.59 on Mon, 17 Oct 2016 04:52:55 UTC All use subject to http://about.jstor.org/terms 102 availability on the growth of seedlings of A. E. T. F. saligna are compared with those of an indigenous Witkowski species, Protea repens (L.) L. (Proteaceae), as single seedlings and as a mixture. Many of the nutrient-poor fynbos areas invaded by A. saligna were previously dominated by shrubs of the Proteaceae, and field observations at a sand-plain lowland fynbos site at Pella, strongly suggest that P. repens is excluded by dense stands of A. saligna. P. repens is a sclerophyllous shrub or small tree, which attains dominance in the mature post-fire period in coastal fynbos (Hoffman, Moll & Boucher, 1987), although it has a relatively wide distribution within the fynbos biome. In contrast, in its native habitat A. saligna dominates during the early post-fire period when competition from other species is reduced, and it is also often found in disturbed sites (Milton & Hall, 1981). The effects of variation in nutrient availability on the uptake of nitrogen and phosphorus were also determined, these nutrients having been shown experimentally to be the most limiting in sand-plain lowland fynbos (Witkowski, 1989ab, 1990; Witkowski, Mitchell & Stock, 1990). In addition, the penetration of the soil by the tap root of seedlings of these species were compared in root observation chambers. Variations in seed size and quality (nitrogen and phosphorus contents) were also determined. It was hypothesized that A. saligna would display a greater absolute growth rate than P. repens, in terms of both shoot and root growth, at practically all levels of nutrient availability found in the fynbos (see Witkowski & Mitchell [1987] for variations in the availability of soil phosphorus). Materials and methods

Influence of temperature and cheatgrass competition on seedling development of two bunchgrasses

Abstract: Rapid seedling growth and ability to compete against cheatgrass (Bromus tectorum L.) are 2 characteristics that perennial grasses must have for successful establishment on semiarid western rangelands. This study was conducted to determine the effects of temperature and competition from cheatgrass on seedling root and shoot growth of 'Hycrest' crested wheatgrass [Agropyron desertorum (Fisch. ex Link) Schult. X A. cristatum (L.) Gaert.] and 'Whitmar' bluebunch wheatgrass (Pseudoroegneria spicata (Pursh) Loeve). For the temperature study, seedlings were grown in growth chambers with alternate 12-h day/night temperatures of 10/5, 15/5, and 20/5 degrees C. Seedlings were destructively harvested on 6 dates and evaluated for 14 root and shoot characteristics. Leaf development, leaf area, total root length, and number and length of the second group of seminal roots were greater (P < 0.05) for cheatgrass than Hycrest and Whitmar at all dates and temperatures. Cheatgrass elongated adventitious roots earlier and at colder temperatures (10/5 degrees C) than Hycrest, and Whitmar did not produce adventitious roots at low temperatures. This would favor the establishment of cheatgrass at low temperatures. For the competition study, seedlings were established in pots in a greenhouse with planting ratios of Hycrest to cheatgrass and Whitmar to cheatgrass of 1:0, 2:0, and 5:0 (32, 64, and 159 plants m(-2) respectively) and compared with planting ratios of 1:1 and 1:4. Fourteen shoot and root characteristics were evaluated 15, 30, and 50 days after seedling emergence. Hycrest had greater shoot and root development than Whitmar for all seedling characteristics. Competition from cheatgrass reduced growth of Hycrest and Whitmar seedlings. At a planting ratio of 1:4, Hycrest-cheatgrass mixtures depleted soil moisture to lower soil water potentials than Whitmar-cheatgrass mixtures. These results indicate that Hycrest seedlings are more effective competitors with cheatgrass than Whitmar seedlings.

Survival of inoculated Laccaria bicolor in competition with native ectomycorrhizal fungi and effects on the growth of outplanted Douglasfir seedlings

Abstract: The survival, development and mycorrhizal efficiency of a selected strain of Laccaria bicolor along with naturally occurring ectomycorrhizal fungi in a young plantation of Douglas fir was examined. Symbionts were identified and their respective colonization abilities were determined. Eight species of symbiotic fungi, which may have originated in adjacent coniferous forests, were observed on the root systems. Mycorrhizal diversity differed between inoculated (5 taxa) and control (8 taxa) seedlings. Ectomycorrhizal fungi which occurred naturally in the nursery on control seedlings (Thelephora terrestris and Suillus sp.) did not survive after outplanting. Both inoculated and naturally occurring Laccaria species, as well as Cenococcum geophilum, survived on the old roots and colonized the newly formed roots, limiting the colonization by other naturally occurring fungi. Other fungi, such as Paxillus involutus, Scleroderma citrinum and Hebeloma sp. preferentially colonized the old roots near the seedling's collar. Russulaceae were found mainly in the middle section of the root system. Mycorrhizal colonization by Laccaria species on inoculated seedlings (54%) was significantly greater than on controls (13%) which were consequently dominated by the native fungi. Significant differences (up to 239%) were found in the growth of inoculated seedlings, especially in root and shoot weight, which developed mainly during the second year after outplanting. Seedling growth varied with the species of mycorrhizae and with the degree of root colonization. Competitiveness and effectiveness of the introduced strain on improving growth performances of seedlings are discussed.

Parental effects in Lychnis flos‐cuculi. I: Seed size, germination and seedling performance in a controlled environment

Abstract: Selection responses in natural plant populations depend on how the phenotypic variation of traits is composed. The contributions of nuclear genetic, maternal, paternal, environmental and inbreeding effects to variation in time to germination, germination percentage, and seed- and seedling size were studied in a population of Lychnis flos-cuculi. It was found that: (1) Maternal effects predominated in the determination of progeny seed size and germination characteristics; (2) Maternal environment during seed development was less important than maternal genotype; (3) Small but significant variation within maternal families could be observed among individuals sired by different fathers; (4) Additive genetic variance was significant for seedling size 4 weeks after germination. In conclusion, selection shortly after emergence will mainly favour particular maternal genotypes, while selection later in the life cycle may act upon zygotic genotypes. Inbreeding depression was significant, especially for vegetative growth. Consistent differences were found among maternal genotypes in the degree of variation in the time to germination, suggesting that selection could operate to favour polymorphic or uniform germination behaviour.

Indirect effects of litter on woody seedlings subject to herb competition

Abstract: We tested the hypothesis that litter indirectly enhances the establishement of woody seedlings in oldfields, because litter reduces the intensity of herb competition. We counted and measured the woody seedlings growing in experimental plots with different amounts of litter of Solidago sp., or leaves of Quercus alba, and in plots with the dominant herb removed. We also conducted a factorial experiment in a green-house to test the effect of water availability, litter, and competition on the growth of seedlings of Ailanthus altissima. The results showed a strong negative effet of herb competition and a positive indirect of litter on woody seedling establishment

Control of seedling emergence by ground cover: a potential mechanism involving seed predation.

Abstract: The presence of ground cover (i.e., plant biomass and litter) could restrict seedling emergence by providing a habitat for seed predators. To test this hypothesis, a field experiment was conducted with three old-field forbs (Daucus carota, Centaurea nigra, Taraxacum officinale). Ground cover was either removed or left in place and sown seeds of the three forbs were either protected from predators or left unprotected. Where seeds were protected from predators, seedling emergence did not improve when ground cover was removed. Where seeds were not protected from seed predators, seedling emergence did improve significantly for all three species when ground cover was removed. Apparently, seed predators removed more unprotected seeds where ground cover was present than absent, thereby reducing seedling emergence. These results are consistent with the hypothesis that ground cover could restrict seedling emergence by providing a habitat for seed predators. Key words: old field, seedling emergence, seed predation.

Are seedlings from small seeds always inferior to seedlings from large seeds? Effects of seed biomass on seedling growth in Pastinaca sativa L.

Abstract: summary Previous field studies of recruitment in Pastinaca sativa L. indicate that more seedlings from small seeds than from large seeds survive short-term droughts. To explore this phenomenon, the effects of variation in seed biomass in Pastinaca sativa on embryo size and seedling characteristics 10, 20, 30 and 40 days after emergence were investigated. On the basis of most characteristics, seedlings from large seeds should be superior to seedlings from small seeds. Embryo length and cotyledon area were positively related to seed biomass, as were above ground biomass, total leaf area, and root biomass in all harvests. Total seedling biomass was positively related to seed biomass in the 10, 20, and 30 day harvests, but not the 40 day harvest. However, the ratio of maximum root length/total leaf area was negatively related to seed biomass in the 10 and 20 day harvests, suggesting that, under drought conditions, seedlings from small seeds may transpire less water than those from large seeds relative to their ability to reach water supplies. Although seedlings from larger seeds had greater root biomass, this may be of little advantage under drought conditions since approximately 90% of the root biomass is in the upper 10 cm of soil which dries out quickly. The advantage that seedlings from small seeds have under drought conditions is short-lived, lasting about 20 days in the glasshouse and an estimated 60–90 days in the field. This advantage is short-term because the relationship between seed biomass and resource allocation patterns changes during early seedling development.

Shrub facilitation of coast live oak establishment in central california

Abstract: Seedlings of Quercus agrifolia were found to be strongly associated with shrubs at two sites in central California. Although shrub cover occupied only 30% of the total cover, over 80% of all Q. agrifolia seedlings were found under shrub canopies. Although one site was grazed by livestock and the other was not, in both sites seedlings under shrubs were less browsed than seedlings in the open grassland. In field experiments where seedlings were grown with and without shrub cover, survivorship after two years in the open was 0, whereas 31% of seedlings survived under shrubs (18% of the experimental shrubs had living seedlings under their canopies). Seedling survival was not the same under all shrub species. Shoot mortality in these experiments attributed to water or temperature stress was 1 7% under shrubs and 63% in the open. These results indicate that Q. agrifolia may have a "nurse plant" interaction with some species of shrubs. Recruitment of young Quercus agrifolia Nee (coastal live oak), is too low to maintain the existing adult populations in much of its range (Muick and Bartolome 1987) and the preservation of this woodland has become a major conservation issue in California, USA. Although the factors that limit regeneration of other oak species in the state are complex, they include drought stress and seedling predation from deer, gophers and livestock (Griffin 1971, 1976; Borchert et al. 1989). The causes of low regeneration of Q. agrifolia have not been studied, but they are likely to be similar to those reported for other species. Quercus agrifolia is an evergreen tree, 10 to 20 m tall, and is endemic to California, USA and northern Baja California, Mexico (Munz 1959). It is widely distributed throughout the central and southern coastal ranges of California and is often adjacent to shrub vegetation or intermixed with shrubs (Sawyer et al. 1977). Acorns mature and drop to the ground and/or are dispersed by vertebrates in autumn and germination occurs in late autumn or early winter. The climate in which the species occurs is mediterranean, with precipitation occurring primarily between September and April. Muick and Bartolome (1987) reported that seedlings and saplings of Q. agrifolia were uncommon in many of their study sites. In 1 Present address: Department of Integrative Biology, University of California, Berkeley, CA 94720 Madrono, Vol. 38, No. 3, pp. 158-169, 1991 This content downloaded from 40.77.167.44 on Fri, 10 Jun 2016 06:13:36 UTC All use subject to http://about.jstor.org/terms 1991] CALLAWAY & D'ANTONIO: LIVE OAK ESTABLISHMENT 159 preliminary surveys we also found that young Q. agrifolia were uncommon, but that seedlings and saplings appeared to be associated with shrub cover. This observation is consistent with the following hypotheses: 1) acorns are not randomly dispersed between open grassland and shrub cover; 2) germination rates are not the same in these microhabitats; and/or 3) seedling survival is restricted to certain microhabitats. The latter two hypotheses have been investigated in other plant associations and have been included in the general phenomenon of "nurse plant" interactions in which established plants ameliorate climatic extremes and /or provide refuge from predators for seedlings of other plant species. Such associations have been previously reported for a wide range of desert taxa and habitats (Steenbergh and Lowe 1977; Everett et al. 1986; Franco and Nobel 1989; McAuliffe 1988), but not for oaks or other species in California woodlands. To test the hypothesis that oak tree seedling recruitment is dependent upon a nurse plant association with established shrubs, we documented the natural distribution and condition of Q. agrifolia seedlings and saplings relative to shrub cover in two sites, each with approximately equal cover of mixed oak savanna and chaparral. We then followed the survivorship of seedlings planted from acorns both in the open and under shrubs at a third site, and documented the probable causes of mortality of these seedlings over two years.

Tree seedling emergence on interactive temperature and moisture gradients and in patches of old-field vegetation

Abstract: Seeds of tree species commonly invading old-fields (Fraxinus americana, Gleditsia triacanthos, Morus rubra, Platanus occidentalis, and Prunus serotina) were germinated at eight temperatures from 5 C to 40 C, with six moisture levels (2% to 18% gravimetric moisture content) at each temperature. For most species, total seedling emergence and emergence rat exhibited approximate bivariate Gaussian response surfaces. (...)

Survival and growth of Virola surinamensis yearlings: Water augmentation in gap and understory.

Abstract: Factors affecting seedling Virola surinamensis (Myristicaceae) survival and growth were investigated on Barro Colorado Island, Panama. Seedlings planted 3 months after germination were monitored in treefall gaps and understory using 2.25 ha irrigated and control plots through the first dry season. During the dry season, irrigated plants in gaps increased total leaf area significantly more than did irrigated plants in the shaded understory. Over the same dry season, control plants in gaps and in the shaded understory lost similar amounts of leaf area. Seedlings in understory were suppressed in stem height and biomass in both irrigated and control plots; these measures were greater in gaps and greatest in irrigated gaps (height). Roots were similar in length in all treatments, but greater in biomass in gaps than understory due to greater proliferation of secondary roots in control and irrigated gaps than in control and irrigated understory. This experiment demonstrates both water and light limitation during the first dry season after germination. V. surinamensis seedlings are capable of survival and modest growth of leaf area in the deep shade of the understory in moist locations; they are severely disadvantaged in shaded understory subject to drought, where most seeds fall and most seedlings establish. The broken canopy of a gap allows shoot and consequently root growth that permits seedlings to survive seasonal drought.

Hydroperoxide Lyase and Other Hydroperoxide-Metabolizing Activity in Tissues of Soybean, Glycine max

Abstract: Hydroperoxide lyase (HPLS) activity in soybean (Glycine max) seed/seedlings, leaves, and chloroplasts of leaves required detergent solubilization for maximum in vitro activity. On a per milligram of protein basis, more HPLS activity was found in leaves, especially chloroplasts, than in seeds or seedlings. The total yield of hexanal from 13(S)-hydroperoxy-cis-9,trans-11-octadecadienoic acid (13S-HPOD) from leaf or chloroplast preparations was 58 and 66 to 85%, respectively. Because of significant competing hydroperoxide-metabolizing activities from other enzymes in seed/seedling preparations, the hexanal yields from this source were lower (36-56%). Some of the products identified from the seed or seedling preparations indicated that the competing activity was mainly due to both a hydroperoxide peroxygenase and reactions catalyzed by lipoxygenase. Different HPLS isozyme compositions in the seed/seedling versus the leaf/chloroplast preparations were indicated by differences in the activity as a function of pH, the K(m) values, relative V(max) with 13S-HPOD and 13(S)-hydroperoxy-cis-9,trans-11,cis-15-octadecatrienoic acid (13S-HPOT), and the specificity with different substrates. With regard to the latter, both seed/seedling and chloroplast HPLS utilized the 13S-HPOD and 13S-HPOT substrates, but only seeds/seedlings were capable of metabolizing 9(S)-hydroperoxy-trans-10,cis-12-octadecadienoic acid into 9-oxononanoic acid, isomeric nonenals, and 4-hydroxynonenal. From 13S-HPOD and 13S-HPOT, the products were identified as 12-oxo-cis-9-dodecenoic acid, as well as hexanal from 13S-HPOD and cis-3-hexenal from 13S-HPOT. In seed preparations, there was partial isomerization of the cis-3 or cis-9 into trans-2 or trans-10 double bonds, respectively.

Mineral nutrition and mycorrhizal infection of seedling oak and birch

Abstract: summary In order to assess the factors influencing the extent and composition of ectomycorrhizal infection, and the consequences for seedling growth, seedlings of oak (Quercus robur L.) and birch (Betula pendula Roth.) were established in both field plots and pots of soil from two forested sites (Thetford Chase and Leith Hill). Seedlings were harvested after one growing season, and analysed for ectomycorrhizal infection. When a single root system of pot-grown oak was analysed in detail seven mycorrhizal types were differentiated. The relative abundance of these types varied depending on whether infection was assessed by counting branched mycorrhizas, or individual mycorrhizal tips. This reflected variation in the pattern of branching of the different mycorrhizal types. Of the 41 mycorrhizal types differentiated in the entire survey of 1800 seedlings, five were consistently recorded as unbranched, and six were found to produce second-order branches. Different mycorrhizal types displayed contrasting patterns of abundance, both with respect to site and host species. For example, 21 types were recorded infecting oak, whereas only 15 were found on birch; only three types were found infecting both oak and birch on both sites. In this survey, seedlings tended to be dominated by between one and three mycorrhizal types. Two types were particularly dominant: those formed by Scleroderma citrinum (Pers.) and Paxillus involutus (Batsch) Fr., which accounted for up to 61 and 92 % of the mycorrhizas of oak and birch respectively. These fungi were exceptional amongst those encountered in the present analysis, in producing both highly branched mycorrhizas and mycelial strands. The relationship between ectomycorrhizal infection and seedling growth was assessed by correlation. Total seedling dry mass was in general found to be positively correlated with the total number of ectomycorrhizal tips, but poorly correlated with the extent of ectomycorrhizal infection, except where seedling growth had previously been shown to be limited by phosphate availability (i.e. in the case of pot-grown birch on soil from Leith Hill). Seedling dry mass was also in general poorly correlated with number of mycorrhizal types, with two exceptions (i.e. field-grown oak and pot-grown birch on soil from Thetford Chase). These results are discussed in the context of the role of ectomycorrhizal infection during seedling establishment, and implications for the methodologies utilized in assessing mycorrhizal infection are highlighted.