Showing papers on "Seedling published in 2003"

Comparative drought-resistance of seedlings of 28 species of co-occurring tropical woody plants.

Abstract: Quantifying plant drought resistance is important for understanding plant species' association to microhabitats with different soil moisture availability and their distribution along rainfall gradients, as well as for understanding the role of underlying morphological and physiological mechanisms. The effect of dry season drought on survival and leaf-area change of first year seedlings of 28 species of co-occurring woody tropical plants was experimentally quantified in the understory of a tropical moist forest. The seedlings were subjected to a drought or an irrigation treatment in the forest for 22 weeks during the dry season. Drought decreased survival and growth (assessed as leaf-area change) in almost all of the species. Both survival and leaf-area change in the dry treatment ranged fairly evenly from 0% to about 100% of that in the irrigated treatment. In 43% of the species the difference between treatments in survival was not significant even after 22 weeks. In contrast, only three species showed no significant effect of drought on leaf-area change. The effects of drought on species' survival and growth were not correlated with each other, reflecting different strategies in response to drought. Seedling size at the onset of the dry season had no significant effect on species' drought response. Our study is the first to comparatively assess seedling drought resistance in the habitat for a large number of tropical species, and underlines the importance of drought for plant population dynamics in tropical forests.

Small-scale Environmental Heterogeneity and Spatiotemporal Dynamics of Seedling Establishment in a Semiarid Degraded Ecosystem

Abstract: In semiarid environments, surface soil properties play a major role in ecosystem dynamics, through their influence on processes such as runoff, infiltration, seed germination, and seedling establishment. Surface soil properties usually show a high degree of spatial heterogeneity in semiarid areas, but direct tests to evaluate the consequences of this heterogeneity on seedling establishment are limited. Using a combination of spatial analysis by distance indices (SADIE) and principal components analysis (PCA) we quantified the spatiotemporal patterns of seedling survival of a Mediterranean native shrub (Pistacia lentiscus) during the first 3 years after planting on a semiarid degraded site in southeastern Spain. We used a variation partitioning method to identify environmental variables associated with seedling survival patterns. Three years after planting, only 36% of the seedlings survived. During the first summer, one-third of the seedlings died, with secondary major mortality in the 3rd summer after planting. The spatial pattern of survival became strongly clumped by the end of the first summer, with clearly defined patches (areas of high survival) and gaps (areas of low survival). The intensity of this pattern increased after subsequent high-mortality periods. Of the 14 variables, the ones most strongly coupled to seedling survival were bare soil cover, sand content, and soil compaction. These findings contribute to our understanding of the linkages between the spatial heterogeneity of abiotic factors and the response of plant populations in semiarid degraded ecosystems and can be used to optimize restoration practices in these areas.

Effects of Monoterpenoids, Acting Alone or in Pairs, on Seed Germination and Subsequent Seedling Growth

Abstract: We compared the potential allelopathic activity of 47 monoterpe- noids of different chemical groups, by estimating their effect on seed germination and subsequent growth of Lactuca sativa seedlings. Apart from individual compounds, eleven pairs at different proportions were also tested. As a group, the hydrocarbons, except for (+)-3-carene, were the least inhibitory. Of the oxygenated compounds, the least inhibitory were the acetates; whenever the free hydroxyl group of an alcohol turned into a carboxyl group, the activity of the resulting ester was markedly lower (against both germination and seedling growth). Twenty-four compounds were extremely active against seedling growth (inhibiting it by more than 85%), but only five against seed germination. The compounds that were most active against both processes belonged to the groups of ketones and alcohols; they were terpinen-4-ol, dihydrocarvone, and two carvone stereoisomers. We used a model to investigate whether compounds acted independently when applied in pairs. The combined effect varied. In half of the cases, it followed the pattern expected under the assumption of independence; in the rest, either synergistic or antagonistic interactions were found in both germination and elongation. However, even in cases of synergistic interactions, the level of inhibition was not comparable to that of a single extremely active compound, unless such a compound already participated in the combination. The specific structural factors that operate and determine the activity of monoterpenoids still remain rather obscure. The same holds true for the combined effect; its character cannot in general be predicted on the basis of individual compounds acting alone.

Seed priming winter wheat for germination, emergence, and yield

Abstract: Insufficient stand establishment of winter wheat (Triticum aestivum L.) is a major problem in the low-precipitation (<300 mm annual) dryland summer fallow region of the inland Pacific Northwest, USA. Low seed zone water potential, deep planting depths with 15 cm or more soil covering the seed, and soil crusting caused by rain before seedling emergence frequently impede winter wheat stands. A 2-yr study involving laboratory, greenhouse, and field components was conducted to determine seed priming effects on winter wheat germination, emergence, and grain yield. Two cultivars were used because of their strong (Edwin) and moderate (Madsen) emergence capabilities. Germination rate was measured in the laboratory by 44 treatment combinations (two cultivars × three priming durations × seven priming media + two checks). Germination rate differed between cultivars as well as by priming duration, priming media, and concentration of priming media. The most promising laboratory treatments were advanced to greenhouse and field experiments where emergence and grain yield (field only) were measured in 10 treatments (two cultivars × four priming media + two checks) from wheat planted deep with 16 cm of soil covering the seed. In the greenhouse, seed primed in potassium chloride (KCI), polyethylene glycol (PEG), and water led to enhanced emergence of Madsen, but not of Edwin, compared with checks. Rate and extent of seedling emergence was greater for Edwin compared with Madsen irrespective of priming media in three of four field plantings at Lind, WA. None of the seed priming media benefited field emergence or subsequent grain yield in either cultivar compared with checks. Overall, results suggest that seed priming has limited practical worth for enhancing emergence and yield of winter wheat planted deep into summer fallow.

Seasonal cycles in the germination capacity of buried seeds of some Leguminosae (Fabaceae)

Abstract: Summary • The influence of seasonal factors on the germination of impermeable seeds of Fabaceae was studied. Seeds of 14 herbaceous species were buried for up to 2.5 yr and exposed to natural seasons. At regular intervals, seeds were exhumed and germination was tested in four different temperature regimes. Some species had a very low percentage germination regardless of season, others germinated predominantly in the autumn. • Six species showed a marked seasonal cycle with high germination percentages in spring and very low percentages in other seasons. The results of the burial experiment were similar to the pattern of seedling emergence in field conditions. • High percentage germination in spring could be simulated in controlled conditions for Melilotus albus, Medicago lupulina, Lotus corniculatus, Trifolium pratense and T. repens. Seeds of T. pratense had a high percentage germination after cold treatment, whereas seeds of the other four species were brought to germination by chilling for 8 wk followed by exposure to daily alternating temperatures (15/6°C or 20/10°C). Without previous chilling, these seeds did not respond to the alternating temperatures. • At least for some legume species, a combination of temperature treatments may act as a signal for seeds and induce water uptake and germination. If the temperature requirements are not fulfilled, seeds remain impermeable and dormant until the next favourable season.

Soil pathogens and prunus serotina seedling and sapling growth near conspecific trees

Abstract: Soil pathogens close to mature Prunus serotina trees reduce the survival of conspecific seedlings. We examined whether the soil community associated withP. serotina trees has continued negative effects on seedling growth. P. serotina seedlings and saplings were harvested after being followed for 1-3 years in the field, and above- and belowground biomasses were measured. Linear regression models including distance to adult P. serotina tree, conspecific seedling density, and seedling age explained 13.9% of the variation in biomass. Distance and age were positively correlated with total biomass, while density was negatively correlated with total biomass. A similar model for saplings established prior to the beginning of the study, and of unknown age, predicted 11.3% of the variation in total biomass, with distance to adult positively correlated with biomass. To separate the effects of distance from density and remove confounding environmental factors, seedlings were grown in the greenhouse at high or low density in soil collected 0-5 m or 25-30 m from adult P. serotina trees. Half of the soil collected at each distance was sterilized by autoclaving. The experiment was repeated in 1998 and 1999, and in both years there was a significant sterilization-by-distance interaction. Sterilization of soil col- lected close to the tree resulted in higher root:shoot ratios, while sterilization of soil collected away from the tree resulted in lower root:shoot ratios. This suggests that soil biota vary with distance to adults and that elimination of soil biota changes plant allocation patterns differentially with distance. Distance-dependent effects of soilborne pathogens may continue beyond seedling mor- tality and suppress juvenile growth. Reduced growth could increase an individual's chance of later mortality, suggesting that initial escape from damping-off pathogens close to pa- rental trees does not ensure longer-term survival.

Breaking physical dormancy in seeds – focussing on the lens

Abstract: ○The influence of seasonal factors on the germination of impermeable seeds of Fabaceae was studied. Seeds of 14 herbaceous species were buried for up to 2.5 yr and exposed to natural seasons. At regular intervals, seeds were exhumed and germination was tested in four different temperature regimes. Some species had a very low percentage germination regardless of season, others germinated predominantly in the autumn. Six species showed a marked seasonal cycle with high germination percentages in spring and very low percentages in other seasons. The results of the burial experiment were similar to the pattern of seedling emergence in field conditions. ○ High percentage germination in spring could be simulated in controlled conditions for Melilotus albus, Medicago lupulina, Lotus corniculatus, Trifolium pratense and T repens. Seeds of T. pratense had a high percentage germination after cold treatment, whereas seeds of the other four species were brought to germination by chilling for 8 wk followed by exposure to daily alternating temperatures (15/6°C or 20/10°C). Without previous chilling, these seeds did not respond to the alternating temperatures. ○At least for some legume species, a combination of temperature treatments may act as a signal for seeds and induce water uptake and germination. If the temperature requirements are not fulfilled, seeds remain impermeable and dormant until the next favourable season.

The effects of the phospholipase D-antagonist 1-butanol on seedling development and microtubule organisation in Arabidopsis.

Abstract: The organisation of plant microtubules into distinct arrays during the cell cycle requires interactions with partner proteins. Having recently identified a 90-kDa phospholipase D (PLD) that associates with microtubules and the plasma membrane [Gardiner et al. (2001) Plant Cell 13: 2143], we exposed seeds and young seedlings of Arabidopsis to 1-butanol, a specific inhibitor of PLD-dependent production of the signalling molecule phosphatidic acid (PA). When added to agar growth media, 0.2% 1-butanol strongly inhibited the emergence of the radicle and cotyledons, while 0.4% 1-butanol effectively blocked germination. When normal seedlings were transferred onto media containing 0.2% and 0.4% 1-butanol, the inhibitor retarded root growth by about 40% and 90%, respectively, by reducing cell elongation. Inhibited plants showed significant swelling in the root elongation zone, bulbous or branched root hairs, and modified cotyledon morphology. Confocal immunofluorescence microscopy of root tips revealed that 1-butanol disrupted the organisation of interphase cortical microtubules. Butanol isomers that do not inhibit PLD-dependent PA production, 2- and 3-butanol, had no effect on seed germination, seedling growth, or microtubule organisation. We propose that production of PA by PLD may be required for normal microtubule organisation and hence normal growth in Arabidopsis.

Amelioration of osmotic stress by brassinosteroids on seed germination and seedling growth of three varieties of sorghum

Abstract: The effect of 28-homobrassinolide and 24-epibrassinolide on the germination and seedling growth of three varieties of sorghum, viz CSH-14 and ICSV-745 (susceptible to water stress) and M-35-1 (resistant to water stress), under osmotic stress conditions was studied Both the brassinosteroids were very effective in increasing the percentage of germination and seedling growth of all the three varieties of sorghum under osmotic stress, the growth promotion being associated with enhanced levels of soluble proteins and free proline Brassinosteroid treatment enhanced the activity of catalase and reduced the activities of peroxidase and ascorbic acid oxidase

Modelling the emergence response of weed seeds to burial depth: interactions with seed density, weight and shape

Abstract: Summary 1 Weeds play an important role in arable and horticultural habitats, and models are being developed to improve our understanding of their population dynamics. The position of a weed seed in the soil profile influences the probability of a seed germinating, emerging successfully and its relative time of emergence. Identifying a relationship between the shape or weight of a seed and its ability to emerge from depth may allow the development of generic models. The aim of this study was to quantify seed response to burial depth, to improve the wider application of existing seedling emergence models. 2 A field experiment used weed seeds sown at different depths and densities in a standard substrate. In addition, two laboratory studies used pre-germinated seeds of the same range of species, buried at a range of depths in optimum conditions using the same substrate. These studies explored the effects of seed size, shape and sowing density on seedling emergence and also enabled reserve-dependent pre-emergence mortality to be quantified. 3 The largest and heaviest of the seeds tested overall, Veronica hederifolia, emerged from the greatest depth (8 cm). In contrast, Tripleurospermum inodorum and Veronica arvensis, the two smallest species, showed a sharp decline when burial exceeded 1 cm. However, the link between seed shape or weight and the ability to emerge from depth suggests a complex relationship. Given optimum conditions, some species (Stellaria media and Chenopodium album) have the physical reserves to emerge from a wider range of burial depths than normally observed in the field, suggesting an ability to exploit opportunities when they occur. 4 For some species, emergence was reduced at high seed densities (e.g. Veronica arvensis). These responses may be associated with traits that have evolved to counteract sibling competition. 5 Synthesis and applications. Generic models identifying the maximum depths for seed germination and emergence have a number of practical applications. For example, they can be used to target cultivation to deplete the weed seed bank or to prescribe the optimum depth of mulches to favour certain species. Our model showed that, in general, larger-seeded species emerged from deeper in the soil, but the relationship between seed size and shape and emergence was complex, possibly species specific. Our germination data may also assist our understanding of the relative importance of different causes of seed losses, particularly from different zones of the soil profile, such as the soil surface. Understanding the component processes of seed behaviour and germination is essential to developing sustainable weed management practices in agriculture and horticulture, and the work reported here contributes to a larger programme modelling weed seed bank population dynamics.

Seedling demography in an alpine ecosystem.

Abstract: Seedling establishment has long been believed to be rare on alpine tundra because of predicted life history trade-offs, the clonality of alpine species, and the harshness of the alpine climate. Contrary to the idea that seedlings are rare on alpine tundra, a 4-yr demographic study of seedlings at Niwot Ridge, Colorado, USA, found seedlings at high densities, particularly in wetter plant communities. Higher germination densities were associated with higher soil moistures both across communities and across time. Mortality of seedlings was highest in the first year and decreased in subsequent years. Species' abundances differed between seedling and adult populations. Many forbs that lacked vegetative reproduction were significantly more abundant among seedling populations, and many monocots and clonal forbs were more abundant among adult populations. In a comparison with published demographic rates, seedling recruitment and mortality rates of Niwot Ridge species fell above or within rates for a wide range of perennial species. Therefore, germination and seedling establishment stages are no more limiting to sexual reproduction in alpine plants than in other perennial plants.

Interactive effects of soil moisture, vegetation canopy, plant litter and seed addition on plant diversity in a wetland community

Abstract: Summary 1 We carried out a factorial experiment to examine how groundwater availability (low and high sites with intermediate or rare flooding), vegetation canopy, leaf litter and seed availability interacted to determine the species richness of a productive wet grassland community in Wicken Fen National Nature Reserve, Cambridgeshire, UK. Seeds of 18 species were added to half the plots in each of eight combinations of elevation, canopy and litter, and seedling emergence was observed for two growing seasons. 2 Both individual and interactive effects on plant diversity and colonization were determined for all four examined factors. Interactive effects explained 41‐63% of the total variation in both species richness and numbers of individuals growing from added seeds. 3 Neither elevation nor vegetation canopy had significant individual effects on total species richness, but their interaction was significant. Litter addition limited seedling emergence at the low elevation but favoured it at the high elevation. 4 The relative importance of vegetation canopy and plant litter in affecting plant community composition varied with the community parameter considered (species richness or number of seedlings), elevation and stage of vegetation development. In general, plant litter was more important in determining species richness, whereas the vegetation canopy was more important in determining seed germination and seedling emergence. Plant litter was also more important than vegetation canopy at an early stage of vegetation development and at low elevation. 5 Seed availability was the most important factor in determining overall species richness in the studied community. The influence of the local seed bank was very limited. Seedling emergence and seedling species richness were generally enhanced by lower elevation and seed addition, but depressed by vegetation and litter addition. 6 The complex relationships observed have considerable implications for ecological modelling and ecosystem restoration. Manipulation of one factor may produce unexpected effects on other factors, which may induce a series of consequences for the whole community. Further knowledge on how natural communities are organized and maintained is needed to guide the management of ecosystems.

Allelopathic Effects of Black Mustard (Brassica nigra) on Germination and Growth of Wild Barley (Hordeum spontaneum)

Abstract: Black mustard [Brassica nigra (L.) Koch.] contains water-soluble allelochemicals that inhibit the germination and growth of other species. This characteristic could be used in weed management programmes. Greenhouse and laboratory experiments were conducted to determine the effects on wild barley (Hordeum spontaneum Koch.) germination and seedling growth of (i) preceding crops, (ii) fresh black mustard residue incorporation, and (iii) black mustard leaf, stem, flower and root water extract concentrations. Growth of wild barley, as indicated by plant height and weight, was significantly reduced when grown in soil previously cropped to black mustard compared with that cropped to wild barley. Soil incorporation of fresh black mustard roots and both roots and shoots reduced wild barley germination, plant height and weight when compared with a no-residue control. In bioassays, black mustard extracts reduced wild barley hypocotyl length, hypocotyl weight, radicle weight, seed germination, and radicle length by as much as 44, 55, 57, 63 and 75 %, respectively, when compared with a water control. Increasing the water extract concentrations from 4 to 20 g per 100 ml of water of all black mustard parts significantly increased the inhibition of wild barley germination, seedling length and weight. Based on 8-day-old wild barley radicle length, averaged across all extract concentrations, the degree of toxicity of different black mustard plant parts can be ranked in the following order of inhibition: leaves > flowers > mixture of all plant parts > stems > roots.

Lichens and mosses promote alternate stable plant communities in the New Jersey Pinelands

Abstract: Severely burned areas in the pitch pine-dominated forests of the New Jersey Pinelands may remain open and only vegetated with mats of lichens and mosses and sparse, scattered vascular plants for many decades. We hypothesize that climate-driven alternation between moss and lichen domination of the cryptogam mats may foster and inhibit, respectively, vascular plant development. We propose that these processes are mediated by the inhibitory effects of lichens on seed germination and seedling establishment versus a facilitating effect of mosses. We tested the hypothesis by 1) detailed surveys of the composition of cryptogam mats and their association with vascular plants, 2) experimental studies of the effects of tissue leachates on seed germination, 3) observations of mycorrhizal infection in field-collected plants, and 4) experimental tests of seedling emergence from mats. Lichen dominance in the mats was correlated with low densities of vascular plants (graminoids and ericoid shrubs), thin organic horizons on the soil, and high levels of light availability; moss dominance was correlated with higher vascular plant densities, thicker organic horizons, and lower light. Tissue extracts of lichens strongly inhibited seed germination, while moss extracts had no effect. Similarly, mycorrhizal infection by both ecto- and endomycorrhizae was lower in plants growing within lichen mats than in moss mats or in bare soil. However, thick mats of both types of cryptogam inhibited seedling emergence. We observed that moss-dominated patches became overgrown with lichens during a series of very dry, hot summers during the study. These observations all support the hypothesis that fluctuating warm and dry versus cool and moist conditions allow alternative stable states (open cryptogam mats vs succession to pine forest) to develop.

Effects of Different Soil Salinity Levels on Germination and Seedling Growth of Safflower (Carthamus tinctorius L.)

Abstract: Safflower (Carthamus tinctorius L.) is rated as moderately salt tolerant and it can produce profitable crops on saline soils. It is slightly more tolerant of salinity than barley but is more tolerant than wheat. Because salinity reduces germination and delays emergence, safflower stands tend to be irregular and crop yield is depressed. However, some varieties are less affected by salinity than others. In this study, we aimed to determine the effects of soil salinity levels (0.8, 2.5, 5.1, 8.7, 13.0, 15.2 and 23.0 dS m-1) on germination and seedling growth of three safflower varieties, one of which is spiny (5-154 cv.) and two of which are spineless (Yenice 5-38 and Dincer 5-118 cv.). Emergence rate, root and shoot length, root and shoot dry weight, root/shoot dry weight ratio, root and shoot dry weight salt stress index were investigated. The results showed that the highest values for the investigated traits were obtained from variety 5-154 (spiny), while they diminished with increasing soil salinity in all cultivars. Among the varieties, the highest emergence rate, root length, shoot length, root dry weight, shoot dry weight and root/shoot dry weight ratio at 23.0 dS m-1 were determined from 5-154 cv. with 23.35%, 3.54 cm, 4.97 cm, 7.23 mg, 26.73 mg and 20.77%, respectively. Moreover, 5-154 cv. gave the higher root and shoot dry weight salt stress index, especially at high salinity levels. The seedling growth of varieties was inhibited by 5.1 dS m-1 although they showed varying responses. At the first development stage, the root growth of safflower was more adversely affected compared to shoot growth by soil salinity. If the cultivation of safflower on saline soils is required, spiny varieties should be preferred.

Spatial distribution of overstory retention influences resources and growth of longleaf pine seedlings

Abstract: Increasingly, overstory retention is being used in forests traditionally managed for single-cohort structure. One rationale for retention is that residual stand structure better resembles the complex structure of forests after natural disturbance, helping to perpetuate ecosystem fuctions dependent on that structure. The benefits of retention come at the cost of reduced survival and growth of regeneration because of competition with residual trees. We argue that inhibition of regeneration depends not only on the number and size of residual trees, but also on their spatial arrangement, which ranges from dispersed to aggregated. We use a model of competition at the scale of seedlings to hypothesize that maximum stand-level resource availability, seedling growth, and seedling survival occur with aggregate retention, rather than dispersed retention, even with constant residual basal area. We test our hypothesis with a silvicultural experiment in longleaf pine (Pinus palustris) in Georgia, USA. Replicated treatments included an uncut control, dispersed retention, small-aggregate retention, and large-aggregate retention. We measured light, soil nitrogen, soil moisture, and growth of longleaf pine seedlings across the full range of overstory conditions in each treatment. Postharvest basal areas in the cut treatments were similar. Gap light index increased from the control to large-aggregate retention, as did nitrogen availability, measured on exchange resins. Nitrogen mineralization did not differ among treatments, nor did soil moisture or temperature. Seedling biomass increment increased significantly from the control to large-aggregate retention. Survival did not differ among treatments. We argue that these results are a consequence of exponential relationships between overstory competition intensity, resource availability, and seedling growth. Given this relationship, resources and seedling growth are low across a wide range of decreasing overstory competitor abundance but increase exponentially only at very low competitor abundance. This seedling-scale model translates into maximum stand scale resource availability and seedling growth with large-aggregate retention, compared to dispersed retention, because the probability of a seedling occupying a site free of overstory competition is greater with the former. Our research shows that one can improve competitive environnments for regeneration by manipulating spatial distribution of residual trees without sacrificing the ecological benefits of overstory retention.

Growth and mortality in high and low light: trends among 15 shade-tolerant tropical rain forest tree species

Abstract: Summary 1Past work on tropical rain forest tree seedlings has been dominated by contrasts between strongly light-demanding and strongly shade-tolerant species. We examined patterns of growth and mortality among shade-tolerant tree seedlings in response to light, and investigated the morphological and physiological correlates of high seedling growth and survival rates across species. 2Seedlings of 15 tree species from Australian tropical lowland forest were grown for up to 1 year in neutral-density shadehouses at three light levels (10%, 0.8% and 0.2% full daylight). All species showed negligible mortality in the 10% and 0.8% shadehouses, but survival was significantly reduced in 0.2% daylight. 3Seedling survival rate in 0.2% daylight showed no significant relationship with either the dry mass of seed reserves (embryo plus endosperm), or relative growth rates in dry mass (RGRM) in 0.8% and 10% light. 4The RGRM values in 0.8% and 10% daylight were strongly positively correlated, and showed a strong negative correlation with the dry mass of seed reserves. Interspecific variation in low-light RGRM was driven by unit leaf rate (rate of accumulation of dry mass per unit area of leaf), whereas interspecific variation in high-light RGRM was most closely correlated with leaf area ratio (leaf area per total plant dry mass). 5Variation in seedling characters in response to light may have important implications for the coexistence of shade-tolerant tropical tree species.

Seed mass and seedling performance within eight species of psychotria (rubiaceae)

Abstract: Large seeds have been hypothesized to confer survival advantages in the shaded forest, and small seeds to confer growth advantages in open habitats. We explored these hypotheses using experimental studies in both field and controlled conditions. Our experiments examined intraspecific effects of seed mass on seedling demography in eight sympatric woody species of Psychotria. We compared the effects of seed mass on seedling size (biomass), maximum relative growth rate (RGR), the proportion of emerged seedlings that survived until one year of age, and the proportion of sown seeds reaching the stage of one-year-old seedlings in gaps compared with shaded rain forest habitats, as well as in contrasting greenhouse light conditions. For each species, seeds were classified into four seed mass categories and introduced to three pairs of shaded forest gap sites. In the greenhouse, emerged seedlings from different seed mass categories were grown in contrasting light environments. A positive effect of seed mass on see...

Root/Shoot Allocation and Root Architecture in Seedlings: Variation among Forest Sites, Microhabitats, and Ecological Groups1

Abstract: I analyzed patterns of variation in root mass allocation and root morphology among seedlings of woody species in relation to environmental factors in four Neotropical forests. Among forests, I explored the response of root traits to sites varying in water or nutrient availability. Within each forest, I explored the plastic response of species to different microhabitats: gaps and understory. Additionally, I explored evidence for life history correlation of root and shoot traits by comparing species differing in their successional group (light-demanding [22 spp.] or shade tolerant [27 spp.]) and germination type (species with photosynthetic cotyledons or species with reserve cotyledons). At each forest site, young seedlings from 10 to 20 species were excavated. A total of 55 species was collected in understory conditions and 31 of them were also collected in gaps. From each seedling, six morphological ratios were determined. Allocation to roots was higher in forest sites with the lowest soil resources. Roots were finer and longer in the most infertile site, while roots were deeper in the site with the longest dry season. Seedling traits did not differ between germination types. Shade tolerant species allocated more to roots and developed thicker roots than light-demanding species. Light-demanding species showed stronger plastic responses to habitat than shade tolerant species, and species with photosynthetic cotyledons showed lower plasticity than species with reserve cotyledons. Overall, these results suggest that among Neotropical species, root allocation and root morphology of seedlings reflect plant adjustments to water or nutrient availability at geographic and microhabitat scales. In addition, life history specialization to light environments is suggested by differences among groups of species in their allocation to roots and in their root morphology.

Soil Texture Involvement in Germination and Emergence of Buried Weed Seeds

Abstract: Laboratory trials were performed to test germination and emergence characteristics of jimsonweed (Datura stramonium L.) seeds buried in 10 different soil types (with or without the control of soil external gas environment) with pronounced sandy or clay texture. The aim of the experiments was to investigate if the physical characteristics of the soils were involved in both buried-seed ecology and emergence dynamics. Germination inhibition due to burial depth was found to be directly proportional to clay content and inversely proportional to sand content. Measurement of soil air permeability showed a dose relation between gas exchange potential and depth-mediated germination inhibition. Comparative analysis of the germination response in nonsoil and soil hypoxia suggested that inhibition is caused not so much by hypoxia per se as by the presence of fermenting metabolites that could not easily be eliminated due to decreased respiratory activity. In situ inspection of buried seeds also revealed that the increased time required for emergence in day soils is primarily due to increased mean germination time rather than greater difficulty in seedling penetration upwards through the soil before emergence. Partial removal of germination inhibition of buried seeds was facilitated by elevated air oxygen availability but only with sandy soils, showing that inhibition is closely linked to soil ability to induce gas exchange with external air. At excessive burial depth (12 cm), seeds exhibited induction of secondary dormancy independent of soil texture. In conclusion, these experiments demonstrated that soil physical properties have a strong effect on buried-seed ecology and consequently on seedbank dynamics in the agroecosystem.

Post-fire aspen seedling recruitment across the Yellowstone (USA) Landscape

Abstract: Landscape patterns of quaking aspen (Populus tremuloides) seedling occurrence and abundance were studied after a rare recruitment event following the 1988 fires in Yellowstone National Park, Wyoming, USA. Belt transects (1 to 17 km in length, 4 m width) along 18 foot trails were surveyed for aspen seedlings on the subalpine plateau of the Park, along gradients of elevation and geologic substrate, during the summer of 1996. Aspen seedling presence and density were characterized as a function of elevation, geologic substrate, slope, aspect, vegetation/cover type, presence of burned forest, and distance to nearest adult aspen stand. Presence of aspen seedlings was best predicted by the incidence of burned forest and proximity to adult aspen; aspen seedlings were only found in burned forest and were more likely to occur closer to adult aspen clones. When tested against independent data collected in 1997, the logistic regression model for aspen seedling presence performed well (overall accuracy = 73%, Taup = 0.41). When present, variation in aspen seedling density at local scales (≤ 200 m) was largely explained by elevation, with higher densities observed at lower elevations. At broad scales (> 1 km), seedling density was a function of cover type, elevation, aspect, slope, and burn severity, with greater seedling density in more severely burned forested habitats on southerly, shallow slopes at lower elevations. Aspen seedling densities ranged from 0 to 46,000 seedlings/ha with a median density of 2,000/ha on sites where they occurred. Aspen seedlings were most abundant in the south central and southwest central regions of the park, approximately an order of magnitude less abundant in the southeast region, and nearly absent in the north central area. Establishment of new aspen stands on Yellowstone's subalpine plateau would represent a substantial change in the landscape. However, the long-term fate of these postfire aspen seedlings is not known.

Sexual reproduction in the invasive species Fallopia japonica (Polygonaceae).

Abstract: Fallopia japonica (Polygonaceae) is an invasive perennial plant, well known in North America for its ability to spread aggressively via vegetative reproduction. The contribution of sexual reproduction to the distribution of this species is not well documented, and as a result, F. japonica is treated solely as a clonal species. To investigate the role of sexual reproduction in this species, germination experiments were conducted using seed collected from 29 parents from field sites in Massachusetts and from four greenhouse-grown cultivars. Results showed that wild F. japonica produce large quantities of seed that typically have high germinability. This seed is viable whether sown immediately after collection or subjected to various conditions during the winter season and germinated the following spring. Cultivars of F. japonica also produce viable seed and can thus contribute to the invasiveness of this species. In addition, wild F. japonica seedlings were observed at several field sites, with several of these seedlings surviving the winter and resprouting the following spring. That sexual reproduction and seedling survival occur in the wild has strong implications for the development of management strategies for this species.

Bridging the gap: influence of seed deposition on seedling recruitment in a primate–tree interaction

Abstract: Frugivores are thought to influence plant recruitment by creating initial seed deposition templates. However, post-deposition processes (e.g., predation) may obscure these initial patterns. Few studies have examined successive life-history stages of animal-dispersed plants to evaluate how initial frugivore seed deposition influences early life stages of plant recruitment. We addressed this issue by quantifying seed removal and seedling recruitment of Monodora myristica (Annonaceae), a tropical, low-fecundity, forest tree in Kibale National Park, Uganda. Fruit morphology suggests that this species is dispersed by only the largest arboreal frugivores. We determined which frugivores removed fruit during focal tree watches and quantified seed fate in experiments designed to mimic natural deposition. We estimated stage-specific transition probabilities of survivorship to seedling establishment and determined expected seedling recruitment from each deposition condition. To evaluate spatial and temporal variation, these methods were conducted at two sites, 15 km apart, over two years. Finally, we compared frugivore abundance between sites to expected seedling and standing seedling, sapling, and pole abundances. We found that large-bodied primates were critical for seed dispersal. They were the only frugivores that opened the hard-husked fruits and were estimated to disperse >85% of mature seeds. Seeds placed away from parents had higher germination and establishment probabilities than those under parents, indicating that seed dispersal is advantageous. Single seeds away from parents (mimicking small-bodied primate seed spitting) had the highest cumulative seedling recruitment probability. However, in three of four site-by-year combinations, 86–94% of estimated recruits came from seeds placed in clumps in dung (mimicking large-bodied primates). High recruitment in the latter condition is due to the large number of seeds estimated to be deposited by large-bodied primates. Despite this concordance, germination and establishment probabilities were highly variable between sites and years. Germination was a limiting step in recruitment, and high seed mortality from beetles and rodents indicates the importance of predation. The site with higher frugivorous primate abundance had higher standing seedling abundance, but lower expected seedling recruitment and lower sapling and pole abundances. Thus, even in a system where frugivores are critical for dispersal, spatial and temporal variation in post-deposition processes reduces predictability of frugivore actions on seedling recruitment. Corresponding Editor: E. S. Menges