Showing papers on "Seedling published in 2000"

Seeds: the ecology of regeneration in plant communities.

Abstract: Reproductive allocation and reproductive effort in plants, F.A. Bazzaz and D.D. Ackerly maternal effects on seeds during development, Y. Gutterman the ecology of seed dispersal, M.F. Wilson animals as seed dispersers, E.W. Stiles fruits and frugivory, P. Jordano seed predators and plant population dynamics, M.J. Crawley longevity, viability and dormancy, A.J. Murdoch and R.H. Ellis the functional ecology of seed banks, K. Thompson seed responses to light, T.L. Pons the role of temperature in germination ecophysiology, R.J. Probert effect of chemical environment on seed germination, C.M. Karssen and H.W.M. Hilhorst the contribution of seedling regeneration to the structure and dynamics of plant communities and larger units of landscape, J.P. Grime and S.H. Hillier.

Pervasive density-dependent recruitment enhances seedling diversity in a tropical forest

Abstract: Negative density-dependent recruitment of seedlings, that is, seeds of a given species are less likely to become established seedlings if the density of that species is high, has been proposed to be an important mechanism contributing to the extraordinary diversity of tropical tree communities1,2,3 because it can potentially prevent any particular species from usurping all available space, either in close proximity to seed sources or at relatively larger spatial scales1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18. However, density-dependent recruitment does not necessarily enhance community diversity14. Furthermore, although density-dependent effects have been found at some life stages in some species3,4,5,6,7,8,9,10,11,12,13, no study has shown that density-dependent recruitment affects community diversity14,15. Here we report the results of observations in a lowland, moist forest in the Republic of Panama in which the species identities of 386,027 seeds that arrived at 200 seed traps were compared with the species identities of 13,068 seedlings that recruited into adjacent plots over a 4-year period. Across the 200 sites, recruit seedling diversity was significantly higher than seed diversity. Part of this difference was explained by interspecies differences in average recruitment success. Even after accounting for these differences, however, negative density-dependent recruitment contributes significantly to the increase in diversity from seeds to seedling recruits.

Soil pathogens and spatial patterns of seedling mortality in a temperate tree

Abstract: The Janzen–Connell hypothesis1,2 proposes that host-specific, distance- and/or density-dependent predators and herbivores maintain high tree diversity in tropical forests. Negative feedback between plant and soil communities could be a more effective mechanism promoting species coexistence because soil pathogens can increase rapidly in the presence of their host3, causing conditions unfavourable for local conspecific recruitment4,5,6. Here we show that a soil pathogen leads to patterns of seedling mortality in a temperate tree (Prunus serotina) as predicted by the Janzen–Connell hypothesis. In the field, the mean distance to parent of seedling cohorts shifted away from maternal trees over a period of 3 years. Seedlings were grown in soil collected 0–5 m or 25–30 m from Prunus trees. Sterilization of soil collected beneath trees improved seedling survival relative to unsterilized soil, whereas sterilization of distant soil did not affect survival. Pythium spp., isolated from roots of dying seedlings and used to inoculate healthy seedlings, decreased survival by 65% relative to controls. Our results provide the most complete evidence that native pathogens influence tree distributions, as predicted by the Janzen–Connell hypothesis, and suggest that similar ecological mechanisms operate in tropical and temperate forests.

A comparative study of seed number, seed size, seedling size and recruitment in grassland plants

Abstract: In this study we analyse relationships between seed number, seed size, seedling size and recruitment success in grassland plants. The often hypothesised trade-off between seed size and seed number was supported by a cross-species analysis and by an analysis of 35 phylogenetically independent contrasts, derived from a data-set of 72 species. Apart from among-species relatedness, we also controlled for possible confounding effect of plant size that may influence both seed size and seed number. A sowing experiment with 50 species was performed in the field. The seeds were sown in a grassland and subjected to two treatments, disturbance and undisturbed sward. Evidence for seed-limited recruitment was obtained for 45 of the species. Disturbance had a significant, or nearly significant, positive effect on recruitment for 16 of the 45 species. The relative recruitment in undisturbed sward increased with increased seed size, and both recruitment success and seedling size were positively related to seed size. We suggest that a trade-off between competitive ability and number of recruitment opportunities follows from the trade-off between seed size and seed number, through a causal chain from seed size via seedling size to recruitment success. The relationships between seed size, seed number and recruitment may be an important underlying mechanism for abundance and dynamics of plant species in grassland vegetation. This is an example of a direct link between evolutionary life-history theory, and theory of plant community structure.

Salinity Effects on Seedling Growth and Yield Components of Rice

Abstract: Flood irrigation practices that are commonly used in California during the early stages of rice (Oryza sativa L.) establishment may contribute to salinity damage and eventually decrease yield. Knowledge of salinity effects on rice seedling growth and yield components would improve management practices in fields and increase our understanding of salt tolerance mechanisms in rice. Salinity sensitivity of rice was studied to determine salinity effects on seedlings and yield components. Plants of rice cultivar M-202 were grown in a greenhouse in sand and irrigated with nutrient solutions of control and treatments amended with NaCl and CaCl 2 (2:1 molar concentration) at 1.9, 3.4, 4.5, 6.1, 7.9, and 11.5 dS m -1 electrical conductivity. Shoot dry weights of seedlings were measured at five harvests in the first month after seeding. Seedling growth was significantly reduced by salinity at the lowest salinity treatment, 1.9 dS m -1 . At 1.9 and 3.4 dS m -1 , significant reduction of seedling growth occurred at longer cumulative thermal time than at higher salt levels. Seedling survival was significantly reduced when salinity was 3.40 dS m -1 and higher. Highly significant linear responses of grain weight per plant, grain weight per panicle, spikelet number per panicle, and tiller number per plant to salinity were observed. There was a common lowest salt level for fertility and pollen germination beyond which they were significantly reduced by salinity. Harvest index was significantly decreased when salinity was at 3.40 dS m -1 and higher. Tiller number per plant and spikelet number per panicle contributed the most variation in grain weight per plant under salinity. Reductions in seedling survival, tiller number per plant, and spikelet number per panicle were the major causes of yield loss in M-202 under salinity. The compensation between spikelets and other yield components was confounded with salinity effects, but was believed to be minor relative to the reduction of spikelets due to salinity and, therefore, not sufficient to offset yield loss even at moderate salt levels.

Recruitment dynamics of a fleshy‐fruited plant (Olea europaea): connecting patterns of seed dispersal to seedling establishment

Abstract: Summary 1 Little is known about the consequences of seed-disperser activity for plant demography. We compared the spatial patterns of seed dispersal generated by frugivorous birds with those of seedling survival for the shrub Olea europaea. We examined the relative importance of dispersal in determining plant recruitment and tested whether the initial dispersal pattern persisted throughout recruitment. 2 We quantified the processes affecting each stage of regeneration (seed within a ripe fruit, dispersed seed, seedling and sapling) in different microhabitats, and evaluated transition probabilities between stages. We could then determine the overall probability of a seed in a ripe fruit becoming a sapling, and compare the probability of such an event occurring in different microhabitats. 3 Only 9.3% of the emerged seedlings reached the sapling stage, whereas 35.3% of the seeds were dispersed; 27.0% of dispersed seeds produced seedlings and 62.9% of saplings survived for 2 years. Seedling survival was therefore the critical link in regeneration. Water stress was responsible for more than 70% of seedling losses, which suggests that abiotic factors (mainly rainfall) may account for most of the fluctuation in recruitment in this species. 4 Neither post-dispersal seed predation nor germination caused changes in the initial spatial distribution of seeds, but differences in the requirements of seeds and seedlings then caused spatial uncoupling. The most favourable places for seeds were the worst for seedlings, and consequently frugivore-generated dispersal patterns differed from the final spatial pattern of recruitment. 5 Recruitment under conspecifics was nearly zero and dispersers are therefore crucial if recruitment is to occur. Their effect on the amount of recruitment was, however, overwhelmed by processes acting on the seedling stage. 6 For Olea europaea, the pattern generated by short-term recruitment dynamics persists in the long-term spatial distribution of saplings.

Is small-scale species richness limited by seed availability or microsite availability?

Abstract: To evaluate the relative roles of seed availability and competitive interactions in creating within-community patterns of species richness in unproductive grassland, we conducted a sowing experiment in a dry calcareous (alvar) grassland, where both the number of arriving seeds and the number of arriving species were approximately doubled compared to the natural seed rain. Also, in half of the plots, 36% of the vegetation and bryophyte cover was removed to simulate disturbance. Sowing significantly increased species richness and the number of seedlings in plots. Disturbance increased the number of seedlings but had no significant effect on species richness. In the first year, the highest number of seedlings was found in disturbed and sown plots. The dynamics of seedling numbers differed among species. Of the 15 sown species, seedlings of nine species were found in some plots. The number of seedlings of two species were not dependent on treatments, those of three species depended only on sowing, and for four species there was a significant positive interaction between sowing and disturbance. The establishment of sown species was not dependent on initial species richness or number of adult ramets in experimental plots. It was concluded that, though the behavior of individual species may differ, the local deficiency of seeds may be an important force generating small-scale community patterns of calcareous grasslands.

Changes in drought response strategies with ontogeny in Quercus rubra: implications for scaling from seedlings to mature trees.

Abstract: We investigated scaling of physiological parameters between age classes of Quercus rubra by combining in situ field measurements with an experimental approach. In the in situ field study, we investigated changes in drought response with age in seedlings, juveniles, and mature trees of Q. rubra. Throughout the particularly dry summer of 1995 and the unusually wet summer of 1996 in New England, we measured water potential of leaves (ΨLeaf) and gas exchange of plants at three sites at the Harvard Forest in Petersham, Massachusetts. In order to determine what fraction of the measured differences in gas exchange between seedlings and mature trees was due to environment versus ontogeny, an experiment was conducted in which seedlings were grown under light and soil moisture regimes simulating the environment of mature trees. The photosynthetic capacity of mature trees was three-fold greater than that of seedlings during the wet year, and six-fold greater during the drought year. The seedling experiment demonstrated that the difference in photosynthetic capacity between seedlings and mature trees is comprised equally of an environmental component (50%) and an ontogenetic component (50%) in the absence of water limitation. Photosynthesis was depressed more severely in seedlings than in mature trees in the drought year relative to the wet year, while juveniles showed an intermediate response. Throughout the drought, the predawn leaf water potential (ΨPD) of seedlings became increasingly negative (-0.4 to -1.6 MPa), while that of mature trees became only slightly more negative (-0.2 to -0.5 MPa). Again, juveniles showed an intermediate response (-0.25 to -0.8 MPa). During the wet summer of 1996, however, there was no difference in ΨPD between seedlings, juveniles and mature trees. During the dry summer of 1995, seedlings were more responsive to a major rain event than mature trees in terms of ΨLeaf , suggesting that the two age classes depend on different water sources. In all age classes, instantaneous measurements of intrinsic water use efficiency (WUEi), defined as C assimilation rate divided by stomatal conductance, increased as the drought progressed, and all age classes had higher WUEi during the drought year than in the wet year. Mature trees, however, showed a greater ability to increase their WUEi in response to drought. Integrated measurements of WUE from C isotope discrimination (Δ) of leaves indicated higher WUE in mature trees than juveniles and seedlings. Differences between years, however, could not be distinguished, probably due to the strong bias in C isotope fractionation at the time of leaf production, which occurred prior to the onset of drought conditions in 1995. From this study, we arrive at two main conclusions.

Seed size, nitrogen supply, and growth rate affect tree seedling survival in deep shade

Abstract: Species differences in seedling survival in deeply shaded understories (i.e., shade tolerance) may depend on both seed size and growth rates, but their relative contributions to survival and how they change with time and with variation in light and belowground resource availability is unknown. With a greenhouse experiment we investigated these relationships by examining responses of growth, growth-related morphology, survival, and their interrelationships to a range of nitrogen (3.4 × 10−9–3.4 × 10−3 mol/L N fertilizer solutions) and low light (0.6–7.3% of open sky) availabilities for young seedlings of 10 North American tree species that vary in observational shade tolerance rankings and seed size (Populus tremuloides, Betula papyrifera, Betula alleghaniensis, Acer saccharum, Larix laricina, Pinus banksiana, Pinus resinosa, Pinus strobus, Picea mariana, and Abies balsamea). Within all species, relative growth rate (RGR) and survival increased with light. RGR and survival also increased with N supply but only at the two highest light levels, and then only for the shade-intolerant, broad-leaved Populus and Betula spp.. In every species, survival was positively related to RGR. Moreover, each species differed in the relationship of survival to RGR, and these differences were related to seed mass: at any given RGR, large-seeded, shade-tolerant species had higher survival than smaller-seeded, intolerant ones. Across species, in most light and N treatments, seed mass was positively related to young seedling survival, but RGR was not. In very low light the relative benefits of greater seed mass to survival were temporary. As seedlings aged, interspecific mortality rates became more dependent on observational shade tolerance rankings than on seed size, but mortality was still unrelated to RGR. Our results indicate potentially important interactions among light, N, and species that could influence regeneration dynamics. For young seedlings in deeply shaded microsites, N supply does not matter, and only shade-tolerant species survive due, in part, to large seeds and physiological traits other than RGR. In moderate shade, RGR is greater, and survival is high for all species, except that small-seeded, broad-leaved, intolerant species have low survival and RGR at low N supply. This suggests that broad-leaved shade-intolerant species compete more effectively in moderate shade on richer soils than on poorer soils. Although we found that both seed mass and vegetative physiology influenced survival in shade, they did not covary tightly, suggesting that they are under somewhat separate selection pressures.

Rhizobial Inoculation Influences Seedling Vigor and Yield of Rice

Abstract: Rice (Oryza sativa L.) is one of the world's most important crops. The present investigation was designed to assess the range of growth-promoting activities of various diazotrophic bacteria on rice seedling vigor, its carryover effect on straw and grain yield, and the persistence of an inoculant strain on rice roots under greenhouse conditions. Growth responses to inoculation exhibited bacterial strain-rice variety specificity that were either stimulatory or inhibitory. Growth responses included changes in rates of seedling emergence, radical elongation, height and dry matter, plumule length, cumulative leaf and root areas, and grain and straw yields. Most notable were the inoculation responses to Rhizobium leguminosarum bv. trifolii E11 and Rhizobium sp. IRBG74, which stimulated early rice growth resulting in a carryover effect of significantly (P = 0.05) increased grain and straw yields at maturity, even though their culturable populations on roots diminished to below detectable values at 60 d after planting. The test strains were positive for indole-3-acetic acid production in vitro, but only some reduced acetylene to ethylene in association with rice under laboratory growth conditions. These studies indicate that certain strains of nonphotosynthetic diazotrophs, including rhizobia, can promote growth and vigor of rice seedlings, and this benefit of early seedling development can carryover to significantly increased grain yield at maturity.

Role of Fire in Regeneration from Seed

Abstract: Fire is a disturbance factor in ecosystems worldwide and affects the reproduction of many plant species. For some species, it is just one of several disturbances that trigger seed germination and subsequent seedling recruitment, whereas in other ‘fire-dependent’ species, fire may be required for seedling recruitment. Fire may trigger seed regeneration directly, through the opening of serotinous fruits or cones or by inducing the germination of dormant soil-stored seed banks. Fire may also indirectly initiate seedling recruitment by opening gaps in closed vegetation, thus providing conditions suitable for colonization. There is a multitude of mechanisms for capitalizing upon such disturbances and the particular mode is a function of fire regime, climate, growth form, phylogeny and biogeography.

Barriers to Forest Regeneration in an Abandoned Pasture in Puerto Rico

Abstract: Sources of forest regeneration (soil seed bank, seed rain) and barriers to seedling establishment were examined in a recently abandoned pasture in eastern Puerto Rico. Few woody species were found in the soil seed bank or in the seed rain. The number of seeds and species in the seed rain and soil seed bank declined with distance from the adjacent secondary forest. Nine species naturally dispersed and colonized plots during the study, with the wind-dispersed tree Tabebuia heterophylla being the predominant colonizer (91% of all seedlings). Barriers to seedling establishment were determined using a blocked field experiment with eleven woody species representative of a variety of life histories. Each species was planted under the pasture vegetation (control) or in areas where all vegetation was removed (removal). Germination was enhanced for four species in the control treatment, five species were not affected, and two species did not germinate under either treatment. Survival to 6 months was higher in the removal treatment for two species. Seedling biomass was greater in the removal treatment at 12 months for one species. Seed mass was a good predictor of germination success and final shoot biomass, but not survival. This study demonstrates that seeding recently abandoned pastures with a mix of known pioneer species may accelerate the rate of secondary succession, but some species will have to be planted in later successional stages in order to overcome strong barriers to establishment.

Smoke as a germination cue

Abstract: Fire is a major environmental selective force that influences plant communities in many parts of the world. Reproductive strategies have evolved as adaptations to the various factors generated by, and/or associated with, fire. This is particularly true for seeds, in which strategies have evolved that respond to both the physical and chemical germination cues that may be associated with fires. Some of these cues are clearly of a primary nature, while others may play a secondary and/or highly specific, or even permissive, role in the germination process. Clearly, there must be considerable interaction between the physical and chemical cues generated by wild fires. Smoke is the most striking chemical factor. Smoke released from burning vegetation contains a chemical signal that triggers germination of both fire climax and non-fire climax species from different parts of the world. It is used in horticulture to stimulate seed germination of wildflower species and can break dormancy and improve germination of vegetable crops, such as lettuce and celery. Smoke can be applied to seeds immediately before sowing, or the seeds may be pretreated and stored until conditions are appropriate for sowing. Both smoke and aqueous smoke-water are active in this respect. The active constituent(s) is volatile, thermostable, water soluble and long-lasting in aqueous solution and in the soil. Attempts to identify the active compound(s) and to determine the mechanism(s) of action have been unsuccessful. Smoke treatment is effective on many seeds that have a light requirement for germination. A very clear concentration effect, resembling that of hormonal responses has been established with aqueous smoke solutions. Smoke extracts interact with gibberellins, cytokinins, abscisic acid and ethylene in photoblastic and in thermodormant seeds. However, despite these interactions it remains questionable whether smoke acts via hormones in stimulating seed germination. There is a clear connection between smoke and life history traits. Smoke may well be the overriding trigger for germination in relation to specific growth habits, regeneration strategies, seed storage, seed sizes, dispersal modes and structures for a large number of species growing in fire-prone habitats. It is becoming increasingly clear that smoke as a germination (or growth regulating) cue must have evolved as part, or as a consequence of fire, as an evolutionary factor. As such, it is probably a very old development and serves as an additional protection mechanism to ensure germination at optimal times for seedling survival.

Factors affecting Campsis radicans seed germination and seedling emergence

Abstract: The effects of environmental factors on germination and emergence of Campsis radicans seeds were examined in laboratory and greenhouse experiments. Campsis radicans pods produced numerous, papery, and small seeds (696 seeds/pod; 4 mg/seed). Seeds exhibited dormancy that was relieved (74% germination) after 2 wk of prechilling. Fluctuating temperatures and a 12-h photoperiod were required for maximum germination. Optimum conditions for C. radicans seed germination (74%) were 35/25 C (day/night, 12/12 h) with a 12-h photoperiod. Temperatures below 25/15 C or above 40/30 C were unfavorable for germination. Germination in constant temperatures or in continuous darkness was less than 15%. More than 59% of C. radicans seeds germinated at pH 5 to 9, but at pH 4 or 10 seed germination was totally inhibited. Germination was totally inhibited at osmotic stress higher than −0.2 MPa. Germination was 60% at 40 mM NaCl and 20% at 160 mM NaCl. Emergence was maximum (68%) for seeds that were placed on the soil s...

Facilitation of cobble beach plant communities through habitat modification by spartina alterniflora

Abstract: Single species can have large effects on entire communities through habitat modification and facilitation. I tested the general hypothesis that the intertidal grass Spartina alterniflora facilitates the establishment and persistence of New England cobble beach plant communities by modifying the shoreline environment. This community is dominated by halophytic forbs and is restricted to estuarine cobble beaches bordered by Spartina. Beds of Spartina can reduce mean water velocity by 50% and maximum velocity by nearly an order of magnitude, and they can substantially stabilize the cobble substrate. Specifically, I determined the importance of five life history stages (seed supply, seed germination, seedling emergence, seedling establishment, and adult survival) and four factors (water velocity, substrate stability, herbivory, and soil quality) in limiting lateral plant distribution. A seed addition experiment demonstrated that seedlings could only emerge behind Spartina, suggesting that seedling emergence is the proximate life history stage limiting adult dis- tribution. Seed germination and adult survival do not appear to be limiting stages, at least in an absolute sense. Although seed supply was much greater (~-10-100X) behind Spartina, a substantial number of seeds were caught in seed traps placed between beds, suggesting that seed supply also does not limit absolute plant distribution. These results are supported by the presence of seedlings buried below the substrate surface between beds at the time when seedlings are naturally emerging behind beds. A manipulative field experiment was performed to test the effects of substrate instability, soil quality, and herbivory on seedling emergence between beds. Seeds of two annual cobble beach species (Suaeda linearis and Salicornia europaea) were added to plots behind Spartina and also between beds with and without substrate stabilization manipulations. This treatment was designed to stabilize the substrate in a manner that would not affect water velocity and soil characteristics or prevent access by potential herbivores. When not buffered by Spartina, seedlings of both species were only able to emerge and survive when the substrate was artificially stabilized. These results indicate that Spartina alternifora facilitates the establishment and persistence of cobble beach plant communities by stabilizing the substrate and enabling seedlings to emerge and survive.

Variation in stand structure, light and seedling abundance across a tropical moist forest chronosequence, Panama

Abstract: We asked whether forest structure and understory light environments across a tropical moist forest chronosequence followed predictions of a 4-phase model of secondary succession (establishment, thinning, transition and steady-state) and whether seedling density and diversity were functions of light availability as predicted by this model Using aerial photographs, we identified eight second-growth stands (two each aged ca 20, 40, 70, and 100 yr) and two old-growth stands within Barro Colorado Nature Monument, Panama Trees and seedlings were sampled in nested, contiguous quadrats in 2 160-m transects in each stand Light was measured as percent transmittance of diffuse photosynthetically active radiation (TPAR) at each seedling quadrat and by estimation of percent total incident radiation during the growing season from hemispherical canopy photographs Basal area, tree density, and canopy height followed predictions of the 4-phase model Percent total radiation, but not TPAR, declined with stand age as did seedling density While seedlings were more likely to occur in quadrats at higher light levels, much variation in seedling density was not related to light availability Seedling patch sizes were small irrespective of light patches, estimated as semivariance ranges Seedling species richness was a function of seedling density; estimates of species diversity unbiased by density did not vary systematically as a function of stand age Proximate seed sources, efficient dispersal mechanisms, and appropriate establishment conditions can promote establishment of species-rich communities early in successions of heterogeneous tropical moist forest

Seed movements and seedling fates in disturbed sagebrush steppe ecosystems: implications for restoration

Abstract: Understanding species establishment patterns and community structure fol- lowing disturbance, and developing effective restoration methods requires knowledge of both the movements and fates of seeds. I used a restoration experiment in a severely disturbed sagebrush steppe ecosystem near Kemmerer, Wyoming to examine the effects of soil surface characteristics and seed morphology on seed entrapment and retention, and the effects of soil surface characteristics on soil water potential and seedling emergence and survival. Seeds of native species with awns, mucilaginous seedcoats, wings, hairy pappi, or no appendages were sown over soil surface treatments consisting of silty loam soil, sand, gravel, surface mulch, shrub mimics, and large and small holes. Seeds that lacked ap- pendages and that had small surface areas did not exhibit significant horizontal movement or redistribution. Seeds with appendages that resulted in exposure of a large surface area to the wind did exhibit significant redistribution, despite apparent adaptations for seed burial or retention. When the entire seed population was considered, the effectiveness of the treatments for trapping and retaining seeds was large holes . small holes $ gravel $ shrub mimics . soil $ sand. Surface mulch neither gained nor lost seeds. The most effective treatments for seedling emergence had among the least negative soil water potentials and included large holes, surface mulch, and sand. Gravel provided an inadequate growing medium, and both shrub mimics and small holes accumulated fine-textured soils resulting in highly negative water potentials and low seedling emergence. Once a seedling emerged, the probability of survival was reasonably high (56.3% over 2 yr) regardless of treatment. This study indicates that soil surfaces that trap and retain high densities of seeds with large surface areas may have little or no effect on seeds with small surface areas, and may or may not result in high seedling emergence and survival. Restoring diverse native ecosystems requires creating soil surface features that can trap and retain seeds with varying mor- phologies as well as provide favorable conditions for seedling establishment.

Experimental tests of the spatiotemporal scale of seed predation in mast‐fruiting dipterocarpaceae

Abstract: Mast-fruiting Dipterocarpaceae exhibit highly synchronous, interspecific seedfall at irregular, multiyear intervals. To investigate how the temporal pattern of seedfall affects dipterocarp seed and seedling survival, in both a logged and a primary lowland tropical forest, we planted Shorea stenoptera Burck seeds in the last three weeks of a 12- wk synchronous dipterocarp seedfall during a major community mast-fruiting event in West Kalimantan, Indonesia. As a result of commercial timber harvest of dipterocarp individuals eight years before, total dipterocarp seed production in the logged site was only 23% of that in the primary forest. At both sites, an average of 35 kg of seed was sown across a large area (?1 kM2) to examine the spatial pattern of seed destruction. During the period in which "natural" community mast seed was available to predators, 92% and 99% of experimentally sown seed escaped predators in logged and in primary forest, respectively. After regional seed resources were exhausted, nomadic vertebrates (primarily the bearded pig, Sus barbatus) were observed in both forest areas, and all ungerminated seed was destroyed. Seed predators arrived earlier in the logged area, before most experimentally sown seed had germinated, and the logged site experienced greater seed loss to vertebrates than did the primary forest. Because nomadic seed predators were absent during peak fruit fall of naturally occurring communities at both study sites, there was no evidence of local predator satiation. Rather, experimentally sown seed escaped predation because of rapid germination before predator arrival, as opposed to being ignored by satiated predators. Seed escape was more dependent on the late arrival of pigs than on the amount of local seed production. There was no significant spatial autocorrelation of seed predation. All remaining seed at the scale of the experiment (>1 kM2) was destroyed by predators. These findings suggest that satiation of nomadic predators occurs at the landscape scale. Postdispersal seed predators caused significantly greater seed destruction in the exper- imentally sown seed populations than in naturally dispersed, mast-fruiting communities at both sites. In both logged and primary forests, there was significantly greater loss of experimentally sown seed to predation than was found in the entire natural mast-fruiting Shorea community combined (21 spp.). Moreover, a naturally occurring, but late-fruiting, Shorea species also exhibited greater seed losses to predation than did all other species within each mast-fruiting community, and these proportional losses were similar to those observed in the experimentally sown seeds. Seeds that escaped predation and vertebrate herbivory on post-establishment seedlings dis- played high survival, indicating that the availability of suitable microsites did not limit re- cruitment. In the primary forest, 65% of the germinated experimental seed that survived early causes of mortality was alive 40 mo post-planting, which coincided with the next mast-fruiting event. The spatial distribution of these seedlings was modified primarily by the foraging behavior of vertebrate seed predators in the first two weeks post-planting. The influence of vertebrate predation on seed and seedling survival suggests that foraging behavior by terrestrial vertebrate seed predators may cause directional and/or stabilizing selection for synchronous, interspecific supra-annual dipterocarp seed production across forest regions in Kalimantan.

Seedling dynamics over thirty-two years in a tropical rain forest tree

Abstract: We examined patterns of seedling recruitment and their underlying mechanisms in a population of Chrysophyllum sp. nov. (Sapotaceae), a shade-tolerant canopy species in mature tropical rainforest in Queensland, Australia. We considered spatial scales ranging from 1 m2 to 1 ha, and temporal scales ranging from 1 to 32 yr. Over the 32-yr study period there were six episodes of very high seedling recruitment (masts) at intervals ranging from 4 to 10 yr. Less than 2% of new recruits were found in the nine censuses in other years. We found no significant correlations between the numbers of seedlings per census and either seasonal or annual rainfall, number of dry months per year, or El Nino Southern Oscillation (ENSO) events but found two correlations with mean seasonal air temperatures in the years preceding flowering. There were long-term changes in abundance in both time and space. In 1965 there were 163 seedling-sized plants in two dense patches, whereas in 1996, there were 4000 in 15 patches. Once a new patch became established, seedlings recruited there in each succeeding mast episode. Adult trees varied in their production of seedlings. Only 25% of the trees in the sample analyzed participated in all mast years; others had few seedlings for up to 28 yr before beginning to produce many seedlings. Seedlings were shade tolerant. They grew extremely slowly in the shaded understory, mean height only doubling in 27 yr. They were also quite long-lived; 6% of seedlings recruited in 1969 were still alive 27 yr later. There was little effect of natural enemies on seeds or seedlings. We found weak positive and negative effects of density on seed germination, seedling mortality, and growth. Mortality rates of Chrysophyllum seedlings did not show any trends with time, nor with distance from conspecific adults. These results suggest that abundance of older stages is determined by recruitment as well as subsequent growth and mortality. Mortality rates of seedlings of species other than Chrysophyllum decreased with distance from adult Chrysophyllum trees. Under present conditions, the Chrysophyllum population may be increasing in relation to that of other species, perhaps leading to a reduction in tree diversity in this tropical rainforest.

Differential effects of tropical arbuscular mycorrhizal fungal inocula on root colonization and tree seedling growth: implications for tropical forest diversity

Abstract: The potential for mycorrhizae to influence the diversity and structuring of plant communities depends on whether their affinities and effects differ across a suite of potential host species. In order to assess this potential for a tropical forest community in Panama, we conducted three reciprocal inoculation experiments using seedlings from six native tree species. Seeds were germinated in sterile soil and then exposed to arbuscular mycorrhizal fungi in current association with naturally infected roots from adults of either the same or different species growing in intact forest. The tree species represent a range of life histories, including early successional pioneers, a persistent understory species, and emergent species, typical of mature forest. Collectively, these experiments show: (i) the seedlings of small-seeded pioneer species were more dependent on mycorrhizal inocula for initial survival and growth; (ii) although mycorrhizal fungi from all inocula were able to colonize the roots of all host species, the inoculum potential (the infectivity of an inoculum of a given concentration) and root colonization varied depending on the identity of the host seedling and the source of the inoculum; and (iii) different mycorrhizal fungal inocula also produced differences in growth depending on the host species. These differences indicate that host-mycorrhizal fungal interactions in tropical forests are characterized by greater complexity than has previously been demonstrated, and suggest that tropical mycorrhizal fungal communities have the potential to differentially influence seedling recruitment among host species and thereby affect community composition.

Patterns of seed mass variation and their effects on seedling traits inAlliaria petiolata (Brassicaceae)

Abstract: Seed mass is considered to be the least plastic component of reproductive yield. Yet, in invasive populations of garlic mustard, Alliaria petiolata, seed mass was highly variable (eightfold among populations, 2.5-7.5 fold within populations, two-threefold within individuals, and 1.4-1.8 fold within fruits). Variation in seed mass among populations explained nearly half of the total variance. Variation among seeds within fi-uits accounted for a further 25% of variance. Individual seed mass within a plant decreased with increased distance from the main stem, suggesting that access to parental resources limits seed size in a predictable manner. MANOVAs and Roy-Bargmann stepdown analyses revealed significant effects of seed mass, but not seed position (within a fruit or within an infructescence), on an array of subsequent seedling traits. Smaller seeds germinated significantly earlier, and seedlings from small seeds produced their first primary leaves significantly later and grew significantly taller. After accounting for seed mass as a covariate, only one seedling trait, date of first leaf emergence, was affected by seed position in a fruit. Differences in seed mass may therefore affect seedling recruitment via effects on early seedling growth in this weedy species.

Effect of chemical environment on seed germination

Abstract: Seeds receive information about the succession of seasons through fluctuations in temperature. In arid and semi-arid zones, the timing of precipitation adds important information. Seeds receive information about their depth in the soil and neighbouring vegetation through the dependency of the germination process on light and fluctuating diurnal temperatures. Therefore, germination of many seed species often only occurs at or close to the surface of the soil and in vegetation gaps. The chemical environment provides seeds with information about the quality of their environment with respect to suitability for growth. In general, chemical factors that promote germination are also beneficial for emergence and seedling growth. The dependence of many species on nitrate for germination is a clear example of this rule. The presence of high soil nitrate levels may even stimulate the germination of the next generation of seeds, via the accumulation of nitrate during seed formation. The dependence of parasitic seeds on chemical promoters excreted by the host plant illustrates the parallelism between the stimulation of germination and seedling growth. Seedlings of the parasite also depend fully on host factors. Similarly, allelopathic substances in the soil, which inhibit germination, are generally deleterious to seedling growth.

Seed abundance versus substrate limitation of seedling recruitment in northern temperate forests of British Columbia

Abstract: We examine the influence of (i) the spatial distribution and abundance of parent trees (as seed sources) and (ii) the abundance and favourability of seedbed substrates, on seedling recruitment for the major tree species in north - western interior cedar-hemlock forests of British Columbia, under four levels of canopy openness (full canopy, partial canopy, large gap, and clearcut). Substrate distribution varied with canopy openness, and substrate favourability was a function of both canopy openness and seedling species. Lack of suitable substrates was the predominant factor limiting seedling density under full canopies. Partial canopy and gap sites provided a broad range of favourable substrates in close proximity to parent trees, resulting in the highest observed seedling densities. There was much higher effective dispersion of seedlings away from parent trees in gaps than in the partially cut stands. Seedling dispersion to clearcut sites was poor with seedlings being tightly restricted to a narrow band along the forest edge. Thus, seedling recruit - ment in these forests was a reflection of the interaction between the abundance of seed and substrate favourability, and the relative importance of these factors varied significantly with canopy structure.

Hydrochory, seed banks, and regeneration dynamics along the landscape boundaries of a forested wetland

Abstract: Following the environmental sieve concept, the setting in which the recruitment of Taxodium distichum occurs in, becomes increasingly restrictive from the seed to seedling stage in an impounded forested wetland. Although a wide elevational band of dispersing seed moves across the boundary of a swamp-field in the water sheet, the zone of germination is relegated to that portion of the forested wetland that draws down during the growing season. Seedling recruitment is further restricted to the uppermost zone of the winter water sheet. These patterns are likely applicable to other species of dominant swamp species, e.g., Cephalanthus occidentalis crossed the boundary of a forested wetland and abandonded field in winter flooding (November–December and November–March, respectively) in Buttonland Swamp. The elevation of the boundary was 101.3 m NGVD. While the seeds of at least 40 swamp species were dispersed across the boundary, few viable seeds were dispersed after the winter season. Kriged maps showed seeds of T. distichum and C. occidentalis dispersed in patches in the water depending on the position of the water sheet. Most species of both water- and gravity-dispersed species had a localized pattern of seed distribution (either spherical or exponential) and this indicated that seeds may not be dispersed for great distances in the swamp. Water-dispersed T. distichum and C. occidentalis had larger dispersal ranges (A0=225 and 195 m, respectively) than Bidens frondosa and B. discoidea (A0=14 and 16 m, respectively). Seed dispersal varied with season depending on the availability of seeds. In Buttonland Swamp, viable seeds typically were dispersed for T. distichum in November–June, and for C. occidentalis in November-July. Low water occurred in August 1993 and high in February 1994 (99.8 and 101.6 m NGVD, respectively). The seed banks along the landscape boundary varied in species composition according to elevation (r2 = 0.996). While the similarity of species richness between water-dispersed seeds and the seed bank at elevations that flooded (during June 1993 through May 1995) was high (10–17%), it was low between water-dispersed seeds and the seed bank at elevations that did not flood (5%). T. distichum seeds had a short germination window in that seeds germinated within a year following their production in zones that were flooded in the winter followed by drawdown during the next growing season. After 1 year, less than 5% of the T. distichum seeds remained viable on the surface of the soil. Germination of T. distichum was confined to specific elevations (above 99.3 but below 101.6 m NGVD) during this study with 4.1% of the seedlings surviving for more than 2 years at a mean of 101.4 m NGVD. All seedlings below this elevation died. To maximize natural regeneration along the boundaries of swamps in abandoned farm fields targeted for restoration, this study suggests a flood pulse regime consisting of high water in the winter to maximize dispersal of live seeds followed by low water in the summer to facilitate seed germination and seedling recruitment. Hydrologic restoration could assist in the natural recovery of damaged wetlands if a seed source exists nearby.