Fitness of sexual reproduction of Toona ciliata var. pubescens natural populations and their sexual reproduction and regeneration

Clonality is a widespread life history trait in flowering plants that may be essential for population persistence, especially in environments where sexual reproduction is unpredictable. Frequent clonal reproduction, however, could hinder sexual reproduction by spatially aggregating ramets that compete with seedlings and reduce inter‐genet pollination. Nevertheless, the role of clonality in relation to variable sexual reproduction in population dynamics is often overlooked. We combined population matrix models and pollination experiments to compare the demographic contributions of clonal and sexual reproduction in three Dicentra canadensis populations, one in a well‐forested landscape and two in isolated forest remnants. We constructed stage‐based transition matrices from 3 years of census data to evaluate annual population growth rates, λ. We used loop analysis to evaluate the relative contribution of different reproductive pathways to λ. Despite strong temporal and spatial variation in seed set, populations generally showed stable growth rates. Although we detected some pollen limitation of seed set, manipulative pollination treatments did not affect population growth rates. Clonal reproduction contributed significantly more than sexual reproduction to population growth in the forest remnants. Only at the well‐forested site did sexual reproduction contribute as much as clonal reproduction to population growth. Flowering plants were more likely to transition to a smaller size class with reduced reproductive potential in the following year than similarly sized nonflowering plants, suggesting energy trade‐offs between sexual and clonal reproduction at the individual level. Seed production had negligible effects on growth and tuber production of individual plants. Our results demonstrate that clonal reproduction is vital for population persistence in a system where sexual reproduction is unpredictable. The bias toward clonality may be driven by low fitness returns for resource investment in sexual reproduction at the individual level. However, chronic failure in sexual reproduction may exacerbate the imbalance between sexual and clonal reproduction and eventually lead to irreversible loss of sex in the population.

<p>Understanding the evolutionary forces that shape populations in the marine environment is critical for predicting population dynamics and dispersal patterns for marine organisms. For organisms with complex reproductive strategies, this remains a challenge. Sponges fulfil many functional roles and are important components of benthic environments in tropical, temperate and polar oceans. They have evolved diverse reproductive strategies, reproducing both sexually and asexually, and thus provide an opportunity to investigate complicated evolutionary questions. This PhD thesis examines sexual and asexual reproduction in two common golf-ball sponges in central New Zealand (Tethya bergquistae and T. burtoni), with particular focus on how the environment influences these modes of reproduction, and further, how they shape species delineations and connectivity patterns. New Zealand waters are projected to experience increases in temperature and decreases in nutrients over the next century, and therefore these species may be experience changes in basic organismal processes like reproduction due to climate change, requiring adaptation to local environments. Therefore, this work has important implications when considering how reproductive phenology, genetic diversity and population structure of marine populations may change with shifts in climate. In my first data chapter, I highlight the difficulty in delineating sponge species by investigating the evolutionary relationship of Tethya spp. in central New Zealand using both morphological and molecular methods. Phylogenetic reconstructions based on two mitochondrial markers (rnl, COI-ext) and one nuclear marker (18S) revealed three genetic clades, with one clade representing T. bergquistae and two clades belonging to what was a priori thought to be a single species, T. burtoni. Morphological analysis based on spicule characteristics allowed T. bergquistae to be distinguished from T. burtoni, but revealed no apparent differences between the T. burtoni clades. These results indicate hidden genetic diversity within T. burtoni, which likely represents a group consisting of incipient species that have undergone speciation but have yet to express clear morphological differences. This chapter supports the notion that cryptic speciation in sponges may go undetected and diversity underestimated when using only morphology-based taxonomy, a result which has implications for conservation and management of marine systems. In my second data chapter, I characterize the reproductive biology for both species of Tethya in relation to potential environmental drivers, including sea surface temperature, chlorophyll-a concentration and rainfall. Using histological methods for sponges collected monthly over two years, Tethya spp. were found to be gonochoristic and oviparous sexual reproducers, with one annual reproductive event occurring in the austral summer from January to March. Differences in oocyte density and reproductive output between both species and sites highlighted both species-specific adaptive responses and environmental influences on reproduction. Temperature and rainfall were found to be correlated with instances of sexual reproduction, and the summer reproductive event occurred each year following the spring bloom of chlorophyll-a. These findings indicate that seasonal fluctuations in the environment may be important for triggering gametogenesis for these species. With shifts in temperature, productivity, and timing of seasons projected for New Zealand, there is a potential for reproductive phenology to become mismatched with the surrounding environment under future climate change scenarios, which has consequences for the frequency, duration and overall output of sexual reproduction for these sponges. My third data chapter characterizes asexual reproduction in both species of Tethya, exploring relationships between reproductive traits and potential environmental drivers that may influence asexual budding events. Two sponge populations, one for each species of Tethya, were monitored over two years by both monthly sampling and periodic in situ observations. Data revealed that budding occurred continuously throughout the year, but had a cyclic pattern where instances of budding and densities of buds were higher during the austral spring and summer. Asexual reproduction coincided with sexual reproduction, and some individuals were found to simultaneously reproduce using both modes. Instances of asexual reproduction were positively associated with temperature and rainfall, but distinct differences between species were difficult to identify. As temperature proved important, an experiment looking at bud production in relation to thermal stress was conducted, where sponges were subjected to stable temperatures treatments of 17°C (control), 19°C and 21°C. No instances of budding were observed under any temperature treatment, and high mortality occurred in the 21°C treatment. These results suggest that temperature changes (i.e., heterogeneous environments) may be more important than temperature alone in driving asexual reproduction, and further, indicate thermal stress will result in increased sponge mortality. Correlations to potential environmental drivers indicate that future shifts in climate may affect instances of asexual reproduction and thus sponge abundance, which has the potential to alter the genetic structure and overall diversity of these populations. In the final data chapter, I developed novel microsatellite markers for Tethya burtoni to characterize the genetic connectivity patterns among four populations in central New Zealand, with particular interest in the roles that sexual and asexual reproduction play in connectivity. I sampled three sites within 10 km of each other in the Wellington Region (WR), and another site on an island (Kapiti Island) approximately 50 km north of the WR. At one of the WR sample sites, I monitored a T. burtoni population over two years to examine the dispersal range of asexually reproduced buds and the ability of clones to sexually reproduce. The WR and Kapiti Island populations were strongly genetically differentiated, but within the WR region, two populations were genetically similar, indicative of high connectivity. For the monitored population, asexual bud dispersal was restricted to no greater than 1 m and clonal individuals had reduced sexual reproductive ability. Asexual reproduction did not appear to play an important role in interpopulation connectivity nor gene flow, as buds had low dispersal ability and rarely reproduced. Population structure and connectivity for T. burtoni appear to be largely driven by sexual reproduction, and asexual reproduction instead aids genotype survivorship and population maintenance. These findings highlight that different reproductive modes can differentially contribute to population dynamics in sessile marine organisms, suggesting that predictions about future population viability under changing environments may be difficult to make. In summary, this PhD thesis uses a combination of genetic, histological, field-based and experimental methods to examine species boundaries, reproduction and connectivity for Tethya spp. on rocky reefs of New Zealand. The sympatric nature, complex reproductive ecology and connectivity patterns observed likely shape the complex evolutionary processes occurring in these sponges, including introgressive hybridization and cryptic species. Individuals that showed evidence of possible introgressive events occurred mainly in populations with more restricted gene flow, while the presence of both cryptic species were more prevalent in well connected populations. Such a trend allows for discussion of under what circumstances both of these processes occur. Furthermore, environmental correlates to both sexual and asexual reproduction indicate that both of these modes of reproduction have the potential to be altered with future changes in the environment. As both modes were found to play different roles in gene flow within and between populations, future shifts in climate are also expected to alter population structure and connectivity for these sponges. Such shifts in gene flow will also likely result in changes to species boundaries and thus the overall diversity of this genus. Many other sessile, benthic marine organisms present reproductive traits and behaviours similar to those of Tethya spp., and therefore these results can aid in the interpretation of results for other marine taxa. Overall, this thesis describes the population dynamics of Tethya spp., which are abundant and ecologically important on New Zealand reefs, and provides insight on how temperate sponge populations may fare with climate change, which has important implications for management and conservation efforts.</p>

PDF HTML阅读 XML下载 导出引用 引用提醒 动物在大别山五针松种群天然更新中的作用 DOI: 10.5846/stxb201710281933 作者: 作者单位: 南京林业大学南方现代林业协同创新中心,南京林业大学南方现代林业协同创新中心,南京林业大学南方现代林业协同创新中心 作者简介: 通讯作者: 中图分类号: 基金项目: 江苏省研究生创新工程（CXZZ12_0548） Role of animals in the natural population regeneration of Pinus dabeshanensis Author: Affiliation: Co-Innovation Center for Sustainable Forestry in Southern China,College of Biology and the Environment,Nanjing Forestry University,Co-Innovation Center for Sustainable Forestry in Southern China,College of Biology and the Environment,Nanjing Forestry University,Co-Innovation Center for Sustainable Forestry in Southern China,College of Biology and the Environment,Nanjing Forestry University Fund Project: 摘要 | 图/表 | 访问统计 | 参考文献 | 相似文献 | 引证文献 | 资源附件 | 文章评论 摘要:大别山五针松（Pinus dabeshanensis）为我国特有树种，其天然植株较少且分布范围狭窄，种群更新困难，已被列为国家二级重点保护野生植物。分别于2015年和2016年的10-12月，在目前已知最大天然种群所在地安徽岳西县河图镇大王沟，研究了大别山五针松球果与种子特征、种子雨与土壤种子库、动物对种子的取食和搬运，幼苗分布格局及其与鼠穴分布的关系，以期探明动物在其天然更新中的作用，分析其天然更新不力的原因。结果显示：大别山五针松种子败育率较高，阳坡个体球果发育情况好于阴坡；种子成熟期间没有明显的种子雨，土壤种子库也未调查到完整种子。共记录到母树与球果的访问动物6目11科16种，其中7种动物确定取食种子；地面种子摆放实验显示超过95%以上的种子被啮齿动物捕食或搬运至他处取食或贮藏，不同的摆放处理对啮齿动物的捕食、搬运没有显著影响；小林姬鼠（Apodemus sylvaticus）盗食现象严重，埋藏实验中人工贮点当夜被发现的概率在90%左右，岩松鼠（Sciurotamias davidianus）和小林姬鼠是其种子主要捕食者。研究地的大别山五针松种群于2015年和2016年分别新增一年生幼苗5株和7株，这表明大别山五针松在当地存在天然更新。幼苗多单独生长在母树周围，点格局分析显示在0-0.6 m的尺度范围内呈随机分布；在 > 0.6 m尺度呈聚集分布。大别山五针松幼苗在小林姬鼠巢穴周边分布，其更新格局受到小林姬鼠贮点位置分布情况影响，小林姬鼠极有很可能为大别山五针松的传播者，啮齿动物对大别山五针松种子的捕食与搬运影响了大别山五针松的天然种群更新。 Abstract:Pinus dabeshanensis is listed as a national second-class protected wild plant species. Because of its rarity and narrow distribution, regeneration of its population is difficult. To determine the role of animals in natural population regeneration of P. dabeshanensis and causes of difficulties with natural regeneration of a P. dabeshanensis population, we studied P. dabeshanensis cone and seed traits, seed rain and soil seed banks, seed predation and removal by animals, spatial distribution patterns of seedlings and their relationships with the distribution of rat nests in the Ta-pieh Mountains, Yuexi County, Anhui Province during the period from November to December in the years 2015 and 2016. The results showed that P. dabeshanensis possessed a high rate of seed abortion, and cones grown on a sunny slope were better developed than those grown on a shady slope. We did not find seed rain or soil seed banks during seed maturation. Sixteen species of animals, belonging to 11 families and 6 orders, visited trees and cones. Seven species were identified as seed predators. A ground seed placement experiment showed that more than 95% of the seeds were preyed upon or removed to other places to be preyed upon or stored. The fates of P. dabeshanensis seeds among different placement locations were not significantly different. Apodemus sylvaticus was a good pilferer in our experiment; the seed discovery and storage rate was about 90% on the first night. Two species of rodents, Sciurotamias davidianus and A. sylvaticus, were the main seed predators. At the study site, we found 5 established seedlings in 2015 and 7 established seedlings in 2016. This indicated that there is a natural regeneration in the local area. The seedings mostly established around the seed tree independently, in a random distribution within 0-0.6 m and a clumped distribution at distances over 0.6 m. The natural regeneration pattern of P. dabeshanensis seedlings was affected by the location of A. sylvaticus nests, and the seedings established for the most part near the nests of A. sylvaticus. In summary, A. sylvaticus may be a seed disperser, and P. dabeshanensis seed predation and removal by rodents could affect P. dabeshanensis natural population regeneration. 参考文献 相似文献 引证文献

A review of research on life‐cycle events in field and laboratory populations of monogonont rotifers shows that there is great variation at multiple levels: (1) degree of sexual dimorphism; (2) occurrence and timing of sex; (3) propensity for sex during sexual periods; (4) factors controlling initiation of sex; and (5) timing and extent of emergence from diapause. There is no regular pattern where: (1) fertilised resting eggs hatch to start the growing season; (2) populations develop via female parthenogenesis during favourable conditions; and then (3) bisexual reproduction with resting‐egg production occurs during later, unfavourable conditions.Sexual reproduction in natural populations can occur throughout much of the growing season, be restricted to some period(s) during the growing season, or be completely absent. During sexual reproduction in both natural and laboratory populations, only some fraction of females produces males or resting eggs. This bet‐hedging strategy can prevent a population crash and permits future population growth via female parthenogenesis. Selection against sexual reproduction, and rapid loss of sex, can occur.Laboratory experiments with pond‐dwelling species have identified specific environmental factors that induce sex in different species: (1) increasing population density; (2) dietary tocopherol (vitamin E) and (3) long photoperiods. These factors generally are associated with favourable conditions for population growth and production of energy‐rich resting eggs: (1) large population size; (2) high probability of contacts between males and fertilisable females; and (3) nutritious diets. Endogenous factors can inhibit responses to these environmental inducers, and thus favour female parthenogenesis.The timing of resting‐egg hatching depends on: (1) occurrence of specific environmental conditions; (2) the minimum duration of obligate diapause; and (3) the genotype and physiology of females producing resting eggs. Hatching may occur shortly after oviposition, after a long diapause before or at the start of a new growing season, or throughout the growing season. Hatching can be massive and contribute substantially to population growth and genetic diversity.Areas for future research include: (1) determining the timing and extent of sex and resting‐egg hatching in more natural populations, especially those that are marine, benthic, sessile, and interstitial; and (2) identifying environmental and physiological factors controlling these events.

Calligonum mongolicum is a successful pioneer shrub to combat desertification, which is widely used for vegetation restoration in the desert regions of northwest China. In order to reveal the limitations to natural regeneration of C. mongolicum by asexual and sexual reproduction, following the process of sand dune stabilization, we assessed clonal shoots, seedling emergence, soil seed bank density, and soil physical characteristics in mobile and stabilized sand dunes. Controlled field and pot experiments were also conducted to assess germination and seedling emergence in different dune soil types and seed burial depths. The population density of mature C. mongolicum was significantly different after sand dune stabilization. Juvenile density of C. mongolicm was much lower in stabilized sand dunes than mobile sand dune. There was no significant difference in soil seed bank density at three soil depths between mobile and stabilized sand dunes, while the emergence of seedlings in stabilized dunes was much lower than emergence in mobile dunes. There was no clonal propagation found in stabilized dunes, and very few C. mongolicum seedlings were established on stabilized sand dunes. Soil clay and silt content, air‐filled porosity, and soil surface compaction were significantly changed from mobile sand dune to stabilized dunes. Seedling emergence of C. mongolicm was highly dependent on soil physical condition. These results indicated that changes in soil physical condition limited clonal propagation and seedling emergence of C. mongolicum in stabilized sand dunes. Seed bank density was not a limiting factor; however, poor seedling establishment limited C. mongolicum's further natural regeneration in stabilized sand dunes. Therefore, clonal propagation may be the most important mode for population expansion in mobile sand dunes. As a pioneer species C. mongolicum is well adapted to propagate in mobile sand dune conditions, it appears unlikely to survive naturally in stabilized sand dune plantations.

Oak forests can naturally regenerate from seed or from sprouts. Both strategies result in the establishment of a tree layer, but they involve a crucial difference: i.e. regeneration from seeds affects population genetics while sprouting assures the recovery of biomass after a disturbance but it does not involve sexual reproduction. In addition the two regeneration mechanisms differ in their complexity and are affected by different constraints: i.e. regeneration from seed is a more intricate pathway with several potential bottlenecks (e.g. seed and micro-sites availability, predation, seedling-saplings conflicts) while sprouting is a much more straightforward process benefiting from the presence of an already established root system and more independent from environmental stochasticity. Ultimately, regeneration from seeds or sprouts will result in contrasting forest structures (respectively, high-forests and coppices) with a different functioning and dynamics and requiring particular forestry practices. When natural regeneration is not possible, oak forest restoration must be done using artificial regeneration by seeding or planting (traditionally, both methods have been recommended), provided that acorn predators are controlled. Although similar results have been obtained with regard to survival, under Mediterranean conditions, shoot growth patterns clearly differ for both methods. Indeed, one-year seedlings often discontinue their shoot elongation shortly after transplanting, especially under drought or competition. At this time, a new taproot and fine lateral roots are formed. This observation suggests that the seeding and planting techniques may bear different consequences with regard to root system development, which may ultimately affect seedling establishment. Survival and growth planted seedlings depends on morphological and physiological attributes (Burdett in Can J For Res 20:415–427, 1990; Villar-Salvador et al. in New For 43:755–770, 2012; Grossnickle in New For 43:711–738, 2012). Cultivation techniques strongly determine the functional attributes of seedlings by manipulating the amount of resources (water, mineral nutrients, light, space) and the conditions (temperature, growing medium pH, photoperiod) for seedling growth. Consequently, how seedlings are cultivated impacts on the performance of forest plantations. Cultivation practices improve the ‘‘seedling physiological potential’’, increasing the chances of survival immediately after field planting. Each of these has an influence and interacts with the others (Ketchum and Rose in Interaction of initial seedling size, fertilization and vegetation control. Redding, CA, pp 63–69, 2000), which should be taken into consideration when evaluating a reforestation proposal; otherwise, artificial forest regeneration often results in unacceptably poor seedling performance. Planting date and site preparation, since they increase water availability, have been shown to be the factors most relevant to the survival of Mediterranean species. However, in less restrictive conditions, the use of less intensive soil preparation, on dates more favorable to the initial growth of the seedlings in the field, might be more efficient. Similarly, the use of tree shelters in oaks plantations is under debate, as its effects are species and environmental dependent. A better understanding of the ecophysiological seedling response under the microenvironment of the tree shelter is needed to improve the management of this protection tool. On the other side, the effects of cultivation practices can be linked closely to newly established seedlings (the post-planting phenological cycle), and such benefits are ephemeral in nature; thus, the effects of cultivation practices have their greatest importance during the initial growing seasons (1–2 years), diminishing with time.

One version of the Red Queen hypothesis suggests that sexual reproduction is maintained in populations because of the need to continually create genotypes that confer resistance against rapidly evolving pathogens and parasites. Here, we report that parthenogenetic individuals of the Heteronotia binoei species complex are much more prone to infection by mites than are their sexual relatives. This accords with a central prediction of the Red Queen hypothesis. The greater susceptibility of the parthenogens is consistent across localities with different combinations of parthenogenetic genotypes and sexual chromosome races and occurs despite the unusually high genetic diversity of the parthenogenetic form. These observations support the contention that clonal reproduction increases the susceptibility of hosts to infection by parasites.

Unseen sex in ancient virgin fungi

Variation of sexual and clonal reproduction in the alpine Geum reptans in contrasting altitudes and successional stages

Vertebrate fauna was studied over 10 years following revegetation of a Eucalyptus tereticornis ecosystem on former agricultural land. We compared four vegetation types: remnant forest, plantings of a mix of native tree species on cleared land, natural regeneration of partially cleared land after livestock removal, and cleared pasture land with scattered paddock trees managed for livestock production. Pasture differed significantly from remnant in both bird and nonbird fauna. Although 10 years of ecosystem restoration is relatively short term in the restoration process, in this time bird assemblages in plantings and natural regeneration had diverged significantly from pasture, but still differed significantly from remnant. After 10 years, 70 and 66% of the total vertebrate species found in remnant had been recorded in plantings and natural regeneration, respectively. Although the fauna assemblages within plantings and natural regeneration were tracking toward those of remnant, significant differences in fauna between plantings and natural regeneration indicated community development along different restoration pathways. Because natural regeneration contained more mature trees (dbh > 30 cm), native shrub species, and coarse woody debris than plantings from the beginning of the study, these features possibly encouraged different fauna to the revegetation areas from the outset. The ability of plantings and natural regeneration to transition to the remnant state will be governed by a number of factors that were significant in the analyses, including shrub cover, herbaceous biomass, tree hollows, time since fire, and landscape condition. Both active and passive restoration produced significant change from the cleared state in the short term.

Reproductive strategies are diverse and a whole continuum of mixed systems lies between strict sexuality and strict clonality (apomixis), including automixis, a parthenogenetic mode of reproduction involving a meiosis and increasing homozygosity over generations. These various systems impact the genetic structure of populations, which can therefore be used to infer reproductive strategies in natural populations. Here, we first develop a mathematical model, validated by simulations, to predict heterozygosity and inbreeding in mixed sexual-automictic populations. It highlights the predominant role of the rate of heterozygosity loss experienced during automixis (γ), which is locus dependent. Whenγis low, mixed populations behave like purely sexual ones until sex becomes rare. In contrast, whenγis high, the erosion of genetic diversity is tightly correlated to the rate of sex, so that the individual inbreeding coefficient can inform on the ratio of sexual/asexual reproduction. In the second part of this study, we used our model to test the presence of cryptic sex in a hybridogeneticCataglyphisant where new queens are produced parthenogenetically, leaving males with an apparent null fitness while they are essential to colony development as sperm is required to produce workers. Occasional sexual production of queens could resolve this paradox by providing males some fertile progeny. To determine whether this occurs in natural populations, we simulated genotypic datasets in a population under various regimes of sexual vs. asexual reproduction for queen production and compared the distribution of inbreeding, expected heterozygosity and inter-individual relatedness coefficients with those observed in a natural population ofCataglyphis mauritanicausing microsatellites. Our simulations show that the distribution of inter-individual relatedness coefficients was particularly informative to assess the relative rate of sexual/asexual reproduction, and our dataset was compatible with pure parthenogenesis but also with up to 2% sexual reproduction. Our approach, implemented in an R script, should be useful to assess reproductive strategies in other biological models.

Studying alternative forms of reproduction in natural populations is of fundamental importance for understanding the costs and benefits of sex. Mayflies are one of the few animal groups where sexual reproduction co-occurs with different types of parthenogenesis, providing ideal conditions for identifying benefits of sex in natural populations. Here, we establish a catalog of all known mayfly species capable of reproducing by parthenogenesis, as well as species unable to do so. Overall, 1.8% of the described species reproduce parthenogenetically, which is an order of magnitude higher than reported in other animal groups. This frequency even reaches 47.8% if estimates are based on the number of studied rather than described mayfly species, as reproductive modes have thus far been studied in only 17 out of 42 families. We find that sex is a more successful strategy than parthenogenesis (associated with a higher hatching success of eggs), with a trade-off between the hatching success of parthenogenetic and sexual eggs. This means that improving the capacity for parthenogenesis may come at a cost for sexual reproduction. Such a trade-off can help explain why facultative parthenogenesis is extremely rare among animals despite its potential to combine the benefits of sexual and parthenogenetic reproduction. We argue that parthenogenesis is frequently selected in mayflies in spite of this probable trade-off because their typically low dispersal ability and short and fragile adult life may frequently generate situations of mate limitation in females. Mayflies are currently clearly underappreciated for understanding the benefits of sex under natural conditions.

Low temperature scanning electron microscopy of the ultrastructural development of zygospores and sporangiospores in Mycotypha africana, and the effects of cultural conditions on sexual versus asexual reproduction

The purpose of this clinical report is to provide pediatricians updated research on evidence-based sexual and reproductive health education conducted since the original clinical report on the subject was published by the American Academy of Pediatrics in 2001. Sexuality education is defined as teaching about human sexuality, including intimate relationships, human sexual anatomy, sexual reproduction, sexually transmitted infections, sexual activity, sexual orientation, gender identity, abstinence, contraception, and reproductive rights and responsibilities. Developmentally appropriate and evidence-based education about human sexuality and sexual reproduction over time provided by pediatricians, schools, other professionals, and parents is important to help children and adolescents make informed, positive, and safe choices about healthy relationships, responsible sexual activity, and their reproductive health. Sexuality education has been shown to help to prevent and reduce the risks of adolescent pregnancy, HIV, and sexually transmitted infections for children and adolescents with and without chronic health conditions and disabilities in the United States.

Scots pine (Pinussylvestris L.) were selectively seeded in 1990, 1991, 1992, and 1994 in bottom layer vegetation of Pleuroziumschreberi (Bird.) Mitt, in a clearcut created in 1989 and in an adjacent undisturbed Scots pine forest of Vaccinium type, in northern Sweden. Seedlings from the 1990 seeding established considerably better on the clearcut than in the forest. In contrast, seeding done in 1991, 1992, and 1994 gave no significant difference in seedling establishment between the two sites. Seedlings that established in the clearcut in 1990 grew significantly larger and had a higher content of macro- and micro-nutrients than those in the forest. For seedlings that established in 1992, biomass and nutrient content of seedlings in the clearcut did not differ from those in the forest. Seedlings established in the clearcut in 1990 increased their nutrient content in relation to seed supply, while seedlings established in the clearcut in 1992 and those in the forest had in general gained less or even lost nutrients (especially P) in relation to seed reserve. These results indicate that Pleuroziumschreberi-dominated microsites can constitute a favourable microhabitat for pine regeneration following stand disturbance, although the positive effect quickly diminishes with time after disturbance. The higher establishment and growth of emerging seedlings the 1st year after cutting compared with later established cohorts in the clearcut may be caused by changes in nutrient availability. We propose that environmental stress after cutting temporarily diminishes the effect of the nutrient barrier formed by Pleuroziumschreberi, ericoid mycorrhiza, and ericaceous dwarfshrubs. We conclude that successful natural pine regeneration in Pleuroziumschreberi-dominnted vegetation induced by clear-cutting can be severely time restricted. This is an important consideration when dealing with forestry methods that depend on natural regeneration.