EFFECTS OF HYBRIDIZATION AND INBREEDING ON FITNESS IN PHLOX

The effects of interpopulation hybridization, and self-fertilization and immigration on fitness in Phlox drummondii were analyzed in 5 natural field sites. Germination rates, survivorship to flowering, fecundity, and net reproductive rates (R_o) were determined for planted populations of natives, hybrids, aliens, and the products of one and two generations of self-fertilization. At all sites, seed germination was 36% for natives, 35% for hybrids and 28% for aliens. Survivorship for natives was 50%, compared to 49% for hybrids and 41% for aliens. The mean fecundity was 34, 35, and 39 seeds per plant for natives, hybrids, and aliens, respectively. The R_o of natives averaged 5.2 vs. 6.4 for hybrids and 4.2 aliens. At all the sites, germination averaged 25% for self-1 plants and 19% for self-2 plants compared to 24% for open-pollinated controls. Survivorship progressed from 50% in the controls to 46% in self-1 and 44% in self-2 plants. Seed production per plant averaged 35% in the control vs. 32% in the self-1 and 25% in the self-2 plants. The mean R_o of the control was 3.8 vs. 3.3 in the self-1 and 2.14 in the self-2 plants. Our results demonstrate that the genetic variable may have a substantial effect on plant fitness in the field.     

THE EFFECT OF SEED SIZE AND MATERNAL SOURCE ON INDIVIDUAL SIZE IN A POPULATION OF LUDWIGIA LEPTOCARPA (ONAGRACEAE)

Seed size is normally distributed for many annual species, while mature plant size is frequently positively skewed. A study was conducted to determine the influence of seed size and the role of genetic differences in determining relative seedling size for Ludwigia leptocarpa. Seed size had a significant effect on percentage germination and time of seed germination but no effect on dry weight or leaf area of seedlings. Seed size and spacing had a significant effect on seedling dry weight for plants grown under competition, while relative day of emergence had no effect. Familial (genetic) differences were found in average seed weight between maternal plants, but not in average number of days to germination, average weight of seeds which germinated, or shoot dry weight. It is concluded that neither seed size alone nor genetic differences between plants are directly responsible for the development of size hierarchies in Ludwigia leptocarpa populations. Large seed size does convey an advantage in growth when plants from seeds of differing initial size interact.     

EFFECT OF SEED DIMORPHISM ON THE DENSITY-DEPENDENT DYNAMICS OF EXPERIMENTAL POPULATIONS OF ATRIPLEX TRIANGULARIS (CHENOPODIACEAE)

The importance of seed size and density in determining individual plant performance and plant population dynamics in experimental populations of the halophyte Atriplex triangularis was studied. Two distinct seed morphs—large, light seeds and small, dark seeds—are produced by individual A. triangularis plants. Experimental populations consisting of seed size monocultures (large or small seeds) and seed size mixtures were established at three different densities, and the time of germination, plant size, plant survivorship, and plant fecundity were monitored. Marked variation in time of germination was observed among treatments and between seed sizes, but germination within any given treatment occurred over a five- to ten-day period. Large seeds produced larger plants than small seeds did, and this dichotomy was maintained over the course of the entire experiment. Germination date and seed size interacted such that larger plants grew from seeds which germinated earlier than those which germinated later, regardless of seed size. Germination date had a more pronounced effect than seed size did on plant mortality in high density populations. At high density, large seed monocultures experienced greater mortality than small seed monocultures did, but in seed size mixtures, the mortality was evenly distributed between plants from the two seed sizes. Regardless of density conditions and parentage, large and small seeds were produced in equal proportion by the plants. Total seed production, however, was dramatically affected by plant density, and to a lesser degree by germination date. Although seed size effects alone did not appear to affect directly final plant biomass and fecundity, effects of seed size early in ontogeny may have contributed to differences in fecundity.     

Population biology of <Emphasis Type="Italic">Kosteletzkya pentacarpos</Emphasis> (Malvaceae) in the Llobregat delta (Catalonia,NE of Spain)

Field experiments and wild population monitoring have been performed to study the population biology of the rare long-lived Kosteletzkya pentacarpos (Malvaceae) in the Llobregat delta (Catalonia, NE Spain). Field experiments explored the fate of seeds in soil at different depths, seedling emergence, and seedling survival, growth and flowering with and without canopy cover during the first 2 years of life. They also were used to ascertain the size-related pattern of seedling survivorship and flowering. Field data concerning mortality, growth and fecundity of adult plants were collected yearly in three wild populations for 7–9 years. In old adults (reproducing long before the beginning of the study), ANOVAR tests were performed to compare maximum diameter, total and fertile shoots, and viable seeds per plant between years and populations. New adults (starting flowering the first year of study or in subsequent years) were used to explore, using linear and polynomial regressions, the association of RGR and both total and fertile shoot production with (i) plant size (maximum basal diameter or its logarithm); (ii) plant age (years in adult stage); and (iii) plant age after removing the effect of size and year-to-year fluctuations. In this case, we examined the age-related pattern of the residuals obtained from the regressions with size and year. The study identified the following main demographic features of K. pentacarpos: (i) transient, shallow soil seed bank; (ii) shade tolerance of seedling emergence; (iii) canopy-facilitation of seedling survival and bolting during the first two years of life; (iv) size-related pattern for seedling survivorship but not flowering; (v) exclusive dependence on a fluctuating seed output for reproduction; (vi) rapid adult growth; and (vii) high adult longevity but (viii) rapid depletion of fecundity with age. Seed output was highly constrained by mining insects. The changing size-structure and the decreasing reproductive success of old adults in several populations suggest that K. pentacarpos might undergo a dynamics of population establishment and extinction in the Ricarda marshes. Because of fluctuating reproduction and the lack of a persistent seed bank, the conservation of standing adult populations appears to be a key factor to ensure the persistence of the species.     

Seed size variation in the palm Euterpe edulis and the effects of seed predators on germination and seedling survival

Intraspecific variation in seed size is common in wild plant populations and has important consequences for the reproductive success of individual plants. Multiple, often conflicting evolutionary forces mediated by biotic as well as abiotic agents may maintain such a variation. In this paper we assessed seed size variation in a population of the threatened, commercially important palm Euterpe edulis in southeast Brazil. We investigated (i) how this variation affects the probability of attack by vertebrate and invertebrate post-dispersal seed predators, and (ii) if seed size influences the outcome of seeds damaged by beetles in terms of seed germination and early survival of seedlings. Euterpe edulis seeds varied in diameter from 8.3 to 14.1 mm. Neither insects nor rodents selected the seeds they preyed upon based on seed size. Seed germination and total, shoot and root biomasses of one-year seedlings were significantly and positively affected by seed size. Root biomass and seedling survival were negatively affected by seed damage caused by a scolytid beetle (Coccotrypes palmarum) whose adults bore into seeds to consume part of the endosperm, but do not oviposit on them. Seed size had a marginally significant effect on seedling survival. Therefore, if any advantage is accrued by E. edulis individuals producing large seeds, this is because of greater seed germination success and seedling vigor. If this is so, even a relatively narrow range of variation in seed size as observed in the E. edulis population studied may translate into differential success of individual plants.     

Long-term variability in seed size and seedling establishment of <Emphasis Type="Italic">Maianthemum bifolium</Emphasis>

Relatively few studies conducted in natural plants populations focus on the relationship between seed size and their germination ability and seedling establishment. Maianthemum bifolium is a perennial herb that spreads vegetatively through rhizomatous growth and reproduces through seeds. However, this species is characterized as seed and microsite limited, and under undisturbed conditions seedlings are not noted. The studies were conducted in two populations of M. bifolium in six subsequent seasons. The mean seed mass was negatively correlated both per ramet as well as in the fruit with the number of seeds, and positively with its height and the number of flowers. The long-term mean annual production of seeds in the populations was 37 and 56 seeds per m². The seeds from both populations had similarly high germination abilities that were approximately 90% under laboratory conditions, 60% in garden, and 55% in the natural habitat. Seeds from four size classes were sown and a positive correlation was noted between seedling establishment and the mass of the seeds from which they grew (r_S = 0.64). Seedling survival was also significantly correlated with seed mass.     

REPRODUCTIVE CAPACITY AND SEED SIZE IN LUPINUS TEXENSIS

The average number of ovules produced per individual of Lupinus texensis is much greater than the average number of seeds per plant. Each plant produces approximately 2,000 ovules but only 2.5% develop into seeds. One fourth of the seeds is lost due to abortion and 0.3% is lost due to predation on the plant. Mature seeds from this population exhibit a five-fold range in weight, from 10 to 56 mg. The distribution of seed weights in the field population is skewed and leptokurtic. Seed wt is positively correlated with both seed germination and seedling survivorship. Heritability of seed wt is 0.09. There is no correlation between average seed wt per plant and total number of seeds per plant, seeds per pod, or legumes per plant.     

BREEDING SYSTEM EVOLUTION IN LEAVENWORTHIA: BREEDING SYSTEM VARIATION AND REPRODUCTIVE SUCCESS IN NATURAL POPULATIONS OF LEAVENWORTHIA CRASSA (CRUCIFERAE)

Populations of Leavenworthia crassa (Cruciferae) studied for 3 years exhibited among- and within-population genetic variation for a suite of floral and reproductive traits (flower width, petal length, anther position, ability to set seeds in the absence of pollinators, time to first flowering) associated with breeding system. We used electrophoretic markers to show that a population with small, monomorphically colored flowers with introrse anthers had a significantly lower outcrossing rate (t = 0.03) than did a population with larger, polymorphically colored flowers with extrorse anthers (t = 0.33). In the more-outcrossing population the correlation between higher maternal plant outcrossing rate and the suite of six traits approached significance (P < 0.067), with greater petal size, greater flower width, and reduced ability to set seeds in the absence of pollinators contributing significantly. Plants in selfing populations had a generally higher reproductive success, with a higher number of flowers per plant, a smaller proportion of unfertilized ovules, a smaller proportion of fertilized ovules aborted, a higher rate of fruit set, and overall a larger number of seeds matured than did plants from the more outcrossing populations. Pollen limitation did not appear to account for lower reproductive success in outcrossing populations. Resource limitation did not differ substantially between populations. However, within-ovary patterns of fertilization, abortion, and seed weight were significantly less random in outcrossing populations than in selfing populations, suggesting that differential gamete and embryo success may be responsible for lower reproductive success in outcrossing populations.     

RESOURCE COMPENSATION AND THE EVOLUTION OF GYNODIOECY IN PHACELIA LINEARIS (HYDROPHYLLACEAE)

Gynodioecy is a dimorphic breeding system in which hermaphrodite and female individuals coexist in populations. Theoretical models have shown that if nuclear genes control sex expression, then gynodioecy can evolve only when females have large advantages in one or more fitness components. These female advantages must be large enough that females' expected lifetime production of viable seeds is more than twice that of hermaphrodites. Previous studies have found that cytoplasmic inheritance and/or a large offspring-vigor advantage of females (caused by hermaphrodite self-pollination and inbreeding depression of selfed seeds) account for this breeding system's evolution. This paper reports studies of gynodioecy in Phacelia linearis, an insect-pollinated annual plant in which gender inheritance appears to be nuclear. Twenty-six P. linearis populations surveyed in northern Utah, USA, contain a majority of perfect-flowered hermaphrodites, but most (22) also contain male-sterile individuals (females), at frequencies of up to 0.16. The hermaphrodite selfing rate is low (0.00–0.20 in four populations). Maternal gender does not consistently affect components of offspring vigor, such as seed size, germination rate, seedling survivorship, and vegetative size. Plants of the two genders do not differ in number of seeds per fruit or mean seed mass. Females produce significantly more fruits and seeds than hermaphrodites in natural populations. The ratio of the mean lifetime seed production of females to the mean lifetime seed production of hermaphrodites ranged from 1.31 to 2.52 in six natural populations. Females have greater shoot biomass than hermaphrodites and produce more seeds at any given shoot biomass than hermaphrodites, suggesting that their seed-production advantage arises from gender-specific patterns of resource allocation to growth and reproduction. The gender difference in plant size varies across environments and across genetic backgrounds. In this species nuclear gynodioecy appears to be evolutionarily stable mainly because of resource compensation by females, without a large outcrossing advantage of females.     

Population dynamics of the shrub Acacia suaveolens (Sm.) Willd.: Survivorship throughout the life cycle,a synthesis

Survivorship in Acacia suaveolens was assessed through seedling and adult stages. Moisture stress was found to be the critical factor limiting early seedling survival. Both seedling and adult populations were characterized by periods of low mortality interspersed with pulses of high mortality. A composite survivorship curve for A. suaveolens based on nine sites predicts that some 20–25 years after afire, established plants should disappear from the above-ground flora if another fire does not occur. Fecundity and survivorship data were used to estimate the flux of seed in the soil over time in a hypothetical A. suaveolens population. From this it was predicted that, following establishment of plants after a fire, the seed-bank would rapidly reach a maximum after 6 years and thereafter slowly decline, until after 60 years there would be only as many seeds as there were original parental plants. The situation would vary with predispersal seed predation, seed predation on the soil surface, seed dispersal by ants to ‘unsafe sites’ and the size of the initial seed-bank prior to establishment. Only after a very long inter-fire period would A. suaveolens be eliminated from a site. Elimination of the species is also possible under very frequent fires. A 2–5 year fire-free period is needed for plants to reach maturity and another 6 years are needed to maximize seed input into the soil seed-bank. In addition, seedling recruitment following cool burns is low to non-existent as dormancy is not broken for most seeds in the soil during such burns.     

Optimum reproduction and dispersal strategies of a clonal plant in a metapopulation: a simulation study with Hieracium pilosella

Clonal spread is favoured in many plants at the expense of seed production in order to expand rapidly into open habitats or to occupy space by forming dense patches. However, for the dynamics of a population in a patchy landscape seed dispersal remains important even for clonal plants. We used a spatially explicit individual-based metapopulation model to examine the consequences of two trade-offs in Hieracium pilosella L: first, between vegetative and sexual reproduction, and second, between short and far-distance dispersal of seeds. Our main question was, what are the environmental conditions that cause a mixed strategy of vegetative and sexual reproduction to be optimal. The model was parameterised with field data on local population dynamics of H. pilosella. Patch dynamics were given firstly by disturbance events that opened patches in a matrix of a clonal grass that were colonisable for H. pilosella, and secondly by the gradual disappearance of H. pilosella patches due to the expanding grass. Simulations revealed opposing selection pressures on traits determined by the two trade-offs. Vegetative reproduction is favoured by local dynamics, i.e. the need for maintenance and expansion of established populations, whereas seed production is favoured by the necessity to colonise empty habitats. Similar pressures act on the proportion of seeds dispersed over short and far distances. Optimum reproductive and dispersal strategies depended on habitat quality (determined by seedling establishment probability), the fraction of dispersed seeds, and the fraction of seeds lost on unsuitable ground. Under habitat conditions supporting moderate to low seedling establishment, between 20% and 40% of reproductive effort in H. pilosella should be devoted to sexual reproduction with at least 10% of the seeds dispersed over distances suitable to attain empty patches. We conclude that in a spatially heterogeneous landscape sexual seed production in a clonal plant is advantageous even at the expense of local vegetative growth.     

Introduced populations of <Emphasis Type="Italic">Genista monspessulana</Emphasis> (French broom) are more dense and produce a greater seed rain in California,USA, than native populations in the Mediterranean Basin of Europe

Some invasive plants perform better in their area of introduction than in their native region, and this is often attributed either to phenotypic responses and/or to adaptive evolution following exposure to new environmental conditions. Genista monspessulana (French broom) is native to Europe, but highly invasive and abundant along the Pacific Coast of the USA. In this study, the population density and age structure, plant growth and reproductive traits, and seed bank characteristics of 13 native (Mediterranean Basin) and 15 introduced (California, USA) field populations of G. monspessulana were compared. Mean population density, plant height and stem diameter were greater in introduced populations, with the latter two traits explained by a greater mean plant age. Age structure also showed a greater percentage of seedling plants in introduced populations. Fecundity was higher in introduced populations when measured in terms of mature seeds per pod, but lower when comparing seed production per plant (number of pods and mature seeds). Thus, seed rain and seed bank size was considerably higher in introduced populations. Results from this study indicate that G. monspessulana performs better in its introduced region. We hypothesize that release from natural enemies and competitors together with more favorable environmental conditions in the introduced region may explain the invasion success of G. monspessulana. As a result, an integrated management approach using introduced seed predators to suppress seed production and selected management practices to reduce seed banks may be needed for effective long-term control in California.     

How detrimental are seed galls to their hosts? Plant performance,germination, developmental instability and tolerance to herbivory in Inga laurina,a leguminous tree

• Gall inducers use these structures as shelters and sources of nutrition. Consequently, they cause multiple physiological changes in host plants.
• We studied the impact caused by seed coat galls of a braconid wasp on the performance of fruits, seeds and seedlings of tree Inga laurina. We tested whether these seed galls are ‘nutrient sinks’ with respect to the fruit/seed of host plant, and so constrain the reproductive ability and reduce seedling longevity. We measured the influence of such galls on the secondary compounds, fruit and seed parameters, seed viability and germination and seedling performance.
• Inga laurina has indehiscent legumes with polyembryonic seeds surrounded by a fleshy sarcotesta rich in sugars. The galls formed inside the seed coat and galled tissues presented higher phenol concentrations, around 7‐fold that of ungalled tissues. Galls caused a significant reduction in parameters such as fruit and seed size, seed weight and the number of embryos. Fluctuating asymmetry (a stress indicator) was 31% higher in leaves of galled seed plants in comparison to ungalled seed plants. However, the negative effects on fruit and seed parameters were not sufficient to reduce seed germination (except the synchronization index) or seedling performance (except leaf area and chlorophyll content).
• We attributed these results to the ability of I. laurina to tolerate gall attack on seeds without a marked influence on seedling performance. Moreover, because of the intensity of seed galling on host plant, we suggest that polyembryony may play a role in I. laurina reproduction increasing tolerance to seed damage.

     

GENDER VARIATION AND SEXUAL DIFFERENCES IN REPRODUCTIVE CHARACTERS AND SEED PRODUCTION IN GYNODIOECIOUS GERANIUM MACULATUM

Variation in sex expression, flowering pattern, and seed production was studied in the self-compatible perennial herb Geranium maculatum in Illinois and Indiana. In a survey of eight populations, female (male-sterile) plants were found in seven (frequencies ranging from 0.5% to 24.3% [median 4.2%]), and intermediate plants (with partly reduced male function) were found in all populations. Gender variation and sexual differences in reproductive characters were studied in detail in two populations. One population consisted of 5% female, 27% intermediate, and 68% hermaphrodite plants; the other consisted of 1% female, 20% intermediate, and 79% hermaphrodite plants. Females produced smaller flowers and began flowering earlier than hermaphrodites. Intermediates produced flowers of an intermediate size and began flowering as early as females. Females and hermaphrodites did not differ in flower number, vegetative size, flowering frequency, survival, or seed size. However, females produced 1.6 times more seeds than hermaphrodites. Intermediates produced 1.3–1.6 times more seeds than hermaphrodites. Some between-year variation in sex expression was observed. Hand-pollination with outcross pollen produced two to four times as many seeds as hand-pollination with self-pollen. A lower outcrossing rate in hermaphrodites than in females may at least partly explain the lower seed set in hermaphrodites. The higher seed production of females, and possibly the high fecundity of the intermediates, should contribute to the maintenance of this sexual polymorphism.     

ENHANCEMENT OF INBREEDING DEPRESSION BY DOMINANCE AND SUPPRESSION IN IMPATIENS CAPENSIS

We investigated the effect of intraspecific competition on the magnitude of inbreeding depression in Impatiens capensis by planting seeds from chasmogamous (CH) and cleistogamous (CL) flowers in three experimental greenhouse treatments: in individual pots, in flats in dense pure stands according to seed type, and in flats with the two seed types intermixed in a checkerboard array. The size distributions of plants grown in flats were significantly more hierarchical than those of plants grown individually, indicating that larger plants competitively suppressed smaller plants in the high-density treatments. The magnitude of inbreeding depression at high density depended upon the planting arrangement of CL and CH seeds. CH advantage was greatest when CH and CL seedlings were grown in competition with one another, suggesting that fitness differences between outcrossed and inbred individuals were intensified by dominance and suppression. For plants grown individually, the effects of maternal parent, seed weight, and emergence date on seedling size disappeared with plant age, whereas at high density these effects remained at the final harvest. Thus, plant density may influence patterns of natural selection both on mating system and on juvenile traits in natural Impatiens populations.     

EFFECTS OF POSTDISPERSAL SEED PREDATION ON SPATIAL INEQUALITY AND SIZE VARIABILITY IN AN ANNUAL PLANT,ERODIUM CICUTARIUM (GERANIACEAE)

By decreasing seed density, ants introduced into flats of uniformly sown seeds of Erodium cicutarium (Geraniaceae) created differences in the neighbor-free area available to individual plants. The changes in spatial patterns brought about by the ants were greater when a higher proportion of seeds was removed but were independent of initial seed density. These spatial changes and differences in seed density were examined for their effects on plant size and reproduction. Gini values were calculated to determine inequalities. As the inequality in space among individual plants increased, the variation in final biomass increased. The number of individuals reproducing was constant among treatments, and yet seed production per plant was significantly greater for populations in which the spatial pattern was influenced by seed predation. The decrease in density and changed spatial pattern, due to previous seed predation, resulted in a few individuals having much more space than others and consequently producing many more seeds. The increase in reproductive effort per flat was much greater than could be explained by the changing density alone. Our experiment demonstrates that spatial inequality, such as that generated by seed predators, can be more important than density in generating size inequalities in plant populations. This result can profoundly alter the competitive interactions between plants and determine which plants produce seed for the next generation.     

Consequences of tropical dry forest conversion on diaspore fate of Enterolobium contortisiliquum (Fabaceae)

Habitat conversion is one of the major threats for biodiversity conservation and viability of natural populations. Thus, habitat disturbance alters distinct ecological processes, such as plant reproductive success and diaspore fate. In this study, we determined the effects of seasonally tropical dry forests (STDFs) conversion by anthropogenic disturbance by assessing diaspore fate of Enterolobium contortisiliquum. We compared 20 adult trees present in a STDFs preserved area and 20 adult trees present in a human-converted area. In general, diaspore fates from both areas were similar, i.e., there was no difference in the reproductive success of trees in STDFs and human-converted area. Habitat disturbance did not affect the length or width of fruits; only fruit thickness was larger in trees of STDFs habitat. None of the biometric seed measures differed between different habitat conditions. Likewise, the number of undamaged seeds, aborted seeds, pre-dispersal predated seeds, and seed production were independent of habitat conditions. Besides, we did not observe any effect of habitat disturbance on germination percentage. However, seeds from preserved STDFs germinated faster than seeds from the human-converted area. Even though the effects of human-modified habitats on the diaspore fate have already been studied, tree species exhibit different responses to habitat conversion regarding seed predation, seed dispersal, seed germination, and seedling establishment. Overall, our results show that habitat disturbance does not affect the diaspore fate of E. contortisiliquum. This study also highlights the importance of remnants trees in converted landscapes as the population’s connectors which maintain plant–animal mutualistic and antagonistic interactions that mitigate the effects of habitat disturbance.

     

Experimental demography of the rare Gentianella germanica: seed bank formation and microsite effects on seedling establishment

The persistence of populations of short-lived species requires regular reproduction and seedling establishment. A persistent seed bank can buffer populations against extinction in unfavourable years. We experimentally investigated seed fate in Gentianella germanica, an endangered biennial species characteristic for species-rich nutrient-poor calcareous grasslands in central Europe. We studied the effect of experimental gaps on seedling establishment from sown seeds and the fate of seeds buried in bags over two years. In December 1993 experiments were established at seven calcareous grassland sites in the Swiss Jura mountains. In spring 1994 seedlings emerged in all plots where seeds had been sown, including previously unoccupied patches. This suggests that limited dispersal within sites contributes to small population sizes. Significantly more seedlings emerged at sites with current populations of G. germanica than at unoccupied sites (5.95% vs 3.40%). Because this difference was not explained by germinations from the natural seed bank it indicates differences in habitat quality. Clipping of the vegetation and disturbance of the soil reduced vegetation cover in the following spring and enhanced seedling emergence. In undisturbed plots 4.5% of seeds sown produced a seedling in spring 1994, whereas in plots with clipped vegetation 9.9% and in disturbed plots 12.7% produced seedlings (p>0.01). This suggests that management measures which create gaps in the vegetation (e.g. grazing) could positively influence population size and persistence of G. geymanica. On average, we recovered 7.55% viable seeds after one year of burial in bags, and 4.05% after two years, indicating that G. geymanica has a persistent seed bank. The demographic data indicate that the number of viable seeds in the seed bank exceeds the number of established plants in a population at least by a factor of 20. Restoration of extinct populations of the species from the seed bank may thus be possible if appropriate management measures are taken within a few years.     

Seed predation,pathogen infection and life-history traits in Brassica rapa

Herbivory and disease can shape the evolution of plant populations, but their joint effects are rarely investigated. Families of plants of Brassica rapa (Brassicaceae) were grown from seeds collected in two naturalized populations in an experimental garden. We examined leaf infection by the fungus Alternaria, seed predation by a gall midge (Cecidomyiidae) and plant life-history traits. Plants from one population had heavier seeds, were more likely to flower, had less fungal infection, had more seed predation and were more fecund. Fungal infection score and seed predation rate increased with plant size, but large plants still had the greatest number of undamaged fruits. Spatial heterogeneity in the experimental garden was significant; seed predation rate and fecundity varied among blocks. An apparent tradeoff existed between susceptibility to disease and seed predation: plants with the highest fungal infection score had the lowest seed predation rate. Alternaria infection varied between populations, but the disease had no effect on fecundity. Seed predation did reduce fecundity. Damaged fruits had 31.4% fewer intact seeds. However, evidence for additive genetic variation in resistance to seed predation was weak. Therefore, neither disease nor seed predation was likely to be a strong agent of genetically based fecundity selection.     

Shrub emergence and seedling growth in microenvironments created by Prosopis glandulosa

Abstract. Facilitation of the establishment of certain plant species by nurse plants is a common phenomenon in arid and semiarid ecosystems. The most commonly reported mechanisms of facilitation include cooler temperatures and increased soil nutrients beneath the nurse plant canopy, which favor establishment of other plant species. During conversion of upland grasslands to thorn woodland in southern Texas, Prosopis glandulosa appears to facilitate establishment of other woody plants, including Celtis pallida, whereas Acacia smallii occurs only in habitats between P. glandulosa canopies. We tested the hypothesis that light intensity and soils under P. glandulosa canopies facilitate seedling emergence and growth of C. pallida but inhibit seedling emergence and growth of A. smallii. In the field, C. pallida and A. smallii seeds were planted under P. glandulosa canopies and in adjacent interspaces. Percent emergence of C. pallida seedlings was greater under the canopy of P. glandulosa, whereas percent emergence of A. smallii seedlings was greater in interspaces. In a greenhouse experiment, seeds of each species were planted in pots filled with soil from under P. glandulosa canopies or from adjacent interspaces. Two treatments, shade and sunlight, were imposed and plants harvested seven weeks later. Seedling mass of both species was greater in canopy soil than in interspace soil in sunlight but mass of the two species did not differ between soil sources in shade. Canopy soils contained more total and available nitrogen than interspace soils. These results suggest that light is more limiting than nutrients under shaded conditions and so neither species can take advantage of the high nutrients beneath P. glandulosa. Shade and greater soil nutrients beneath P. glandulosa do not appear to be the major factors that facilitate C. pallida or inhibit A. smallii. Aggregation of C. pallida beneath P. glandulosa canopies appears to be a complex process that involves both passive facilitation (seed dispersal by birds) and active facilitation (reduction of seed dormancy by under-canopy temperatures) operating only during the seed germination stage with successional mechanisms other than facilitation operating during later stages of shrub establishment and growth.