摘要:

The effect of seed mass on emergence, survival, and recruitment to flowering inPastinaca sativa, a species that produces progressively smaller seeds on the primary, secondary, and tertiary umbels, was analyzed in two field experiments begun in 1982 and 1983. In both experiments, overall emergence was positively related to seed mass, but the effects of seed mass on emergence in fall cohorts was relatively unimportant. Initial seed mass was positively related to overwintering survival in one experiment but not the other, possibly due to differences in winter conditions. Survival to flowering was positively related to seed mass in the 1982 experiment but negatively related to seed mass in the 1983 experiment. This difference is due to greater survival of seedlings from small seeds during short‐term droughts, a previously unrecognized advantage of small seeds. Because of differences in survival in the two experiments, recruitment (number of flowering plants/number of seeds sown) was positively related to seed mass in the 1982 experiment but not in the 1983 experiment. The effects of timing of emergence on survival generally disappeared within 12 months. Our results suggest that the relationship between initial seed mass and recruitment inPastinacarepresents a distribution of outcomes, variable from year to year. However, because short‐term droughts in Iowa are common, we hypothesize that in most years, seed mass has little impact on recruitment. Furthermore, small scale heterogeneity may often overwhelm any potential effects of seed mass and timing of emergence on recruitment.

ISSN:0002-9122    

DOI:10.1002/j.1537-2197.1992.tb14563.x

出版商:Wiley    

年代:1992

数据来源: WILEY

摘要:

Reproductive and growth parameters ofAtriplex canescenswere examined in a common garden study. Cloned ramets of male, female, and hermaphrodite individuals from two natural populations were planted in irrigated and nonirrigated portions of the garden. Sexual differences in growth and flowering schedules suggest tradeoffs in resource allocation between growth and reproduction. Males flowered with the greatest frequency and intensity while producing the smallest growth parameters. Females had the largest measures of vegetative growth, but flowered with the least frequency and intensity. Hermaphrodites appear to have a biology distinct from males and females, being more like females in growth and more like males in reproduction.

ISSN:0002-9122    

DOI:10.1002/j.1537-2197.1992.tb14564.x

出版商:Wiley    

年代:1992

数据来源: WILEY

摘要:

Sexual systems of 139 tree species from a tropical rain forest at Los Tuxtlas, Mexico were investigated to: 1) estimate the relative proportions of hermaphroditic, monoecious, and dioecious species; 2) describe flowers, fruits, and seeds in terms of size and weight; 3) describe flowering and fruiting phenology; and 4) correlate sexuality to pollination and dispersal syndromes, and the successional status occupied in the forest. Hermaphroditism occurred in 63% of the species, monoecism in 9%, and dioecy in 27%. Nondioecious species had larger flowers, but dioecious species had more seeds per fruit. The dioecious condition was associated with small flowers pollinated with unspecialized insects and fleshy fruits dispersed by animals at both species and generic levels. Reproductive traits were more correlated among nondioecious species than dioecious species. Pioneer species had more seeds per fruit, and longer flowering and fruiting periods, but persistent species produced heavier seeds and fruits. Flower and fruit morphological traits, sexual systems, and tree guilds are related in a comprehensive way, and a flow model based on data from this study is presented.

ISSN:0002-9122    

DOI:10.1002/j.1537-2197.1992.tb14565.x

出版商:Wiley    

年代:1992

数据来源: WILEY

摘要:

The abiotic and biotic factors causing mortality ofSalix lasiolepis(Salicaceae) seedlings were studied along a seasonally flowing stream in northern Arizona. Mortality of first‐year seedlings approached or equaled 100% for the four cohorts observed. Water‐addition experiments demonstrated that lack of soil surface moisture was the primary cause of mortality for seedlings through their first 2 years with>75% of seedlings dying in the first month of growth in unwatered plots. Biotic factors, including herbivory and inter‐ and intraspecific competition were evaluated, but played a minor role in the dynamics of seedling populations during this study. Shade conditions between sites were inferred to influence growth and survival, and experiments verified the effect of shade in decreasing seedling size. Smaller seedlings were shown experimentally to have a greater likelihood of dying during the winter. The probability of survival between years increased with seedling age, and seedlings that survived three growing seasons were likely to join the population of mature, reproductive plants. Studies of seedling survivorship through the establishment phase have seldom reported mortality levels as extreme as those reported here.

ISSN:0002-9122    

DOI:10.1002/j.1537-2197.1992.tb14566.x

出版商:Wiley    

年代:1992

数据来源: WILEY

摘要:

Little bluestem grassSchizachyrium scoparium([Michx.] Nash) plants were grown under field conditions for 2 years in soils fumigated with methyl bromide and chloropicrin, or in unfumigated soil, and treated with supplemental inorganic nutrients (bases calcium and magnesium) phosphorus, nitrogen, and potassium. Most differences in measured plant responses were due to interactions between fumigation and nutrient treatments. These included biomass production, root mass per unit length (μg/cm), root lengths, flowering culm production, percent colonization, colonized root length, and spore production in rhizosphere soil. Plants generally responded to mycorrhizal fungal colonization by reducing total root length and producing thicker roots. Treatment of plants with bases appeared to profoundly affect the mycorrhizal association by reducing sporulation of vesicular‐arbuscular mycorrhizal fungi and increasing colonization. When fumigated or unfumigated soils were considered separately, base‐treated plants produced more biomass than other treatments. Base‐treated plants grown on unfumigated soil had more flowering culms and longer colonized root lengths than all other plants. Percent colonization by mycorrhizal fungi and colonized root length were positively correlated with phosphorus/nitrogen ratios, but the ratio was not correlated with plant biomass production. This suggests that phosphorus is not a limiting nutrient in our soil and investment in a mycorrhizal association may not result in enhanced plant growth. The base‐nutrient effects may indicate a need to reevaluate earlier studies of macro nutrient effects that did not take into account the role played by calcium and magnesium in assessing fungus‐host plant interactions.

ISSN:0002-9122    

DOI:10.1002/j.1537-2197.1992.tb14567.x

出版商:Wiley    

年代:1992

数据来源: WILEY

摘要:

Diurnal and seasonal patterns of CO2concentration ([CO2]) in leaf gas spaces were measured to better understand the relationship of sediment‐derived CO2to photosynthesis in the emergent wetland species,Typha latifoliaL. (cattail). Leaf [CO2] was above 2,000 μl/liter at dawn on all but the first sampling date. At all sampling dates, leaf [CO2] declined to near atmospheric [CO2] at midday and rose to well above atmospheric [CO2] in the late afternoon. The maximum leaf [CO2] varied with sampling date and was over 18 times atmospheric levels (over 6,300 μl/liter) in August. Based on measurement of photon flux density and temperature, the diurnal pattern in leaf [CO2] may be generally controlled by expected photosynthetic rates. It is hypothesized that seasonal variation in leaf [CO2] may be a function of variation in microbial (soil) respiration. Using dye and slight pressurization, it was confirmed that gas spaces in rhizomes were interconnected with the gas spaces in leaves through the rhizome‐shoot transition. From anatomical measurements, it was also estimated that over 50% of total leaf volume was occupied by gas spaces and that most of the total gas‐space volume in plants was in the shoot. Photosynthetic rate in C3 plants, like cattail, can increase with increasing [CO2] under natural conditions. For this reason, cattail and other emergent wetland plants possessing continuous gas‐space pathways appear to have a significant carbon supplement as compared to other C3 plants growing in well‐aerated soils.

ISSN:0002-9122    

DOI:10.1002/j.1537-2197.1992.tb14568.x

出版商:Wiley    

年代:1992

数据来源: WILEY

摘要:

We measured outcrossing rates of several North Carolina populations of the annual weedDatura stramoniumincluding both natural populations and experimental populations in which we manipulated plant spatial arrangement. Because capsules ofD. stramoniumtypically produce hundreds of seeds and we used an easily scored genetic marker for flower and hypocotyl color, we could measure outcrossing rates accurately for both individual plants and single flowers. The population‐wide estimates of outcrossing rates were surprisingly low for a species with showy, entomophilous flowers and ranged from 1.9% in an experimental population with a “clumped” spatial arrangement to 8.5% in an experimental population with a “dispersed” arrangement. These low values were not produced by pollinator discrimination among flower color morphs, as determined by outcrossing measurements on test plants of different colors and by direct observations of pollinator behavior. For individual plants and single flowers in the experimental populations, variation in outcrossing rates was significantly affected by such population‐wide characteristics as plant spatial arrangement and nightly fluctuations in total floral abundance. However, by far the most important factor was stigma position. Flowers with stigmas above the anthers had significantly higher outcrossing rates than did flowers with overlapping stigma and anthers. The strong effect of floral morphology suggests that the very low population‐wide levels of outcrossing inD. stramoniummay represent a persistent mixing mating system rather than a transition to complete selfing.

ISSN:0002-9122    

DOI:10.1002/j.1537-2197.1992.tb14569.x

出版商:Wiley    

年代:1992

数据来源: WILEY

摘要:

The potential influence of pollen‐tube competition on offspring “quality” has received considerable attention in recent years. Yet the prevalence of pollen competition in natural populations is largely unknown because few investigators have actually measured rates of pollen deposition on stigmas. In this study, we assess the potential for pollen‐tube competition in natural populations of the self‐compatible, pollinator‐dependent herbaceous perennial,Hibiscus moscheutos.Individual flowers averaged two to four visits per 15 min by potential pollinators (PtilothrixandBombus), and about 34% of these visits involved contact with a stigma. The median number of pollen grains deposited on virgin stigmas per contact visit was 70 grains (values ranged from 0 to 889), and flowers averaged about four contact visits per hour. Approximately 360 pollen grains must reach stigmas for full seed set to occur in a typical flower (an average ovary has 139 ovules, and 2.6 pollen grains are required per seed). Within 2 and 3 hr exposure to pollinators, 65% and 97% of the flowers received excess pollen (>360 grains) and median stigmatic pollen loads exceeded the number of ovules by a factor of 4.0 and 5.4, respectively. Based on 3 yr observations, it is concluded that pollen competition may frequently occur in this species.

ISSN:0002-9122    

DOI:10.1002/j.1537-2197.1992.tb14570.x

出版商:Wiley    

年代:1992

数据来源: WILEY

摘要:

Pollination biology at the community level was investigated using quantitative techniques, comparative methodology, measurements of the physical habitat, and consideration of floral characteristics. The frequency of insect visits to flowers was investigated in three contrasting communities: a deciduous woodland‐meadow site in eastern Massachusetts, alpine tundra in New Hampshire, and Mediterranean scrub (fynbos) in South Africa. Visits to flowers were most common in woodland‐meadow, followed by alpine tundra, and least frequent in fynbos. Bees were the most common visitor in the woodland‐meadow and the fynbos, but flies were the most common visitor in the tundra. Flower color often influenced visitation rates and had a weak but significant effect on the type of insect that visited flowers. Preferences for color by different types of insects often changed in different communities, which suggests that floral syndromes may be community‐specific. In all communities, tubular flowers were visited less often than open flowers. Less specialized insects were more common on open than tubular flowers, but large variances made few differences statistically significant. Combining measurements of temperature, light, humidity, wind speed, time of day, and season (using cluster analysis) with the shape of a flower, it was predicted that a 10‐minute observation would include at least one visit. Flower shape, temperature, light, and season appear to be the most important variables influencing insect visitation rates. Results of this study indicate that relationships between insects and flowers are nonspecific and vary among communities.

ISSN:0002-9122    

DOI:10.1002/j.1537-2197.1992.tb14571.x

出版商:Wiley    

年代:1992

数据来源: WILEY

摘要:

A detailed examination was made of the numbers of pollen grains and microgametophytes occurring on stigmas of a dioecious, wind‐pollinated species,Staberoha banksii.The number of pollen grains per pistil ranged from 0 to over 100; and the mean pollen load size per pistil per plant varied from 4.4 to 28. Distance to the fourth nearest male plant was used as a measure of local pollen availability and accounted for 63.5% of the between‐plant variation in pollen load size. In contrast, the number of microgametophytes did not show a leptokurtic decline with increasing distance from a pollen source. On average there were 5.19 microgametophytes per ovule, which is higher than all comparable data for insect‐pollinated taxa, and 66% of the stigmas had two or more microgametophytes. Given that there is a single ovule per flower, there is considerable potential for gametophytic competition. Seedset was very high, averaging 93%, and showed no significant decline in isolated female plants. We suggest that the quantity of pollen is not limiting fertilization in this population and that wind may therefore be highly effective as a pollen vector.

ISSN:0002-9122    

DOI:10.1002/j.1537-2197.1992.tb14572.x

出版商:Wiley    

年代:1992

数据来源: WILEY