Showing papers on "Seed dispersal published in 1976"

Causes of Juniper Invasion in Southwestern Idaho

Abstract: Invasion of western juniper into vegetation dominated by mountain big sage- brush and perennial bunchgrass on the Owyhee Plateau of southwest Idaho appears to be directly related to cessation of periodic fires Evidence from adjacent climax juniper stands indicates that fires were frequent for at least several hundred years preceding white settlement Fires have been much less frequent during the past century due to active fire control, develop- ment of roads and other fire barriers, and reduced fuel because of heavy grazing and a shift towards decreased precipitation Physical and biotic factors affecting the establishment of juniper, seed dispersal mechanisms, and the fire history of the study area were investigated Results indicated that range condition as such had a negligible effect on juniper establishment Juniper seedlings became established most readily on areas supporting well-developed herba- ceous and shrubby vegetation Seed dispersal was primarily localized, and accomplished by gravity and disturbance by animal trampling Abundant evidence of fire in the form of charred stumps and fire scars on living trees was found throughout the study area Old juniper stands are confined to rocky ridges where understory vegetation is sparse and fires less intense Juniper was apparently kept out of the denser vegetation of deeper soils by more intense fires Most herbaceous and shrubby species survived this treatment due to greater tolerance to fire, or rapid reproduction from seed

Dispersal ecology of carex pedunculata (cyperaceae), a new north american myrmecochore

Abstract: Carex pedunculata is the first North American species of the Cyperaceae that is identified as a myrmecochore. Many morphological and phenological features of this species and its breeding system are interpreted as adaptive for seed dispersal by ants. In laboratory tests, workers of the ant species Aphaenogaster rudis carry the diaspores to the nest, eat the elaiosomes, carry larvae to the elaiosomes to feed, and deposit diaspores whose elaiosomes have been eaten with other nest debris. The achenes then germinate. Achenes will also germinate without any handling by ants. Workers will also transport diaspores with uneaten elaiosomes when the nest is disturbed. Greenhouse tests show that seedling growth is greatly inhibited if a diaspore remains near the parent plant and cohort seedlings. Field studies of natural populations identify rotting logs (the location of ant nests) as forest floor microsites for colonization of C. pedunculata and other myrmecochores. Ant nesting behavior may pattern much of the herb stratum. This species is self-compatible, and single seeds may start successful new populations. Three processes contribute to population growth: vegetative growth, germination of untransported diaspores, and germination of ant-transported diaspores. THE GENUS CAREX has had a remarkable evolutionary radiation, with hundreds of species recognized in America (Mackenzie, 1935; Hermann, 1974) and Eurasia (Kuikenthal, 1909). Despite this, little is known about the distribution or ecology of most species, perhaps because the superficial vegetative similarities of these sedges make identification difficult. As part of a study of the comparative popula

Natural food requirements of the large milkweed bug, Oncopeltus fasciatus (Hemiptera: Lygaeidae), and their relation to gregariousness and host plant morphology

Abstract: The life ofOncopeltus fasciatus centers on the seeds of milkweeds (Asclepiadaceae,Asclepias syriaca in this study). Adults reproduce prolifically on these seeds, but they engage in only half as much copulation and lay only a few eggs when fed milkweed buds and flowers instead. They can not maintain body weight on a diet of just vegetative plants. Vegetative shoots support only slow growth and produce adults only in certain circumstances. Seeds are often inaccessible to nymphs in the three youngest instars, since their mouthparts are too short to penetrate the thick walls ofA. syriaca pods. Nymphs feeding from the outside of closed pods develop more slowly than those feeding on exposed seeds. Since mortality occurs at a constant rate, slow growth results in fewer surviving. Even adults, which can feed through almost any pod wall, prefer to feed where it is thinnest. Thus the pod wall effectively protects many seeds from this herbivore. Nymphs in larger groups (20 individuals) suffer much lower mortality than those in small groups (5 individuals), when they are feeding from the outside of closed pods. However, group size does not affect survival when nymphs are fed seed. Gregariousness apparently partly compensates for the less nutritious diet attainable from the outside of pods and thus is related to this bug's extreme specialization of diet. Other characteristics coordinated with the seed requirement include the timing of migration and egg laying, the female's choice of oviposition site, and the nymphs' balanced tendencies to be sedentary and to disperse. Among the milkweeds, such traits as thick pod walls, wide spacing, and rapid seed dispersal could have been selected for by a seed predator such asO. fasciatus. As a specialist and a probable agent of such selection,O. fasciatus is committed to keeping up with changes in its host plant. This requires maintaining some flexibility in its behavior or gene pool. K. Evans (personal communication) finds thatO. fasciatus in California onA. fascicularis lays eggs so early that the first nymphs hatch as the first flowers are opening. In that more equitable climate and on that host, adult reproduction is apparently not as dependent on pods being present in the colonized patch, and the nymphs must rely on non-seed food more often than they do in the association withA. syriaca in the East and Midwest. This very specialized species apparently retains enough adaptability to exploit milkweeds of various forms and phenologies living in a wide range of climates. This adaptability could promise its continued success in the coevolutionary race with its host plants.

White Spruce Seed Dispersal in Central British Columbia

Abstract: Seed dispersal of white spruce (Picea glauca) (Moench) Voss) from stands bordering a large clearcut and a strip cut was studied. Seedfall fell sharply with distance from the clearcut edge to 100 m, but even at 300 m the average dispersed seed density exceeded 740 000 seeds/ha or 3% of that recorded within the stand. Dispersed seed density in the middle of the 200-metre-wide strip cut exceeded 1 300 000 seeds/ha or 20% of that recorded within the bordering stands. About one-third of the seeds was disseminated by the end of September.

The viable seed contents of ant‐hill and pasture soil *

Abstract: SUMMARY The viable seed contents of ant-hill and pasture soils were estimated in samples from acidic grassland on Rhossili Down, Gower peninsula, South Wales. The pasture soils contained 9150 seeds per m2 and the same mass of ant-hill soil produced 6121 seedlings per m2, including 4030 of Aira praecox L. Twelve species had at least four times as many seeds in pasture soil than in ant-hill soil. Most of these species are more abundant in the pasture than on the ant-hills, perhaps partly through inefficient seed dispersal.

Consequences of limited seed dispersal within simulated annual populations

Abstract: Theoretical analyses of the consequences of dispersal have been almost entirely limited to studies of colonization. A simulation model is constructed and analyzed to study the effects of dispersal within local plant populations. Rules for the growth and reproduction of individuals are deterministic and related only to competition with other plants. Rules for the dispersal of individual seeds are stochastic; the generalized seed shadow is negatively exponential. The analysis has several interesting results. 1) In small, dense populations of annuals, mortality and growth behavior are significantly affected by small-scale changes in dispersal. 2) Stable population size, at any life stage, is not environmentally fixed (e.g. by the number of safe sites) but is also a function of reproductive capacity and dispersal. 3) There is a minimum productive capability which must be exceeded for a population to be able to respond to increased dispersal. The responsive range of the parameters may be related to the grain of safe site distribution. 4) When the total number of seeds per generation is large, high dispersal decreases the number of germlings that are able to establish. 5) For plants that reach the growing stage, however, expected reproduction can exceed that of genotypes with greater reproductive capacity but less dispersal. 6) An uneven distribution of seeds is essential to a spatial patterning of mortality which can deter competitive extinction.

Dispersal and survival of seed of eastern dwarf mistletoe

Abstract: Seed dispersal of eastern dwarf mistletoe (Arceuthobiumpusillum) parasitizing black spruce (Piceamariana) in the Fond du Lac State Forest, Carlton Co., Minnesota, U.S.A., began in the 1st week of September and continued until the first frost. Most seeds were discharged during the first 3 h after sunrise. Seed expulsion was a function of rising temperature during that time. The mean horizontal distance travelled by discharged seeds was 1.65 m. One seed travelled 13.7 m. Freezing temperatures on August 31, 1970, caused nearly complete fruit abortion in that year of the study and caused a substantial reduction in seed discharge in the other 2 years. Of 3200 seeds placed on the needles of healthy black spruce in September 1971, only 6.5% of the seeds had germinated by August 1972.

Maturation of Noble Fir (Abies procera) and Grand Fir (Abies grandis) Seed

Abstract: approved: The objectives of this investigation were two-fold: (1) to examine possible seed and cone characteristics and their usefulness as seed maturity indices, and (2) to examine the effects of artificial ripening on seed maturation and germination. Cone fresh weight, cone length, cone specific gravity, cone color, seed fresh weight, seed development, seed wing color and embryo/embryo cavity ratio were studied to determine their usefulness as maturity indices. The artificial ripening treatments incLuded: (1) detached cones stored outdoors in the shade, and (2) detached cones stored outdoors in the shade with their basal end setting in moist perlite. With maturity, grand fir seed germination increased linearly without any leveling off prior to seed dispersaL Due to the erratic nature of noble fir seed germination no definitive pattern was observed. Artificial ripening techniques were beneficial for both species, increasing germination above that of the control and also AN ABSTRACT OF THE THESIS OF JEFFREY L. SNYDER for the degree of MASTER OF SCIENCE Title: MATURATION OF NOBLE FIR (Abies procera) AND GRAND FIR (Abies grandis) SEED presented on December 17, 1976 increasing the rate of germination. Seedling vigor, as measured by oven dry weight, was also improved by artificially ripening the immature cones. Dry outdoor storage of cones produced more total sound seed that moist otitdoor storage. The ratio of embryo length to the length of the embryo cavity was the single most useful index of seed maturity found. Seed wing color and degree of seed wing attachment also appear to be useful in assessing seed maturity. Cone color and cone specific gravity were beneficial cone characteristics found for determining seed maturity. The most marked changes in bioclemical constituents of ripening seed were the steady accumulation of reserves, i.e crude fat and protein nitrogen, and structural components. Soluble sugar, amino acid and starch contents decreased with maturity, while crude fat and nitrogenous compounds increased. During artificial ripening an I even more significant increase in crude fat and nitrogenous compounds occurred, thus reflecting a, cOnverson from mobile to storage forms. The decrease in organic materials of cone scales also was associated with mobilization of these materials by maturing seeds. An even more significant decrease in these materials occurred after the cones had been picked and stored outdoors, suggesting translocation of substances from cone scales to ripening seeds.

The Angiosperm Flower And Related Structures

Abstract: One of the main factors in the rapid rise and evolutionary diversification of the angiosperms* during the Cretaceous era was the development of the flower into a highly efficient structure for cross-pollination and seed dispersal. These features permitted a far greater amount of genetic recombination and rapid colonisation of habitats which were comparatively unsuitable for their ancestors.