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  • 1
    Electronic Resource
    Electronic Resource
    Springer
    Oecologia 72 (1987), S. 589-596 
    ISSN: 1432-1939
    Keywords: Seed dormancy ; Genetic variation ; Variable selection ; Erodium
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Summary The periodic occurrence of summer/early autumn precipitation in the California annual grassland can result in the formation of early and late emerging cohorts of Erodium botrys and E. brachycarpum. The occurrence of early rainfall and the timing of such rainfall are highly variable from year to year. A series of field watering experiments in 1980–81 were used to simulate early emergence conditions that would result from significant rainfall (1 cm) occurring in mid-July, late August, and mid-September. Net reproduction was used to estimate fitness differentials between Erodium cohorts emerging in response to a watering treatment (early emerging cohorts) and Erodium cohorts emerging with the onset of winter rains in mid-October (late emerging cohorts). Survival was lower and gross reproduction was higher among early emerging cohorts than late emerging cohorts. For both species, net reproduction of the early cohort was lower than that of the late cohort under the July watering treatment and higher than that of the late cohort under the August watering treatment. Early cohorts, formed in response to rainfall in mid-September, 1982, were also compared demographically to later cohorts emerging in October. Compared to late cohorts, net reproduction, gross reproduction and survival were higher for the early cohorts. Common garden experiments indicate that differences in the duration of seed dormancy between the progenies of early and late emerging plants reflect a significant genetic component. Progency produced by early cohorts of E. brachycarpum from all three watering treatments possessed more extended seed dormancy than progeny of late cohorts. In E. botrys, progeny from early cohorts emerging in response to the July watering treatment were also more dormant than late progeny. In contrast, early cohorts of E. botrys emerging in response to the September watering treatment produced seed less dormant than seed produced by late cohorts. When combined with demographic data, indicating that fitness differentials between early and late cohorts varied with changes in the date of early emergence, genetic results suggest that year to year variation in early rainfall may act to retain genetic variation in the duration of seed dormancy.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1432-1939
    Keywords: Neighborhood competition ; Quercus douglasii ; Root morphology ; Soil water potential ; Plant water relations
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Summary We examined the competitive effects of two annual species on soil water potential and blue oak (Quercus douglasii Hook & Arn.) seedling growth and water relations. Two densities of the annual grass Bromus diandrus (Roth.) (100/dm2, 3.6/dm2) and one density of the annual forb Erodium botrys (Cav.) (3.6/dm2) comprised plant neighborhoods around the oak seedlings grown in 1 m deep boxes. Rates of soil water depletion differed among neighborhoods. Soil in the Erodium neighborhoods dried significantly more slowly than did soil in the Bromus neighborhoods at either density. Differences in the rates of soil water depletion were correlated both with the 30% lower root biomass developed by Erodium, and the lower water extraction rates of Erodium relative to Bromus roots at constant root biomass. These results suggest that the annual species are not equivalent competitors for water: fibrous grass roots had greater competitive effect than did forb tap-roots. In a control container without an annual neighborhood, soil water potentials remained high for the duration of the experiment. Oak seedling emergence and growth responses were significantly affected by annual plant density. High density of annual plants suppressed oak root growth and shoot emergence. Only 20% of the acorns planted in high density Bromus neighborhoods showed aboveground shoot growth; 56% of those planted in low density Bromus or Erodium emerged. Ninety percent emerged in the control box. Relative growth rates of oak seedling roots and shoots were directly dependent on soil water potentials. Soil water was also closely correlated with oak seedling predawn water potentials and gas conductance measurements. Higher soil water potentials greater dry weights, and longer growing seasons were found for oak seedlings in the Erodium neighborhood and the container with no annuals than in Bromus neighborhoods of either density. These results suggest that competition for soil water with introduced annual species contributes to the increased rate of blue oak seedling mortality currently observed in California woodland systems.
    Type of Medium: Electronic Resource
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