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  • Quercus douglasii  (2)
  • Competitive interactions  (1)
  • 1
    ISSN: 1432-1939
    Keywords: Ecological genetics ; Phenotypic plasticity ; Plant competition ; Quercus douglasii ; Stable carbon isotope discrimination
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract Blue oak (Quercus douglasii) is a deciduous tree species endemic to California that currently exhibits poor seedling survival to sapling age classes. We used common garden techniques to examine how genetic variation at regional and local scales affected phenotypic expression in traits affecting oak seedling growth and survival. Between-population variation was examined for seedlings grown from acorns collected from a northern, mesic population and a southern, xeric population. Within-population variation was examined by comparing seedlings from different maternal families within the mesic population. Acorns were planted into neighborhoods of an annual dicot (Erodium botrys), an annual grass (Bromus diandrus), and a perennial bunchgrass (Nassella pulchra). By varying the species composition of herbaceous neighborhoods into which acorns were planted, the interactive effects of competition and acorn germplasm source on phenotypic expression could also be examined. Potential maternal effects, expressed as variation in acorn size, were assessed by weighing each acorn before planting. Probability of seedling emergence increased significantly with acorn size in the xeric population but not in the mesic population. Similarly, the effect of acorn size on seedling leaf area, stem weight, and root weight was also population-dependent. At a within-population level, acorn size effects on seedling traits varied significantly among maternal families. In addition to acorn size effects, rates of oak seedling emergence were also dependent on an interaction of population source and competitive environment. Interactions between maternal family and competitive environment in the expression of seedling leaf characters suggest the possibility of genetic variation for plasticity in traits such as specific leaf area. Using carbon isotope discrimination (Δ) as an index of relative water-use efficiency (WUE), higher water use efficiency was indicated for oak seedlings grown in the annual plant neighborhoods compared to seedlings grown in the bunchgrass neighborhood. This trend may represent an adaptive plastic response because, compared to the bunchgrass neighborhood, soil water depletion was more rapid within annual plant neighborhoods.
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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.
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  • 3
    ISSN: 1432-1939
    Keywords: Soil nitrogen competition ; Competitive interactions ; Oak savannas ; Annual and perennial grasses ; Quercus
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Summary The spatial overlap of woody plant root systems and that of annual or perennial grasses promotes competition for soil-derived resources. In this study we examined competition for soil nitrogen between blue oak seedlings and either the annual grassBromus mollis or the perennial grassStipa pulchra under controlled outdoor conditions. Short-term nitrogen competition was quantified by injecting15N at 30 cm depth in a plane horizontal to oak seedling roots and that of their neighbors, and calculating15N uptake rates, pool sizes and15N allocation patterns 24 h after labelling. Simultaneously, integrative nitrogen competition was quantified by examining total nitrogen capture, total nitrogen pools and total nitrogen allocation.Stipa neighbors reduced inorganic soil nitrogen content to a greater extent than didBromus plants. Blue oak seedlings responded to lower soil nitrogen content by allocating lower amounts of nitrogen per unit of biomass producing higher root length densities and reducing the nitrogen content of root tissue. In addition, blue oak seedlings growing with the perennial grass exhibited greater rates of15N uptake, on a root mass basis, compensating for higher soil nitrogen competition inStipa neighborhoods. Our findings suggest that while oak seedlings have lower rates of nitrogen capture than herbaceous neighbors, oak seedlings exhibit significant changes in nitrogen allocation and nitrogen uptake rates which may offset the competitive effect annual or perennial grasses have on soil nitrogen content.
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