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  • 1
    Electronic Resource
    Electronic Resource
    Springer
    Oecologia 113 (1998), S. 447-455 
    ISSN: 1432-1939
    Keywords: Key words Resource competition ; Allometry of growth and resource uptake ; Plasticity ; Spatial patterns ; Competition in clonal plants.
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract When plants are competing, larger individuals often obtain a disproportionate share of the contested resources and suppress the growth of their smaller neighbors, a phenomenon called size-asymmetric competition. We review what is known about the mechanisms that give rise to and modify the degree of size asymmetry in competition among plants, and attempt to clarify some of the confusion in the literature on size asymmetry. We broadly distinguish between mechanisms determined primarily by characteristics of contested resource from those that are influenced by the growth and behavior of the plants themselves. To generate size asymmetric resource competition, a resource must be “pre-emptable.” Because of its directionality, light is the primary, but perhaps not the only, example of a pre-emptable resource. The available data suggest that competition for mineral nutrients is often size symmetric (i.e., contested resources are divided in proportion to competitor sizes), but the potential role of patchily and/or episodically supplied nutrients in causing size asymmetry is largely unexplored. Virtually nothing is known about the size symmetry of competition for water. Plasticity in morphology and physiology acts to reduce the degree of size asymmetry in competition. We argue that an allometric perspective on growth, allocation, resource uptake, and resource utilization can help us understand and quantify the mechanisms through which plants compete.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    Springer
    Oecologia 80 (1989), S. 349-355 
    ISSN: 1432-1939
    Keywords: Asymmetric competition ; Local interference ; Neighborhood analysis ; One-sided competition ; Plant-plant interactions
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Summary Although considerable evidence exists that plant competition is generally asymmetric or “one-sided”, with larger plants having a disproportionate competitive effect on smaller plants, currently employed measures of local interference generally assume that competition is “two-sided”. We describe a simple measure of competitive asymmetry in which the effects of neighbors smaller than a focal individual are discounted by a constant factor, and include this variable in a composite measure of local interference. In this model competition varies between complete asymmetry (the effects of smaller plants are entirely discounted) and complete symmetry (the competitive effect of a neighbor is proportional to its size). The proposed method is applied to two natural populations and one experimental monoculture. In all cases an asymmetric model provides the best fit to the data. Completely two-sided models account for 26–39% of the variance in relative growth rate, while relatively one-sided models account for 44–57%. The increases in r 2 values resulting from the inclusion of asymmetry are significant in the two cases in which the data permit randomization tests. Our results suggest that interference is completely asymmetric in a population of Impatiens pallida, a species with very low root allocation and a shallow crown, and somewhat less asymmetric in an experimental monoculture of Ambrosia artemisiifolia and a natural stand of Pinus rigida, cases in which competition for water and nutrient resources is likely to be of greater importance.
    Type of Medium: Electronic Resource
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  • 3
    ISSN: 1432-1939
    Keywords: Competition ; Demography ; Modules ; Impatiens pallida ; Size
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Summary We compared the size distributions of leaves on naturally-occurring crowded and experimentally thinned “uncrowded” individuals of Impatiens pallida in southeastern Pennsylvania. Crowding decreased the number of leaves on individual plants and altered the distribution of leaf size. Crowded individuals had smaller leaves, but the size (length) inequality of the leaf population did not change. The relationships between the height of a plant and the mean and maximum length of its leaves were significantly different for crowded and uncrowded plants. There were weak positive relationships between height and total leaf area, and height and total number of leaves for uncrowded plants, whereas crowded plants showed tighter but curvilinear relationships between these variables. Our results point out the strengths and the limitations of viewing canopies as populations of modules.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    Springer
    Oecologia 62 (1984), S. 57-60 
    ISSN: 1432-1939
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Summary We looked at the relationship between “available area”, as defined by Thiessen polygons around individual plants, and plant size and mortality in even-aged green-house populations of Lapsana communis L. Polygon area was a good predictor of plant weight in these populations. After nine weeks growth, just prior to the onset of self-thinning, the dry weight of plants was directly proportional to the square root of polygon area. After the onset of selfthinning, plant weight appeared to be directly related to polygon area to the 3/2 power. Plants in small polygons were much more likely to die than those in larger areas. Thinning changed the frequency distribution of polygon sizes from highly skewed and unequal to normal and more equal, while inequality in surviving plant sizes did not appear to be affected by thinning.
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    Springer
    Oecologia 61 (1984), S. 334-336 
    ISSN: 1432-1939
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Summary The term “size hierarchy” has been used frequently by plant population biologists but it has not been defined. Positive skewness of the size distribution, which has been used to evaluate size hierarchies, is inappropriate. We suggest that size hierarchy is equivalent to size inequality. Methods developed by economists to evaluate inequalities in wealth and income, the Lorenz curve and Gini Coefficient, provide a useful quantification of inequality and allow us to compare populations. A measure of inequality such as the Gini Coefficient will usually be more appropriate than a measure of skewness for addressing questions concerning plant population structure.
    Type of Medium: Electronic Resource
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  • 6
    ISSN: 1573-5052
    Keywords: Ericaceae ; Interspecific competition ; Mediterranean shrubland ; Neighborhood models ; Removal experiment ; Resprouting
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract To study the effects of competition in Mediterranean shrubland regeneration following disturbance, we used a neighborhood approach to assess the influence of mature Rosmarinus officinalis neighbors on the resprouting of Erica multiflora individuals after clipping. Sprout biomass of target plants 2 years after clipping was regressed against various measures of neighbor abundance within a 2 m radius around target E. multiflora individuals in which all vegetation except R. officinalis had been removed. The largest single influence on the biomass of sprouts produced was the previous biomass of the resprouting plant. The abundance of R. officinalis neighbors had a weak but detectable effect on resprouting of E. multiflora. Abundance of neighbors within 60 cm from target plants was the best predictor of regrowth. At this distance, two simple measures of neighbor abundance within the neighborhood, the number of neighbors and the sum of their heights, were significant in accounting for variation in resprouted biomass. None of the combinations of neighbor variables performed significantly better than single variables. The best models accounted for around 24 percent of the variation in resprout biomass. As in other studies, angular dispersion of neighbors never had a significant effect on performance of target plants. The weak but significant response of resprouting to variation in R. officinalis abundance suggests that the intensity of competition in the experiment was low because of the removal of other species.
    Type of Medium: Electronic Resource
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