Showing papers by "Sean C. Thomas published in 1989"

Including competitive asymmetry in measures of local interference in plant populations.

Abstract: 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 r2 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.

Growth, death and size distribution change in an impatiens pallida population

Abstract: SUMMARY (1) The growth and fate of individual plants in a natural self-thinning stand of Impatiens paNida located along a disturbed forest edge in southeastern Pennsylvania were followed from canopy closure to the onset of flowering (eight weeks). The primary study population consisted of 161 plants measured at the beginning and end of this period. Survivorship, lodging and herbivory of these individuals were monitored weekly, and flowering status was recorded at the end of the study. A second set of twenty representative but easily accessible plants at the same location were measured repeatedly the following year to determine the shape of the growth curve of individual plants during this same time interval. (2) Mortality in the primary population was 25%. Lodging was an important cause of death. Smaller plants were more likely to lodge and to suffer herbivore damage than were larger plants. Lodging, independent of plant size, accounted for 16% of mortality explained by a path analysis model; the effects of herbivory on mortality (independent of plant size) were not significant, however. Death caused by lodging in a natural self. . thinning population supports the hypothesis that biomechanical constraints may have substantial effects on self-thinning relationships and patterns of size structure change in herbaceous plant populations. (3) A decrease in above-ground biomass (negative growth) was found to occur in 21 % of the surviving plants in the main study population. Although models of plant competition usually assume that plants with zero or negative growth die, 88% of the surviving plants displaying negative growth flowered by the end of the study. Growth curves for the twenty repeatedly measured plants were approximately linear over the course of the study period. (4) The survival of plants with negative growth affects the function relating biomass (B) to density (D) and the function relating size inequality (G) to mean plant mass (m) over the course of self-thinning. Had these plants died during the study period, the observed log Blog D slope (- 1.73) would have been very close to the mean thinning slope observed across many species (-0.85). Because many non-growing and shrinking plants survived, size inequality did not decrease during self-thinning. We suggest that the ability of plants to survive sustained suppression may have a major and predictable effect on both the selfthinning trajectory and on changes in size variability over the course of stand development. Our results emphasize the importance of field studies aimed at understanding the behaviour of suppressed plants.