Effects of competition mode on spatial pattern dynamics in plant communities

Abstract The effects of the mode of competition between individual plants (symmetric versus asymmetric) and the gap formation caused by natural disturbances on the dynamics of spatial configuration pattern of individual size were investigated theoretically based on an individual growth model incorporating competitive effects of neighbouring individuals. The degree of spatial heterogeneity in local size distribution was represented by the CV (coefficient of variation) of averages of local size distributions (CVav), the average of CVs of local size distributions (AVcv) and the CV of CVs of local size distributions (CVcv). Without gap formation, CVav, AVcv and CVcv were larger under asymmetric competition than under symmetric one, suggesting a fine-scale mosaic spatial pattern in asymmetrically competing populations. With gap formation, these statistics under symmetric competition approached those values under asymmetric competition. The spatial configuration pattern of individual size also showed the same trend in terms of the semivariogram. The patchiness index was almost the same in both gap and non-gap cases irrespective of the mode of competition. The semivariogram and patchiness index showed the presence of more uniform and larger patches under symmetric competition than asymmetric competition. Gap formation therefore increased spatial heterogeneity in local size distribution especially under symmetric competition, but there was still a difference in spatial heterogeneity between the two modes of competition even in the gap formation case. The effects of gap formation on spatial pattern dynamics were larger under symmetric competition than under asymmetric competition; under asymmetric competition, the spatial pattern dynamics were similar in both gap and non-gap cases. Therefore, against spatial disturbances (i.e. gap formation), symmetric competition brings about a more variable system than asymmetric competition. These theoretical results can explain spatial pattern dynamics of natural forests (northern coniferous and temperate hardwood forests). In conclusion, both the disturbance regime (gap formation process) and the mode of competition between individuals should be investigated to study the spatial pattern dynamics and species diversity of plant communities. The implications of the mode of between-individual competition for conservation biology are discussed. It is suggested that symmetrically competing plant communities (e.g. northern coniferous forests) should be preserved in larger areas than asymmetrically competing ones (e.g. temperate hardwood forests) if the plant communities are subject to frequent natural disturbances.

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