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Relation of the reproductive biology of plants to the structure and function of four plant communities Pojar, Jim


Four plant communities of southwestern British Columbia were studied in an attempt to answer the following related questions: (1) do communities of harsh physical environments exhibit any characteristic phytosociological features? (2) are there any correlations between environmental harshness and certain synecological properties of such communities ? (3) are species of such communities selected for reproductive specializations that tend to reduce their genetic variability? The four communities (representing three types of herbaceous or semi-shrubby vegetation) were a salt marsh, two coastal sphagnum bogs, and a subalpine meadow. In answer to the first two questions, the findings of this investigation indicate that: (a) species population structure becomes more aggregated as environmental heterogeneity and physical stress increase, and less aggregated as succession proceeds and interspecific competition increases. (b) interspecific association and correlation, both negative and positive, increase as environmental heterogeneity and competition increase. (c) levels of polyploidy within communities appear to be correlated with environmental rigor (broadly defined). (d) the most abundant species within a community are the most variable and presumably have the largest niches ; niche size and population variability decrease as interspecific competition increases. (e) within a community, ecological distinctiveness reduces interspecific competition; communities under the least stress (especially seasonal stress) have the most ecologically dissimilar1 species. (f) dominance decreases as species diversity increases, and species diversity is roughly correlated with overall environmental severity. In answer to the last question: (a) all four communities are dominated by predominantly outcrossing species; there is no major shift to self-pollination or apomixis in any of the communities. (b) an index of potential recombination was devised, embodying a number of aspects of reproductive biology, according to which there is no significant difference in potential recombination, on the average, between species of the four different communities. Plant communities and their constituent species both respond to evolutionary forces, but more or less independently, at different rates, and often in different or even opposite ways. Environmental stress has a powerful effect on the structure and function of plant communities, but in an evolutionary sense there is little difference between normal (mesic, zonal) environments and extreme or azonal environments to an individual species. Different selection pressures have been operating in salt marshes, sphagnum bogs, and subalpine meadows, but the present study indicates that, on the average, the resultant evolutionary strategies of the species of these communities are equivalent.

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