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We studied effects of fertilization and time of cutting on the regeneration of cutleaf teasel (Dipsacus laciniatus L.) at the Mascoutin Recreation Area at Clinton Lake, Dewitt County, Illinois, for two growing seasons. D. laciniatus is an exotic European invasive species that infests natural habitats across much of the United States and southern Canada. We established fourteen 5.0-m × 5.0-m paired plots, seven fertilized and seven unfertilized. Each set of paired plots was assigned one of seven cutting treatments, including no cutting, or one of six cutting times from 3 June to 10 August 2002. We measured stem height, number of heads per individual plant, number of seeds per head, and seed viability. We found no significant difference between plant heights or number of heads per individual in fertilized versus unfertilized plots. Reproductive success significantly increased when D. laciniatus had more time to grow after cutting. If cutting occurred before flower buds opened in early July, no sig...

This research was conducted to test two predictions about niche and competitive relationships in the structure of plant communities. It has been demonstrated that early successional species of plants have broad, overlapping niche occupation on many gradients whereas later succes- sional species show more niche differentiation. We predicted that, if the differences in resource use were for contested resources in limited supply, the more niche-differentiated species should: (1) experience less biomass reduction in pairwise interspecific competition than should broadly overlap- ping species of early succession, and (2) be able to convert relatively more of the available resources into plant matter as a heterospecific assemblage than can the nonspecific stands from that assemblage. Six species from each of early (annual), mid- (early perennial), and late- (prairie) successional plant communities were grown in containers singly, in con- and heterospecific pairs, and in pure and mixed stands. Competitive interactions were assessed by comparing biomass production of species under the various treatment conditions. The late-successional community, in which we have previously shown more niche separation on several axes, showed a significantly higher mean ratio for the mass of individuals grown in heterospecific pairs divided by the mass of individuals of the same species grown in nonspecific pairs than did the early successional community. All heterospecific pairs in the prairie and early perennial communities had a winner (ratio of mass in heterospecific pair to mass in nonspecific pair > 1.0) and a loser (ratio < 1.0), while for most pairs in the annual community both competitors had ratios of < 1.0. Annuals showed the most reduction in total mass and prairie species the least when the mass of an individual grown in a heterospecific pair was compared to the mass of an individual of the same species grown by itself. Therefore, there was less competitive reduction in the more niche-differentiated community, the prairie. There was some evidence that mixtures of the more niche-differentiated late-successional species may be able to convert relatively more of the available resources into plant matter. The prairie assemblage had a slightly greater ratio of mixed stand to pure stand biomass with more variation among species than did the annual assemblage. There was also a tendency for the pairs of prairie species to have higher relative and absolute yields in mixture than in nonspecific pairs. Thus members of the late-successional community experienced less competitive reduction in the presence of heterospecific neighbors than did species of early succession. This is strong evidence that niche reduction and separation are effective in reducing competition within a community and that selection to reduce competition could have been more important in the evolution of late-successional species than for early successional species.

Weekly root extension of six successional annuals in glass-faced boxes was measured over a growing season in 42 soil resource states. Niche breadth and overlap were calculated and compared to values obtained from published root diagrams of eight tall grass prairie species. Differences in use of soil resources by the successional species were found and interpreted as primitive niche separation to avoid competition. Temporal displacement in absorption of moisture and nutrients is apparently important only to one species (Ipomoea hederacea). The six species showed marked differences in total composition of N, K, Ca, and Mg, although proportional similarities in nutrient use were high. The dominants of the successional community, Setaria faberii, Polygonum pensylvanicum and Ambrosia artemisiifolia had broader niches than the species with lower biomass and density. Two-dimensional overlap matrices in time and space showed Chenopodium album and Setaria to be highest in mean overall overlap, Polygonum and Ambrosia to be inter- mediate, and Abutilon theophrasti and Ipomoea to be lowest. The use of multidimensional resource states in the construction of an overlap matrix incorporating more than one parameter of the niche hypervolume was shown to be more useful than the multiplication of overlaps along each of the two parameters, space and time. Mean overlap in the use of underground space was considerably lower in the mature community (prairie) than in the successional community (oldfield). This corroborates the hypothesis that competition is less important in the evolution of fugitive species of the oldfield than of equilibrium species of the prairie.

Alpha-niche parameters involving pollination resources were examined for plants in communities of 3 different successional ages. Niche breadths and overlaps were compared in early successional annual, early perennial, and prairie communities. Niche axes examined were seasonal time of flowering, species of flower visitor, daily time of visit, and 2-dimensional parameter, species of visitor x daily time. Seasonal flowering was clumped in all communities. It occurred fairly synchronously in midsum- mer in the annual community and in the fall in the perennial community. Flowering in the winter annual and prairie communities occurred at 3 distinct seasonal times; spring, summer and fall. In this annual community, the different flowering times reflected varied life history strategies demonstrating adaptations mainly to the physical environment. In the prairie, the same 3 seasonal flowering aggre- gations were most likely formed through adaptation to the biotic environment. The prairie species had smaller seasonal flowering niche breadths, but showed no reduction in seasonal niche overlap. In fact, there was some evidence for convergence in flowering time of species attracting similar vector species. There was little difference in daily time of flowering for most species pairs in all communities. Only in early and midsummer was there evidence for niche separation in daily time of visitation. In early spring and fall, mean overlaps between species were > in comparable random data sets, sug- gesting convergence in daily time of insect visitation. The relative proportions of general types of visitor differed among the communities. There were tendencies for annuals to be visited more by halictid bees and syrphid flies and there were strong tendencies for prairie species to be visited more by apid and megachilid bees and by lepidopterans. Because the latter groups are considered to be more effective pollinators, significantly more of the visits could have effected cross-pollination in the prairie than in annual communities. Niche overlaps of species of visitor were significantly lower than random for all communities, indicating some niche differentiation on that axis. The mean similarities of species of visitor and the 2-dimensional visitor species x daily time for the prairie plants were significantly lower than the means for the annuals, and showed a tendency to be lower than the means of the early perennials. Niche breadths on the same 2 parameters were also lower in the prairie. Therefore, species of the late successional community, the tallgrass prairie, showed more niche specialization and separation than did species of early successional communities. We conclude that a-scale differentiation on these pollination parameters does occur between plant species, and the degree to which it occurs increases with the successional age of the community. These results support the hypothesis that selection to reduce competition has resulted in greater niche differentiation in late successional communities.

Differences in major selective forces important in early and late successional communities should influence niche breadth and degree of overlap. Early successional species may not experience consistent, strong selection against competition and can be expected to have broader niches with more overlap than later successional species. This paper presents data from 2 early successional winter annuals that show clear niche separation under conditions in which coevolution is unlikely. Lactuca scariola L. and Erigeron annuus (L.) Pers. flower in mid-summer and attract the same insect visitors. Erigeron, the more common species, begins to flower in mid-morning when insect visitors are quite active. Lactuca flowers open earlier in the morning and close just before Erigeron flowers open. The early morning hours are not optimum for Lactuca visitation, as most of the visits occur just before floral closing. Both species can selffertilize without a vector, and Lactuca was introduced from Europe. It is, therefore, not possible that niche separation inflowering time and subsequent time of visit in Erigeron and Lactuca is a result of coevolutionary niche differentiation between the two species. This separation is likely to be a preadaptation resulting from coevolution with other species.

Siblings of Abutilon theophrasti, were grown on a nutrient gradient. The plants grown at higher nutrient levels were larger and produced larger and more seeds than plants grown at lower soil nutrient concentrations. There were no differences in germinability of seeds, but the competitive abilities of resulting plants were markedly different. In two different competition experiments designed to eliminate the effects of genotype, seed size, and germination time, by using synchronously germinated seedlings derived from similar size seed from plants grown at different nutrient levels, we found that plants from seeds produced at higher nutrient levels consistently, outperformed plants from seeds produced at the lower nutrient levels. The dominance of seeds produced at higher nutrient levels may be explained by the fact that they had markedly higher concentrations of nitrogen than did seeds produced at lower soil nutrient levels. The additional advantage of increased seed quality to plants controlling more of the nutrient resource than their neighbors would be expected to accelerate their contributions to the gene pool of the population.