Underground Niche Separation in Successional Plants

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.