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<br /> <br />,... w.,........:. <br />9 <br />N decreases during early secondary succession and compositional <br />changes occur, at least in part, because of differing abilities of <br />plant species to tolerate this decrease, then there should be a <br />relationship between seral position of a species and N content of <br />that species at different available soil N levels (potential <br />competitive stress levels). For example, the decrease in tissue N <br />concentration as soil N decreases should be greater for early-seral <br />species than for later-seral species. The N concentrations of the <br />five major seral species in our study showed such a relationship. <br />The N concentration of each species on the N treatment should <br />be a reasonable estimate for the species when N is not limiting <br />(near optimum conditions for soil N). As N becomes more limited <br />(nitrogen > control > sucrose), competition for this resource might <br />be expected to increase. The degree of decrease in N concentration <br />within the plant tissue of these species along this gradient is one <br />measure of the sensitivity of each species to N stress. <br />The two early-seral species displayed both the highest N <br />concentration under high soil N levels and the greatest decrease in <br />N concentration as soil N became less available (Figure 2). The <br />first response is an indication of the ability of these two species <br />to rapidly utilize this resource when availability is high, an <br />advantage to species with high potential growth rates and partially <br />explaining how these species dominate sites during the earliest <br />stages of secondary succession. However the second response <br />suggests that these species are not good competitors for the <br />resource when it becomes limited. Mid-seral herbaceous species, <br />