SPECIES REPLACEMENT DURING EARLY SECONDARY SUCCESSION - THE ABRUPT DECLINE OF A WINTER ANNUAL

The factors that contribute to species establishment and decline deter mine the rate and pattern of successional change. We tested a commonly held assumption that competitive displacement is responsible for the loss of species during succession. Manipulative field experiments were used to examine the effects of interspecific competition on the popul ation dynamics of Senecio sylvaticus, a winter annual that briefly dom inates postharvest sites in the western Cascade Range of Oregon. Senec io increased in density 400-fold from the first to the second growing season after disturbance but decreased precipitously in year 3 to 10% of the density and 0.5% of the biomass per plot of the previous year. Although interspecific competition reduced the cover and biomass of Se necio during its peak year, it had little or no effect on either the p opulation increase or decline; the pattern of change was similar among all treatments. These counterintuitive results underscore the importa nce of testing, not simply assuming, that interspecific competition is responsible for the replacement of a species during succession.