CHANGES IN VEGETATION AND NUTRIENT POOLS DURING RIPARIAN SUCCESSION

Changes in vegetation composition, structure, biomass, and the nutrient pools of phosphorus, nitrogen, and potassium are described for a riparian sere on the floodplain of the lower Yellowstone River, Montana. Community dominance progressed from seedlings of Great Plains cottonwood (Populus deltoides) and sandbar willow (Salix exigua), to a thicket of sandbar willow and cottonwood, to cottonwood forest, to shrubs, and finally to grassland. Sandbar willow and cottonwood were lost because they died without regeneration. Community height and structural complexity increased to a maximum in the cottonwood stages and decreased in the latter stages. Change from the cottonwood seedling to cottonwood forest and grassland stages for total above-ground biomass (1 Mg/ha to 193 Mg/ha to 2 Mg/ha, respectively), below-ground biomass (6 Mg/ha to 94 Mg/ha to 29 Mg/ha, respectively) and soil organic matter (31 Mg/ha to 216 Mg/ha to 177 Mg/ha, respectively) was similar, initially increasing then decreasing. Total ecosystem phosphorus mass increased from 7000 kg/ha to 9000 kg,/ha as the cottonwoods matured and remained relatively constant thereafter. In contrast, nitrogen and extractable potassium mass increased as the cottonwood forests matured (3000 kg/ha to 8000 kg/ha and 500 kg/ha to 3000 kg/ha for N and K, respectively) and declined slightly as the cottonwood forests died out.