DIRECT AND INDIRECT EFFECTS OF CALCIUM-SULFATE AND NITROGEN ON GROWTHAND SUCCESSION OF TREES ON THE TANANA RIVER FLOODPLAIN, INTERIOR ALASKA

Seedlings of Alaskan floodplain species (Populus balsamifera L. (balsa m poplar), Alnus tenuifolia Nutt. (thinleaf alder), and Picea glauca ( Moench) Voss (white spruce)) and an upland species (Populus tremuloide s Michx. (trembling aspen)) were grown in early-successional floodplai n soils treated with a floodplain salt (calcium sulfate, CaSO4), an os moticant (polyethylene glycol), and nitrogen. CaSO4 reduced the growth of aspen relative to controls but also reduced the growth of some typ ical floodplain colonizers (alder at low nitrogen and poplar at high n itrogen). Aspen and poplar were the most rapidly growing species, even when grown with salt or polyethylene glycol. Effects of CaSO4 on grow th, therefore, do not explain why aspen is less abundant on the floodp lain than are typical floodplain colonizers. CaSO4 reduced growth dire ctly in salt-sensitive species, judging from the insensitivity of wate r potential, transpiration, and photosynthesis to CaSO4 addition. Tiss ue concentrations of nitrogen and phosphorus were unaffected by CaSO4, suggesting that the declines in nutrient accumulation by salt-sensiti ve species in response to CaSO4 addition reflected a decline in nutrie nt demands for growth rather than being the cause of the reduction in growth. Growth and nutrient accumulation were stimulated by nitrogen a ddition in all species. We suggest that floodplain salts may be import ant in succession by slowing the establishment and growth of alder, wh ich is responsible for most of the nitrogen acquired by plants during succession.