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

Seedlings of Alaskan floodplain species (Populus balsamifera L. (balsam poplar), Alnus tenuifolia Nutt. (thinleaf alder), and Picea glauca (Moench) Voss (white spruce)) and an upland species (Populus tremuloides Michx. (trembling aspen)) were grown in early-successional floodplain soils treated with a floodplain salt (calcium sulfate, CaSO4), an osmoticant (polyethylene glycol), and nitrogen. CaSO4 reduced the growth of aspen relative to controls but also reduced the growth of some typical floodplain colonizers (alder at low nitrogen and poplar at high nitrogen). Aspen and poplar were the most rapidly growing species, even when grown with salt or polyethylene glycol. Effects of CaSO4 on growth, therefore, do not explain why aspen is less abundant on the floodplain than are typical floodplain colonizers. CaSO4 reduced growth directly in salt-sensitive species, judging from the insensitivity of water potential, transpiration, and photosynthesis to CaSO4 addition. Tissue concentrations of nitrogen and phosphorus were unaffected by CaSO4, suggesting that the declines in nutrient accumulation by salt-sensitive species in response to CaSO4 addition reflected a decline in nutrient demands for growth rather than being the cause of the reduction in growth. Growth and nutrient accumulation were stimulated by nitrogen addition in all species. We suggest that floodplain salts may be important in succession by slowing the establishment and growth of alder, which is responsible for most of the nitrogen acquired by plants during succession.