Growth of balsam poplar and black cottonwood in Alaska in relation to landform and soil

While constructing site index curves for balsam poplar (Populus balsamifera L.) and western black cottonwood (Populus trichocarpa Torr. & A. Gray) for interior and southcentral Alaska, we found variations in growth patterns t hat appeared to be related to landform and soil properties. We characterize d soils for 42 of 65 site index plots in an attempt to explain site product ivity variation. We found significant negative correlations between site in dex and elevation. Region, landform, and floodplain characteristics (especi ally sediment deposition) significantly affected poplar growth rate and soi l development patterns. Nutrient availability and recycling appear to be me diated by flooding through scouring or burial of surface organic layers. So il pH patterns related to O-horizon development and salt crust formation an d dissolution described previously for the Tanana River floodplain do not h old for all floodplains in Alaska. At similar latitudes and elevations, upl and locations may have higher site indices than frequently sedimented flood plain locations because upland soil development is relatively uninterrupted . Floodplain locations experiencing little or no sediment accumulation afte r establishment of poplar stands tend to have higher site indices than thos e experiencing frequent sediment accumulation. At some floodplain locations , site index was positively correlated with rooting depth.