THE ROLE OF BALSAM POPLAR SECONDARY CHEMICALS IN CONTROLLING SOIL NUTRIENT DYNAMICS THROUGH SUCCESSION IN THE ALASKAN TAIGA

The vegetation mosaic of the Alaskan taiga is produced by patterns of disturbance coupled to well-defined successional patterns. In primary succession on river floodplains, one of the critical transitions in su ccession is that from thinleaf alder (Alnus tenuifolia) to balsam popl ar (Populus balsamifera). This is the shift from a Nz-fixing shrub to a deciduous tree. Through this transition there are major changes in N cycling including a decrease in Nz-fixation, mineralization, and nitr ification. Most models of plant effects on soil processes assume that these changes are caused by shifts in litter quality and C/N ratio. Th is paper reviews several studies examining the effects of balsam popla r secondary chemicals on soil nutrient cycling. Balsam poplar tannins inhibited both N-2-fixation in alder, and decomposition and N-minerali zation in alder soils. Other poplar compounds, including low-molecular -weight phenolics, were microbial substrates and increased microbial g rowth and immobilization, thereby reducing net soil N availability. Th us, substantial changes in soil N cycling through succession appear to have been mediated by balsam poplar secondary chemicals.