Physiological Controls Over Seedling Growth in Primary Succession on an Alaskan Floodplain

Abstract

Competitive interactions were more important than facilitative processes in controlling seedling growth in primary succession on the Tanana River floodplain in interior Alaska. Low soil and plant nitrogen levels in early succession were ameliorated by stands of alder associated with nitrogen—fixing bacteria. Facilitation was demonstrated in the greehouse where alder (Alnus tenuifolia) soil enhanced growth of willow (Salix alaxensis) and poplar (Populus balsamifera) seedlings. Similarly, in the field, nutrient addition experiments and plant nutrient analyses demonstrated that all field—grown seedlings were least nutrient limited in alder stands. However, naturally occurring spruce (Picea glauca) seedlings and transplanted seedlings of alder and poplar grew least in alder stands where nitrogen levels were highest. Trenching and clear—cutting experiments demonstrated that alder stands inhibit seedling growth by root competition and shading. Physiological differences among the study species help explain their changing dominance through succession. Seedlings of species that establish in early succession (willow and poplar) grew more rapidly than alder. Late successional spruce seedlings had the slowest rates. In addition, spruce seedlings were more tolerant of the shaded conditions in alder stands than were early successional species. Consequently, only spruce seedlings continued to establish in later stages. We conclude that this floodplain succession is a result of a complex interaction of life history, facilitative, competitive, and stochastic processes. To understand succession in such sites, the relative importance of these interactive processes must be investigated.