The Processes Controlling Primary Succession On An Alaskan Flood Plain

Abstract

Life history and competitive processes were more important than facilitative processes in controlling seedling establishment during primary succession on an Alaskan river flood plain. These experimental results contrast with the widely held assumption that facilitation is essential to primary succession. Low soil and plant nitrogen levels in early succession are ameliorated by stands of alder associated with nitrogen-fixing bacteria. Yet seedlings transplanted into each successional stage grew least in alder stands where available nitrogen was highest. Increases in litter depth, root competition, and shade from alder limit most natural seedling establishment to river banks where alder stands have not yet formed but where mineral soil seed beds are available for germination. Colonization of the river banks is also influenced by stochastic factors such as seed production, seed dispersal, and fluctuating river levels. Distinct growth responses of seedlings to successional changes in light, water, and nutrient regimes alter the relative dominance of species in each successional stage. Rapidly growing willow, alder, and poplar seedlings were more tolerant of flooding and silt deposition but less shade-tolerant than the slowly growing spruce. Consequently, only spruce seedlings continued to establish in later stages. Relative longevities are therefore important in explaining the succession: the short-lived willow and alder die first, followed by poplar, and finally the long-lived spruce. Life history (regeneration, growth rate, and longevity), competitive, stochastic, and perhaps facilitative processes were important in the flood plain succession. Understanding plant succession requires examination of the role of each of these processes during establishment, maturation, and senescence phases of the life cycle of each species, rather than differentiation among complex but mutually exclusive successional models.