The dynamics of white spruce populations on a boreal river floodplain

Studies of forest development on river floodplains in interior Alaska have asserted that succession is linear and directional. Beginning with the invasion of willows on newly formed silt bars, subsequent fluvial deposition builds terraces of increasing height and distance from the river on which successive communities of alder, balsam poplar, white spruce, and eventually black spruce develop. This classical model assumes that primary succession is a deterministic autogenic process in which early successional species facilitate the establishment of late successional species through environmental modification.;I focused on the dynamics of white spruce establishment and growth in this successional environment. My primary objective was to describe boreal floodplain white spruce forests and the major environmental and biotic constraints on their development. I examined factors affecting the age, growth and spatial structure of white spruce populations across the successional sere.;Ecosystem processes on the Tanana River floodplain are closely linked to fluvial processes, and these in turn are directed by climate. Patterns of deposition and erosion resulting in the building and removal of successional terraces are functions of the climate controlled river discharge fluctuations, and are neither continuous nor directional.;White spruce occurs as seedlings, saplings, and seed-producing trees throughout the primary successional sequence, and its age structure reflects past variation in recruitment and mortality rates. Successful seedling establishment is episodic and correlated with a combination of interacting environmental and biotic factors, including silt deposition accompanying floods, seed production and dispersal, and herbivory of seedlings by snowshoe hares. Both herbivory and low light under canopies reduce seedling height growth. The relative influence of some of these factors changes through succession because of interactions with the developing vegetation.;Radial growth patterns of mature floodplain white spruce trees differ from those of nearby upland trees in their reduced sensitivity to climate variability because of the high water table on the floodplain.;Although elements of the classic facilitation model of succession are consistent with some of my results, much of the spatial and temporal variability in patterns of white spruce establishment and growth can be attributed to episodic environmental and biotic factors throughout the succession.