Controls over Forest Production in Interior Alaska

State factors provide a powerful conceptual basis for understanding current patterns and potential changes in forest productivity (Chapter 1). The BNZ-LTER program has focused on investigations of ecosystem structure and function related to state factors (time, topography, and climate) that account for dramatic spatial variation in productivity and provide a basis for predicting future temporal variation (e.g., climate change). Other state factors are either relatively uniform across the region (e.g., potential biota) or co-vary with topography (i.e., parent material) and are difficult to study as clearly independent factors. State factors have many direct and indirect effects on productivity, and these controls may co-vary in a complex fashion across the landscape. The nitrogen productivity concept provides a mechanistic framework for understanding the effects of environmental variation on forest productivity (Ågren 1985). The nitrogen productivity of a tree or forest stand is defined as the amount of production per unit of foliar nitrogen (gram biomass production per gram foliar nitrogen) in the canopy of the tree or stand. At steady-state nutrition, the growth rate is proportional to the amount of foliar nitrogen and the N-productivity. Biological and chemical processes that occur in soils are an excellent example of the way in which multiple interacting factors influence productivity through their effects on N supply. In interior Alaska, several state factors have a hierarchical influence on forest production. These factors are time (Chapter 7), parent material (Chapter 3), topography (Chapter 2), and macroclimate (Chapter 4). These factors have both direct and indirect effects, many of which vary over time and space. In this chapter we emphasize the influence of the four relatively direct state factors: parent material, topography, time, and climate and a critical “resource” (Chapter 1), soil, which represents the indirect interaction of multiple state factors. The parent material in lowland locations is primarily alluvium or loess over alluvium; thick silt, glacial deposits, or eolian sands are present in some areas (Chapter 3). Organic soils also occur on level surfaces that rarely or never flood. Both alluvial and organic soils usually contain a fine-grained mineral substratum. In addition, limited lowland areas in interior Alaska contain very thick loess deposits.