Successional Processes in the Alaskan Boreal Forest

Superimposed on the topographic and climatic gradients in vegetation described in Chapter 6 are mosaics of stands of different ages reflecting the interplay between disturbance and succession, that is, the ecosystem changes that follow disturbance. The nature of disturbance governs vegetation succession, and vegetation properties, in turn, influence disturbance regime. Both disturbance and succession are controlled by state factors and by stochastic variation in local conditions such as weather and the abundance of herbivores. Even in this relatively simple biome, the interactions among site, chance, and disturbance history result in a vast array of possible successional trajectories following a disturbance event, generating at least 30 forest types in interior Alaska (Viereck et al. 1992). Despite this broad range of possible dynamics, certain patterns recur more frequently than others (Drury 1956, Viereck 1970). In this chapter, we discuss selected successional pathways that commonly occur on river floodplains and on permafrost-free or permafrost-dominated upland sites in interior Alaska. River floodplains occupy only 17% of interior Alaska, but they account for 80% of the region’s commercial forests and therefore have attracted considerable attention from forest managers (Adams 1999). These forests provide an excellent example of primary succession, that is, the succession that occurs on surfaces that have not been previously vegetated. Although many successional pathways are possible on interior Alaska’s floodplains (Fig. 7.1; Drury 1956), the trajectory that actually occurs in a particular place is usually determined by the patterns of colonization during the first decades (Egler 1954). This, in turn, depends primarily on physical environment, flood events, and seed availability. For example, fine-textured sediments, which are common along the gradual grade of the Tanana River near Fairbanks (Chapter 3), retain more moisture than gravelly substrates and favor establishment of thinleaf alder (Alnus incana subsp. tenuifolia) following the initial colonization by willow (Salix). Alder is therefore a more important component of this successional sequence than along some other rivers. In this chapter, we focus on the alder-mediated pattern of floodplain succession, which has been the major focus of the LTER research program. The common and scientific names of species mentioned in this chapter are given in Table 6.1.