Dynamics and restoration of Australian tropical and subtropical rainforests

At a stand level and as a generalization, the dynamics of moist tropical and subtropical rainforests following disturbance can be explained by a few interrelated factors (Swaine and Whitmore 1988; Hopkins 1990; Finnegan 1996; Richards 1996; Terborgh et al. 2002; Wright 2002). First, mature rainforest has a closed canopy and a shady understory. Second, the longlived trees that dominate mature rainforest are relatively shade tolerant and slow growing, and their seeds typically germinate on dispersal. Third, rainforests also support a suite of shade-intolerant, fast-growing plants (“pioneers”) whose seeds persist in the soil and whose germination is stimulated by the increase in heat and light that accompanies disturbance to the canopy. Fourth, animals are key agents in the pollination, dispersal, and consumption of rainforest plants. Consequently, in rainforests with an intact fauna, moderate-scale disturbance of the canopy (large tree-fall gaps to blowdowns of several hectares) typically gives rise to the following successional sequence: Pioneers recruit from the seed bank, grow rapidly to form a closed canopy, but do not recruit under their own shade. As the pioneers senesce, they are replaced by slower-growing, shade-tolerant, long-lived trees either present as seedlings at the time of disturbance or subsequently dispersed to the regenerating forest from surrounding areas of rainforest (fig. 14.1). We note that this successional model considerably simplifies variation in plant traits and ecological strategies and does not address issues concerning the maintenance of diversity in rainforests (Tilman 1994; Westoby et al. 2002; Wright 2002). Nevertheless, this model has been widely used to describe the ecology of rainforests and often forms the basis of strategies developed for their exploitation (Hartshorn 1989; Hopkins 1990; Kooyman 1996; Richards 1996; Lugo 1997; Duncan and Chapman 2003; Ganade 2007; Holl 2007). In this chapter, we consider the utility of successional and alternate models (e.g., King and Hobbs 2006; Cramer 2007) for describing forest dynamics and informing the conduct of restoration projects in rainforest landscapes subject to broadscale anthropogenic disturbance. Such landscapes are characterized by extensive areas of cleared land dominated by grasses, crops and other exotic vegetation, patches of remnant forest, areas of secondary forest on marginal agricultural land, and various types of tree plantations (Richards 1996). Our examples are drawn largely from Australian rainforests, the focus of our research experience. We begin with a brief description of intact Australian rainforests before discussing disturbed systems.