Dynamics and restoration of Australian tropical and subtropical rainforests
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In rain forests 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 rain forest. This successional model has been widely used to describe the ecology of rain forests and often forms the basis of strategies developed for their exploitation Successional models have strongly influenced the development of techniques for restoring rain forests to cleared land. These techniques have been successful insofar as restored sites can develop a rain forest-like structure and provide habitat for some rain forest-dependent biota within one to two decades of establishment. However, few restoration projects have been established long enough to progress to the stage where planted trees have been replaced by species dispersed to the restored site let alone converge on target conditions. As successional models address only some of the factors influencing the dynamics of restored sites in disturbed landscapes, it may be useful to consider alternate models of forest dynamics to inform the long-term management of restored sites. A state-and-transition model of the dynamics of restored sites suggests that these sites will often require long-term intervention, including the control of exotic plants and the addition of native species, to converge on the condition of intact forest. Because the costs of intervention can be high, particularly if a problem such as weed invasion is not controlled at an early stage, the routine monitoring of restoration projects may need to become a necessary part of their management. At present, most Australian rain-forest restoration projects are not monitored in any formal way. Once the long-term outcomes of restoration projects are better known (e.g., through more systematic monitoring), there may need to be a reassessment of what constitutes appropriate "target conditions" for restored sites. It may become apparent, for example, that most restoration projects are so small and isolated and so heavily influenced by the disturbed matrix that the structure, composition, and dynamics of restored sites will always tend to diverge from the condition of intact rain forest. If so, it may be necessary to accept stable assemblages of native and exotic plants ("emerging" or "new forests") as the target for most restoration projects. Alternatively, investment in projects aimed at reestablishing assemblages typical of intact rain forests may need to be restricted to high-value applications (e.g., habitat for threatened species or plantings strategically positioned to act as future seed sources) rather than being used as the default approach across the landscape, as currently tends to be the case. All models of rain-forest dynamics recognize the importance of landscape context in affecting the trajectory of restored sites. To date, much restoration effort in Australian has been expended on the creation of corridors to facilitate dispersal between remnants. However, corridors do not address other landscape-scale factors affecting the persistence of biota in remnants, such as the influence of the surrounding matrix. A better understanding of the relative importance of the extent and configuration of rain-forest cover at the landscape scale and integration of this knowledge into models of rain-forest dynamics is required to inform future restoration projects aimed at conserving rain-forest biota.
New Models for Ecosystem Dynamics and Restoration