Multiple successional pathways and precocity in forest development: can some forests be born complex?

Background. In forests subject to stand-replacing disturbances, conventional models of succession typically overlook early-seral stages as a simple re- organization/establishment period. These models treat structural development in essentially ‘relay floristic’ terms, with structural complexity (three-dimensional heterogeneity) developing primarily in old-growth stages, only after a closed-canopy ‘self-thinning’ phase and subsequent canopy gap formation. However, is it possible that early-successional forests can sometimes exhibit spatial complexity similar to that in old-growth forests – i.e. akin to an ‘initial floristic’ model of structural development? Hypothesis. Based on empirical observations, we present a hypothesis regarding an important alternative pathway in which protracted or sparse forest establishment and interspecific competition thin out tree densities early on – thereby precluding overstorey canopy closure or a traditionally defined self-thinning phase. Although historically viewed as an impediment to stand development, we suggest this process may actually advance certain forms of structural complexity. These young stands can exhibit qualities typically attributed only to old forests, including: (1) canopy gaps associated with clumped and widely spaced tree stems; (2) vertically heterogeneous canopies including under- and mid-stories, albeit lower stature; (3) co-existence of shade-tolerant and intolerant species; and (4) abundant dead wood. Moreover, some of these qualities may persist through succession, meaning that a significant portion of eventual old-growth spatial pattern may already be determined in this early stage. Implications. The relative frequency of this open-canopy pathway, and the degree to which precocious complexity supports functional complexity analogous to that of old forests, are largely unknown due to the paucity of naturally regenerating forests in many regions. Nevertheless, recognition of this potential is important for the understanding and management of early-successional forests.