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

Background/Question/Methods

Fire exclusion and laws requiring the timely re-establishment of conifers following harvest have left many landscapes in the Pacific Northwest effectively devoid of naturally regenerating forests for several decades. As a result, distinctions between naturally protracted conifer establishment and accelerated conifer establishment and their impact on forest development have not been adequately addressed. Conventional models of succession often assume that lasting structural complexity develops only after a closed-canopy ‘self-thinning’ phase and subsequent canopy gap formation, processes which are typically associated with a dense tree cohort. Based on empirical observations, we present a hypothesis regarding an important alternative pathway in which a protracted establishment period and interspecific competition thin out tree densities early on—thereby precluding overstory canopy closure or a traditionally defined self-thinning phase.

Results/Conclusions

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) coexistence 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 pre-determined in this early stage. The degree to which precocious early-successional complexity supports functional complexity analogous to that of old-growth forests is 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 following natural and anthropogenic disturbances.