A canopy perspective of community dynamics of an old-growth Pseudotsuga-Tsuga forest
Old-growth Pseudotsuga-Tsuga forests of the western Cascade Range are characterized by presence of large, old trees of Pseudotsuga menziesii (Mirb.) Franco var. menziesii. Colonizing soon after a stand-replacing disturbance, P. menziesii is a long-lived pioneer species that persists in these forests for centuries. In this study of canopy dynamics of a 450-year-old Pseudotsuga-Tsuga forest, four canopy processes were found to contribute to the long-term coexistence of P. menziesii with late-successional species and to long-term survival of individual trees. (1) Vertical stratification: P. menziesii dominates the upper-most part of the canopy, comprising nearly 70% of trees above 50m in height. Measurement and projection of current height-growth patterns indicate that this dominance will continue for at least another century. (2) Decreasing crown competition in the upper canopy: Height growth and crown expansion culminate in large trees of all species, indicating that crown competition decreases with increasing tree size. This allows the existing large trees of different species to coexist in the upper canopy. (3) Morphological acclimation: Old-growth P. menziesii have deep crowns with low branch density, and symmetric branching pattern that result in efficient shoot and foliage display. In comparison to young and mature trees, old-growth P. menziesii have marked differences in crown form and branching pattern. Morphological plasticity and acclimation enable P. menziesii to persist into later stages of succession. (4) Crown maintenance: Epicormic shoot production in old-growth P. menziesii results in adaptive reiteration of shoot cluster units, subbranch units, and entire branches. This is an important mechanism for maintaining productivity of the established crown after height growth and crown expansion have culminated, and contributes to prolonging individual tree life-span.Architectural characteristics generally associated with early-successional species do not apply to old-growth P. menziesii. Characteristics associated with persistence of P. menziesii may be linked with the long-term community, dynamic and disturbance regime of this region. Architectural characteristics of trees change during their development, and various aspects of the species' ecology as well as its role in community dynamics need to be considered to understand the ecological significance of tree architecture.
- Forestry