Investigating scale-dependent stand heterogeneity with structure-area-curves

The creation of stands that exhibit characteristics similar to old-growth forests has become one of the most important objectives in forest management and structure has become the tool of choice for evaluating this condition. Attributes of forest structure, however, are extremely heterogeneous and differ considerably in spatial pattern and extent due to the patchy and variable nature of disturbances. Silvicultural research that aims to better understand structural outcomes of natural disturbances must be conducted at spatial scales that are appropriate to capture the variation in both the disturbance regimes and in these structures. Scale-dependent variation in structural attributes, including the index of old-growth (Iog) and the structural complexity index (SCI), were evaluated in unmanaged mature, transitional, and old-growth Douglas-fir (Pseudotsuga menziesii (Mirbel) Franco) forests in the south central Cascade Range of Oregon. Moving windows of 100, 500, 1000, 2500, 5000 m2 were overlain onto stem maps of each of ten 1-ha stands and structural attributes evaluated at each spatial scale. Multiple iterations revealed that structural attributes and indices varied with subplot size, with variation stabilizing with subplots 2500 m2 and larger. Structural heterogeneity across subplot sizes was consistently highest in old-growth stands. These findings have implications for minimum acceptable plot sizes for research projects that aim to characterize the effect of silvicultural treatments on these structural attributes. For instance, a quarter hectare may be a minimum acceptable plot size to evaluate these structural attributes and potentially even larger plot sizes may be necessary in more complex old-growth forests.