Abstract:

Managing mature Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] stands to emulate the structural complexity of natural old-growth forest requires identifying structural targets at the sub-stand level at which neighborhood dynamics and patchy disturbance shape structure. We therefore sought to describe the archetypal shapes of predominant sub-stand diameter distribution types (DDTs) observed in natural tree neighborhoods in stands comprising a chronosequence (ca. 120–450+ yrs) encompassing mature, vertical diversification, and horizontal diversification development stages. The ten 1.0 ha stands are located in the southcentral Oregon Cascades, USA. Building on the known spatial position of each tree, natural tree neighborhoods were identified using the floating neighborhood approach based on spatial tessellation connecting neighboring trees at the first- (mean 61 m²) through fifth- (mean 2058 m²) order scales. Cluster analysis was then used to objectively identify the most predominant DDTs among the relative tree size distributions observed in the trans-scale neighborhoods within each stand. Repeated measures regression was used to classify each DDT to one of six observed archetypal shapes: negative exponential, unimodal, rotated sigmoid, bimodal, concave, or multi-modal. Only three of the 81 observed DDTs deviated by < 10% from the stand average, while every stand had at least one DDT that deviated by > 50% (maximum 83%). Within each stand, five to ten predominant DDTs were observed, which deviated from the stand average by 30–48% and were characterized by two to five different archetypal shapes. Consequently, in some stands the majority of tree neighborhoods were best characterized by a different shape from that assigned at the stand level. Deviation from the stand average increased from the youngest stand in the mature development stage through the middle stands in the vertical diversification stage to the oldest stands in the horizontal diversification stage. The complexity of DDT shapes tended to increase along the chronosequence (from negative exponential and uniform toward concave and multi-modal), with shape richness highest mid-sequence and evenness peaking in the oldest stands. The high diversity of sub-stand structural complexity reduces the utility of stand-level diameter distributions as old-growth restoration targets. However, the presence in early-sequence stands of sub-stand diameter distributions common to later-sequence stands may facilitate active management at the neighborhood level to promote future old-growth condition. Restoration of sub-stand diameter structures would likely require combining spatial perspectives, such as by augmenting tree-level crop-tree management with diameter distribution targets for the resulting tree neighborhoods.

Key words: Diameter distribution type, Floating neighborhood, Restoration silviculture, Small-scale, Structural complexity