Abstract
The forest stand structure class “old-growth” has previously been qualitatively described as having several distinct “sub-structures.” Species composition, diameter distribution, and other structural features commonly associated with old-growth in the Pacific Northwest are quite variable. We determined which quantitative stand structure variables are commonly found together using the Spearman correlation and non-metric multidimensional analysis. Some features were more commonly found together than others, indicating different old-growth stand types, or sub-structures. Cluster analysis classified the old-growth forests into four groups: Douglas-fir dominance, shade tolerant species dominance, and intermediate groups. The intermediate groups were split by the density of large logs and large shade tolerant trees. The old-growth sub-structures appear to change from one to another as the old forest develops.
Highlights
Old-growth forests can contain a variety of structural features that require a long time to develop, such as large trees, snags, downed logs, and multiple canopy layers [1,2,3]
Density of trees ≥100 cm diameter at breast height (DBH) showed the greatest number of significant correlations with other variables, followed by density of Douglas-firs ≥100 cm DBH
Stands may change from group D to group S, with the ratio of Douglas-fir basal area to stand basal area decreasing and the ratio of shade tolerant species basal area to stand basal area increasing, showing shade tolerant species replacing the long-lived Douglas-fir pioneer species
Summary
Old-growth forests can contain a variety of structural features that require a long time to develop, such as large trees, snags, downed logs, and multiple canopy layers [1,2,3]. Forests 2015, 6 physiognomy has been described as varying from containing a relatively continuous upper canopy of shade intolerant species, to a fragmented canopy, to a continuous canopy of primarily shade tolerant species [4,5]. There appear to be several “sub-structures” of old-growth. Several sub-structure classifications of old-growth forests in the Pacific Northwest have been developed [4,6,7]. The classification varied in which structural attributes were associated with each old-growth sub-structure [4,8]. Franklin et al [5] reviewed the different classifications and proposed that old-growth be divided into three sub-structures: “Vertical Diversification,” “Horizontal
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