Model Origin Development Strategy Current (and Future) Development

Model Origin

SDMDs are based on the theories of the -3/2 power law of self-thinning developed by Japanese scientists for agricultural crops in the 1960’s and 1970’s (Yoda et al. 1963). This law states that "…for any given density, there is a maximum average biomass that an individual plant can attain…"and "…any further increase in average plant biomass can only be achieved at a lower density, therefore some individual plants must die." This law was first applied to forest crops in North America by Drew and Flewelling (1979) to suggest that there is a highly predictable relationship between the average tree size and the maximum number of such trees that can be sustained on any site for a given species.

Early in stand development when all trees have sufficient growing space, there are typically only low levels of mortality that are essentially unrelated to inter-tree competition. As stands continue to grow, they begin to approach the maximum size-density limit, and increasing levels of inter-tree competition result in the weakest/smallest trees dying. This process continues as long as the stand continues to increase in height, with fewer and fewer larger and larger trees occupying all of the available growing space.

SDMDs illustrate this growth pattern on a logarithmic grid, most typically with density on the horizontal axis and mean tree volume on the vertical axis (note that some diagrams developed in the United States use a different format based on Reineke’s Stand Density Index). Within this framework are sets of iso-lines depicting increments of top height and quadratic mean diameter. Also depicted on some diagrams are isolines for merchantable volume and expected patterns of mortality/survivorship.

For more complete illustrated descriptions of the diagrams, how they work and how to use them, see Archibald and Bowling (1995) or Farnden (1996 ), 1998, 1999).

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Development Strategy

SDMDs for British Columbia species have been developed primarily by Craig Farnden, originally as an employee of the Canadian Forest Service (lodgepole pine, white spruce and interior Douglas-fir), and later under contract to the BC Ministry of Forests, Forest Practices Branch (coastal Douglas-fir, western hemlock, western redcedar and Sitka spruce).

SDMDs for British Columbia are based on the format proposed by Drew and Flewelling (1979), with a few minor revisions. The diagrams are based on data from the managed stand yield tables produced by TIPSY (version 1.3 and 2.0 beta) and thus reflect similar growth patterns.

Credit for the concept of building SDMDs from TIPSY managed stand yield tables belongs to Ian Moss R.P.F.

Current (and Future) Development

Stand Density Management Diagrams have been developed for the following species in British Columbia:

• lodgepole pine: planted and natural
• white (interior) spruce: planted and natural
• interior Douglas-fir: planted and natural
• coastal Douglas-fir: planted and natural
• western redcedar: planted and natural
• western hemlock: planted and natural
• Sitka spruce: planted and natural

In addition to the basic diagrams, overlays have been developed depicting merchantable volume, and an interpretive guide has been developed for assessing windthrow risk.

SDMDs have also been developed for other regions of North America including Ontario for jack pine, red pine, white pine, black spruce and aspen (Archibald and Bowling, 1995; Smith and Woods, 1997; Woods et al. 1998; Woods, 1998), Newfoundland for black spruce and many parts of the United States.