SDI-Flex: A New Technique of Allocating Growing Stock for Developing Treatment Prescriptions In Uneven-Aged Forest Stands

One of the difficulties of apportioning growing stock across diameter classes in multi- or uneven-aged forests is estimating how closely the target stocking value compares to the maximum stocking that could occur in a particular forest type and eco-region. Although the BDQ method had been used to develop uneven-aged prescriptions, it is not inherently related to any maximum stocking guide. Adapting Stand Density Index (SDI) to uneven-aged silviculture by apportioning stocking equally in all diameter classes has been proposed. SDImax is the maximum possible stocking possible for a given species and region and provides a consistent and reliable benchmark on which to base silvicultural prescriptions, which can be expressed as a percentage of maximum SDImax. However, allocating a consistent percentage of maximum SDI desired after thinning across all diameter classes raises or lowers the resulting stocking curve, but does not change its shape, as could be done by changing the Q value in BDQ stocking control. This results in high numbers of small trees being retained, regardless of the %SDImax prescribed, and has been a major criticism of using SDI-based stocking control for fuels treatment biomass estimates. The SDI-Flex procedure presented here combines the flexibility of the BDQ method with an SDI-based stocking guide. In this method, a Flex Factor can is used to proportionally reduce the amount of SDI assigned to successively smaller dbh classes. The simple spreadsheet program below can be used to iteratively manipulate both the Flex Factor and %SDImax values to arrive at a desired stocking configuration for a particular silvicultural objective.

SDI-Flex Spreadsheet

Example Contract Specs

Example Prescriptions

Wayne D. Shepperd - Research Silviculturist
Rocky Mountain Research Station
240 W. Prospect Rd.
Ft. Collins, CO 80526
(970) 498-1259
wshepperd@fs.fed.us