Operational Definitions of Old Growth

Tyrell and others (1998) proposed an operational definition for old-growth that provides a tool for identifying the degree of "old-growthness" that a particular stand exhibits. This old-growth concept may be diagrammed as a wheel with a central hub and radiating spokes which represent defining characteristics. There are 15 defining characteristics, hence 15 spokes. The scales are defined so that the closer a value is to the hub on each spoke, the closer the value of that characteristic is approximating the core concept of old-growth (Tyrell and others 1998).

Tyrell Model of Old-Growth Forests

The core concept for old-growth used for this model is:

"an old, uneven-aged forest with large trees and lots of dead wood, located on productive sites at low elevations, connected to other old-growth sites via forested corridors, and not likely to experience large-scale disturbance, embodies the quintessential old-growth concept" (Tyrell and others 1998).

The process of using this model to estimate the degree of "old-growthness" of any stand is straight forward. The target forest is assigned a value for each spoke (characteristic). This value is marked on the spoke of the wheel diagram at the appropriate place. All marks are then connected to produce a polygon. For example, compare an ideotypical northern hardwood forest with an ideotypical aspen forest. The smaller the polygon, the more closely the target forest resembles the core old-growth concept. The ideotypical aspen forest diverges from our core old-growth concept on several spokes (characteristics), thus resulting in a larger polygon.