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Dynamics and Stocking-Level Relationships of Multi-Aged Ponderosa Pine Stands

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Ponderosa pine (Pinus ponderosa Dougl. ex Laws.) stands in western Montana and central Oregon were sampled to determine total leaf area index (LAI), leaf area distribution among age classes or cohorts, and stem volume increment. Stands included from one to five cohorts. LAI was assumed to represent occupied three-dimensional growing space. Within multi-aged stands, cohorts varied in their total LAI, with older cohorts generally having more LAI, or growing space, than younger cohorts. Older cohorts also produced stem volume growth more efficiently per unit of leaf area than younger cohorts. Higher individual tree leaf area, or growing space occupancy, was strongly related to increased stem volume growth in all cohorts. A general increasing relationship between growing space efficiency and number of cohorts per stand was also observed.

A model, termed PP-MASAM for Ponderosa Pine-Multi-Aged Stocking Assessment Model, was developed to assess multi-aged stocking relationships. Results indicate that residual age structures with large numbers of young trees reduce stand production from structures with fewer young trees because they grow more slowly than older trees and will die, need to be thinned, and will not become merchantable. Stocking relationships which appeared most productive are characterized by a positive linear relationship between LAI and cohort age, and a negative linear relationship between number of trees per cohort and age. These options produced the most gross and recoverable stem volume increment, produced acceptable tree vigors, and maintained sufficient growth rates in younger cohorts to sustain the structure.

Formal stocking guidelines for uneven-aged silviculture typically define stocking and residual stand structure using diameter distributions. The most common of these procedures, the q-factor, attempts to create and maintain "balanced" sustainable structures with a negative exponential diameter distribution. These size structures designed with the q-factor approach are assumed to be sustainable and to represent equal occupancy of growing space by each diameter or age class. PP-MASAM results indicate residual size structures defined by the q-factor are no more sustainable than other structures that, for example, provide linearly increasing growing space to each older age class. The q-factor approach also perpetuates ponderosa pine size structures which are atypical of structures which developed under a typical presettlement fire regime. For. Sci. Monograph 33:1-34.

Keywords: Montana; Oregon; Pinus ponderosa; Uneven-aged; even-aged; leaf area index; q-factor; stand density index; stand dynamics; stand management; stand structure

Document Type: Journal Article

Affiliations: Associate Professor of Silviculture, School of Forestry, University of Montana, Missoula, MT 59812

Publication date: November 1, 1996

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