Precommercial thinning : implications of early results from the Tongass-Wide Young-Growth Studies experiments for deer habitat in southeast Alaska

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This report documents the results from the first "5-year" round of understory responses to the Tongass-Wide Young-Growth Studies (TWYGS) treatments, especially in relation to their effects on food ...

This report documents the results from the first "5-year" round of understory responses to the Tongass-Wide Young-Growth Studies (TWYGS) treatments, especially in relation to their effects on food resources for black-tailed deer (Odocoileus hemionus sitkensis). Responses of understory vegetation to precommercial silviculture experiments after their first 4 to 8 years posttreatment were analyzed with the Forage Resource Evaluation System for Habitat (FRESH)-Deer model. The studies were conducted in western hemlock (Tsuga heterophylla)-Sitka spruce (Picea sitchensis) young-growth forests in southeast Alaska. All four TWYGS experiments were studied: (I) planting of red alder (Alnus rubra) within 1- to 5-year-old stands; (II) precommercial thinning at narrow and wide spacings (549 and 331 trees per hectare, respectively) in 15- to 25-year-old stands; (III) precommercial thinning at medium spacing (420 trees per hectare) with and without pruning in 25- to 35-yearold stands; and (IV) precommercial thinning at wide spacing (203 trees per hectare) with and without slash treatment versus thinning by girdling in >35-year-old stands. All experiments also included untreated control stands of identical age. FRESHDeer was used to evaluate the implications for deer habitat in terms of forage resources (species-specific biomass, digestible protein, and digestible dry matter) relative to deer metabolic requirements in summer (at two levels of requirements-- maintenance only vs. lactation) and in winter (at six levels of snow depth). Analyses for both summer and winter indicated that in all cases except for Experiment I (red alder planting in 1- to 5-year-old stands), habitat values of all treatments exceeded untreated controls (P < 0.05), and earlier treatments yielded greater benefits than did later treatments (i.e., treating at 15 to 25 years of age was more effective than at 25 to 35 years, and at >35 years was least effective). When compared to a wide range of old-growth stands from throughout the region, it was apparent that in summer and winter with low snow depths (<20 cm) early treatments (15- to 25-year-old stands) yielded better food resources than did old-growth forest, while later treatments (25- to 35-, and 35+ year-old stands) yielded poorer habitat than old growth. These results, however, are from only the first 4 to 8 years posttreatment. The next study of TWYGS responses is scheduled to occur at 9 to 13 years posttreatment.

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