PARENT SESSION
Poster Session 37: Forest Ecology.

Thursday, August 5 Presentations from 5:00 PM to 6:30 PM, Exhibit Hall A 1.

Effects of thinning on forest understory vegetation structure in southeast Alaska.

Alaback, Paul^*,1, Casey, Dee¹, ¹ University of Montana, Missoula, MT, United States

ABSTRACT- The ecological effects of conventional (fixed-spaced) and experimental (variable-spaced) thinning protocols were evaluated in second growth hemlock-spruce (Tsuga heterophylla-Picea sitchensis) forests on Prince of Wales Island in southeastern Alaska. Understory composition, vertical structure, overstory structure and plant species diversity were examined in both control and thinned permanent plots before and 6-9 years after treatment. Before thinning, the pit-and-mound microtopography and low site-potential of poorly drained sites was associated with greater cover of forage species, plant species diversity, and vertical structural complexity than either moderately or well drained sites. On two out of the three well-drained sites a positive response to thinning was noted with variable-spaced thinning. This was particularly pronounced for shrubs. Fixed-spaced plots were less successful in stimulating understory growth than variable-spaced thinning plots. Thinned plots were generally more diverse than untreated plots. Well drained sites had an increase in species richness more significant than the poorly-drained sites (61% vs. 37% increase). No significant effects of thinning on species equitability (evenness) were detected on poorly-drained sites. All three well-drained sites by contrast had significant increases in species richness and equitability, suggesting that rare species became more common following thinning. Two out of the three well-drained study sites showed a very significant increase in the height and complexity of vertical foliage distribution on thinned plots. Detailed pre-treatment data showed that initial vegetation conditions can have a major influence on plant response to thinning. Forbs and ferns were particularly sensitive to initial conditions with 46-50% of variation in cover in 2002 associated with initial cover (regardless of thinning treatment). Variable-spaced thinning was effective in stimulating tree growth and improving overall stand structure, similar to fixed-spaced thinning. More experimentation is needed with variable-spaced thinning protocols to better understand mechanisms controlling variation in vegetation diversity, which is requisite for developing more effective wildlife habitat restoration strategies in the region.

Key words: thinning, southeast alaska, restoration, understory vegetation