Dead wood dynamics in white spruce plantations following alternative commercial thinning

Background/Question/Methods

Coarse woody debris (CWD) and snags constitute important structural and functional components in forest ecosystems. We examined dead wood dynamics in intensively managed white spruce (Picea glauca (Moench) Voss) plantations in northern New Brunswick, Canada. The objective was to assess changes in CWD and snag volume in response to four alternative commercial thinning treatments that created varying amounts of deadwood; collaborators are examining effects on birds, small mammals, plants, and beetles. Six large (>20 ha) white spruce plantations were selected on the basis of similar stand characteristics, divided into  four blocks, and subjected to four treatments: 1) an unthinned control; and three 40% basal area commercial thinning removals, with 2) slash and tops remaining on site (status quo), 3) branches and tops extracted from site (biomass removal), and 4) clumps of unthinned trees left, and one-half girdled to create snags (enhanced). Five permanent sample plots (PSPs) per treatment block were measured before and after thinning, and line intersect sampling was used to measure CWD on 25 m transects from the center of each PSP in all cardinal directions. Snags that occurred within PSPs, and girdled trees in unthinned clumps in the enhanced treatment were also measured.

Results/Conclusions

Two years after thinning, volume of CWD increased by 11-18% in the status quo and enhanced thinned treatments, by 3% in biomass removal, versus 0.3% in unthinned. The thinned treatments added smaller-sized CWD which showed little or no signs of decay two years post-thinning.  Mean snag volume was  0.1-0.4 times higher in the thinned treatments compared to pre-treatment levels, and  girdled trees in the enhanced treatments added a mean of 1.6 m³/ha.  Total deadwood (CWD and snags) averaged 15-22 m³/ha for the 30 PSPs of each of the four treatments, but was variable, ranging from 0-102 m³/ha in individual PSPs. Volume increase of deadwood in the thinned treatments was highest in the enhanced (3.8 m³/ha), followed by status quo (2.6 m³/ha), and biomass removal (0.7 m³/ha) treatments, respectively. The results indicate that commercial thinning in 20-25 year old spruce plantations produced small increases in CWD volume. Girdling trees during commercial thinning is unlikely to produce sufficient snags for functional habitat for all deadwood-requiring wildlife, and leaving islands and clumps during the harvest before plantation establishment would be a more feasible way of adding structure.