Capacity for recovery from defoliation in spruce-fir forests and the influence of commercial thinning

Langley, CJ; Day, Michael

From the introduction: "Maine’s timber land is a valuable economic resource that has been plagued by the spruce budworm for centuries. The spruce budworm (Choristoneura fumiferana (Clemens)) reduces forest productivity by feeding on newly expanding foliage of spruce and, preferably, fir trees. An increase in tree mortality results after prolonged outbreaks of severe defoliation caused by the heavy feeding of these insects. A tree's capacity for recovery after a defoliation event is linked to the pool of non-structural carbohydrates (NSC) available to stimulate grown of latent buds to recover leaf area. Past studies have shown that survival and recovery are related to species composition at the stand level, where higher mortality and slower post outbreak recovery is experienced by stands with an increased fir composition. In addition, current forest management practices, such as commercial thinning, have been shown to have a complicated effect on outbreak severity and duration. This study examines possible physiological mechanisms that could explain these phenomena. Four organ systems of red spruce (Picea rubens) and balsam fir (Abies balsamea) were analyzed for NSC content from thinned and unthinned stands. We found that balsam fir maintains a higher concentration of NSC than spruce, with both species having the highest available reserves within their fine branches. Outside of these fine branches, fir stores the majority of NSC within large branches, close to the site of production, putting few reserves into the roots for longer term storage. Spruce, on the other hand, stores equal amounts of NSC within the large branches and the roots. Both species store very few NSC within the stem. Commercial thinning was found to decrease the amount of NSC available at the stand level. Both species, within thinned stands, had decreased NSC available within large branches. The effect on all other organ systems was not statistically significant. Continuing research will explore the potential for decreased palatability of new foliage produced post-defoliation characterized by a higher tannin concentration."