Conversion of natural, mixed-species forests to conifer plantations: Implications for dead organic matter and carbon storage

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Abstract
Aboveground carbon storage of stands of natural forest was compared with that of conifer plantations of various age in New Brunswick, Canada. The stands of natural forest were dominated by balsam fir (Abies balsamea), red spruce (Picea rubens), red maple (Acer rubrum), and yellow birch (Betula alleghaniensis). The plantations were dominated by planted black spruce (Picea mariana). The stands of natural forest ranged in age from 55 to 105 years, while the plantations represented a chronosequence of 3 to 21 years. Carbon storage in the aboveground living tree biomass of mature natural stands averaged 74.9 t C/ha-1 (range 57.1 to 92.7 t C/ha-1), compared with a projected 82.5 t C ha-1 in a 60-year-old spruce plantation. Forest-floor biomass was similar among stands of natural forest and plantations (overall average 20.4 t C/ha-1). Woody detrital biomass (snags and coarse woody debris) in mature natural stands averaged 19.1 t C/ha-1, considerably more than in the older plantations studied (0.20 t C/ha-1 in a 21-year-old plantation). Because of clear-cut harvesting and precommercial and commercial thinning, the plantations receive few inputs of snags or coarse woody detritus. The longer-term paucity of these biomass components in plantations has important implications for carbon storage. Overall, our study suggests that a landscape managed as a shifting mosaic of plantations on a 60-year rotation would store only about 22% as much aboveground carbon (i.e., in live trees, snags, coarse woody debris, and the forest floor) as a landscape covered in older-growth natural forest in which gap-phase disturbance dynamics were the primary mechanism of disturbance.