Carbon storage in old-growth western larch (Larix occidentalis) forests of western Montana

Background/Question/Methods There is limited understanding of the carbon (C) storage capacity and patterns in old-growth forests of western Montana, leaving little ability to evaluate the role of old-growth forests in regional C cycles and ecosystem level C storage capacity. Although old-growth studies are commonly found in the literature, few studies focus on the ecological significance of long-term carbon storage associated with specific components of these ecosystems. In fact, old-growth forests are often considered insignificant C sinks and near equilibrium with respect to C exchange. The C balance of these ecosystems may actually be far more dynamic than previously assumed. To illustrate the variance of carbon storage between equivalent stands of contrasting age classes and management histories, we surveyed paired old-growth and second growth western larch (Larix occidentalis) stands in northwestern Montana. The objectives of this study were to: 1) compare the rate and nature of C storage in soils, coarse woody debris, litter, and standing timber of old-growth western larch stands to that in paired second growth western larch stands, 2) determine whether soil disturbance due to harvest significantly impacts soil structure and long-term C storage of managed stands. We surveyed stands using a modified USFS FIA protocol, focusing on ecological components related to soil, forest floor, and stand carbon. We then analyzed all downed wood, forest floor litter, and soil for total C and total nitrogen. Results/Conclusions Average total soil C was not significantly different in second-growth stands (mean = 2.48%, se = 0.24) compared to old-growth stands (mean = 2.43%, se = 0.23); total carbon of forest floor organic matter, however, was significantly greater in old-growth (mean = 47.5%, se = 0.49) than in second growth stands (mean = 42.9%, se = 0.81). Understory species richness was greater on second growth than on old-growth stands (130 species vs. 115 species, respectively), and abundance of coarse woody debris was significantly greater on old-growth stands (1951.3 m3/m2 total volume, mean = 130.09, se = 19.19) than on second-growth stands (202.29 m3/m2 total volume , mean = 13.49, se = 5.65). Carbon concentrations also increased with increasing degree of decay in coarse woody debris, regardless of species sampled (decay class 1 = 50.59% to decay class 4 = 57.19%). Results suggest ecological components specific to old-growth western larch forests, such as large amounts of coarse woody debris and forest floor litter, significantly contribute to long-term carbon storage within these ecosystems.