Understanding how climate, disturbances, and carbon storage interact in subalpine forests is critical for assessing the role of this ecosystem in the global carbon budget under altered climate scenarios. Most research to date has focused on wildfire effects on forest carbon storage, but the current extensive insect outbreak in western North America suggests that such disturbances are also an important driver of carbon dynamics. I measured carbon storage across outbreak severity and stand age along replicated lodgepole pine chronosequences in the Greater Yellowstone Ecosystem (GYE) to examine how carbon storage varies with forest stand development following mountain pine beetle (MPB) outbreaks. I measured aboveground and coarse root biomass using site-specific allometric equations, and directly measured carbon stocks in dead wood, forest floor, fine roots, and soil.
Results/Conclusions
Differences in carbon storage and distribution occur with time since outbreak, but these differences are muted and short-lived because MPB selectively kill host trees in a stand such that mortality is rarely 100%. C stocks in all stands experiencing MPB-caused mortality are shifted towards dead wood and forest floor pools, as C in large killed trees is redistributed from the live C to the dead C pool. Dead wood and forest floor represented a significantly higher proportion of total carbon in stands recently attacked than those attacked over 15 years ago. As expected, the rate of recovery of C stocks in live biomass depends on the number of surviving trees following an outbreak, such that live C stocks recover more quickly to pre-outbreak levels when outbreak severity is lower. The balance of carbon stored in live biomass and the forest floor and carbon released from decomposing dead wood are closely linked to carbon storage in these forests. Therefore, at a landscape scale, the re-distribution of forest carbon among living and non-living components by insect outbreaks has strong implications for whether a given forested landscape acts as a source or sink of carbon to the global carbon budget.
