OOS 64-3 - Dynamics of standing and downed woody biomass in post-bark beetle forests in the Rocky Mountains, USA

Severe tree mortality, following unprecedented drought and bark beetle (BB) outbreaks in the in the past two decades, has altered the structure and function of wide swaths of western US forests. New opportunities have emerged to quantify these changes over time: the USDA Forest Service’s Forest Inventory and Analysis (FIA) program has entered its second round of remeasurement across much of the eight-state Rocky Mountain Research Station-FIA unit. Combining FIA data with aerial detection survey (ADS) data, we asked (1) When and where have BB outbreaks led to forest mortality? (2) How has BB mortality altered standing dead and downed woody biomass? and (3 What is the timing of coarse woody debris accumulation after BB mortality, accounting for salvage logging and removals? Outbreak mortality peaked from 2006 through 2009 (after annualized inventories were established in most states), affecting nearly 24 million acres. Bark-beetle affected forest contained more downed woody material per hectare than non-affected forests (78% on average) and the differences were largely attributable to coarse woody debris and less to fine woody material. Most standing dead and snagfall woody debris were lodgepole pine. Changes in snagfall and coarse woody debris were also greater (up to 70% more) in plots with bark beetle mortality. Snagfall rates generally peaked 15-20 years after outbreaks. Nearly 20 years ago, researchers warned of carbon cycle consequences of the large tree mortality events and predicted those events to become more widespread across the western US. Our analysis of FIA data confirmed a sizable transfer of live tree C to dead and downed wood C, yet we are still not experiencing the full implications of such a large influx of dead wood in our forests. Compounded disturbances such as wildfires following beetle outbreaks may exacerbate C loss in the future.