COS 7-7
Examining the potential for a widespread severe bark beetle outbreak to restrict future outbreak
The spruce beetle (Dendroctonus rufipennis) is a native bark beetle that has caused extensive tree mortality across North America over the past 30 years. These outbreaks have dramatically altered forest structure and composition across North America. Increasingly, managers, the public, and scientists are questioning the capacity of bark beetle-affected systems to absorb this change without potentially shifting to alternate stable states. Here we examine how the ecological resilience of beetle-affected systems may be maintained through a long-term negative feedback on beetle populations exerted through host depletion.
Spruce beetle epidemics are hypothesized to collapse when host populations become depleted. This host-depletion negative feedback on beetle populations has been hypothesized to persist until understory host trees reach a size at which they are again susceptible. Here, for the first time we empirically examine how a prior widespread and severe spruce beetle outbreak, which depletes susceptible hosts, affects the landscape template for future outbreaks. This negative-feedback was studied using a combination of GIS and stand structure data for the Flat Tops region of northwestern Colorado, which experienced severe outbreak during the 1940s.
Results/Conclusions
We found that recent (1998-2012) spruce beetle activity in the Southern Rocky Mountains occurred in areas characterized by warmer and drier summers, advanced forest structural stages, and large diameter trees, all of which promote spruce beetle infestation. We found that the 1998-2012 outbreak within the Flat Tops was not constrained by climate but limited by a low proportion of large trees (> 23 cm DBH). Areas affected by the 1940s outbreak were characterized by a high proportion of medium diameter trees (12.5 – 23 cm DBH) in the 2000s, which suggests that trees have not had enough time to grow large enough to become susceptible to spruce beetles. Concordantly, we found no overlap between areas affected by the 1940s outbreak and the current outbreak. These findings suggest that ecological resilience of beetle-affected systems may be maintained through a long-term negative feedback on beetle populations exerted through host depletion.