Linked and compound disturbances in subalpine forests: Bark beetles and fire
Extensive drought and warmer temperatures have been linked to increased area burned by wildfire and synchronous outbreaks of native bark beetles, causing widespread tree mortality in western forests over the last 15 years. While the causes and consequences of these keystone forest disturbances are often examined separately, disturbance interactions are an important frontier in predicting forest dynamics as climate continues to change. We develop a conceptual framework about key disturbance interactions in subalpine forests that considers: 1) linkages between fire and bark beetles, where one disturbance alters the probability or severity of another, and 2) compound effects of fire and bark beetles, where linked disturbances affect successional trajectories and capacity of the forest system to recover.
Disturbance linkages: While there has been considerable debate about linkages between bark beetles and fire, we present a scaled framework that clarifies some of the apparent lack of consensus. Landscape-scale studies indicate that bark beetles have only weak effect on the probability of large fire occurrence; dry conditions remain the primary trigger in subalpine forests. Stand-scale modeling and observational studies suggest that once a fire occurs, the stage (time since attack) and severity of the bark beetle outbreak can alter fire behavior non-linearly, but apparently less so under extreme burning conditions. Reciprocal linkages between fire and bark beetle are less tightly linked in time, yet fairly clear: stand-replacing fire reduces the probability and severity of bark beetle outbreaks for about 60-80 years.
Compound disturbance effects: Few field studies have quantified the subsequent effect of linked disturbances on regeneration in subalpine forests. Regeneration following fire burning red and grey stages of mountain pine beetle was largely affected by levels of canopy seed storage (serotiny), rather than the timing or severity of the two disturbances. In areas where serotiny is low, disturbances are large and severe, and bark beetle-fire interactions are tightly linked in time, ecological surprises will likely prevail. Warmer, drier post-fire conditions will limit successful post-fire forest regeneration. In contrast, advanced regeneration of established trees persisting in the understory after bark beetle outbreaks, in areas where severe fire had burned almost a century earlier, is expected to be less sensitive to warmer, drier post-disturbance conditions. Overall, the order, timing and severity of disturbances affect the strength and character of disturbance linkages, while subsequent effects of these interactions on forest resilience are primarily mediated by the abundance and age of residuals and/or early-successional climate conditions.