Emergent Properties: Bark Beetles, Forest, & Fire Dynamics

Bark beetle infestations are an interesting emergent complication in forest management in a changing climate. As temperature and precipitation patterns change, we a seeing a subsequent shift in ranges of not only plants, but animals as well. The trouble for [most] plants, is that they are not able to physically move northward or upslope as their preferred weather regime slips away year after year. Instead, long-lived plants must stay in place and soldier on through droughts and heat waves which substantially stress the plant and make it susceptible to pests and disease. Conversely, these weather changes are good fortune for most insects through northerly expanded ranges, less lethal winters, and landscapes of sickly trees for easy pickings. After an unusually warm winter most insect populations expand, and critters like bark beetles are no different, multiplying like crazy through a protracted breeding period in new expanses where the vegetation have no effective defenses. The result: acres upon acres of dead and dying trees. 

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Early in the infestation, fine surface fuels may increase as the weakest and oldest trees succumb to beetle predation. Fire during this stage might be low-intensity, low-severity depending on the resilience of the associate vegetation. As the infestation progresses, the proportion of shed limbs and snags increases, increasing fuel continuity and the presence of ladder fuels. Fire at this point has high-intensity potential because the incidence for crown fires increases substantially. Late in the infestation most trees are dead and depending on how many years it took the stand to succumb, a significant portion of these dead trees could now litter the ground increasing both fuel continuity and fuel density. Fire at the end stages of bark beetle infestation would have the highest potential for severity as a surface fire as most of the aerial fuels have likely been removed by this time.

Another consequence of beetle-kill is the elimination of the patchy nature of most wildland fires. A landscape covered with large, continuous fuels may not have any natural fire breaks and will probably be stand-replacing. The regeneration following such a fire will be even-aged and of a different composition than pre-infestation, since the productivity and recruitment of the host species was handicapped for so long. Successive infestations and fires may facilitate the extirpation of once-abundant species as ranges and climate patterns continue to shift.