Coupling Mountain Pine Beetle and Forest Population Dynamics Predicts Transient Outbreaks that are Likely to Increase in Number with Climate Change
by Micah Brush and Mark A. Lewis
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Mountain pine beetle (MPB) have spread well beyond their historical range, with destructive consequences for forests in Canada. We here present and analyze a new model that couples forest growth to MPB population dynamics to address long term questions about the risk of further spread and inform management strategies, particularly under climate change. This model captures key aspects of MPB biology, including a threshold for the number of beetles needed to overcome tree defenses and beetle aggregation that facilitates mass attacks. These mechanisms lead to a demographic Allee effect, which is known to be important in beetle population dynamics. We show that as forest resilience decreases, a fold bifurcation emerges and there is a stable fixed point with a non-zero MPB population. We derive conditions for the existence of this equilibrium. We then simulate biologically relevant scenarios and show that the beetle population approaches this equilibrium with transient boom and bust cycles with period related to the time of forest recovery. As forest resilience decreases, the Allee threshold also decreases. Thus, if host resilience decreases under climate change, for example under increased stress from drought, then the lower Allee threshold makes transient outbreaks more likely to occur in the future.
Image caption:
A graphical representation of the model dynamics. Pine beetles infest trees by mass attack, overcoming host defenses. They overwinter under the bark of the tree before emerging and dispersing the following summer. This is connected to a forest growth model where saplings grow everywhere there is available light before becoming susceptible to pine beetle attack. After they are infested, they die. Tree needles turn red the year following infestation, and are lost the following year, clearing room on the forest floor for new growth.
