Interactions among spruce beetle disturbance, climate change and forest dynamics captured by a forest landscape model

The risk of bark beetle outbreaks is widely predicted to increase because of a warming climate that accelerates temperature-driven beetle population growth and drought stress that impairs host tree defenses. However, few if any studies have explicitly evaluated climatically enhanced beetle population dynamics in relation to climate-driven changes in forest composition and structure that may alter forest suitability for beetle infestation. We synthesized current understanding of the interactions among climate, spruce beetles (Dendroctonus rufipennis) and forest dynamics to parameterize and further advance the bark beetle module of a dynamic forest landscape model (LandClim) that also integrates fire and wind disturbance and climate-driven forest succession. We applied the model to a subalpine watershed in northwestern Colorado to examine the mechanisms and feedbacks that may lead to shifts in forest composition and spruce beetle disturbance under three climate change scenarios. Simulation results suggest increased drought- and beetle-induced reduction of large Engelmann spruce (Picea engelmannii) trees while Douglas-fir (Pseudotsuga menziesii) and ponderosa pine (Pinus ponderosa) increased in dominance throughout the study area under all climate change scenarios. This shift in forest composition and structure counterbalances the enhancing effects of accelerated beetle population development and increased drought-induced susceptibility of spruce to beetles. As a result, we projected a long-term decrease in beetle-induced spruce mortality to below historical values under all climate scenarios at low elevations (<2800 m asl). Beetle-induced spruce mortality above 2800 m asl and under moderate climate change was slightly higher and more variable than under historical conditions but decreased to 36% and 6% of historical values under intermediate and extreme climate change, respectively. Because mechanisms driving beetle disturbance dynamics are similar across different bark beetle species, we argue that the depletion of host trees due to drought and beetle disturbance may also be important in other climate-sensitive beetle-host systems. We advocate for the consideration of climate-driven shifts in forest and disturbance dynamics in devising adaptive management strategies.