Derek Johnson Population Ecology Lab
Global Change Biology
Anthropogenically-driven changes are among the greatest problems facing the world over the coming century. Whether global change will increase or decrease the frequency and magnitude of forest insect outbreaks has been debated. My past and current research investigates climatic effects on insect outbreaks. Larch budmoth (Zeiraphera diniana Gn.) outbreaks in the European Alps are detectable through tree ring analysis of the host, the European larch (Larix decidua Mill.). Defoliation by the larch budmoth reduces growth of the host tree, resulting in smaller and denser growth rings. The analysis revealed that larch budmoth populations defoliated host trees every 8-10 years almost without fail for at least 1,200 years before the pattern largely ceased after the early 1980s. Warming trends in the Alps over the last few decades coincided with the disappearance of the outbreaks, suggesting a causal linkage. I analyzed what is perhaps the most spatiotemporally-extensive time series of population dynamics (200 years across 65 locations), which were derived from larch tree ring analysis. I found evidence that larch budmoth outbreaks spread in traveling waves up and down mountain slopes. Annual fluctuations in the outbreak epicenter (i.e. elevations where traveling waves began) correlated with annual changes in mean winter temperature (Johnson et al. 2010 PNAS). In the last century, the outbreak epicenter underwent directional elevational shifts up the mountain slopes, approaching the tree line, before periodic defoliation largely disappeared. I used a tri-trophic model of larch budmoth population dynamics to demonstrate that epicenters are associated with optimal habitat, i.e., elevations with high population growth rates of the moth. These results suggest that the recent warming trend drove an upward shift in the outbreak epicenter and, with winter warming of as little as 2° C, outbreaks were greatly dampened in simulated population cycles. This study suggests that winter warming caused the collapse of outbreak cycles. Insect outbreaks are most common at high latitudes, as are the effects of global climate change, which suggests that outbreaking insects may be inordinately affected by climate change. This publication was the product of a working group at the Centre for Ecological and Evolutionary Synthesis at the University of Oslo, Norway.
Global change over the next century is hypothesized to involve increases in the frequency and severity of extreme climatic events. Atypical climate can be a stressor to trees, potentially making them more susceptible to natural enemies and inviting outbreak. I am currently completing analysis of the effects of drought on outbreak intensity of the southern pine beetle, a native bark beetle in the southeastern US (Johnson et al., In prep). I have found that the effect of drought on southern pine beetle outbreaks is dependent on beetle density the previous year. Specifically, low density populations will not increase in response to drought; however, drought will induce greater outbreaks if beetle populations are already high. This result is consistent with the biology of the system. Pine trees resist southern pine beetle attack by producing resin. The beetles aggregate on host trees through release of a pheromone, which attracts conspecifics. The beetles attack the host tree en mass. Drought acts to increase the thickness of the resin, thus, can increase resistance to low density beetle populations. However, if beetle density is high enough, the beetles can overcome the effects of resin and attack an already stressed tree. This study demonstrates that the effects of climate change on insect outbreaks depends on the ecology of the system.