The spatial patterning of population fluctuations is a fundamental emergent property of biological populations. Pestilence, an important class of biotic disturbance in forests, is almost defined by large population fluctuations with high spatial synchrony. However, we lack a general understanding of what produces the range of spatial patterning in population dynamics. The three general processes that can influence spatiotemporal patterns of abundance are dispersal, climate, and community interactions. We have been attempting to develop and test process-based models of spatiotemporal dynamics for an episodically abundant primary consumer, the southern pine beetle (SPB). SPB population fluctuations are synchronized over hundreds of kilometers, well beyond that expected by dispersal distances, and the east-west correlations exceed the north-south correlations (anisotropy). This implicates climatic variation as a driver but there are numerous plausible mechanisms.
Results/Conclusions
Extreme winter temperatures are of demonstrable importance to northern SPB populations. Average interannual change in beetle abundance matched theoretical predictions of a process-based model of winter mortality from low temperatures; an increase in average minimum winter temperature (3.3̊C from 1960-2004) has apparently contributed to a spate of recent outbreaks near its northern limit where it has been historically rare. However, winters do not get cold enough to kill beetles in most forests where SPB occurs. Mortality from extreme summer temperatures is a more promising possibility for climatic effects on beetle abundance in the southern U.S. In fact, the hot summers of 1980 and 1996 coincided with the termination of beetle outbreaks in Texas and a model that includes the number of days >32̊C explained 59% of the variation in interannual growth rates of SPB in Texas. Spatiotemporal variation in temperatures also affects SPB phenology, most importantly the synchrony of dispersal, aggregation, and attacks of host trees, which affects population fluctuations. Through the combination of these three processes, climatic trends may be permitting outbreaks in northern regions, where SPB has historically been rare, reducing outbreaks in the south where lethally warm temperatures are now common, and producing sustained risks of outbreaks in forests in the southern Appalachian region where outbreaks have been only occasional in the past.
