Climatic Inferences from Dendroecological Reconstructions - Dendroclimatology - page 273

The subject of potential climate entrainment of insect outbreaks—and on animal
population dynamics in general—has been debated in the ecological literature for
decades (e.g., White 1976 ; Royama 1984 ; Martinat 1987 ; Mattson and Haack 1987 ) .
A variety of theories have emerged, including the famous 'Moran effect,' based
on Moran's theorizing about the oscillatory behavior and regionally synchronized
lynx-hare populations of Canada (Moran 1953 ) . In the case of the western spruce
budworm, it seems likely that favorable moisture conditions may act through an
increase in the quantity and improvement in the quality of tree foliage, the food base
for these defoliators (Swetnam and Lynch 1993 ) . Although the exact mechanisms
remain unidentified, we think the consistent, coincident patterns in the tree-ring data
of wet periods/outbreaks, and droughts/endemic phases over multiple centuries and
large regions are quite compelling evidence that decadal moisture variability is an
important driver of budworm populations.
Notable features of forest insect outbreak reconstructions from tree rings are
the remarkable cycles or quasi-cycles that are evident in these time series (e.g.,
see Speer et al. 2001 ) . The famous larch budmoth ( Zeiraphera diniana ) of central
Europe is the clearest example of relatively strong cyclicity in forest insect popula-
tions. These outbreaks produce a distinct tree-ring signature of defoliation events,
evident as sharply reduced ring widths and densities that have allowed for recon-
structions of up to 1200 years in length (Weber 1997 ; Esper et al. 2007 ) . The recent
work of Esper et al. ( 2007 ) demonstrates a remarkable regularity and stability of
larch budmoth cycles in the European Alps over the past millennium, with an aver-
age period of 9.3 years between outbreaks. The most notable finding in this paper
is that the larch budmoth reconstruction (832-2004 CE) reveals the recent period
(since 1981) is the most unusual in the entire reconstruction, with an unprecedented
absence of outbreaks. Esper et al. ( 2007 ) show that this period corresponds to an
unprecedented increase in temperatures in this region (from an independent tree-ring
reconstruction), and conclude that nutrient cycling and other ecosystem processes
operating in the Alps may be undergoing a drastic alteration. Although the exact
mechanisms of this change are not known, they speculate that warming winter tem-
peratures may have led to earlier emergence of larvae from their eggs in the late
winter or spring before larch tree needles have emerged, leading to starvation and
failure of the larch budmoth populations to enter an outbreak phase.
Our regional time series of western spruce budworm typically show more vari-
able oscillatory behaviors than is the case with larch budmoth, with considerable
variability in the periods between budworm outbreaks. Hence, budworm population
fluctuations appear not to be very strongly cyclic. Nevertheless, as much as 50% to
60% of the variance of the regional composite time series is explained by cycles of
about 25-35 years (Swetnam and Lynch 1993 ; Ryerson et al. 2003 ) . Considering
that these data are derived from tree rings, where cycles of this strength are vir-
tually never observed in raw ring-width series from non-host trees, this in itself is
a remarkable characteristic of defoliated host trees. Moreover, in cross-correlation
and cross-spectral analyses there appears to be some coherence of these moderate-
strength outbreak periodicities, and much weaker but similar periodicities in climate
reconstructions (Swetnam and Lynch 1993 ) .