outbreak waves in northern fennoscandia
In northern Fennoscandia, the combined outbreak areas of E. autumnata and O. brumata has a wide east–west extension, i.e. from north-eastern Fennoscandia, over northern Norway and Sweden to north-western Norway, where it reaches into the Ofoten–Vesterålen–Lofoten island chain, i.e. the extreme western offset of northern Scandes. It has been shown (Tenow et al. 2007) that the interperiodic outbreaks in north-western Norway in 1998–2000 were the westernmost endpoints of an outbreak wave moving along this stretch and which branched off from the main wave moving down the Scandes during period XV.
Can the interperiodic outbreaks around 1979 and 1989 be explained in the same way? In midwinter, the average E. autumnata egg freezes at −36 °C (Nilssen & Tenow 1990), which has prevented outbreaks on the cold Finnmarksvidda and in northernmost Sweden (Tenow 1972). In 1987–93, however, the winters were unusually mild in northern Norway (Vaer and Klima, 1989–1995), and hence outbreaks had time to develop on the Finnmarksvidda during period XV (Tenow 1996). This made it possible to track the east–west wave of outbreaks (or population peaks) from north-eastern Fennoscandia via the Finnmarksvidda to north-westernmost Norway (Tenow et al. 2007). In contrast, winters during periods XIII and XIV were normally cold, with egg-killing air temperatures Anonymous 1979–87. Hence, outbreaks could not develop and be tracked on the Finnmarksvidda (cf. lack of reported outbreaks between 22° and 26° E in Fig. 1b). However, with the documented east–west wave in the 1990s (Tenow et al. 2007) as a representation, we interpret the interperiodic outbreaks around 1979 mainly as the outer ripples of a similar western side-wave branched off from the main outbreak wave down the Scandes. In the 1980s, the spatio-temporal pattern differed, i.e. outbreaks at the north-westernmost coast occurred not only at the decadal shift (1988–89), but also in the middle of the decade (1985–86). Presumably, the 1988–89 outbreaks were locally delayed outbreaks or the result of a prolonged outbreak period over most of the north-western coastal area or, even, local populations independent from a common periodicity (see Ims, Yoccoz & Hagen 2004).
synchrony over the scandes
The Scandes and neighbouring areas offer a great variety of living conditions for E. autumnata, O. brumata and other birch-defoliating geometrids (see Materials and methods). Despite these differences, historical documentation shows that outbreaks have been synchronized in distinct outbreak periods along the entire range of the Scandes, and in northern Scandes approximately in the middle of each decade during the period 1895–1968 (outbreak periods V–XII: Tenow 1972). Three types of E. autumnata/O. brumata outbreaks were recognized (see Introduction). Outbreak periods XIII–XV were congruent with this recognition of types. Of these, the ‘moving type’ seems similar to what has been named recently ‘travelling waves’, e.g. outbreak waves of the lepidopteran Zeiraphera diniana Gn. in the European Alps (Bjørnstad et al. 2002).
We could show that the interperiodic local outbreaks at the north-western coast of Norway in 1998–2000 belonged to outbreak period XV (Tenow et al. 2007). Consequently, the interperiodic outbreaks around 1979 and 1989 in the same area should have belonged to periods XIII and XIV, respectively. In contrast, none of the few interperiodic local outbreaks south of about 67° N (Fig. 1) can be referred to any outbreak period, preceding or succeeding it.
outbreak waves and sunspot activity
Are outbreak periods of the moving type compatible with forcing by sunspot activity? None of the studies claiming a forcing of E. autumnata outbreaks by sunspot activity (see Introduction) has posed this question. This may be explained by the fact that these studies were either world-wide surveys concerning several defoliating insects (including E. autumnata: Myers 1998) or studies restricted to parts of the Scandes. The study of Ruohomäki et al. (2000) was limited to outbreaks north of 64° N and that of Selås et al. (2004) to population changes at one single locality, Budal, in central Norway. However, the compatibility of moving population densities and sunspots has been discussed by Williams (1954) concerning the 10-year periodicity in numbers of North American grouse. He argued that if there is an extra-terrestrial forcing, there should be no interregional time lag (of 4–7 years) as there appears to be in grouse occurrence between the Maritime Provinces, the rest of Canada, and Alaska. We agree with this conclusion and apply it to E. autumnata: if there is sunspot forcing of E. autumnata numbers there should be no waves from north to south (or the reverse) in outbreaks as happened, for instance, during period XV. Further, in cases when local outbreaks occur synchronized along the Scandes about mid-decade (in theory possibly synchronized by sunspot activity) there should be no delayed, prolonged or isolated outbreaks in the north at the subsequent decadal shift, as happened during period XIV.
outbreak periods and sunspot activity
We have shown that a data set covering at least 110 years is needed for a reliable test on the dependence of E. autumnata outbreaks on sunspot activity (see ‘Outbreaks and sunspot activity’). During the time-period 1888–2001, i.e. 114 years, outbreaks of E. autumnata were out-of-phase, in-phase and out-of-phase with sunspot numbers at the scale of the entire Scandes (Fig. 3). In addition, the observed interval between two successive in-phase positions of the two cycles, about 100 years, agrees with the expected interval. This incompatibility of the sunspot cycle with the E. autumnata outbreak cycle indicates that the E. autumnata (and probably O. brumata) outbreak periods are not related to sunspot activity. A similar incompatibility of the sunspot series with the 9–10-year cyclic Canadian lynx (Lynx canadensis Desm.) series has been demonstrated (Lindström, Kokko & Ranta 1996; see also Elton & Nicholson 1942: 242).
This observation is at variance with the results of Selås et al. (2004) and Ruohomäki et al. (2000). Selås et al. (2004) compared an E. autumnata data set from Budal (1972–2000) with the four latest sunspot cycles (1971–2000). As in our study, they found a strong correlation between caterpillar maxima and solar activity minima during this (relatively) short time-period. A causal connection was also suggested. During sunspot minima, ultraviolet-B (UV-B) radiation is maximal and the metabolic costs of producing UV-B protective pigments in leaves should reduce the resistance of trees to herbivores.
Ruohomäki et al. (2000) analysed the same set of 12 E. autumnata outbreak periods as in the present study (but with non-quantitative data on E. autumnata occurrence) against the same suite of 11 sunspot cycles. They found that the first year of the 12 outbreak periods were associated significantly with the decline phases of sunspot activity, but causal connections remained unclear. We interpret the results of Ruohomäki et al. (2000) so that the outbreak periods coincided more or less with sunspot minima, i.e. in essence the same conclusion as that of Selås et al. (2004). Because the surveyed time-period (1885–1999) embraced twice as many outbreak starts during sunspot decrease phases as during increase phases (cf. Ruohomäki et al. 2000; Fig. 3), the test method used (G2-test) gave a significant, but spurious, relation between outbreak periods and sunspot minima. If outbreak starts are ordered chronologically, the temporal shifts of the first year of outbreaks will parallel almost exactly the out-of-phase, in-phase and out-of-phase shifts of outbreak and solar cycles over time, as presented here (cf. Pearson's r values in Fig. 3). Hence, outbreaks > 64° N, such as the outbreak over the entire Scandes, seem not to have been forced by sunspot activity.
Eidmann (1931) compared sunspot frequencies and outbreaks of folivorous Lepidoptera on Scots pine in Germany during the period 1830–1925. He concluded that sunspot maxima, particularly since 1870, coincided with outbreak maxima. Myers (1998) analysed the same relation for the same type of insects from a diverse of trees (including conifers) around the Northern Hemisphere. She surveyed the period from the 1930s to the 1990s. Her conclusion was the reverse of Eidmann's, i.e. outbreaks occur during the troughs of sunspot frequencies. In these cases, opposite results were reached depending on which time-period was considered.
Thus, shifts between phases have occurred as predicted for cycles of different periods. Besides, the E. autumnata long-term population fluctuation does not show any apparent sign corresponding to the intermittent periods of phase lock in the 10-year cycle of the Canadian snowshoe hare (Lepus americanus Erxl.), which may indicate a synchronization by sunspot forcing (see Sinclair et al. 1993). Rather, the E. autumnata cycle has drifted continuously (Fig. 3).
A synchronization by sunspot activity demands a climatic link to biosystems. One important cause for interannual and perhaps interdecadal climate variability in Europe, especially during boreal winters, is the North Atlantic Oscillation (NAO), an index of the strength of the westerly airflow across the sea (Hurrell & Dickson 2004). However, the NAO does not relate to the number of sunspots (Selås et al. 2004). It covarys with spring air temperatures in Norway (Selås et al. 2004), but has small or insignificant effects on budburst in B. pubescens (Post & Stenseth 1999). This may imply little or no influence on optimal timing of budburst with egg hatch and, hence, little or no influence on the performance of neonate caterpillars in spring. On the other hand, eggs freeze during cold winters, these in turn being correlated with a low NAO (Klemola et al. 2003; cf. Yoccoz et al. 2002).
Altogether, sunspot activity seems not to be a synchronizer of E. autumnata (and O. brumata) cycles, but NAO could be a candidate. The most commonly suggested mechanisms to explain spatial synchrony include dispersal and regional stochasticity, i.e. the ‘Moran effect’ (Peltonen et al. 2002). Dispersal of E. autumnata and O. brumata seems less likely (Tenow et al. 2007). In addition, interspecific synchrony cannot be explained by dispersal, while weather which is correlated over large areas clearly could (Liebhold et al. 2004 and references therein), possibly in interaction with density-dependent dynamics of similar kind among populations.
Selås et al. (2004) asked: can sunspot activity explain cyclic outbreaks of forest moth pest species? In some systems there may be a significant congruence between population fluctuations and sunspot numbers. That was demonstrated by Klvana et al. (2004) for a 130-year series of indices (feeding scars on tree trunks) of porcupine (Erethizon dorsatum L.) occurrence in a particular part of the Canadian conifer forest region (cf. the snowshoe hare cycle: Sinclair et al. 1993; Selås 2006) Solar–climatic mechanisms are complex and still unclear (e.g. Raspopov, Dergachev & Kolström 2004). A definitive answer must therefore wait. However, according to the data available we answer: sunspot activity cannot explain cyclic outbreaks of E. autumnata and O. brumata.