Current issue: 54(2)
There is no doubt that tree survival, growth, and reproduction in North America's boreal forests would be directly influenced by the projected changes in climate if they occur. The indirect effects of climate change may be of even greater importance, however, because of their potential for altering the intensity, frequency, and perhaps even the very nature of the disturbance regimes which drive boreal forest dynamics. Insect defoliator populations are one of the dominating disturbance factors in North America's boreal forests and during outbreaks trees are often killed over vast forest areas. If the predicted shifts in climate occur, the damage patterns caused by insects may be considerably changed, particularly those of insects whose temporal and spatial distributions are singularly dependent on climatic factors. The ensuing uncertainties directly affect depletion forecasts, pest hazard rating procedures, and long-term planning for pest control requirements. Because the potential for wildfire often increases in stands after insect attack, uncertainties in future insect damage patterns also lead to uncertainties in fire regimes. In addition, because the rates of processes key to biogeochemical and nutrient recycling are influenced by insect damage, potential changes in damage patterns can indirectly affect ecosystem resilience and the sustainability of the multiple uses of the forest resource.
In this paper, a mechanistic perspective is developed based on available information describing how defoliating forest insects might respond to climate warming. Because of its prevalence and long history of study, the spruce budworm, Choristoneura fumiferana Clem. (Lepidoptera: Tortricidae), is used for illustrative purposes in developing this perspective. The scenarios that follow outline the potential importance of threshold behaviour, historical conditions, phenological relationships, infrequent but extreme weather, complex feedbacks, and natural selection. The urgency of such considerations is emphasized by reference to research suggesting that climate warming may already be influencing some insect lifecycles.
The European Pine Sawfly (Neodiprion sertifer Geoffroy) is one of the most serious defoliators of Scots pine (Pinus sylvestris L.) in northern Europe. We studied the pattern in the regional occurrence of the outbreaks of N. sertifer in Finland in years 1961-90, and made predictions about the outbreak pattern to the year 2050 after predicted winter warming. We tested whether minimum winter temperatures and forest type and soil properties could explain the observed outbreak pattern. We analysed outbreak patterns at two different spatial levels: forest board- and municipal-level.
The proportion of coniferous forests on damage-susceptible soils (dry and infertile sites) explained a significant part of the variation in outbreak frequency at small spatial scale (municipalities) but not at large spatial scale (forest boards). At the forest board level, the incidence of minimum temperatures below -36 °C (= the critical value for egg mortality) explains 33% of the variation in the outbreak pattern, and at the municipal level the incidence of cold winters was also the most significant explaining variable in northern Finland. Egg mortality due to cold winters seems to be the most parsimonious factor explaining why there have been so few N. sertifer outbreaks in northern and north-eastern Finland. We predict that climate change (increased winter temperatures) may increase the frequency of outbreaks in eastern and northern Finland in the future.