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GEO-6 Regional Assessment for North America


Box 2.8.2: Mountain pine beetle outbreaks: natural selection for climate change adaptation?


Insect outbreaks are increasing in size and severity on a global scale, attributable to climate change (Allen et al 2010; Bentz et al. 2010). In North America, the current mountain pine beetle (Dendroctonus ponderosae) outbreak is an order of magnitude larger than any bark beetle outbreak previously recorded (Mitton and Ferrenberg, 2012; Raffa et al. 2008). Outbreaks have played a major role in the evolution of coniferous forest ecosystems, as bark beetles are largely dependent on availability of stressed, dying, or recently-dead host trees. As well, insect populations typically decline as the supply of such trees is exhausted (Schowalter 2014). Historically, bark beetle outbreaks end with cold weather at crucial points in a population’s potential eruptive dynamic (Six et al. 2014). Outbreaks erupt only when multiple thresholds–involving temperatures, tree defences, and brood productivity–are all surpassed, allowing positive feedbacks to amplify across several scales (Raffa et al. 2008). While outbreak development is complex, the primary elements that must exist are an abundance of suitable hosts and a trigger (Bentz et al. 2010).


The warming North American climate and the severe drought conditions in the West and Northwest have supported, and triggered, the mountain bark beetle outbreak in two ways (Allen et al. 2015). First, the hot and dry conditions stress trees, leaving them vulnerable to infestation; and second, the year-round warming conditions encourage an extension of the breeding season and the breeding territory (Showalter 2015; Mitton and Ferrenberg 2012). By 2010, researchers verified an extent eastward into Alberta, with an unprecedented species jump to infest jack pine (Pinus banksiana), the dominant boreal forest species in North America (Cullingham et al. 2011). This eastward progress suggests the pest may reach across the continent’s northern boreal forest and prey on eastern forests (Natural Resources Canada 2015; Cullingham et al. 2011; Carroll et al. 2004; see Figure 2.8.5).


Since the outbreak began in the 1990s, standard management practices have involved various methods of removing trees, either to isolate infested stands or to thin them. A number of recent studies looking at the trees remaining after a beetle infestation asked whether the forest is better off with management or without (Kayes and Tinker 2012; Diskin et al. 2011). These studies highlight an elusive, but intriguing, outcome after a beetle outbreak: that an outbreak seldom removes all mature trees and can act as a natural agent to thin a stand of trees. These outcomes could be an important part of the ecological role that the beetle plays in western pine forests (Hansen 2014). The trees that survive the infestation in unmanaged areas remain because of inherent resilience, usually the result of genetic variation. In contrast, the trees that survive in managed areas remain because foresters selected which trees to remove. The possibility presents itself: stands that survive beetle outbreaks may be a variety with resistance to beetle outbreaks and to hotter climate (Six et al. 2014; Millar et al. 2012).


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