Résumé - CAFD

Ecological feedbacks of forest composition manipulations; consequences on insect outbreak regimes and ecosystem stability.

Guillaume Sainte-Marie.

Forest protection against natural disturbance is an important part of forest management. Yet the overall effectiveness of this approach is sometimes questioned in the light of its potentially negative effects on forest ecosystem dynamics. Treatments that change forest composition indirectly or directly (e.g. for simplicity, forest protection, economic or productivity purposes) can affect a whole suite of ecosystem processes like forest succession, productivity and disturbance regimes. They can thus affect the stability of ecosystems, thereby increasing the uncertainty related to forest resource exploitation.

Different cases of unexpected drastic changes in ecosystem dynamics or regime shifts following composition modifications are reviewed in this report. That of Anticosti Island in eastern Canada is particularly relevant for studying insect outbreak regimes and ecosystem stability. The formerly dominant balsam fir-white birch forest ? typically host to the spruce budworm (SBW) and hemlock looper (HL) insect defoliators ? is now intensively browsed by predator-free white-tailed deer and is giving way to pure white spruce forests which are uncommon to eastern North America. Consequently, the abundance of insect parasitoids has decreased as a result of the low diversity of food and habitat availability. This novel ecosystem already gave rise in the early 2000s to a spruce beetle (SB) outbreak of unprecedented duration. The impact on SBW and HL remain uncertain; although white spruce is a primary host for these insects, stands dominated by white spruce do not usually bring these pests to outbreak levels.

The potential impacts on SBW outbreak regime and the interactions with host plant and natural enemy populations are explored using a tritrophic conceptual model. The model suggests that SBW outbreaks in such conditions could be of shorter duration and amplitude, thereby generating lower damage to a white spruce-dominated ecosystem. Due to the decreasing abundance of natural enemies, the role of SB will potentially increase in the future. In terms of forest dynamics, the composition change that occurred is comparable to what occurs in intensively managed forests where forest composition is modified. Forests may become more resistant to the irruptive insect of focus, but not necessarily to other pests. Thus forest composition modifications could trigger regime shifts on susceptible ecosystems, and in such cases increase the uncertainty linked with natural forest management, as opposed to forest protection initial objectives.