Insects are part of our ecosystems and are well integrated in the dynamics of forest landscapes. However, high-intensity episodes, known as outbreaks, can have strong negative impacts to host plants and, consequently to the whole forest ecosystem. Direct impacts of forest insects are widely studied, but knowledge of their indirect effects through interactions with other disturbances is still lacking. A recent review paper shed some light on this topic, which is crucial in an era of climate change and increasing disturbances.  

When you walk in a forest it seems like nothing is happening. Trees are there, still and firm, immobile, slowly growing of a few centimetres per year. However, forest landscapes are a highly dynamic environment. They are chronically affected by a series of abiotic factors, such as climatic regimes, soil characteristics, topography, which influence the way they develop over time. But such drivers can also enter into acute phases, such as windstorm, fire and drought. They are known as forest disturbances, abrupt event that cause sudden structural and compositional changes of a forest ecosystem. Among them, biotic disturbances, such as outbreaks of insect pests (e.g., defoliators, wood and bark borers), can also drastically alter our forests. Insect outbreaks interact in one way or another to all other disturbance events. Additionally, given rapid climate change and global trade, new invasive insect species can lead to new interactions and impacts on forest ecosystems never experienced before.

A recently published article in the journal Landscape Ecology, provide a systematic literature review on the interactions between insect pests and other forest disturbances. In this paper, authors detailed the processes determining how an insect pest can interact with other disturbances, highlighting the main elements conditioning these interactions and identifying potential future interactions due to global change.

What interactions are more frequently studied?

An international team of researchers led by scientists from the InFOREST JRU, a joint research venture by Forest Sciences and Technology Centre of Catalonia (CTFC) and CREAF in Spain, collected 216 articles published between 1990 and 2019 related to forest insect outbreaks and their interaction with fire, drought, windstorms, snow, pollution and others forest disturbances.

They found many studies focusing on the interaction between forest insects and fire. This has been extensively studied. Forests affected by insect pests typically accumulate amounts of fuel that promotes the ignition and spread of fire. Some articles argue that the relationship is not synergistic and that, precisely, tree mortality reduces fuel connectivity and therefore the spread of fire. Interactions in which insects act as a result of fires are also described, as burned trees have fewer defenses against a pest attack. Another interaction that have been receiving lots of attention in the scientific literature is the one between insects and drought. Water scarcity can increase insect´s attraction to trees, which are weakened in their defense system and thus more susceptible. But if drought is so strong to kill trees and shaping structure and composition, insects might not be able to establish.

Then there are many other disturbances that interact with insect pests but that have received less attention in research studies. Forest management, for example, a human-induced disturbance that typically alters forest stand composition and structure, may favor or hinder the presence of insects. But also other diseases and pests (e.g.. pathogens), either because insects play a role as a vector or through interspecific competitive relationships, or pollution, storms, avalanches, and landslides.

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What defines such interactions and what tools can we use?

Most of the studies reviewed indicated that the interaction between disturbances was significant and synergistic. However, some research papers concluded that such connections did not occur. This review attempted to identify the factors that define the existence or non-existence of interactions. The researchers analyzed whether the frequency of interactions reported is different according on the temporal and spatial scale, but also the trophic group of the insects involved, the order of the interaction (i.e., if the insect outbreaks occurs before or after the other disturbance), and the type of response reported. Their analysis showed that, although such factors were studied in different frequency (see figure below), there was no clear relationship between any of them and the presence or absence of interaction. Several authors, however, highlighted the importance of differentiating interactions in terms of probability of occurrence and in terms of severity of the latter disturbances. This differentiation is essential both in the study of forest disturbances and in their management.

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The role of simulation models

The review also dedicates a specific section to those articles that used simulation models to investigate these interactions. Only a few papers (23, 11%) applied such tools to explore this topic, and most of them focused on insect outbreaks vs fire. Since disturbances are usually studied at landscape scale, the models most applied were spatially explicit forest landscape simulation models. These models, although representations of real systems constructed from the combination of theoretical knowledge and mathematical concepts and based on empirical information, they are likely the most appropriate to study forest disturbances, their interactions and, in particular, those involving insect pests. This because they allow the study of impacts at different spatial scales simultaneously (e.g., effects on trees individually and in the landscape in general), portraying long-term dynamics and anticipating them in the future. Although the use of simulation models has been increasing in recent decades, only a handful of articles used this methodology to approximate the interaction between insects and other disturbances. Therefore, the researchers called for the use of simulation models and highlighted their usefulness in the study of disturbances.

An example of a model able to simulate multiple insect outbreaks is the modelling platform LANDIS-II. This modelling framework allows using different extensions, which are maintained by a large community by scientists, mostly based in North America. Here below an example of multiple insect outbreaks being simulated in LANDIS-II for a landscape in Quebec, Canada. Outbreaks are simulated according to the severity of outbreak damage across the landscape, which depends on the abundance of available tree species host being simulated according to multiple scenarios of climate change and management. Another promising approach is BITE, a general model to simulate the dynamics of biotic disturbance agents, with a special focus on being able to accommodate also emerging pests and pathogens. BITE can be easily coupled with the forest landscape model iLand.

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New interactions and challenges

As impacts due to insect pests are expected to increase with the intensification of global change, it is now all the more important to study these topics. A better understanding of potential interactions between insect pests and other disturbances, as well as the emergence of new interactions, would also allow us to better plan management interventions to prepare our forest landscapes to cope with such future events.

Read here the paper: Canelles, Q., Aquilué, N., James, P. M., Lawler, J., & Brotons, L. (2021). Global review on interactions between insect pests and other forest disturbances. Landscape Ecology, 1-28.

Authors of the post

Quim Canelles Trabal is a forest ecologist from Centre de Ciència i Tecnologia Forestal de Catalunya (CTFC). His research is related to landscape dynamics and forest disturbances in a context of global change. He worked mainly on forest insect pest impacts and the arrival of new alien species, using simulation landscape models approaches to describe current dynamics and to project future environmental impacts.

Marco Mina is a forest ecologist, currently research fellow at Institute of Alpine Environment of Eurac Research, Italy. His research interests include the dynamics of temperate forest ecosystems and anticipating the impacts of global and climate change using simulation models of forest dynamics to explore interactions of trees with their changing environment and to investigate adaptive management strategies.

Featured image: a forest landscape affected by bark beetle outbreaks (Pixabay).