The great heat waves and accompanying droughts put forests at the limit of their resilience and expose them more drastically to attack by insect pests. This combination is the trigger for the mortality of large forest masses due to pests. This is why the strength of a forest against massive insect attack depends to a large extent on the intensity and duration of droughts. The only situation that helps forests to resist these pests to a greater extent is living in an optimal environment.
However, a recent study led by CREAF and published in Global Change Biology points out that climate change is increasing the fact that even forests located in optimal environments are affected by the infestation of boring beetles. “The effects of drought are cumulative: forests have memory and the more episodes of lack of water they experience during their history, the more likely they are to succumb,” according to Luciana Jaime González, CREAF researcher and first author of the study.
The global drought is encouraging certain pests that are particularly aggressive against trees to develop intensely, such as the pine borer beetles of the genus Tomicus and the genus Ips. And, at the same time, it can lead to more intense droughts, such as the one we are experiencing in the summer of 2022. According to the researcher, the result of this combination is a lethal cocktail for conifers: “Of the thousands of trees studied, almost 30% are infested by a scolytid, the family of pine borer beetles that attacks them most frequently, and 6% have already died”.
In addition to drought and temperature, there are also other factors that predispose trees to be attacked by insect pests, such as the structure of the forest itself and the composition of its forest species.
Northern, Central and Eastern European forests at risk
In recent years, the number of coniferous forests in Europe that have died directly or from causes associated with insect attacks has increased worryingly. For this reason, this study led by CREAF and the Universitat Autònoma de Barcelona (UAB), has investigated 130 plots across Europe monitored since 2010 to understand how trees respond to insect attack and which are the most exposed.
Besides Luciana Jaime González, the team also includes CREAF researcher Enric Batllori, CREAF researcher and professor of Ecology at the UAB Francisco Lloret, and Marco Ferretti from the Swiss Federal Institute (WSL), has identified that central, northern and eastern Europe are the areas where the pests are becoming more aggressive and infesting more forests. In particular, the scolitid species that are causing the most problems are those that complete more than one life cycle per year, known as multivoltines, because they take advantage of rising temperatures to reproduce more and more times per year. These data “are very important for generating risk maps and hot spots where the scolitids will be more favoured by the high temperatures and associated droughts and can put the integrity of the forest at risk,” explains Luciana Jaime González.
Jaime L, Batllori I, Ferretti M, Lloret F (2022). Climatic and stand drivers of forest resistance to recent bark beetle disturbance in European coniferous forests. Global Change Biology.