Effects of drought and heat on forest insect populations in relation to the 2003 drought in Western Europe

Abstract

-Although drought affects directly tree physiology and growth, the impact of secondary factors (insect pests, pathogens and fire) is often greater than the impact of the original stress and can lead to important tree mortality. In 2003, Western and Central Europe experienced a drought and heat waves that led to extensive forest damage. This paper reports on the impacts of drought and high temperatures on forest insect populations in the context of this exceptional event. Observations of changes in population levels of the main European forest insect pests during and after the drought are presented and discussed in the light of current knowledge and theories of interactions between drought and insects. We investigated the direct effects of drought on life history traits and indirect effects through physiological changes experienced by host trees. Forest pest insects were separated in 4 feeding guilds: woodborers, leaf-chewers, leaf-miners and leaf-suckers. The impact of water stress varied according to feeding guilds. Woodborers were positively influenced by prolonged water stress and the decline of host resistance. In contrast, defoliators profited better from the increased nitrogen in plant tissues linked to moderate or intermittent water stress. Field observations showed the importance of the soil water status in tree resistance against pest attacks. Thus, the 2003 drought confirmed observations from earlier droughts that, is case of bad choice of tree species in some plantations, site matching becomes a prominent and primary cause of the development of pest outbreaks. This exceptional drought may give us some indication of the impacts of extreme climatic events. However, observations of the performance at the individual level were not sufficient for predicting long-term insect population dynamics, which depends on complex interactions between biotic and abiotic factors.climate change / drought / heat wave / forest pest insect / population dynamic Résumé -Effet de la sécheresse et de la canicule de 2003 sur les populations d'insectes ravageurs forestiers en Europe centrale et occidentale. Bien que la sécheresse affecte directement la physiologie et la croissance des arbres, l'impact de facteurs secondaires (insectes ravageurs, pathogènes et feu) est souvent plus important que le stress original et peut conduire à la mortalité des arbres. En 2003, une sécheresse et des vagues de chaleur ont provoqué des dégâts importants dans les forêts d'Europe centrale et occidentale. Cet article rend compte de l'impact de la sécheresse et de la canicule sur les populations d'insectes forestiers dans le contexte de cet évènement exceptionnel. Les observations des fluctuations de populations des principaux insectes ravageurs des forêts européennes sont présentées et discutées en regard des connaissances actuelles et des théories des interactions entre sécheresse et insectes. Nous avons recherché les effets directs et indirects de la sécheresse, respectivement sur les traits d'histoire de vie et au travers des modifica...

Temperate forest trees and stands under severe drought: a review of ecophysiological responses, adaptation processes and long-term consequences

“…As a consequence, tree ring width or leaf area is frequently smaller during several years following a severe drought [12,83]. Moreover, physiological disorders increase tree vulnerability to secondary stresses like insect damage [120], frost or another drought. Fungi may invade weakened trees [37].…”

Interactive effects of drought and pathogens in forest trees

“…This is particularly relevant for insect-vectored diseases such as bark beetle-Ophiostomatoïd fungi [101,171] and Dutch elm disease. Gibbs and Greig [75] reported a high invasion success of the large elm bark beetle in elms during the exceptionally hot, dry summer of 1976 in England.…”

Modelling bark beetle disturbances in a large scale forest scenario model to assess climate change impacts and evaluate adaptive management strategies

“…A shift in climatic conditions could trigger a dramatic expansion of herbivores to host areas currently not susceptible to these disturbance agents. Additionally, an increase in environmental stress factors for the host species may reduce host resilience to infestation by insects (e.g., Rouault et al 2006). Drought stress, for instance, has been identified as a major factor increasing predisposition of Norway spruce to attacks of I. typographus (e.g., Wermelinger 2004;Rouault et al 2006) and is likely to increase under warmer climate conditions.…”