Our researchers involved in a study demonstrating that enhanced insect herbivory under global change can alter climate
Future warmer climates in northern latitudes facilitate dispersal of many insect herbivores and lead to overall greater plant herbivore damage. Upon herbivore feeding, plants start to emit a plethora of stress-induced volatiles, such as green leaf volatiles that are associated with damaged membranes and de novo synthesized volatile terpenoids that serve as attractants to herbivore enemies. Apart from the immediate biological significance of the stress-induced volatiles, these volatiles could be involved in formation of atmospheric particles, the secondary organic aerosols (SOA), that reduce solar transmission, thereby potentially cooling climate. So far, the research on the role of plant emissions in atmospheric processes has mainly concentrated on constitutive emissions present in only some constitutive emitters, but emissions induced under herbivory stress could quantitatively play a major role too, especially upon severe infection.
To gain quantitative insight into the herbivore effects on SOA formation, Estonian and Finnish teams led by Prof. Ülo Niinemets (Estonian University of Life Sciences) and Prof. Jarmo Holopainen (University of Eastern Finland) (UEF) investigated jointly the responses of northern tree species mountain birch (Betula pubescens subsp. czerepanovii) and silver birch (Betula pendula) to feeding by the larvae of autumnal moth (Epirrita autumnata). Epirrita autumnata larvae are omnivorous and infestations by this moth often result in complete defoliation of extensive forest areas. Over the recent years, the area of dispersal of the moth has extended further to the north and it is expected that in future climates the northern ecosystems will be increasingly impacted by this moth. Upon moth infection, major emissions of green leaf volatiles, and mono- and sesquiterpenes were emitted from impacted birch leaves and the emissions were quantitatively associated with the amount of foliage consumed by herbivores. New particle formation was up to 150 times higher in the air with volatiles from herbivory-infected leaves. This is the first study demonstrating that herbivore feeding enhances SOA formation by plants, and heavier herbivore load predicted in future climates might actually slow down global climate warming.
Citation: Yli-Pirilä P, Copolovici L, Kännaste A, Noe S, Blande JD, Mikkonen S, Klemola T, Pulkkinen J, Virtanen A, Laaksonen A, Joutsensaari J, Niinemets Ü, Holopainen JK (2016) Herbivory by an outbreaking moth increases emissions of biogenic volatiles and leads to enhanced secondary organic aerosol formation capacity. Environmental Science & Technology 50: 11501-11510 (link to full text)
In addition to climate warming, greater herbivore pressure is anticipated to enhance the emissions of climate-relevant biogenic volatile organic compounds (VOCs) from boreal and subarctic forests and promote the formation of secondary aerosols (SOA) in the atmosphere. We evaluated the effects of Epirrita autumnata, an outbreaking geometrid moth, feeding and larval density on herbivore-induced VOC emissions from mountain birch in laboratory experiments and assessed the impact of these emissions on SOA formation via ozonolysis in chamber experiments. The results show that herbivore-induced VOC emissions were strongly dependent on larval density. Compared to controls without larval feeding, clear new particle formation by nucleation in the reaction chamber was observed, and the SOA mass loadings in the insect-infested samples were significantly higher (up to 150-fold). To our knowledge, this study provides the first controlled documentation of SOA formation from direct VOC emission of deciduous trees damaged by known defoliating herbivores and suggests that chewing damage on mountain birch foliage could significantly increase reactive VOC emissions that can importantly contribute to SOA formation in subarctic forests. Additional feeding experiments on related silver birch confirmed the SOA results. Thus, herbivory-driven volatiles are likely to play a major role in future biosphere-vegetation feedbacks such as sun-screening under daily 24 h sunshine in the subarctic.