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Herbivore-induced emissions of biogenic volatiles from arctic plants under climate warming

Periodic Reporting for period 1 - HIVOL (Herbivore-induced emissions of biogenic volatiles from arctic plants under climate warming)

Período documentado: 2018-04-01 hasta 2020-03-31

Biogenic volatile organic compounds (BVOCs) are reactive trace gases with significant impacts on air quality and ecosystem processes. Terrestrial BVOC emission and composition can respond strongly to climate change. The Arctic is warming at twice the rate of the global average. Early studies by my host have shown that warming can increase Arctic BVOC emissions several folds, transforming the naturally low-emitting Arctic to a stronger BVOC source. Yet, it remains largely unknown what controls BVOC emissions from Arctic ecosystems, particularly about the impacts of insect herbivory. This greatly limits our understanding of the complex abiotic and biotic processes controlling Arctic BVOC emissions.

Therefore, HIVOL built on previous Arctic BVOC studies from my host, long-term climate change manipulation sites in the Arctic maintained by my host and collaborators, and recent advances in BVOC analytical methods and modeling to rigorously investigate how Arctic warming and associated changes in insect herbivory pressure interact to influence Arctic BVOC emissions. Specifically, it attempted to assess 1) how climate warming influences insect herbivory on dominant woody plants in the Arctic, 2) how insect herbivory, both alone and in conjunction with warming, influences BVOC emissions, and 3) how to integrate herbivory effects into the existing emission model to estimate these effects at landscape scale.

The results of HIVOL have improved our comprehension of the factors driving Arctic BVOC emissions as well as the ecological and atmospheric functioning of BVOCs in the rapidly changing Arctic. Furthermore, they have direct applications in developing sustainable pest management strategies in Arctic ecosystems, particularly in boreal mountain birch forests, which are increasingly threatened by frequent outbreaks of geometrid moths due to ongoing Arctic warming.
Due to parental leaves, the project indeed covered three seasons (2018-2020). Thus far, four publications have been delivered with more being peer-reviewed or prepared. The results of HIVOL have been discussed at two congresses and several seminars. Due to COVID-19, an invited oral presentation on the International Congress of Entomology in 2020 has been postponed to 2021.

Toward Task 1 in WP1, I did a three-season field observation to study the impacts of warming on insect herbivory. I found that experimental warming consistently and substantially increased leaf damage by chewing insects, irrespective of warming duration. This work is under preparation for publication.

Toward Task2 in WP1, I conducted several field experiments to study combined impacts of warming and insect herbivory on BVOC emissions. Plant species involved included dwarf birch, dwarf willow, and mountain birch. Insect species included geometrid moth caterpillars and gall-inducing mites. I leveraged long-term field sites with experimental warming for varying durations (1 month, 9 years, 19 years, and 30 years). In dwarf birch-caterpillar systems, I found that both warming and insect herbivory substantially increased birch BVOC emissions, with a strong synergistic effect between warming and herbivory. This is the first documentation showing the strong herbivory effect and the synergy between warming and herbivory on BVOC emissions in the Arctic. In the dwarf willow-mite system, I found a clear warming effect on willow isoprene emissions, but did not detect an effect of gall infestation and its interaction with warming. In the mountain birch-caterpillar system, I found that caterpillar feeding significantly increased mountain birch BVOC emissions, particularly at low elevations where herbivory pressure was higher in the early season. BVOC emissions varied considerably over the season, following seasonal variation in temperature and light. One publication have stemmed from these experiments with one manuscript being under review and three manuscripts being submitted soon.

Toward Task3 in WP2, I did two field measurements investigating the quantitative relationship between BVOC emissions and herbivory severity. The first one investigated BVOC responses to background herbivory (0–10% defoliation), while the second one covered a broader range of herbivory severity (0–100% defoliation). I found that mountain birch BVOC emissions increased linearly with herbivory severity. One manuscript was recently accepted with one more under preparation.

Toward Task4 in WP2, I run a laboratory experiment in growth chambers using mesocosms collected at our field site to assess joint effects of warming and insect herbivory on BVOC emissions and CO2 exchange at micro-ecosystem scale. The experiment was conducted from 2020 June–August due to delayed delivery of customized growth chambers. I have not processed the data, but my visual inspection indicated that warming strongly increased ecosystem isoprene emissions, particularly during heat wave.

Toward Task5 in WP3: I used remote sensing to map geometrid moth outbreaks in boreal mountain birch forests and integrated it, along with field measurements of quantitative BVOC vs herbivory responses, with the MEGAN model to assess the impacts of moth outbreaks on BVOC emissions at regional scale. Data are still under processing and will be published in the coming year. Preliminary simulation indicated a large effect of birch leaf defoliation on BVOC emissions during outbreaks.
As the Arctic is undergoing unprecedented change, it is very important to characterize terrestrial BVOC emissions and determine the primary drivers of BVOC emissions. HIVOL is the first to provide empirical evidence that insect herbivory, a hitherto underappreciated engineer of Arctic ecosystems, can be a crucial determinant of Arctic BVOC emissions. It also gives first indication that ongoing Arctic warming will amplify plant responses to insect herbivory in terms of BVOC emissions. These findings beg the question if similar processes are occurring in other plant-insect interaction systems in the Arctic. BVOCs are known to mediate species interactions and act as direct and indirect plant defenses against herbivores. In addition, BVOC emissions may create a negative climate forcing feedback by contributing to aerosol and cloud formation - and thereby to cooling of climate. Due to the minimal anthropogenic pollution, BVOCs have a higher relative importance in the Arctic than in more inhabited areas. In this regard, the results of HIVOL open up new questions about the likely consequences of BVOC emissions on trophic interactions in Arctic ecosystems and the potential feedbacks on regional climate.

Insect outbreaks are extreme events and typically occur sporadically. However, ongoing climate changes are the frequency and severity of insect outbreaks, particularly in the northern hemisphere where insects have not reached their optimal growth. This can potentially cause long-lasting effects on terrestrial carbon dynamics, including BVOC emissions. The boreal mountain birch forests have been suffering from periodic outbreaks of different geometrid moth species. This project provides an urgently needed quantitative understanding of mountain birch BVOC emissions in response to insect outbreaks. It sheds new insights into the role birch foliar chemistry plays in controlling moth population dynamics, and the mechanistic interpretation of differential susceptibility of different birch populations to geometrid moth outbreaks.
Graphical summary of project main findings