An understated player of Climate Change - increased air humidity - impact on volatile signaling compound emission at northern forests

Informations projet

BioVOLHum

N° de convention de subvention: 798293

DOI

10.3030/798293

Projet clôturé

Date de signature de la CE 9 Avril 2018

Date de début 1 Janvier 2019

Date de fin 31 Decembre 2020

Financé au titre de

• EXCELLENT SCIENCE - Marie Skłodowska-Curie Actions

Coût total

€ 191 325,60

Contribution de l’UE

€ 191 325,60

Coordonné par

HELSINGIN YLIOPISTO
Finland

Periodic Reporting for period 1 - BioVOLHum (An understated player of Climate Change - increased air humidity - impact on volatile signaling compound emission at northern forests)

Période du rapport: 2019-01-01 au 2020-12-31

Résumé du contexte et des objectifs généraux du projet

Global climate scenarios predict that differently from southern regions, where more drought occasions will occur in future, in northern regions climate will be wetter with higher rates of precipitation. We cannot predict very accurately, how wet and cloudy the future climate will be in north, if we have no idea of the effect of forest canopies on future climate. The formation of clouds is not merely important because of increased precipitation probability but also because the more there will be cloudy days, the less sunlight reaches the vegetation. Forests emit highly reactive particles (volatile organic compounds – VOC and nitric oxides - NOx) which play an important role in cloud formation above forests. Just as atmospheric properties (temperature and humidity) affect the emissions of those reactive particles, reactive particles in turn affect atmospheric properties as well. In order to make any assumptions of how cloudy and humid future climate in north will be, we need to understand, how climate affects the emissions of different reactive particles from northern forests. Also, for general society it is important to know about the reactive compound emission potential of different forest species, while in order to mitigate the effect of human-induced warming, the emission potential of different tree species might be an important factor to consider, if one decides, which species of trees will be planted in forestry section.

In current project, our aim was to understand, how increased air humidity and soil moisture affects the emissions of reactive compounds from canopies of silver birches (which is a very common tree species in northern Europe). We studied the changes in reactive compound emissions, that are directly related with higher moisture conditions. We also investigated, how wet climate indirectly affects the physiology of birches and thus also affects the production of volatile reactive compounds. Special interest of this project was to reveal, if the inability of plants to use produced sugars for growth due to non-optimal climate conditions, might increase the emissions of carbon containing reactive particles as carbon is in excess.

Travail effectué depuis le début du projet jusqu’à la fin de la période considérée dans le rapport et principaux résultats atteints jusqu’à présent

During preliminary phase of the study, in spring of 2019 we investigated the datasets available from Finnish experimental site SMEAR II, where VOC emissions from a few birches are continuously measured. We aimed to understand, which humidity–related climate and plant parameters would be best correlated with reactive compound emissions from birches. Soil moisture seemed to affect more clearly VOC emissions, than air humidity, however as variation in air humidity in ambient conditions is high and rarely consistent for longer periods, such dataset did not provide us with substantial information about the effect of humid air on VOC emissions from birches.

As a secondary phase of the study we aimed to measure reactive compound emissions from silver birches at a unique experimental site FAHM (in Estonia), where air humidity is artificially increased in an open-air setting at planted forest plots. Before starting the actual measurements, we needed to construct a system, which would pump manufactured standard air through shoot-chambers in order to absorb reactive compounds, that were emitted exclusively by birch shoots. From the end of May till July 2019 the fluxes of VOC and NOx. Every day the leaf area of intact leaves within chambers was estimated via photos of the shoots. The measurements were not executed during rainy and foggy weather as during those days reactive compounds may dissolve in water droplets. Measurements were done in three control plots and three plots, where air was artificially increased. Additionally also climate factors and different parameters, describing plant physiological characteristics, were measured.

At the third phase of the study we calculated the VOC and NOx fluxes per leaf area, analyzed leaf osmotic potential, performed statistical analysis of the data and presented the preliminary and final results of the project in different seminars and conferences. The data of the project has been made publicly available and the results are going to be published as a scientific paper.

Progrès au-delà de l’état des connaissances et impact potentiel prévu (y compris l’impact socio-économique et les conséquences sociétales plus larges du projet jusqu’à présent)

We found that changed atmospheric humidity, soil moisture and tree physiology had different impact on emissions of different volatile compounds. Most abundant VOC from silver birches where monoterpenes. Monoterpene emissions were highest during drought period, when soil moisture was low, being consistent among different monoterpene compounds. Camphene (a monoterpene) emissions showed strong dependence on leaf hydraulic properties as well. The emissions of sesquiterpenes from birches was much lower in quantity, however, occasionally sesquiterpene oxidation products may still be dominant particle formation precursor (and thus cloud formation precursor) in northern forests. Sesquiterpenes did not show such strong dependence on soil moisture as monoterpenes did, but during times of highest emissions, they were significantly affected by air humidity. In our study sesquiterpene emissions were highest in the middle of the summer, when leaves were fully developed. During that period sesquiterpene emissions were much lower in plots with increased air humidity. Nitric oxide emissions were higher during periods of higher evaporation rates from trees and were significantly affected by leaf water potential as well.
We also found that the abundance of non-structural carbohydrates ( ̴ sugars) within leaves (estimated as leaf osmotic potential) had opposite effect on emissions of monoterpenes versus sesquiterpenes. Monoterpene emissions from birch shoots were positively related to leaf osmotic potential, suggesting that more volatile carbon compounds are emitted when sugars in leaves are not used for plant growth and are accumulating in leaves. However, in case of sesquiterpenes, the relationship was completely opposite as more sesquiterpenes were emitted when leaf osmotic potential was low. This difference suggests different hormonal function of sesquiterpenes within plants when compared to monoterpenes. The finding is somewhat contradicting with previous findings of Norway spruce, where carbohydrate depletion resulted in lower levels of sesquiterpenes. This result shows us, that we are still lacking of knowledge on the function of sesquiterpenes in trees. However, those results also revealed how differently different reactive compounds might be affected by increased humidity.

In conclusion, we found that more moist climate produces significant changes in the ratio of different reactive compounds that are emitted into the atmosphere by birch forests which may have significant impact on atmospheric chemistry in future. Also we concluded that the effect of more humid birch forest on composition of atmosphere in future might depend also on the rate of carbohydrate ( ̴ sugars) accumulation within trees due to the climatic changes as emissions of monoterpenes and sesquiterpenes are differently affected by the latter.

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