One of the characteristics of global climate change is the increase in extreme climate events, e.g., droughts and floods. Forest adaptation strategies to extreme climate events are the key to predict ecosystem responses to global change. Severe floods alter the hydrological regime of an ecosystem which influences biochemical processes that control greenhouse gas fluxes. We conducted a flooding experiment in a mature grey alder (Alnus incana (L.) Moench) forest to understand flux dynamics in the soil-tree-atmosphere continuum related to ecosystem N 2 O and CH 4 turn-over. The gas exchange was determined at adjacent soil-tree-pairs: stem fluxes were measured in vertical profiles using manual static chambers and gas chromatography; soil fluxes were measured with automated chambers connected to a gas analyser. The tree stems and soil surface were net sources of N 2 O and CH 4 during the flooding. Contrary to N 2 O, the increase in CH 4 fluxes delayed in response to flooding. Stem N 2 O fluxes were lower although stem CH 4 emissions were significantly higher than from soil after the flooding. Stem fluxes decreased with stem height. Our flooding experiment indicated soil water and nitrogen content as the main controlling factors of stem and soil N 2 O fluxes. The stems contributed up to 88% of CH 4 emissions to the stem-soil continuum during the investigated period but soil N 2 O fluxes dominated (up to 16 times the stem fluxes) during all periods. Conclusively, stem fluxes of CH 4 and N 2 O are essential elements in forest carbon and nitrogen cycles and must be included in relevant models.Greenhouse gases (GHG), in particular, methane (CH 4 ) and nitrous oxide (N 2 O) contribute 16% and 6% to global warming, respectively 1 . In addition, N 2 O is a dangerous stratospheric O 3 layer depleting agent 2 . Due to the increasing emissions, both gases have high radiative forcing potential. In principle, terrestrial biosphere may be seen as a net source of GHG to the atmosphere 3 . Temperate as well as tropical forest soils (in general) seem to be a central natural emitting source of N 2 O, on the one hand, a natural sink of CH 4 on the other 4-9 . Flux estimations of N 2 O and CH 4 in forest systems are mainly based on studies of forest soil measurements, usually excluding exchange potential of vegetation 5,7,10 . Nevertheless, investigations on GHG fluxes from plants in wetland or riparian ecosystems show that plants, especially trees, can be essential sources of CH 4 and N 2 O 9,11-13 . However, recent studies uncover the relevance of tree stem surfaces playing an important role in understanding GHG dynamics in different forest ecosystems 8,9,14 .Grey alder (Alnus incana (L.) Moench)) is a fast-growing, pioneer tree species with excellent potential for short-rotation forestry in the Northern hemisphere [15][16][17][18] . Due to the symbiotic Frankia bacteria which fix atmospheric nitrogen, alder forests are important nitrogen sequestering ecosystems 19,20 . Decomposition of nutrient-rich alder litter improves soil properti...

Abstract. Secondary organic aerosols (SOAs) formed from biogenic volatile organic compounds (BVOCs) constitute a significant fraction of atmospheric particulate matter and have been recognized to significantly affect the climate and air quality. Atmospheric SOA particulate mass yields and chemical composition result from a complex mixture of oxidation products originating from a diversity of BVOCs. Many laboratory and field experiments have studied SOA particle formation and growth in the recent years. However, a large uncertainty still remains regarding the contribution of BVOCs to SOA. In particular, organic compounds formed from sesquiterpenes have not been thoroughly investigated, and their contribution to SOA remains poorly characterized. In this study, a Filter Inlet for Gases and Aerosols (FIGAERO) combined with a high-resolution time-of-flight chemical ionization mass spectrometer (CIMS), with iodide ionization, was used for the simultaneous measurement of gas-phase and particle-phase oxygenated compounds. The aim of the study was to evaluate the relative contribution of sesquiterpene oxidation products to SOA in a springtime hemiboreal forest environment. Our results revealed that monoterpene and sesquiterpene oxidation products were the main contributors to SOA particles. The chemical composition of SOA particles was compared for times when either monoterpene or sesquiterpene oxidation products were dominant and possible key oxidation products for SOA particle formation were identified for both situations. Surprisingly, sesquiterpene oxidation products were the predominant fraction in the particle phase in some periods, while their gas-phase concentrations remained much lower than those of monoterpene products. This can be explained by favorable and effective partitioning of sesquiterpene products into the particle phase. The SOA particle volatility determined from measured thermograms increased when the concentration of sesquiterpene oxidation products in SOA particles was higher than that of monoterpenes. Overall, this study demonstrates that sesquiterpenes may have an important role in atmospheric SOA formation and oxidation chemistry, in particular during the spring recovery period.

Abstract. Establishment of the SMEAR Estonia at a hemiboreal mixed deciduous broad-leaved-evergreen needle-leaved forest at Järvselja, South-Eastern Estonia, has strongly enhanced the possibilities for national and international cooperation in the fi elds of forest ecosystem -atmosphere research and impacts of climatic changes on forest ecosystems, atmospheric trace gases, aerosols and air ions. The station provides a multitude of comprehensive continuously measured data covering key climatic and atmospheric characteristics (state and dynamics of solar radiation, trace gases, aerosols and air ions, meteorological parameters) and forest ecosystem traits (net primary productivity, individual tree growth, gas-exchange characteristics, soil variables). The station follows a multidisciplinary and multiscale approach covering processes in spatial dimensions ranging from nanometres to several hundred square kilometres, being thus able to signifi cantly contribute to worldwide measurement networks and the SMEAR network. Here we present an overview of the station, its data produced and we envision future developments towards sustainable research and development of the large-scale scientifi c infrastructure SMEAR Estonia.

Aims The forest floor is a significant contributor to the stand-scale fluxes of biogenic volatile organic compounds. In this study, the effect of tree species (Scots pine vs. Norway spruce) on forest floor fluxes of volatile organic compounds (VOC) was compared in boreal and hemiboreal climates. Methods Monoterpenoid and sesquiterpene flux rates were determined during 2017-2018 using dynamic (steady-state flow-through) chambers placed on permanent soil collars on boreal and hemiboreal forest floors, where the canopy was formed by Scots pine (Pinus sylvestris), Norway spruce (Picea abies) or dominated by Scots pine with small coverage of Norway spruce and birches (Betula pendula and Betula pubescens). Results The total monoterpenoid fluxes were higher from the Scots pine forest floor (boreal 23 μg m −2 h −1 ) and from the mixed forest floor (hemiboreal 32 μg m −2 h −1 ) compared to the Norway spruce forest floor in both boreal (12 μg m −2 h −1 ) and hemiboreal (9 μg m −2 h −1 ) climates. Due to higher litterfall production, the forest floor seems to be a greater source of monoterpenoids and sesquiterpenes in the hemiboreal mixed stand dominated by Scots pine compared to the boreal Scots pine stand, although the difference was not statistically significant. Forest floor VOC fluxes followed a similar seasonal dynamic in different forest stands, with the highest flux rates in spring and summer. Significant VOC sources in the boreal forest floor were synthesis and release from vegetation and living roots together with litter decomposition of fungi and other microbes, where VOCs are released from needle storage pools. Ground vegetation in the hemiboreal forest is scanty under the dense tree canopy, meaning soil processes, such as litter decomposition, microbial metabolism and root release, were likely the dominating VOC sources. VOC fluxes from the hemiboreal forest floor were reduced by increases in soil moisture.Conclusions This study indicates that if the warming climate changes tree species' abundance and stand biomass, by increasing tree growth and coverage of broadleaf species, it may affect VOC fluxes from the forest floor and impact the total VOC emissions from northern soils.

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The potential of carbonyl sulfide (COS) flux measurements as an additional constraint for estimating the gross primary production depends, among other preconditions, on our understanding of the soil COS exchange and its contribution to the overall net ecosystem COS flux. We conducted soil chamber measurements of COS, with transparent chambers, in three different ecosystems across Europe. The in situ measurements were followed by laboratory measurements of soil samples collected at the study sites. The soil samples were exposed to UV radiation to investigate the role of photo-degradation for COS exchange. In situ and laboratory measurements revealed pronounced intersite and intrasite variability of COS exchange. In situ COS fluxes were primarily governed by radiation in the savannah-like grassland (SAV), soil temperature and intrasite heterogeneity in the deciduous broadleaf forest, and soil water content and intrasite heterogeneity in the evergreen needleleaf forest. The soil of the ecosystem with the highest light intensity incident on the soil surface, SAV, was a net source for COS, while the soils of the other two ecosystems were COS sinks. UV radiation increased COS emissions and/or reduced COS uptake from all soil samples under laboratory conditions. The impact of UV on the COS flux differed between soil samples, with a tendency toward a stronger response of the COS flux to UV radiation exposure in samples with higher soil organic matter content. Our results emphasize the importance of photo-degradation for the soil COS flux and stress the substantial spatial variability of soil COS exchange in ecosystems.

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The development of fertility of soils on different parent rocks is considered for different types of spruce forests in the Khibiny Mountains. The spruce forests of Mts. Kuel'por, Vud'yavrchorr, Chil'mana, and Saami were the objects for the study. The results showed that the fertility level of the soils of the Khibiny Mountains was determined by the combined influence of the parent rock's composition and the vegetation. The differences in the soil properties are mainly explained by the composition of the parent rocks. The pod burs differ from the podzols by the higher contents of organic matter, nitrogen, and available nutrients. The podzols are the most acid soils there. The podburs of Mt Kuel'por developing on base rich parent rocks are the most fertile. The differences in the fertility of the soils on the intrabiogeocenotic (tessera) level are related to the vegetation. The soils of the spruce and tall grass tesseras are richer in nitrogen, calcium, and manga nese as compared to the soils of the dwarf shrub-green moss, low grass-dwarf shrub-green moss, and tus sock grass-dwarf shrub tesseras.