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Emissions of biogenic volatile organic compounds (BVOCs) are a crucial component of biosphere-atmosphere interactions. In northern latitudes, climate change is amplified by feedback processes in which BVOCs have a recognized, yet poorly quantified role, mainly due to a lack of measurements and concomitant modeling gaps. Hence, current Earth system models mostly rely on temperature responses measured on vegetation from lower latitudes, rendering their predictions highly uncertain. Here, we show how tundra isoprene emissions respond vigorously to temperature increases, compared to model results. Our unique dataset of direct eddy covariance ecosystem-level isoprene measurements in two contrasting ecosystems exhibited Q10 (the factor by which the emission rate increases with a 10 °C rise in temperature) temperature coefficients of up to 20.8, that is, 3.5 times the Q10 of 5.9 derived from the equivalent model calculations. Crude estimates using the observed temperature responses indicate that tundra vegetation could enhance their isoprene emissions by up to 41% (87%)-that is, 46% (55%) more than estimated by models-with a 2 °C (4 °C) warming. Our results demonstrate that tundra vegetation possesses the potential to substantially boost its isoprene emissions in response to future rising temperatures, at rates that exceed the current Earth system model predictions., We are grateful to ICOS Sweden, the Abisko Scientific Research Station (ANS), and the Finse Alpine Research Centre for providing excellent logistics for the work, and we are grateful in particular to Mikkel Sillesen Matzen, John Hulth, Andreas Westergaard-Nielsen, and Rune Maigaard for field assistance and to Jutta Holst for assistance with data. High-frequency wind data from Abisko were provided by ICOS Sweden, which has been cofinanced by the Swedish Research Council (grant 2019-00205); Abisko-Stordalen has been hosted by ANS and SITES "Swedish Infrastructure for Ecosystem Sciences" (cofinanced by the Swedish Research Council). This research has been supported by the European Research Council (TUVOLU "Tundra Biogenic Volatile Emissions in the 21st Century", grant 771012) under the European Union’s Horizon 2020 research and innovation program, the Independent Research Fund Denmark/Natural Sciences (grant DFF–4181-00141), and the Danish National Research Foundation (CENPERM grant DNRF100). R.S. acknowledges a Ramón y Cajal grant (RYC2020-029216-I) funded by the Spanish Ministry of Science and Innovation and the State Research Agency (MCIN/AEI/10.13039/501100011033) and by the European Social Fund “ESF Investing in Your Future”. IDAEA-CSIC is a Severo Ochoa Centre of Research Excellence (MCIN/AEI, project CEX2018-000794-S). This work is a contribution to the strategic research initiative LATICE "Land–ATmosphere Interactions in Cold Environments" (Faculty of Mathematics and Natural Sciences, University of Oslo, project UiO/GEO103920) and was supported by the EMERALD "Terrestrial ecosystem–climate interactions of our EMERALD planet" project funded by the Research Council of Norway (project 294948).

This review presents how the boreal and the tropical forests affect the atmosphere, its chemical composition, its function, and further how that affects the climate and, in return, the ecosystems through feedback processes. Observations from key tower sites standing out due to their long-term comprehensive observations: The Amazon Tall Tower Observatory in Central Amazonia, the Zotino Tall Tower Observatory in Siberia, and the Station to Measure Ecosystem-Atmosphere Relations at Hyytiäla in Finland. The review is complemented by short-term observations from networks and large experiments.The review discusses atmospheric chemistry observations, aerosol formation and processing, physiochemical aerosol, and cloud condensation nuclei properties and finds surprising similarities and important differences in the two ecosystems. The aerosol concentrations and chemistry are similar, particularly concerning the main chemical components, both dominated by an organic fraction, while the boreal ecosystem has generally higher concentrations of inorganics, due to higher influence of long-range transported air pollution. The emissions of biogenic volatile organic compounds are dominated by isoprene and monoterpene in the tropical and boreal regions, respectively, being the main precursors of the organic aerosol fraction.Observations and modeling studies show that climate change and deforestation affect the ecosystems such that the carbon and hydrological cycles in Amazonia are changing to carbon neutrality and affect precipitation downwind. In Africa, the tropical forests are so far maintaining their carbon sink.It is urgent to better understand the interaction between these major ecosystems, the atmosphere, and climate, which calls for more observation sites, providing long-term data on water, carbon, and other biogeochemical cycles. This is essential in finding a sustainable balance between forest preservation and reforestation versus a potential increase in food production and biofuels, which are critical in maintaining ecosystem services and global climate stability. Reducing global warming and deforestation is vital for tropical forests.

Supplemental Material, SupplementaryMaterial_Virkkalaetal for The current state of CO 2 flux chamber studies in the Arctic tundra: A review by Anna-Maria Virkkala, Tarmo Virtanen, Aleksi Lehtonen, Janne Rinne, Miska Luoto in Progress in Physical Geography: Earth and Environment

We used a monoterpene volume mixing ratio dataset measured from 12 June 2006 to 24 September 2007 and from 1 June 2008 to 3 March 2009 at the SMEAR II station to quantify the magnitude of anthropogenic monoterpene emissions aside from biogenic origins, to examine the anthropogenic sources, and to look at other associated pollutants. We discuss the relations between increased monoterpene mixing ratios and particle concentrations. We also characterize chemical properties of aerosol particles during two monoterpene pollution episodes in case studies. Out of 580 days analyzed, anthropogenic monoterpene pollution episodes were found on 341 (58.8%) days. The average monoterpene mixing ratio increased from 0.19 to 0.26 ppbv due to the presence of anthropogenic monoterpenes, which is equal to an increase of 36.8%. The observed anthropogenic monoterpenes were mostly from the Korkeakoski sawmill. Other gas pollutants might occasionally be emitted during the episodes, but did not show clear association with anthropogenic monoterpenes. Aerosol particle concentrations substantially increased during episodes, and monoterpene mixing ratios showed strong connections with Aitken mode particles both in number and volume concentrations. Particles associated with monoterpene episodes reached a CCN (cloud concentration nucleus) size. The chemical characterizations of aerosol particles in case studies show that the increase in aerosol particle mass was mainly from secondary organic aerosol.

Microbial activity in soils is the main source of nitrous oxide (N2O) to the atmosphere. Nitrous oxide is a strong greenhouse gas in the troposphere and participates in ozone destructive reactions in the stratosphere. The constant increase in the atmospheric concentration, as well as uncertainties in the known sources and sinks of N2O underline the need to better understand the processes and pathways of N2O in terrestrial ecosystems. This study aimed at quantifying N2O emissions from soils in northern Europe and at investigating the processes and pathways of N2O from agricultural and forest ecosystems. Emissions were measured in forest ecosystems, agricultural soils and a landfill, using the soil gradient, chamber and eddy covariance methods. Processes responsible for N2O production, and the pathways of N2O from the soil to the atmosphere, were studied in the laboratory and in the field. These ecosystems were chosen for their potential importance to the national and global budget of N2O. Laboratory experiments with boreal agricultural soils revealed that N2O production increases drastically with soil moisture content, and that the contribution of the nitrification and denitrification processes to N2O emissions depends on soil type. Laboratory study with beech (Fagus sylvatica) seedlings demonstrated that trees can serve as conduits for N2O from the soil to the atmosphere. If this mechanism is important in forest ecosystems, the current emission estimates from forest soils may underestimate the total N2O emissions from forest ecosystems. Further field and laboratory studies are needed to evaluate the importance of this mechanism in forest ecosystems. The emissions of N2O from northern forest ecosystems and a municipal landfill were highly variable in time and space. The emissions of N2O from boreal upland forest soil were among the smallest reported in the world. Despite the low emission rates, the soil gradient method revealed a clear seasonal variation in N2O production. The organic topsoil was responsible for most of the N2O production and consumption in this forest soil. Emissions from the municipal landfill were one to two orders of magnitude higher than those from agricultural soils, which are the most important source of N2O to the atmosphere. Due to their small areal coverage, landfills only contribute minimally to national N2O emissions in Finland. The eddy covariance technique was demonstrated to be useful for measuring ecosystem-scale emissions of N2O in forest and landfill ecosystems. Overall, more measurements and integration between different measurement techniques are needed to capture the large variability in N2O emissions from natural and managed northern ecosystems. Typpioksiduuli (N2O) eli ilokaasu on voimakas kasvihuonekaasu alailmakehässä ja osallistuu otsonia tuhoaviin reaktioihin yläilmakehässä. Ihmistoiminnan ansiosta typpioksiduulin pitoisuus alailmakehässä on jatkuvasti kohonnut. Maan mikrobitoiminta on merkittävin typpioksiduulin lähde ilmakehään, mutta maaperän prosessit typpioksiduulin lähteinä ja nieluina ja typpioksiduulin kuljetusmekanismit maasta ilmakehään tunnetaan huonosti. Tämän tutkimuksen tavoitteena oli määrittää N2O-päästöjä erilaisissa pohjoisen ilmastovyöhykkeen ympäristöissä, ja tutkia typpioksiduulin tuotto- ja kuljetusprosesseja. Typpioksiduulipäästöjä mitattiin maatalous- ja metsämailla sekä kaatopaikalla. Mittauspaikoiksi valittiin sellaisia ekosysteemejä, joiden arvioitiin olevan merkittäviä typpioksiduulin päästölähteitä, mutta joista oli olemassa vain vähän aikaisempaa mittaustietoa. Laboratoriokokeissa havaittiin, että maatalousmaiden typpioksiduulin tuotto lisääntyy radikaalisti maan kosteuden kasvaessa. Maan mikrobiologisten prosessien, nitrifikaation ja denitrifikaation, osuudet typpioksiduulin tuotossa olivat riippuvaisia maalajista. Tutkimuksen toisessa laboratoriokokeessa havaittiin, että puiden taimet pystyvät kuljettamaan maaperässä tuotettua typpioksiduulia ilmakehään. Nykyiset metsäekosysteemien päästöarviot perustuvat maaperän päästöjen mittaamiseen. Mikäli merkittävä osa maassa tuotetusta typpioksiduulista kulkeutuu ilmakehään puiden kautta, nykyiset arviot metsämaiden N2O-päästöistä voivat olla aliarvio todellisista metsikkötason päästöistä. Typpioksiduulipäästöt metsämailta ja kaatopaikalta olivat hyvin vaihtelevia ajan ja mittauspaikan suhteen. Boreaalisen vyöhykkeen kangasmetsän N2O-päästöt olivat erittäin pieniä ja ajoittain metsämaa toimi typpioksiduulin nieluna. Pääosa typpioksiduulin mikrobiologisesta tuotosta ja kulutuksesta tapahtui maan orgaanisessa pintakerroksessa. Kaatopaikkojen typpioksiduulipäästöt olivat yhdestä kahteen kertaluokkaa suuremmat kuin päästöt maatalousmailta, jotka ovat merkittävin typpioksiduulin päästölähde Suomessa. Kaatopaikkojen pienen pinta-alan vuoksi niiden merkitys Suomen kokonaispäästöihin on kuitenkin pieni. Tutkimus osoitti, että N2O-päästöt erilaisista ympäristöistä vaihtelevat huomattavasti ajan ja paikan suhteen. Päästöjen suuren vaihtelun vuoksi tarvitaan lisää päästömittauksia sekä päästövertailuja eri menetelmillä, jotta typpioksiduulin virrat maasta ilmakehään voidaan arvioida tarkemmin.

Understanding the interaction between precipitation and vegetation growth in water-limited ecosystems is vital for various livelihoods that depend on water resources. Precipitation is the primary driver of vegetation growth in dry ecosystems, while fog deposition is essential for the microclimate at dry coastal ecosystems and cloud forests. The analysis of soil moisture, which incorporates the action of climate, soil, and vegetation, is the key to understanding the carbon and water relations and the interaction between precipitation and vegetation. This thesis examines the impacts of precipitation variability on carbon and water relations in African savannas and the similarities in rainfall and fog deposition. The ecosystem-scale transpiration was estimated from eddy covariance measurements based on annually fitted water use efficiency and optimality hypothesis. The soil moisture measurements were analyzed using a hierarchy of soil moisture models with precipitation, NDVI, and potential evapotranspiration (PET) variability. The statistics of fog and rainfall were analyzed using an analogy with self-organized criticality. The annual evapotranspiration (ET) was comparable to the annual precipitation at the grazed savanna grassland. While the annual precipitation was highly variable, the estimated annual transpiration was nearly constant 55 % of ET. The transpiration (T) was reduced only during the drought year due to grass dieback-regrowth and possibly due to other changes in soil surface properties that enhanced evaporation. The annual net CO2 exchange (NEE) had large variation ranging from –58 (sink) to 198 (source) gC m-2 yr-1. The annual NEE was related to the maximum of remotely sensed vegetation index (NDVI), and the annual ecosystem respiration was strongly correlated with early season rainfall amount. The analysis of measured soil moisture across savannas showed that NDVI and PET adjustments to daily maximum ET are necessary for modeling depth averaged soil moisture. The soil moisture memory timescale, a rough measure of the time it takes for a soil column to forget the initial soil moisture state, was linearly related to daily mean precipitation intensity at semi-arid savannas. Both rainfall and fog time series showed approximate power-law relations for dry period and event size distributions consistent with self-organized criticality prediction. The spectral exponents of the on-off time series of the fog and rainfall exhibited an approximate f(-0.8) scaling, but the on-off switching was not entirely independent from the amplitude intermittency in fog and rainfall. The results show the role of short and long-term variability in precipitation and its consequences for the carbon and water cycle of semi-arid savannas with significant tree cover. These findings can be used to develop minimalist water balance models to understand how vegetation state affects water resources. Sateen ja kasvien vuorovaikutuksen ymmärtäminen on tärkeää kuivissa ekosysteemeissä, joissa kasvien aktiivisuus riippuu vesivaroista. Sade määrittää ensisijaisesti kasvillisuuden kasvua kuivissa ekosysteemeissä. Sumu on myös tärkeä tekijä kuivien rannikkoekosysteemien ja pilvimetsien mikroilmastoille. Maankosteutta tutkimalla voidaan ymmärtää veden ja hiilen kiertoa, sekä sateen ja kasvillisuuden vuorovaikutusta. Tässä väitöskirjassa tarkastellaan sateen vaihtelun vaikutusta hiilen ja veden kiertoon Afrikan savanneilla, sekä sateen ja sumun samankaltaisuutta. Maankosteutta tutkittiin neljällä eri Afrikan savannilla käyttäen malleja, joissa sadanta, normalisoitu kasvillisuusindeksi (NDVI) ja potentiaalinen haihdunta (PET) otettiin asteittain huomioon. Etelä-Afrikan laidunnetulla heinäsavannilla ekosyysteemin transpiraatio arvioitiin mikrometeorologisista kovarianssimittauksista (EC-menetelmä) määrittämällä vuosittainen vedenkäytön tehokkuus ja olettamalla optimaalinen suhde hiilen ja veden diffuusiolle ilmaraoista. Sumun ja sateen mittauksia analysoitiin nojautumalla itseorganisoituvasti kriittiseen teoriaan (self organized criticality, SOC). Vuosittainen haihdunta (ET) oli saman suuruinen vuosisateen kanssa laidunnetulla heinäsavannilla. Vuosittainen sademäärä vaihteli suuresti, mutta arvioitu vuosittainen transpiraatio oli kaikkina vuosina noin 55% vuosittaisesta haihdunnasta. Transpiraatio väheni vain erityisen kuivana vuonna, jolloin heinien lakastuminen ja uudelleen kasvu sekä muut maanpinnan ominaisuuksien muutokset lisäsivät haihduntaa maaperästä. Ekosysteemi vaihteli hiilen nielusta (–58 gC m-2 yr-1) hiilen lähteeksi (198 gC m-2 yr-1). Vuosittainen nettohiilenvaihto korreloi kaukokartoitusaineistosta arvioidun maksimaalisen kasvillisuusindeksin (NDVI) kanssa, kun taas vuosittainen kokonaishengitys korreloi alkukauden sademäärän kanssa. Eri savannien maankosteusmittauksien analysointi osoitti, että NDVI ja PET muuttujien vaikutus päivän maksimaaliseen haihduntaan täytyy ottaa huomioon syvyyskeskiarvoistetun maankosteuden mallintamisessa. Maaperän kosteuden aikasarjalle laskettiin niin kutsuttu muisti, joka kuvaa keskimääräistä ajanjaksoa, jossa maankosteus poikkea alkuarvostaan. Tämä muisti korreloi lineaarisesti päiväkeskiarvoisen sateen intensiteetin kanssa kuivilla savanneilla. Sateen ja sumun aikasarjoissa havaittiin likimääräinen potenssilaki kuivien ajanjaksojen ja sadetapahtuman sademäärän jakaumissa. Tämä havainto on yhdenmukainen SOC mallin kanssa. Binäärisen sade/ei-sade ja sumu/ei-sumu aikasarjojen tehospektri noudatteli likimääräisesti f(-0.8) potenssilakia, jossa f on taajuus. Kuitenkaan binääriset aikasarjat eivät olleet täysin riippumattomia sateen ja sumun amplitudin epäsäännöllisyydestä. Väitöskirjan tulokset osoittavat sateen lyhyen ja pitkän aikavälin vaihtelun seuraukset hiilen ja veden kierrolle kuivilla savanneilla. Näitä tuloksia voidaan jatkossa käyttää minimaalisten vesitasemallien kehittämisessä ja arvioidessa kasvillisuuden vaikutusta vesivaroihin.