Snow cover dynamics and its impact on greenhouse gas fluxes in drained peatlands in Estonia
Date
2020-09-11
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Abstract
Maa keskmine temperatuur on tööstusrevolutsiooni algusest tänaseks tõusnud ligikaudu ühe kraadi võrra. Talvise keskmise temperatuuri muutus Põhja-Euroopas on olnud suurem kui globaalne keskmine muutus. Õhutemperatuur on peamine tegur, mis määrab lumikatte kestuse ja paksuse, lumikate omakorda võib mõjutada talvist kasvuhoonegaaside (KHG) lendumist. Doktoritöö eesmärgiks oli analüüsida lumikatte muutlikkuse mõju KHG talvisele voole Eestis.
Territoriaalsed erinevused lumikatte parameetrites Eestis on suured: selgelt erinevad teineteisest Lääne-Eesti saared ja rannikualad ning sisemaa. Lumikatte kestus varieerus 61 päevast Vilsandil kuni 130 päevani Kagu-Eesti kõrgustikel. Eestis on 66 aastaga lumikatte periood lühenenud keskmiselt 27 päeva võrra, eelkõige kevadise varasema sulamise tõttu.
Välitöödel mõõdeti KHG süsihappegaasi (CO2), metaani (CH4) ja dilämmastikoksiidi ehk naerugaasi (N2O) lendumist kahest jääksoost (Ess-soo ja Laiuse) ning Järvselja kõdusookuusikus ja –kaasikus. Jääksoo on turbakaevandamise lõppemist tekkinud taimkatteta ala, mis ei seo süsihappegaasi ning on seetõttu oluline kasvuhoonegaaside allikas. Kõdusoometsadest ehk kuivendatud soometsadest lendus oluliselt nii CO2 (talvel 15–20% aastasest voost) kui ka N2O. Metaani voog oli mõlemas koosluses suhteliselt väike, kuid talvine voog moodustas 31–52% aastasest emissioonist jääksoodes ning 33–49% aastasest tarbimisest kõdusoometsas.
Talvine N2O voog oli kõdusoometsas märkimisväärselt kõrgem kui jääksoos, moodustades nii kõdusookuusikus kui -kaasikus 87% aastasest lendumisest. Eriti suur oli lendumine külmumis/sulamistsüklite ajal: arvestades kõigi kolme gaasi kliimamõju, moodustas kõdusookaasikute talvine N2O voog põhilise osa selle ökosüsteemi aastasest KHG bilansist.
Lumikatte paksuse ja erinevate KHG voogude vahel selge seos puudus, kuid puuduva või õhukese lumikatte korral oli voogude varieeruvus suur. Lumikatte kestuse jätkuv lühenemine ning paksuse ulatuslikum varieerumine suurendab tulevikus talviste KHG voogude osatähtsust veelgi.
Since the beginning of Industrial Revolution Earth´s average temperature has risen by about one degree centigrade. The change in average winter temperatures in northern Europe has been greater than the global average. Air temperature is the main factor which determines the duration and depth of snow cover. Snow cover, in turn, affects winter greenhouse gas (GHG) emissions. The aim of this dissertation was to analyze the dynamics of snow cover in Estonia and its effect GHG emissions in winter. Territorial differences in snow cover parameters in Estonia are considerable. Snow cover duration varied between 61 days in Western-Estonia and 130 days on uplands of south-eastern Estonia. As an Estonian average, the snow cover period has been shortened by 27 days in 66 years, mainly due to the earlier spring melting. GHG were measured from two abandoned peat extraction areas and two drained peatland forests. Both ecosystems are important GHG sources. The values of carbon dioxide (CO2) emissions were similar between abandoned peat extraction areas and drained peatlands, the emission was positively correlated with soil and air temperatures. Wintertime CO2 efflux made up 10–25% of the annual release. Drained forests were methane (CH4) sinks and abandoned peat extraction areas emitters (sources). Higher CH4 emission values were measured at high groundwater depths (<20 cm). Wintertime CH4 fluxes made up 31–52% of annual release at the abandoned peat extraction areas and 33–49% of annual consumption in the drained forests. Nitrous dioxide (N2O) fluxes from drained forests were significantly higher than those from the abandoned peat extraction areas. Wintertime N2O release from the drained forests accounted for 87% of the total annual emission. Emissions during freezing/thawing events were particularly high. Our hypothesis on the snow cover effect was only partly supported, showing both a positive and negative impact on different sites and gases. In general, thin and scattered snow cover during the freeze-thaw periods initiates GHG emissions. Regarding trends in snow cover duration and thickness, more wintertime emissions of GHGs are expected in future.
Since the beginning of Industrial Revolution Earth´s average temperature has risen by about one degree centigrade. The change in average winter temperatures in northern Europe has been greater than the global average. Air temperature is the main factor which determines the duration and depth of snow cover. Snow cover, in turn, affects winter greenhouse gas (GHG) emissions. The aim of this dissertation was to analyze the dynamics of snow cover in Estonia and its effect GHG emissions in winter. Territorial differences in snow cover parameters in Estonia are considerable. Snow cover duration varied between 61 days in Western-Estonia and 130 days on uplands of south-eastern Estonia. As an Estonian average, the snow cover period has been shortened by 27 days in 66 years, mainly due to the earlier spring melting. GHG were measured from two abandoned peat extraction areas and two drained peatland forests. Both ecosystems are important GHG sources. The values of carbon dioxide (CO2) emissions were similar between abandoned peat extraction areas and drained peatlands, the emission was positively correlated with soil and air temperatures. Wintertime CO2 efflux made up 10–25% of the annual release. Drained forests were methane (CH4) sinks and abandoned peat extraction areas emitters (sources). Higher CH4 emission values were measured at high groundwater depths (<20 cm). Wintertime CH4 fluxes made up 31–52% of annual release at the abandoned peat extraction areas and 33–49% of annual consumption in the drained forests. Nitrous dioxide (N2O) fluxes from drained forests were significantly higher than those from the abandoned peat extraction areas. Wintertime N2O release from the drained forests accounted for 87% of the total annual emission. Emissions during freezing/thawing events were particularly high. Our hypothesis on the snow cover effect was only partly supported, showing both a positive and negative impact on different sites and gases. In general, thin and scattered snow cover during the freeze-thaw periods initiates GHG emissions. Regarding trends in snow cover duration and thickness, more wintertime emissions of GHGs are expected in future.
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Keywords
snow cover, greenhouse gases, winter, emission (ecology), cut-over peatlands, peatland forests, Estonia