Boreal pine forest floor biogenic volatile organic compound emissions peak in early summer and autumn
Research highlights
► The boreal forest floor showed the clear emissions of BVOCs. ► Monoterpenes were the most emitted group, sesquiterpene emissions being minor. ► The fluxes of most terpenoids peaked in early summer and autumn. ► Association of BVOC fluxes with soil physical factors and trace gas emissions were weak.
Introduction
Biogenic volatile organic compounds (BVOCs) include a broad spectrum of atmospheric hydrocarbons, commonly excluding methane (CH4), that are emitted by natural sources (Kesselmeier and Staudt, 1999) and constitute the largest part of the volatile chemicals produced and emitted by the biosphere (Guenther et al., 2006). Terpenoids, a common and large BVOC group, consists of hemiterpenes, monoterpenes and sesquiterpenes. Isoprene (a hemiterpene) globally is the most abundant (Guenther et al., 2006) and intensively studied BVOC compound, containing five carbon and eight hydrogen atoms. Monoterpenes are based on two isoprene units and sesquiterpenes on three isoprene units, both of them including numerous compounds.
The boreal zone is the second largest forested region in the world, after tropical forests (Global Forest Resources Assessment 2000, 2001). BVOCs emitted from the branch and canopy levels in boreal forests have been rather well characterised (Hakola et al., 2003, Hakola et al., 2006, Ruuskanen et al., 2005), in contrast to soil BVOC exchanges. Since BVOC emissions are technically challenging and costly to measure, they are relatively little known compared with other trace gases emitted from soils. We measured BVOCs emitted from a boreal forest floor, including ground vegetation and soil. Sampling focused especially on the early and late parts of the growing season, since they are the most important parts based on an earlier study by Hellén et al. (2006). Soil as a truly heterogeneous emission source needs temporally and spatially representative sampling series to assess the wide variation in soil BVOC exchange.
The importance of the forest floor in carbon and nutrient cycling is commonly undervalued, even though many studies have reported e.g., significant photosynthesis of ground vegetation (Swanson and Flanagan, 2001, Kolari et al., 2006, Kulmala et al., 2008). Mosses and vascular plants growing on the forest floor have adapted to lower light conditions, photosynthesising efficiently for most of the growing season (Kolari et al., 2006). Actively functioning forest floor plant ecosystems also maintain diverse and effective soil microbial populations, both factors of which are crucial to forest floor BVOC emissions.
Isoprene is mainly produced in chloroplasts and is closely associated with photosynthesis (Kesselmeier and Staudt, 1999) (and references therein), and thus is highly dependent on temperature and photosynthetically active radiation (PAR). Emissions of monoterpenes and sesquiterpenes from plant shoots also follow temperature and PAR in many plant species (Guenther et al., 1993); however, terpenoids stored in special organs or cells after synthesis (basically in the conifers) respond mainly to changes in temperature. Even though the actual BVOC sources in soils are poorly understood, decomposition processes, roots (Janson, 1993, Hayward et al., 2001, Asensio et al., 2008a) and microbes (Asensio et al., 2008a, Leff and Fierer, 2008) are known to produce and emit these compounds. In boreal forest soil, BVOC emissions are highest in spring and autumn (Hellén et al., 2006), but the processes behind the seasonal variations remain uncertain. However, soil temperature and moisture conditions are directly associated with many physical and biological processes of soil BVOC formation (Asensio et al., 2007a, Asensio et al., 2008a).
BVOCs are closely associated with air chemistry in the troposphere. However, for emissions of compounds with short lifetime (e.g., mono- and sesquiterpenes) originating from the forest floor, the chemistry below the canopy becomes more important (Rinne et al., 2007). Conditions for chemical reactions below the canopy differ from those above the canopy e.g., in lower light levels and lower turbulence. VOCs react with ozone (O3), OH or NO3 radicals the reaction products formed may condense into aerosol particles or affect the growth of atmospheric aerosols (Kulmala et al., 2000). The exact conditions needed for aerosol formation and growth are not fully understood, but it has been observed that in boreal regions these events peak in early summer and autumn (Dal Maso et al., 2005).
Our objective was to qualify and quantify snow-free period BVOC emissions from the boreal forest floor, as well as to determine spatial and temporal changes in the BVOC fluxes. The aim was also to identify the environmental factors controlling soil BVOC exchange and to determine the associations of BVOC fluxes with intra-annual patterns of other trace gases carbon dioxide (CO2), CH4 and nitrous oxide (N2O) from the soil and with the photosynthesis of ground vegetation. Our hypothesis was that the sources of BVOC emissions change during the measurement period, thus affecting both, the total amount and the emission pattern of forest floor BVOC emissions. Since a clear association with photosynthetic activity was observed in BVOC emissions from Scots pine (Pinus sylvestris L.) shoots (Bäck et al., 2005), one of our goals was also to determine whether similar behaviour occurs in forest floor BVOC emissions.
Section snippets
Measurement site
We measured BVOC fluxes from a Scots pine forest floor at the SMEAR II (Station for Measuring Forest Ecosystem–Atmosphere Relations II) station (61°51′N, 24°17′E, 180 m a.s.l.), located in the vicinity of the Hyytiälä Forestry Field Station in southern Finland (Hari and Kulmala, 2005). The forest stand at the SMEAR II station is 46 years old and dominated by Scots pine (>60% of the trees); some Norway spruce (Picea abies (L) H. Karst.), aspen (Populus tremula L.) and birch (Betula L. spp.) also
Environmental conditions
At the SMEAR II station, April–November 2008 was cloudy and rainy (Fig. 2), and midsummer temperatures were lower than the 30-year average (Drebs et al., 2002). From late May to early June, there was a two-week-long warmer period with no precipitation. During sampling on 4 June, the below canopy PAR was at its highest (694 μmol m−2 s−1), whereas the ambient temperature reached the highest value at 26 °C during sampling on 5 June.
In early summer, the litter deposited in the previous autumn covered
Sources of BVOC emissions
Although the synthesis pathways of the main BVOCs (isoprene, monoterpenes and sesquiterpenes) in plants are well known, the purposes of these compounds for plants are still under debate. The function of BVOCs emitted from flowers or fruits is more or less well-defined, but the functions of compounds emitted from the green parts of plants or from roots are less understood (Kesselmeier and Staudt, 1999). In the case of soil BVOC emissions, the emission sources and formation processes are likewise
Conclusions
The boreal forest floor showed clear emissions of BVOCs, most of them monoterpenes, although some other terpenoids were also emitted. α-Pinene was the most abundant compound, followed by Δ3-carene and camphene; these three compounds together constituted over 90% of the total monoterpene emissions. Isoprene and sesquiterpenes were also detected, but their emissions were minor. The fluxes of most of the terpenoids followed seasonal variation, with the emissions peaking in early summer and autumn.
Acknowledgements
We thank the staff of the SMEAR II and Hyytiälä Forestry Field station for help and for use of the facilities in the study. This project was financially supported by the Vilho, Yrjö and Kalle Väisälä Fund, by the Academy of Finland projects 218094, 130984 and by the Academy of Finland Centre of Excellence programme (project number 1118615). Financial support by the EU projects NitroEurope-IP and Infrastructure for Measurements of the European Carbon Cycle (IMECC) is gratefully acknowledged.
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