BIOGENIC VOC EMISSIONS IN THE CONTEXT OF BOREAL

1.4.1 Characteristics of biogenic VOCs

Biogenic volatile organic compounds (BVOC) are non-methane hydrocarbons (Kesselmeier and Staudt 1999) and secondary metabolic compounds synthesized in plants, animals and microorganisms by different biochemical processes and are released into the Earth’s atmosphere. BVOCs comprise mainly isoprene, monoterpenes, other reactive VOCs and other VOCs according to their atmospheric lifetimes (Laothawornkitkul et al.

2009). They are also categorized into chemical groups such as terpenoids (a class of organic compounds composed of one or

more isoprene units), phenylpropanoids/benzenoids, fatty acid derivatives and amino acid derivatives according to their biosynthetic origin (Dudareva et al. 2013). BVOCs, in terms of modelling their contribution to emissions of gases and aerosols in nature (MEGAN2.1), are divided into four main groups:

isoprene, terpenoids, stress VOCs and other VOCs (Guenther et al. 2012). Isoprene and monoterpenes are the most dominant species of BVOCs (Kesselmeier and Staudt 1999; Guenther et al.

2012). Of the 100,000 known plant-derived compounds, over 1,700 have been identified as BVOCs that are released from nearly 100 different plant families including both flowering plants and non-flowering gymnosperms (Knudsen et al. 2006;

Dicke and Loreto 2010). Although, terrestrial plants, in some cases, release up to 10% of the total carbon assimilated from atmospheric CO² during net primary production back into atmosphere as BVOCs (Peñuelas and Llusià 2003), a more typical figure is 1—4% (Kesselmeier et al. 2002; Pressley et al.

2005; Rinne et al. 2009).

Emissions of volatile compounds from plants can be constitutive (continuously synthesized compounds that are emitted from mesophyll cells and are dependent on temperature and light or compounds released via diffusion from specialized plant storage structures) (Grote et al. 2013; Niinemets et al. 2013) or induced (emissions of de novo synthesized volatiles activated in response to various stresses) (Paré and Tumlinson 1997;

Niinemets et al. 2013). Constitutively emitted BVOCs from plants have several ecological and metabolic functions. In conifers, volatile terpenes are constitutively released mainly from resin ducts terpene storage structures both in stressed and unstressed conditions but they can also be synthesized de novo in damaged or nearby tissues to fulfil a higher demand for defensive compounds to compensate for stress (Ghirardo et al.

2010). In addition to immediate stress-induced emissions, biotic stress also elicits emissions that trigger secondary induction responses in plants, which possibly affect systemic emission responses (Niinemets et al. 2013). Terpenoids are released both from above-ground and below-ground plant parts, such as

stems (Martin et al. 2002), needles and foliage (Martin et al.

2002), and roots (Lin et al. 2007) of conifers.

1.4.2 Typical conifer VOCs

Conifers emit a diverse range of volatile terpene compounds from their above-ground parts. Monoterpenes dominate the volatile blend emitted by Scots pine, with SQTs constituting a minor component (Komenda and Koppmann 2002), while isoprene, monoterpenes and some SQTs are typical for Picea spp. (Hayward et al. 2004; Kivimäenpää et al. 2013;

Bourtsoukidis et al. 2014).

The most common monoterpenes emitted by Pinus sylvestris are -pinene, 3-carene, camphene, and -pinene (Komenda and Koppmann 2002), of which -pinene and 3-carene are the most abundant compounds in branch, canopy and above-canopy measurements (Räisänen et al. 2009; Bäck et al. 2012). Other dominant terpenes include the monoterpenes myrcene, limonene, -phellandrene and terpinolene, and the sesquiterpenes muurolene, longifolene, cadinene, -caryophyllene and -bourbonene, which were reported as shoot and bark emissions of young Scots pine seedlings (Heijari et al.

2011; Kovalchuk et al. 2015). The monoterpenes -pinene, 3carene and pinene and sesquiterpenes bergamotene, -farnesene, -farnesene and -caryophyllene were emitted by branches of mature pine trees (Tarvainen et al. 2005; Helmig et al. 2007). The variation in monoterpene profiles emitted by Scots pine are known to be caused predominantly by genotypic variation between pine individuals (Komenda and Koppmann 2002) and pine provenances (Semiz et al. 2007). Furthermore, monoterpene diversity of Scots pine, particularly of the two dominant MTs, -pinene and 3-carene depends on the stand history and seed origin (Bäck et al. 2012).

Isoprene and the monoterpenes -pinene, camphene, limonene and -pinene were the most common VOCs in the shoot emissions of young Norway spruce seedlings (Blande et al. 2009; Kivimäenpää et al. 2013), while two other monoterpenes ( -phellandrene and 3-carene) and three

sesquiterpenes (longifolene, longipinene and -bourbonene) were also reported in the study by Kivimäenpää et al. (2013). In addition to isoprene and monoterpenes, other compounds (including acetic acid, acetone, acetaldehyde, methanol) and OVOCs (hexanal and methyl salicylate) were detected in the foliar emissions of young Norway spruce (Filella et al. 2007). In addition to isoprene and the monoterpenes -pinene, 3-carene, camphene, limonene, -pinene and -phellandrene, total SQTs and other oxygenated BVOCs (including formaldehyde, ethanol, formic acid, methyl ethyl ketone, pinonaldehyde and caronaldehyde) were reported in the emissions of mature Norway spruce trees (Bourtsoukidis et al. 2014). Similarly, the monoterpenes tricyclene, pinene, 3carene, camphene and -pinene dominated the BVOC profiles of tree bark emissions of mature Lodgepole pine (Amin et al. 2012) and Engelmann spruce (Amin et al. 2013).

BVOCs produced by the belowground tissues of conifers are similar in diversity to those produced by aboveground organs.

Conifer roots were found to mainly emit volatile terpenoids which have important and diverse roles in the rhizosphere, and impact on soil ecology and atmospheric chemistry. The monoterpenes -pinene and -pinene were dominant VOCs in the root emissions of Pinus halepensis, whereas -pinene, camphene and limonene were major monoterpenes in the rhizosphere emissions of Scots pine (Lin et al. 2007). With the dominance of -pinene and 3-carene, other common monoterpenes detected in the rhizosphere emissions of Scots pine seedlings were camphene, sabinene, -pinene, myrcene, limonene, -phellandrene, -terpinene and terpinolene (Rasheed et al. manuscript). Mono-and sesquiterpenes and some non-isoprenoids were the common VOC species released from Pinus pinea roots with -pinene, -pinene and limonene the dominant MTs (Lin et al. 2007). Below-ground VOC emissions include not only the emissions from plant roots but also the emissions from dead organic matter and microorganisms. Methanol (a highly dominating compound) and some MTs were released from microbial decomposition of

needle litter of Pinus contorta and Pinus ponderosa species (Gray et al. 2010). The monoterpenes -pinene, -pinene, camphene, 3-carene and limonene were detected in the needle litter emissions of Scots pine and Norway spruce during decomposition in a natural environment (Isidorov et al. 2010). Of the dominant MTs, -pinene was emitted from the roots and needle litters of Scots pine, whereas -pinene and myrcene were mainly found in the root and litter emissions of Norway spruce (Ludley et al.

2009).

1.4.3 Synthesis of biogenic VOCs

The two main precursors for the biosynthesis of volatile terpenoids are isopentenyl diphosphate (IPP) and its allylic isomer, dimethylallyl diphosphate (DMAPP) (McGarvey and Croteau 1995). Biosynthesis of BVOCs is driven by the energy provided by primary metabolism and depends on the availability of carbon and nutrients such as nitrogen and sulphur in plants. Primary metabolism in plants includes the production of fundamental compounds including fatty acids, amino acids, and sugars. Different BVOCs originate from different biosynthetic pathways. The lipoxygenase (LOX) pathway activates the biosynthesis of green leaf volatiles (GLVs) and methyl jasmonate (MeJA) in the cytosol (Matsui et al. 2012;

Dudareva et al. 2013). The methylerythritol phosphate (MEP) pathway is located in plastids of plant cells and is responsible for the synthesis of hemiterpenes (C⁵), monoterpenes (C¹⁰), diterpenes (C²⁰) and the homoterpene TMTT (4,8,12-trimethyltrideca-1,3,7,11-tetraene) (Pichersky et al. 2006). The mevalonic acid (MVA) pathway, which is mostly cytosolic, but also linked to endoplasmic reticulum and peroxisomes synthesizes volatile sesquiterpenes (C¹⁵), triterpenes (C³⁰) and the homoterpene DMNT (4,8-dimethylnona-1,3,7-triene) (Dudareva et al. 2013).

1.4.4 Role of BVOCs

Plants emit diverse and complex blends of VOCs, which have both metabolic and ecological functions. Interactions between

organisms, such as plant-to-plant, plant-to animal or microbe and microbe-to-microbe interactions, are mediated by biogenic VOCs. Plants have developed physical and chemical defence systems to protect themselves against various biotic and abiotic stressors. BVOCs function in oxidative protection and thermal tolerance for plants (Peñuelas and Llusia 2003; Dudareva et al.

2006) and in signalling with other plants and organisms above-ground (Dicke and Baldwin 2010; Heil and Karban 2010;

Holopainen et al. 2013) and below-ground (Wenke et al. 2010).

BVOCs promote plant growth and reproduction (Knudsen et al.

2006; Dudareva et al. 2013), and defence against herbivores (Unsicker et al. 2009; Dicke and Baldwin 2010; Holopainen and Gershenzon 2010), pathogens (Huang et al. 2012) and abiotic stresses (Loreto and Schnitzler 2010; Possell and Loreto 2013).

Some of the key abiotic stress factors against which plants have to defend during their growth and developmental processes are high temperature (Possell and Loreto 2013), ozone (Pinto et al.

2010) and low soil nitrogen availability (Oliet et al. 2013).

1.4.5 BVOCs have impacts on Earth’s atmosphere and climate BVOCs, particularly mono-and sesquiterpenes and isoprene emitted by boreal ecosystems are highly reactive components of the lower atmosphere. BVOCs react with hydroxyl (OH) and nitrate (NO³) radicals and O³ (Atkinson and Arey 2003). In photochemical reactions with nitrogen oxides (NO^X), BVOCs yield ozone in the troposphere (Matyssek and Sandermann 2003; Pinto et al. 2010). Many BVOCs are highly reactive with ozone, which is decomposed in the lower atmosphere during O³ photolysis, and results in the formation of OH radicals that oxidise VOCs. Products of BVOC oxidation by ozone and OH radicals include extremely low volatility organic compounds (ELVOCs), which contribute to secondary organic aerosol (SOA) growth and the production of atmospheric nanoparticles and cloud condensation nuclei (CCN) (Hao et al. 2011; Ehn et al.

2014; Jokinen et al. 2015). The compounds -pinene, -pinene and limonene are the main BVOC species that result in the formation of highly oxygenated peroxy radicals (RO²) in

oxidation reactions with O³and OH (Ehn et al. 2014; Rissanen et al. 2014; Jokinen et al. 2015). The formation of -pinene-derived ELVOC is much greater in ozonolysis reactions than in reactions with OH radicals, and the total yield of ELVOC from -pinene ozonolysis was higher than that from -pinene ozonolysis (Ehn et al. 2014).

The main natural aerosol precursor, dimethyl sulphide (DMS), is mainly obtained as oceanic emissions by marine algae (Watts 2000) and from BVOCs emitted from the terrestrial biosphere (Boucher et al. 2013). Globally, a major fraction of atmospheric SOA is expected to originate from biogenic VOCs, and the current atmospheric SOA budget is estimated to be 12–

1820 Tg y¹(Spracklen et al. 2011). BVOC emissions from boreal forests contribute almost 50% of cloud condensation nuclei at the regional scale (Spracklen et al. 2008). Modelling of monoterpene emission rates predicted that 10% defoliation in the boreal insect outbreak area would result in an increase of 480% in total particulate mass and 45% in cloud condensation nuclei at the global scale compared to the emissions of non-outbreak forest area (Joutsensaari et al. 2015). Excessive yield of organic aerosols in the atmosphere will affect temperature balance and global climate of the Earth by reflection and adsorption of solar and terrestrial radiation and through the alteration of cloud albedo over the boreal forest canopy (Spracklen et al. 2008).

1.5 EFFECTS OF CLIMATE CHANGE ON CONIFER VOC