Bioenergy plantations: assessment of biogenic volatile organic compounds from short-rotation forests and their potential impact on UK air quality

Abstract

The UK government has committed to a net-zero greenhouse gas (GHG) emission target by 2050. Bioenergy technologies are a dominant feature in the pathways to achieving this target. It is suggested that 0.7 million hectares of land could be used for bioenergy crops by 2050. Planting fast-growing tree species as short-rotation forest (SRF) or short-rotation coppice (SRC) is one potential source of biomass. However, trees emit biogenic volatile organic compounds (BVOCs) such as isoprene and monoterpenes, which are important precursors in the formation of the atmospheric pollutants ozone and secondary organic aerosols (SOA). Therefore extensive additional tree planting may have implications for air quality. In this thesis, BVOC emissions were measured from four tree species grown in UK SRF and SRC plantations: hybrid aspen (Populus tremula L. x tremuloides Michx.) and Italian alder (Alnus cordata Desf.) (broadleaf deciduous trees), Eucalyptus gunnii (broadleaf evergreens) and a common plantation species Sitka spruce (Picea sitchensis, a conifer evergreen) were selected. Emissions were quantified using a chamber method and off-line thermal desorption-gas chromatography-mass spectrometry analysis. In addition, a further five eucalypt species were assessed during a scoping study of pot-grown young trees in outside conditions. Sitka spruce, although not a species selected for development as SRF, is the dominant plantation tree species within the UK and therefore served as a comparison. Field measurements for BVOCs were made on foliage (branch scale) and from the forest floor of SRF plantations to assess the contribution of these sources to emissions from the plantation as a whole. Mean isoprene emission rates were highest from hybrid aspen (22.8 μg gdw-1 h -1 ). In comparison, isoprene emissions from Sitka spruce (10.9 μg gdw-1 h -1 ) and Eucalyptus gunnii (7.5 μg gdw-1 h -1 ) were around half that of aspen. Only trace amounts of isoprene were emitted from Italian alder (0.03 μg gdw-1 h -1 ). This ranking was almost the reverse for total monoterpene emissions, with Sitka spruce (3.4 μg gdw-1 h -1 ) being the largest emitter, followed by Eucalyptus gunnii (1.2 μg gdw-1 h -1 ), Italian alder (0.86 μg gdw-1 h -1 ) and hybrid aspen (0.17 μg gdw-1 h -1 ). BVOC emissions from a range of cold-tolerant eucalypt species (Eucalyptus globulus subsp. bicostata, Eucalyptus subcrenulata, Eucalyptus pauciflora subsp. debeuzevillei, Eucalyptus cordata subsp. quadrangulosa, Eucalyptus Johnstoni) deemed suitable for the UK climate are also reported here for the first time. The mean isoprene emissions of the young pot-grown trees ranged between 1.3 μg gdw-1 h -1 and 10 μg gdw-1 h -1 , which means the UK-grown eucalypts in this initial scoping study can be classified as ‘medium’ emitters of isoprene, contrary to the ‘high’ (>10 μg gdw-1 h -1 ) emitter classification suggested by previous studies. Emissions of total monoterpenes were an order of magnitude smaller than the isoprene emissions in most cases. The contribution of the forest floor to total forest BVOC emissions is often not deemed significant and has in the past been overlooked for some forest types. This work highlighted the changes in magnitude and the variation in composition of the BVOC emissions. In the case of Eucalyptus gunnii, in particular, monoterpene emissions change by an order of magnitude as a result of the changes associated with the SRC management cycle. The contribution of monoterpenes (and isoprene when measured) are presented here for aspen, alder and Eucalyptus gunnii plantations for the first time. In the evergreen plantations (Sitka spruce and Eucalyptus gunnii), where leaf litter was present all year round, the monoterpene emissions peaked during the summer months, reaching up to 70 μg m-2 h -1 when temperatures were high and soil moisture was low, although the drivers for forest floor emissions under field conditions are complex. Total monoterpene emissions from the forest floor in these plantations contributed a maximum of 10% compared to canopy emissions. In the deciduous plantations (Italian alder and hybrid aspen) contributions of monoterpenes from the forest floor were attributed to leaf fall in the autumn or ground vegetation growth during the summer. However, the BVOC emissions to the atmosphere from the forest floor still remains a relatively minor source in comparison to the canopy. Modelled emissions (MEGAN2.1) using measured leaf area index data and two contrasting weather conditions for the UK (two consecutive years in south and north UK) produced mean annual isoprene and total monoterpene emissions for the four different tree types at the plantation scale. Isoprene emissions from the two bioenergy SRF species, hybrid aspen (15.5 kg C ha 1 ) and Italian alder (0.02 kg C ha-1 ), and bioenergy SRC species Eucalyptus gunnii (2.2 kg C ha-1 ), may have BVOC emissions no larger than those of commercial UK conifer forests of Sitka spruce (15.7 kg C ha-1 ). Total monoterpene emissions followed a similar pattern with emissions from Sitka spruce being the largest. A first assessment of the contribution of SRF and SRC bioenergy crops to UK scale emissions was made based on the proposed expansion of bioenergy crops (assuming all as SRF or SRC) to 0.7 Mha by 2050. The relative percentage increase in UK BVOC emissions from this bioenergy expansion ranged widely – between <1% up to 35% – both due to tree species planted but also because of the wide variation in previous literature estimates of existing UK BVOC emissions. UK-wide air quality simulations carried out using the EMEP4UK atmospheric chemistry transport model for a range of fairly extreme SRF planting scenarios of one of the 4 species mentioned above showed annual average ozone increases of up to 7% in some parts of the UK (particularly the south). On the other hand significant benefits of planting these trees on atmospheric composition were also observed with reductions of up to 11% in annual mean PM2.5 concentrations. As well as further work on aspects of research described in this thesis, further work is also required to understand other aspects of increased planting of bioenergy forests on air quality such as from the emissions of sesquiterpenes and pollen.