The revised European Union (EU) Climate and Energy Framework and subsequent Effort Sharing Proposals (COM/2016/482) propose an emission reduction target of 30% in 2021–2030, relative to a baseline year of 2005 for Ireland. The European Green Deal also aims for climate neutrality by 2050 and seeks to increase the EU’s green house gas (GHG) reductions target for 2030 towards 55%, compared with 1990 levels (EC, 2020). Under the effort sharing proposals, the EC has set an ambitious net removal target of 310 MtCO2eq for the entire EU land use, land use change and forestry (LULUCF) sector.
Land-use, including dominant habitats and landscape features (e.g. hedgerows) will thus play an important role in achieving net removal targets. Whilst some research has been undertaken on hedgerows in relation to assessing their contribution to a variety of ecosystem services, the role of hedgerows in relation to climate regulation and their contribution to net removal of carbon is less well researched. It is widely acknowledged that hedgerows have the potential to sequester CO2 from the atmosphere (Drexler and Don, 2020; Drexler et al., 2021, Axe et al., 2017; Black et al., 2014; Follain et al., 2007; Lacoste et al., 2016), however, Denmark is the only EU country which currently includes hedgerow biomass carbon stock change estimates in National Inventory Reports (Nielsen et al., 2021), submitted to the United Framework Convention on Climate Change (UNFCCC). National systems throughout the EU need to be developed to collect activity data as well as soil and biomass carbon (C) stock changes. However, these data are difficult to measure because of the complex structure of hedgerows. Moreover, hedgerows vary in terms of their structure and extent of management, which is suggested to have a significant impact on the accumulation of C stock over time (see Drexler et al., 2021 for review).
Hedgerows are a prominent feature of farmed landscapes in Western Europe and are particularly prevalent across the Irish agricultural landscape (Hickie, 2004). In 2010, Teagasc, the Agriculture and Food Development Authority of Ireland, developed a hedgerow map, the Teagasc Hedge Map, which indicated an area of 482,000 hectares (ha) or 6.9% of total land area and included scrub, trees and hedges (Green, 2010). More recently, the third National Forest Inventory (NFI) (Forest Service, 2017) stated that the national hedgerow and non-forest other wooded land is estimated to occupy 347,690 ha with methodological differences considered as the primary reason for the difference between this and the earlier work (DAFM, 2020). Despite their prevalence throughout Ireland, Larkin (2019) found that hedgerow studies are not as common in Ireland as in other countries where hedgerows are prominent. Considering the abundance of hedgerow throughout the country, and their potential to sequester carbon, hedgerows may have a significant impact on GHG removals within the LULUCF sector in Ireland.
The contribution of hedgerows to GHG removal is likely to be influenced by the quantity and quality of existing hedgerows. A recent study by Green et al., (2019) indicated a decline in total hedgerow length, ranging from between 0.16% and 0.30% decline per annum, over the periods 1995, 2002 and 2015. A re-survey of the 2010 County Monaghan hedge survey showed a higher annual removal rate of 0.9% of which 75% were attributable to agriculture (Mac Elwain et al., 2021). In addition to the net decline in hedgerow area, there is also a risk of hedgerow C sink reversal, due to intensive management of hedgerows. Cross compliance obligations (under the Basic Payment Scheme (BPS)) currently incentivises farmers to maintain hedgerow width to less than 4m, in order to limit encroachment on utilisable field area (DAFM, 2020). Research in the UK highlights that compared to increasing hedge height, extending hedge width has greater capacity to sequester C into hedge above-ground biomass (Axe et al., 2017). Additional potenital policy drivers of intensive hedgerow management include Irish agri-environment schemes (Rural Environemental Protection Scheme (REPS), the Agri-Envrionment Options Scheme (AEOS), and Green Low-Carbon Agri-Environment Scheme (GLAS)). These schemes incentivised the establishment of new hedgerows, but also incentivised coppicing and laying of existing hedgerows.
Overall, these example highlight a competing demand in relation to policy objectives between primary productivity and environmental objectives. The positive benefits and value associated with hedgerow width for carbon, and other ecosystem services including biodiversity or water quality benefits) are effectively superseded under current policies, with farmers penalised for hedgerow encroachment under area-based payments., Intensive management could, in fact, result in premature sink saturation of established hedgrows and even a reversal from the net removal to a net emission.
To date, few studies have undertaken direct biomass measurements to quantify hedgerow C in temperate climates (for review see Drexler et al., 2021), with no direct measurements of hedgerow biomass to measure C identified in the literature for an Irish context. Axe et al. (2017) directly sampled and measured biomass in hawthorn dominated hedges in the UK. Previous studies in Ireland have used either Laser diode array (LIDAR, Black et al., 2014) or photogrammetry and X-ray remote sensing techniques (Green et al., 2019) to indirectly quantify hedgerow aboveground biomass. Levin et al., (2020) adopted a dual sampling approach using destructive biomass sampling and LIDAR surveys to derive hedgerow biomass based on hedge volume to biomass ratio conversion factors. Clearly, direct measurements of hedgerow biomass are required to develop models so that remote sensing technologies can be used in national inventory frameworks to quantify carbon stock changes (CSCs). The inventory framework must also complement national land use definitions and hierarchical classification rules applied in national greenhouse gas inventories (Duffy et al., 2021, IPCC 2006).
This study attempts to develop a national framework to assess changes in hedgerow above- and below- ground biomass stocks using a two point sampling approach involving: targeted destructive sampling of hedgerows; and remote collection of 3-D digital elevation model (DEM) data from drones and aircraft. The developed inventory framework was then applied to a pilot study area of 419,701 ha in counties Wexford and Waterford in Ireland to assess if hedgerows are a net removal or emission of CO2 under current management practices.