BIO0050

A Response from the British Ecological Society to the Environmental Audit Committee “Ecosystems and Biodiversity” Inquiry

September 2020

Pairing nature-based solutions to climate change with biodiversity (Section 5):

Which nature-based solutions are most effective in achieving both climate and biodiversity goals?

In order to better understand the potential of different nature-based solutions (NbS) across major habitat types and landscapes in the UK, the BES assembled an extensive group of scientific experts from academia, statutory agencies and the NGO community to produce a report. The report is still in production, but the following interventions appear to be the most effective NbS in achieving climate and biodiversity goals. Where possible, we have sought to represent carbon sequestration as tonnes of carbon per hectare per annum (t. C/ha/y).

Key points:

• NbS are not a panacea and will not be an effective substitute for emissions reductions. NbS operate synergistically to most emissions reduction scenarios and work through different mechanisms (i.e. less economic impact)[1]. However, monitoring of emissions reductions and carbon sequestration delivered by NbS should be kept separate. This will drive emissions reductions and allow accurate evaluation of which NbS are successful or where improvements are required.

• Peatland restoration is likely to be the most effective NbS for climate and biodiversity, and should be a priority.

• Afforestation and protection of native broadleaf woodlands bring great climate change mitigation and biodiversity benefits, and this should make them another NbS priority. However, we acknowledge that well-managed commercial forestry can also provide significant carbon and some biodiversity benefits, while reducing demand for overseas timber products and the UK’s international environmental footprint.

• Hedgerows, agroforestry and field margins offer significant climate mitigation, adaptation and biodiversity benefits - with considerable delivery potential at scale across the UK’s agricultural landscapes, and these should be a priority for Environmental Land Management schemes (ELMS). 

Peatlands

Peatlands are highly valued for their biodiversity, nationally and internationally. They are a UK Biodiversity Action Plan priority habitat, due to their extent and relative lack of fragmentation[2]. The highly distinctive conditions created by most UK peatlands (water-logged, acidic, low nutrient)[3],[4] mean many of the birds, mammals, invertebrates and plants found in them are specialised and dependent on the existence of these habitats2.  Many of these species are regionally or nationally rare, such as the large heath butterfly and the swallowtail butterfly which is restricted to the peatlands of the Norfolk Broads. Peatlands also form the main centre of distributions for all our carnivorous plants.

Peatlands are the most carbon dense ecosystems on Earth, with the UK’s containing at least 2,300 Megatonnes (Mt) of carbon, of which 584.4 Mt is found in English peatlands[5]. Covering at least three million hectares, existing peatland habitats make-up around 10% of the UK land area[6],[7],[8], meaning they can potentially make significant contributions to the UK’s net-zero target, if they are brought back into good condition.

However, much of the UK’s peatland is no longer actively sequestering carbon8,[9] and is instead emitting 16 million tonnes of CO2 e (equivalent) each year. Not only is this nearly three times the amount released through the nation’s agricultural sector[10], but emissions from peat soil subject to lowland arable make it one of the greatest, if not the greatest, contributors to UK land-use carbon emissions9. This switch from a carbon sink to source is mainly due to drainage, air pollution, fire, and other land-use pressures[11],9,8,[12],[13],[14]. Only 20% of the UK’s peatlands are now considered in a “near-natural” state[15].

It is possible to return a proportion of these degraded areas to carbon sinks through restoration involving rewetting and revegetation and this should be a NbS priority for the government. This could involve “carbon farming” projects, or sustainable traditional farming on peat soils – termed ‘paludiculture’[16],[17],[18],[19].

Continuing to strengthen the Peatland Code would help in this regard. To ensure investment in peatland restoration is not undermined, land managers need to see policy reinforcing the view of peatlands as valuable assets to society.

Woodlands

The UK’s forests currently sequester about 19 Mt of CO2 from the atmosphere per year, which equates to about 4-5% of total emissions[20],[21]. If an additional 23,000ha of forest had been planted every year since 2010 and in successive years to 2050, it is estimated they would eventually sequester an additional 12 MtCO2e per year, taking until 2060 to reach that peak[22]. Woodland creation therefore has an important role in meeting climate objectives but is not a panacea due to the relatively slow rates climate benefits are delivered.  

The UK’s existing network of native woodlands are a vital habitat for biodiversity, with England’s native woodlands supporting a fifth of the UK’s priority species for conservation, while ancient woodlands are the UK’s richest and most complex terrestrial habitat, containing more threatened species than any other. Managing woodlands to create structurally diverse canopies is vital to providing niches in which native plants, fungi and animals flourish[23],[24],[25]. This involves harvesting trees and removing biomass carbon from the system, so is not optimal for carbon sequestration in the short-term, although further research is needed into this matter.

There are opportunities to create bigger, better-connected native woodlands though planting and natural regeneration, subsidised under the public money for public good principle, which could develop into significant carbon sinks over decadal timescales. Native species are genetically diverse and should continue to be a core component of future tree planting. However, in some situations, southerly provenances or non-native species may be an appropriate choice to promote long-term forest resilience.

More commercial forestry is needed to reduce the UK’s reliance on imported wood and fibre products and its overseas environmental footprint, given that the UK is the second biggest importer of forest products in the world. Plantations of exotic conifers sequester carbon more rapidly than native tree plantations[26]. As existing plantations are felled, there are opportunities to replant multispecies woods with greater structural complexity, creating forests that are resilient to climate change and the onslaught of novel pathogens and pests. Plantations can deliver multiple ecosystem services. Understanding the different outcomes and trade-offs for both biodiversity and climate resulting from different tree species and management practices is critical for decision-makers. For example, allowing patches of conifers to reach maturity is beneficial to several groups of organisms in production systems that have a reputation for being bad for wildlife[27], but reduces timber yields. 

Deciding where new plantations should be planted is complex. Evidence is gathering that plantations should not be situated on damp carbon-rich soils, because disturbance of the soil during site preparation creates an initial carbon debt that takes time repay[28],[29], and tree alteration of soil conditions result in further carbon being lost[30]. Conversion of domestic grassland to woodlands is possible and would draw down CO2 from the atmosphere, but it would also reduce the UK’s capacity to rear livestock, and without a commensurate reduction in meat consumption it would displace production overseas, perhaps accelerating deforestation in Amazonia. Such a move might result in a net increase in global atmospheric carbon. New plantations on arable land will be particularly effective at carbon sequestration, but also has knock-on consequences for food production.

In relation to climate adaptation, increasing tree cover has a small statistically significant effect on reducing channel peak discharge[31] and influencing fluvial flood peaks[32]. Other studies show the importance of woodland, especially old broadleaved trees, in increased infiltration (5-6 times higher than adjacent grassland[33]), and increased surface roughness[34]. However, there is a lack of direct evidence and overall uncertainty in the evidence31.

We would therefore consider afforestation and protection of native broadleaf woodlands to bring climate change mitigation and biodiversity benefits, and this should make them an NbS priority. However, we acknowledge that mature conifer plantations also create structural diversity and provide climate and biodiversity benefits. Additionally, the creation of new native woodlands to enlarge nature reserves, and replanting of forestry plantation with mixtures of species adapted to drier climates and resistance to known pests and pathogens should also not be neglected.

Hedgerows

Hedgerows are a highly effective NbS. Evidence from Britain, Germany and France, suggests that established hedge networks may store roughly 100 t C ha-1 [35]. A meta-analysis of data from 60 studies found that soil carbon stocks are 22% higher under hedgerows and 6% higher next to the hedgerow than in fields without hedgerows[36]. They are highly effective at increasing water infiltration and storing runoff, thus helping to reduce the flooding risk and soil erosion in agricultural landscapes.

Hedgerows support a high number of plants, animals and fungi[37],[38] and provide nectar and pollen resources for many pollinators[39]. At least 21 Section 41 bird species are associated with hedgerows and for 13 of these, hedgerows are a primary habitat. Similarly, as many as 16 out of the 19 birds used by Government to assess the state of farmland wildlife are associated with hedgerows, with 10 using them as a primary habitat[40]. Hedgerows may also provide landscape connectivity which facilitates the movement of species in response to a changing climate, although the effectiveness of these corridors is not yet established[41].

Crucially, hedgerows are relatively easy to deliver and scale-up because they are relatively fast growing, many rural landowners have experience of creating or maintaining them, governments have experience of incentivising them, and there is approximately 19 million ha of agricultural land in the UK[42]. Existing hedgerows should be strictly protected and the creation of new hedgerows should be a high-priority for future post-CAP environmental subsidies in the UK. Evidence for the effectiveness of hedgerows is high.

Agroforestry

Agroforestry is another potentially highly effective NbS. The average carbon storage by agroforestry systems has been estimated at up to 63 t. C/ha in climates such as the UK’s[43]. Agroforestry can help to make arable cropping more resilient through greater protection against wind and associated soil erosion, increased water conservation through reduced evapotranspiration, and effective utilisation of nutrients via roots and leaf fall and microclimate benefits[44],[45]. 

Tree rows can provide habitat akin to well managed hedgerows with increases in agronomically beneficial species such as spiders and ground carabid beetles that provide pest control within the arable crop[46]. However, some species can have negative impacts, such as slugs emanating from tree rows damaging crops and lowering yields[47].

The impacts on arable yields need not be severe with the chief impact being competition for light. However, benefits have been observed in overall wheat yields relative to wheat in an open field, attributed to a reduction in evapotranspiration rate in the alleys[48]. More importantly for the farmer, and depending on the choice of trees, agroforestry can be as, or more, profitable than monoculture systems[49], and although the farm business becomes more complex, the diversification of income streams brings benefits, alongside wider opportunities for the local economy[50]. Delivery potential and scalability of agroforestry are high, and future land-subsidies should be designed to incentivise a significant increase in agroforestry.

Field margins 

Grassy field margins alongside annual crops have 37% higher soil carbon in the upper 30 cm soil layer, compared to arable fields without a grass margin36. Studies from temperate grasslands show that increasing plant species diversity increases soil organic matter[51],[52],[53],[54] with carbon accumulation increasing over time during grassland restoration54; increased species diversity can also reduce carbon loss from deeper soils53.

Like hedgerows, grass margins also prevent pollution and soil erosion, intercepting nitrogen from the surface and subsurface flow, and phosphorus and soil sediment from the surface flow36. 

There are proven benefits to biodiversity from taking field margins out of production[55]. For example, grassy field margins have been shown to host more species and higher numbers of insects, spiders, wild plants, birds and mammals, compared to control cropped field edges. Margins sown with wild flowers, or specific varieties of nectar-rich plants for pollinators, are particularly beneficial for flower-feeding insects such as bees, butterflies and flies. Vegetated field margins have also been shown to increase pollination services, pest regulation, nutrient cycling in the soil and off-site soil erosion[56]. 

Taking land out of production for hedgerows, field margins, buffer strips or agroforestry, potentially generates a short-term trade-off with crop yields. However, a number of studies have shown that these effects can be offset by, for instance, long term increased pollination and pest control[57],[58] that leads to higher yields, but the effect is not consistent[59]. Yield benefits may take time to accrue and habitat measures must be carefully designed for specific systems to avoid trade-offs. 

Saltmarsh

Saltmarshes provide an ideal carbon store, with typically high plant productivity, slow organic deposition in anaerobic sediments[60], and a low energy environment which traps a lot of organic matter[61]. As such, saltmarshes sequester more carbon than other coastal habitats with rates in the UK around 1.20–1.50 t.C/Ha/y[62].

Moreover, newly restored areas have rapid rates of sequestration of up to 1.04 t.C/Ha/y during the first 20 years (slowing to a steady rate of around 0.65 t.C/Ha/y thereafter)[63] meaning they could play an important role in removing carbon from the atmosphere in the near-term. Total carbon storage of restored and natural saltmarshes is assumed to be around 65-70 t.C/Ha63 after 100 years.

Saltmarsh restoration is well established with 3000 ha created in the UK through managed realignment between 1990 and 2015[64]. The cost of restoration is relatively high, but is countered by reduced coastal protection costs. The UK Environment Agency generally works to a guideline figure of £10,000 per hectare (2006 prices). However, the costs can run to over twice this figure, particularly when significant engineering is required64, with figures of US$67,000 / Ha reported from projects occurring worldwide[65].

Saltmarshes also contribute to coastal protection through dissipation of wave energy, reducing wave attenuation by 60-80%[66]. Saltmarsh often forms part of flood plains, reducing the risk of coastal flooding by storing flood water, although little data exist to quantify these benefits[67]. Managed realignment processes, normally involving the creation of new saltmarsh, can protect areas of the coast under risk from rising sea levels[68].

Saltmarshes are also an important resource for birds and other wildlife. They act as high tide refuges for birds feeding on adjacent mudflats, as breeding sites for waders and gulls, and the seeds of annual saltmarsh plants provide a source of food for passerine birds, particularly in autumn and winter. In winter, saltmarshes with shorter vegetation (often also grazed by livestock) are used as feeding grounds by large flocks of wildfowl. Areas with high structural and plant diversity, particularly where freshwater seepages provide a transition from fresh to brackish conditions, are particularly important for invertebrates. Saltmarsh creeks and flooded areas at high tide also provide sheltered nursery sites for several species of fish, which exhibit a high degree of site fidelity and a degree of seasonal use[69]. UK saltmarshes are internationally important for redshank and two saltmarsh habitats found in the UK are listed in Annex I of the Habitats Directive reflecting their importance for nature conservation.

Heathland Restoration

Heathlands are one of the most valuable habitats for biodiversity in the UK, with lowland heathland in England hosting 133 priority species and upland heathland 35 priority species[70]. Good condition heathland with healthy ericaceous plant coverage (e.g. heather) also contains among the highest amounts of carbon per unit area of any habitat, and considerably more than grassland, with soil stocks of up to 374 t .ha -1 [71]. Restoration and expansion of heathland should therefore be an NbS priority, and heathland should not be converted into woodlands.

What would constitute clear indicators of progress and cost-effectiveness of nature-based solutions?

Monitoring carbon sequestration from NbS is complex due to uncertainties regarding how much land-use and land-cover change contributes to fluxes in carbon stores[72]. These uncertainties restrict our ability to predict the contribution of NbS towards emissions targets and it is crucial that a robust standardised monitoring system for evaluating the carbon sequestration of different NbS is established.

Regular and ongoing monitoring must demonstrate the short-term effects of an NbS in terms of carbon sequestered annually, as well as the total carbon stored and its contribution to long-term goals[73] such as net-zero. Monitoring should take place at the same time each year to minimise the impact of seasonal variation on the carbon sequestration rates. There needs to be a centralised database that captures data and allows for aggregation, otherwise the contribution of small sites will be lost[74].

Satellite remote sensing may provide information on land-use and cover which, when integrated with carbon-cycle process models and regional climate databases, could provide spatially explicit information on carbon storage and flux[75]. However, monitoring strategies should also aim for the least amount of specialist equipment and knowledge so they can be used widely with standardised performance indicators74. Developing effective yet accessible monitoring methodologies should be a priority research area.

Targets and timeframes for active reductions in emissions (e.g. through behavioural changes) should be handled entirely separately from carbon sequestration delivered by NbS[76]. This enables accurate evaluation of NbS performance. Given the relative novelty of delivering NbS at scale, an adaptive management approach will almost certainly be necessary[77].

How can funding be mobilised to support effective nature-based solutions to climate change? How can the private sector be encouraged to contribute to funding?

Payments for environmental land management contracts have great potential for delivering NbS, but need careful design, implementation and monitoring[78]. They should be used in conjunction with baseline environmental standards, enforced by regulation, so that the land management contracts only reward management that goes beyond these standards.

Results-based contracts have some clear advantages over action-based contracts, both in terms of outcomes and the degree of engagement by land managers, but finding suitable result indicators is difficult and more work is required before a wholesale switch to this type of contract is feasible[79].

Reverse auctions can be useful in circumstances where there is choice about where land management measures take place. They could be more useful if they were adapted to encourage competition on both quality and price. Evidence suggests both action and results-based contracts work best when accurately targeted on specific environmental outcomes[80],[81].

Privately funded carbon offsetting has considerable potential but must be accompanied by a robust regulatory framework for emissions reductions. There must also be separate targets for emission reductions and carbon removal, to ensure a real contribution is made[82].

Permit/credit markets have the advantage of flexibility and can bring in private and third sector resources. Green tax breaks could provide an additional incentive for the adoption of NbS.

Regardless of the finance mechanism, legal mechanisms that ensure the longevity or, ideally, the permanence of the intervention are vital. Conservation covenants offer a solution but only if they are accompanied by a robust, well-financed system[83] of monitoring and enforcement.