Tree planting in a climate and ecological emergency

Dr. Gareth Parry addresses the importance of Gloucestershire's woodlands and encourages us to think about where we plant new trees, could they affect the ecosystem around them?

The last 18 months has seen a surge in public awareness and expectation for action on climate change and biodiversity declines. The activism of Extinction Rebellion and the Student Climate Network has brought these issues to the front pages and pushed the UK Government and other public bodies into declaring ‘Climate and Ecological Emergencies’. Gloucestershire Wildlife Trust has been working with local decision makers to the develop the policies and tools that can tackle these issues.   

All of Gloucestershire’s Local Authorities have declared climate and ecological emergencies. Declaring an emergency and making a commitment to carbon neutrality is highly commendable but it is the easy part. Putting these words put into action is more challenging.  Everyone loves a ‘quick win’ and tree planting is emerging as a popular idea to tackle climate change. Political parties are falling over each other to commit to the biggest tree-planting scheme, whilst many local councils are planning their own initiatives.  Environmental originations, such as Gloucestershire Wildlife Trust, have been inundated with offers of trees and requests for planting sessions on our nature reserves.   

It is great news that society is recognising the significant existential threats of climate change and biodiversity declines and that people want to harness the power of nature to tackle this. Trees provide fantastic habitat, store lots of carbon and can help to tackle air pollution, flooding and soil retention. So why on earth should there be any delay in planting as many trees as possible? Well, we must remember that nature is complex and fundamentally works as an interlinked network of species and the physical environment – otherwise known as an ecosystem.  Planting trees changes an ecosystem and the more trees you plant the more substantial these changes are. The obvious difference is that you are adding new living features, which provide habitat that suits some species but not others. Trees also have a significant impact on the non-living characteristics of an ecosystem, influencing levels of shading, soil nutrients and pH and water cycling. Whilst many of these changes are beneficial, in some places tree planting would have a negative impact on biodiversity.  

The UK urgently needs to increase its woodland cover, and this should be a big part of the action to deliver nature’s recovery. Establishing mosaics of wooded and open habitats can play an important role in re-building ecological networks. There is a target to increase tree canopy cover in the UK to 20%, which for some areas is a massive uplift. This is a great ambition and should be achievable without degrading other important wildlife habitats. It is all about the right tree in the right place, with a plan in place to look after it over its lifetime.  

Before trees are planted anywhere, it is important to consider whether they will negatively affect the quality, quantity or connectivity of existing valuable wildlife habitat. There is a growing body of evidence that large patches of high quality habitat is  the most important factor determining the diversity and quantity of wildlife. These areas are the wildlife ‘cities’, vital to the long-term viability of the species they support and precious source sites for nature’s recovery. One example where tree planting would be detrimental is on the UK’s flower-rich meadows and grasslands, 97% of which were destroyed during the 20th century. Gloucestershire is fortunate to still have some flower-rich grasslands, such as Crickley Hill, Rodborough Common and Cleeve Common, all of which are designated for their fantastic wildflowers and associated fauna, and all touted by some as possible locations for mass tree planting.  These grasslands are one of the most wildlife-rich habitat types in Europe supporting an abundance of life, including many rare plants, insects and fungi. Planting trees would reduce the quality and quantity of the habitat that these specialist species need, thereby, undermining their future.  

Tree planting could also have a negative impact on important wetlands and heathlands which are great at locking in carbon too. Gloucestershire Wildlife Trust spends a lot of time managing tree growth on wetlands and heathlands to maintain some open habitat for the large number of species that require this. Breeding wetland birds often avoid wooded edges, probably due to the association with predators, so planting in these habitats could affect breeding success. Species such as Nightjar can benefit from a heathland mosaic that includes woodland, but they avoid dense woodland blocks. The best opportunities for delivering appropriate biodiversity enhancements through tree planting are probably on marginal farmland, although care is still required to protect threatened farmland birds which are adverse to breeding near woodland edges. 

Maintaining and restoring large areas of high-quality habitat is the most important thing we need to do to reverse biodiversity declines, but wildlife also needs to be able to move between good bits of habitat. Many species exist as groups of connected populations. From a survival perspective this is a smart strategy, reducing the risk of extinction. If a chance event, such as flooding or disease, wipes out one population it can be replaced by individuals from a connected population. Think of it as a collection of villages. If lots of housing becomes available at a bargain price in one village, people from a nearby village are likely to move in and the risk of all villages becoming empty is low. Water voles (Arvicola amphibius) are one species whose survival relies upon these groups of connected populations.   

Habitat fragmentation occurs when the connections between wild places are broken. These connections are known as ecological networks and humans are very adept at breaking them. Roads, housing, industrial estates and intensive farming can all act as barriers to wildlife movement if they are created without this in mind. Whilst landscapes containing a mosaic of different habitats normally strengthen ecological networks, introducing large blocks of woodland in the wrong place could act as a barrier for specialist species of grasslands, heathlands and wetlands. Once again, the smaller less mobile species are the most vulnerable, some insects will not cross woodland even if corridors are provided. Ecological networks also play a vital role in supporting species to adapt to climate change, enabling them to move north, south, east and west as weather conditions change. 

The fragmentation of ecological networks is a particular problem for some wildflowers and insects and can lead to something called an ‘extinction debt’. This describes a situation where everything appears fine, but fragmentation has fundamentally undermined the long-term future of many species. There could be a substantial delay, sometimes more than a century before the debt gets ‘paid’ and lots of species go extinct. Extinction debt is a long-term phenomenon which is difficult to detect and plan for, making it one of the biggest challenges for wildlife conservation. Gloucestershire Wildlife Trust has been mapping Gloucestershire’s ecological networks to identify where we need to reconnect the best wild places and create new ones. This will be Gloucestershire’s Nature Recovery Network. The maps show a level of habitat fragmentation within the county that is nothing short of scary.  Who knows how much extinction debt we have already accumulated in our ledger?  

When it comes to tree planting, there is one rather large elephant in the room – ash dieback; a disease caused by the fungus (Hymenoscyphus fraxineus), which affects one of the most common trees in the UK and has a fatality rate of 70-85% over a 20-year period. To date, the disease has been recorded in 84% of the 10 km grid squares which wholly or in part cover Gloucestershire. This is an increase of 63% since 2016, so it is reasonable to assume that ash dieback will soon be ubiquitous across the county. It is estimated to account for more than 10% of the canopy in 90% of woodlands in Gloucestershire and in some cases forms 80-100% of the canopy. For more information it is worth visiting the Forest Research website. Essentially, the UK will lose tens of millions of trees to ash dieback and current tree planting commitments for carbon offsetting are unlikely to offset these losses, never mind deliver a net gain. The UK needs to increase its woodland cover, but ash dieback means we may need to recruit more than 100 million new trees to achieve the target of 20% canopy cover. At this point it is worth noting that with current tree planting pledges, the UK’s existing tree nurseries will quickly run out of native sourced and grown trees, so there is a supply chain issue. The ash dieback fungus was almost certainly introduced through imported sapling trees for planting in the UK. We must not introduce another disease by importing trees to satisfy our vigor for tree planting.   

Whilst the loss of ash trees creates opportunities for planting, which is highly desirable where hedgerow, landscape and urban trees are lost, it is not desirable in all instances. The decline in woodland management is a big challenge for the UK’s native broadleaved woodlands as this reduces habitat quality. Not replacing some ash lost to the disease could help to open up undermanaged woodlands, letting in more light and improving the structural complexity of the habitat, benefiting woodland plants, insects, birds and mammals. Where replacement trees are needed, current best practice is to allow natural regrowth from seed banks in the soil. This not only maintains the local genetic diversity of tree stocks, it also reduces the risk of introducing new diseases and is cheaper than planting new trees.  Natural tree regeneration is possible in most habitats and should be a serious consideration when it comes to establishing new areas of woodland.   

The other big challenge for tree planting is land. Many people who want to plant trees do not own land for planting and there is limited suitable public land. Gloucestershire Wildlife Trust’s reserves seldom require extra trees and our preference is to grow our woodlands through natural regeneration. Tree planting is likely to deliver the biggest benefits in marginal agricultural land, where it creates a mosaic of open and wooded habitats and does not impede ecological networks for rare grasslands, heathlands and wetlands. This is of course at odds with current land use, so it is vital that farmers and landowners are engaged in tree planting discussions, such as through the ‘matchmaking’ initiatives being run by the NFU and Woodland Trust here in Gloucestershire.  

Furthermore, no tree should be planted without a long-term plan for how it is going to be managed. Whether a tree is planted or naturally seeded it will need care if it is to become a healthy mature tree. Deer and squirrels are very fond of munching through young trees and whilst some of the methods for tackling this are controversial, protecting your new trees needs consideration and non-lethal options are available. Trees in public places may also need regular maintenance and insurance for public safety purposes. Grants currently available for woodland management just aren’t large enough for many farmers and landowners to see tree planting as something that is financially viable. It might be more sensible for political commitments to provide better long-term support to landowners for establishing and managing new areas of public woodland, whilst letting natural regeneration provide most of the trees for free.    

At GWT, our three golden rules for tree planting are: 

  1. Make sure the location is ecologically suitable 

  1. Use locally sourced trees of appropriate species 

  1. Have a plan and funding to look after the tree as it grows.  

The current tree planting rush is mainly driven by public bodies trying to deliver offsetting so they can become carbon neutral, a commitment that Gloucestershire Wildlife Trust is fully supportive of. Tree planting is a great tool for tackling the climate and ecological emergency, but there are other tools in the tool box that may be more suitable in some locations, such as establishing wetlands and permanent grassland. Trees do capture and store a lot of carbon, but the amount captured falls over time, which is not the case for peatlands and other wetland habitats. In a county such as Gloucestershire, wetlands could also deliver benefits for flood prevention and create new habitat for the wildlife of the Severn estuary, one of our internationally important ecosystems.  

As part of our work with the Gloucestershire Local Nature Partnership we are creating a Nature Recovery Network for Gloucestershire, which among other things will identify the best places for tree planting. We are promoting two areas where we think tree planting at scale makes sense in Gloucestershire: a mass ‘southern forest’ linking Westonbirt to our incredible reserve at Lower Woods; and a huge and ambitious project linking the Forest of Dean with the Wyre Forest in Worcestershire. These two projects could significantly increase tree cover in our county, but there’s a long way to go engaging farmers and other landowners with these ideas. 

The Local Nature Partnership is also developing a Natural Capital map to show where nature can deliver the biggest benefits for challenges such as carbon offsetting and flood protection. The GFirst Local Enterprise Partnership has shown extraordinary leadership by putting Natural Capital at the heart of its Local Industrial Strategy, acknowledging that adapting to climate and ecological threats will be vital for economic growth and productivity.  

In taking action on the climate emergency, let’s also take action on the ecological emergency.  Stopping and reversing biodiversity declines cannot be ignored. While adaptation to climate change will be difficult, humans are unlikely to be able to adapt to ecosystem collapse. We do have to act now and act quickly. But we also need to make sure that ‘quick wins’ don’t have unintended negative impacts on the environment, despite our best intentions. 

Ref.

1.Chen, L.-N. et al. Impacts of afforestation on plant diversity, soil properties, and soil organic carbon storage in a semi-arid grassland of northwestern China. Catena 147, 300–307 (2016). 

2.Chirino-Valle, I., Davis, M. R. & Condron, L. M. Impact of different tree species on soil phosphorus immediately following grassland afforestation. J. Soil Sci. Plant Nutr. 16, 477–489 (2016). 

3.Öckinger, E. et al. The landscape matrix modifies the effect of habitat fragmentation in grassland butterflies. Landsc. Ecol. 27, 121–131 (2012). 

4.Villemey, A. et al. Mosaic of grasslands and woodlands is more effective than habitat connectivity to conserve butterflies in French farmland. Biol. Conserv. 191, 206–215 (2015). 

5.Poniatowski, D., Stuhldreher, G., Löfflera, F. & Fartmannab, T. In agricultural landscapes, permanent grassy habitats are often fragmented and partly composed of linear elements. Landscape-level connectivity of both grassland patches and grassy linear elements actively contributes to biodiversity conservation in farml. Biol. Conserv. 225, (2018). 

6.Thomas, J. A. et al. The quality and isolation of habitat patches both determine where butterflies persist in fragmented landscapes. Proc. R. Soc. B Biol. Sci. 268, 1791–1796 (2001). 

7.Hodgson, J. A., Moilanen, A., Wintle, B. A. & Thomas, C. D. Habitat area, quality and connectivity: Striking the balance for efficient conservation. J. Appl. Ecol. 48, 148–152 (2011). 

8.Swaay van, C. A. . The importance of calcareous grasslands for butterflies in Europe. Biol. Conserv. 104, 315–318 (2002). 

9.Wilson, J. D. et al. Modelling edge effects of mature forest plantations on peatland waders informs landscape-scale conservation. J. Appl. Ecol. 51, 204–213 (2014). 

10.Sharps, K., Henderson, I. A. N., Conway, G., Armour-chelu, N. & Dolman, P. M. Home-range size and habitat use of European Nightjars Caprimulgus europaeus nesting in a complex plantation-forest landscape. Ibis (Lond. 1859). 157, 260–272 (2015). 

11.Gilroy, J. J. Ã., Anderson, G. Q. A., Vickery, J. A., Grice, P. V & Sutherland, W. J. Identifying mismatches between habitat selection and habitat quality in a ground-nesting farmland bird. Anim. Conserv. 14, 620–629 (2011). 

12.Sutherland, C., Amherst, M., Elston, D. A. & Lambin, X. A demographic , spatially explicit patch occupancy model of metapopulation dynamics and persistence. Ecology 95, 3149–3160 (2014). 

13.Ries, L. & Debinski, D. M. Butterfly responses to habitat edges in the highly fragmented prairies of Central Iowa. J. Anim. Ecol. 70, 840–852 (2001). 

14.Öckinger, E. & Smith, H. . Do corridors promote dispersal in grassland butterflies and other insects? Landsc. Ecol. 23, 27–40 (2008). 

15.Butaye, J., Adriaens, D. & Honnay, O. Conservation and restoration of calcareous grasslands: A concise review of the effects of fragmentation and management on plant species. Biotechnol. Agron. Soc. Environ. 9, 111–118 (2005). 

16.Oliver, T. H. et al. Interacting effects of climate change and habitat fragmentation on drought-sensitive butterflies. Nat. Clim. Chang. 5, 941–946 (2015). 

17.Darvill, B. et al. Cryptic differences in dispersal lead to differential sensitivity to habitat fragmentation in two bumblebee species. Mol. Ecol. 19, 53–63 (2010). 

18.Vellend, M. et al. Extinction Debt of Forest Plants Persists for More than a Century following Habitat Fragmentation. Ecology 87, 542–548 (2006). 

19.Kuussaari, M. et al. Extinction debt : a challenge for biodiversity conservation. Trends Ecol. Evol. 24, 564–571 (2009). 

20.Coker, T. L. R. et al. Estimating mortality rates of European ash (Fraxinus excelsior) under the ash dieback (Hymenoscyphus fraxineus) epidemic. Plants, People, Planet 1, 48–58 (2019). 

21.Alonso, I., Weston, K., Gregg, R. & Morecroft, M. Carbon storage by habitat: Review of the evidence of the impacts of management decisions and condition of carbon stores and sources. (2012). doi:ISSN 1754-1956