This blog post aims to discuss the key insights from a report by The Future Landscapes Forum, a group of leading academics and practitioners, shedding light on why managing bracken is becoming an increasingly urgent issue in the UK.
We'll explore what makes bracken so successful, how much of it there is, the problems it poses, and the challenges we face in controlling it now that our most effective tool is no longer widely available.
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What Exactly Is Bracken?
Bracken (Pteridium aquilinum) is a fern and a remarkably successful one at that, thriving on almost every continent except Antarctica. What's its secret? A few clever adaptations give it a real edge:
Shady Superstar: In spring and summer, bracken unfurls large fronds, creating a dense canopy that blocks sunlight and outcompetes other plants.
Underground Powerhouse: It boasts an extensive underground rhizome system, acting like a food and nutrient store, fuelling frond growth and protecting it from frost, fire, and disturbance. This also allows it to regenerate rapidly if the top growth is damaged.
Litter Layer Lockdown: When the fronds die-back in autumn, they form a thick litter layer, preventing other plant species from getting a foothold.
Chemical Warfare: Bracken is packed with a range of secondary plant chemical compounds, some of which are not so friendly. These include substances that can be carcinogenic, cytotoxic, mutagenic, and teratogenic. These chemicals likely serve as a defense against herbivores and microbes, and might even inhibit the growth of other plants (allelopathy).
The type of bracken most common in the UK is Pteridium aquilinum subsp. aquilinum, while another variant, P. esculentum, is more prevalent in the Southern Hemisphere.
The Great Bracken Census: How Much Is There?
You'd think knowing how much bracken covers the UK would be straightforward, perhaps using satellite imagery. However, it's surprisingly complicated.
Blending In: It's tough to consistently distinguish bracken from grasslands in remote sensing data. In fact, since 2007, bracken has often been grouped with grasslands in UK land cover maps.
The Sparse Problem: While dense patches of bracken are easier to spot, many areas have sparse bracken cover (less than 25% fronds), which is very difficult to detect remotely.
Hidden Underneath: Bracken often grows under trees, completely hidden from aerial or satellite view. This hidden bracken can rapidly expand if the trees are removed, skewing any change assessments.
Linear Invaders: Bracken thriving in hedges, along fences, and on road verges is hard to quantify but acts as a source for spreading to new areas.
The Growth Cycle: Bracken's frond density can fluctuate naturally through growth phases, making it hard to determine if changes in cover are real expansions or just part of a cycle.
Early estimates of bracken cover in Great Britain varied significantly, from 4729 km² down to 2360 km². More recent surveys, like the UK Countryside Survey, provided estimates with considerable standard errors. These surveys, however, have not been run since 2007, and newer data isn't yet available.
The Land Cover Map also attempted to map bracken, but faced challenges in obtaining reliable data and consistent imagery timing. Since 2007, for consistency, the bracken class has been incorporated into "acid grassland".
Interestingly, a 1990 survey estimated that the total area of land infested with some bracken was a staggering 17072 km² (7.3% of GB), more than four times the area classified as dense bracken. Additionally, there were 122,000 km of linear features containing bracken. This highlights that focusing only on dense bracken in open areas underestimates the true extent and potential for spread.
Measuring change in bracken cover is even more challenging. Bracken primarily spreads through slow rhizome invasion (typically less than 1 metre per year), with sexual reproduction via spores being rare in most UK areas. Therefore, many reported rapid increases are likely due to changes in frond density rather than actual colonization of new land.
Bracken's Future: Climate Change and Land Management
Looking ahead, climate change is predicted to favour bracken. Models from the mid-1990s indicated that warmer temperatures and longer frost-free periods are likely to increase bracken's productivity and spread. Bracken is sensitive to frost, so a longer growing season allows for more photosynthesis.
Changes in land management, such as reduced grazing pressure due to extensification or rewilding, could also lead to increased bracken cover and spread. Experiments have shown that bracken recovers much faster after control treatments in ungrazed areas compared to grazed ones.
It's also worth noting that bracken acts as a significant carbon store, with estimates around 0.5 Gt of carbon in the UK.
The Bracken Problem: Why Land Managers Worry
While bracken can provide habitat for some species of conservation interest, particularly in areas that were formerly woodland, its expansion generally reduces biodiversity compared to the plant communities it replaces. It's even considered one of the over-dominant native species linked to reduced diversity in British Broad-Leaved Woodland.
Beyond conservation, bracken causes significant problems for:
Agriculture: It reduces available grazing land and increases the costs of managing livestock. It's also poisonous to grazing animals, causing vitamin deficiencies and cancers.
Human Health: Correlative data suggests a possible link to some human cancers. A carcinogenic compound (ptaquiloside) produced by bracken has been detected in potable water supplies, raising concerns.
Disease: Bracken-dominated areas can harbour ticks, which can transmit diseases to both humans (like Lyme disease and Tick-borne encephalitis) and animals (like Louping Ill).
Water Quality and Soil: Bracken negatively affects water quality and likely also impacts soil carbon stocks by altering soil chemistry.
A survey in England and Wales found that almost half of farmers in Less Favoured Areas reported bracken as a problem, and a majority favoured a national control scheme.
The Control Challenge: Life After Asulam
For many years, the primary focus of bracken control has been the use of the herbicide asulam. It was considered the safest and most effective option, particularly for large areas and difficult terrain where helicopter-based application was possible.
However, in 2011, the EU decided not to re-register asulam due to concerns about its safety data. Although the UK secured annual Emergency Authorisations (EAs) for its use until 2022, the manufacturers stopped research needed for re-regulation in October 2023 for commercial reasons. Consequently, no EA application could be submitted for 2024, leaving a major gap in bracken control options.
Now, without asulam, land managers are left with essentially three main approaches:
Mechanical Control: This includes cutting, pulling, bruising, and ploughing/discing.
Cutting: Needs to be done multiple times per year for many years (at least 8 years of 2-3 cuts annually, or even 14-25 years of twice-yearly cuts in experiments) to achieve good control. Specialized machinery like small Alpine tractors and self-powered Robocutters can help on steep slopes, though rocky terrain remains a challenge.
Bruising: An older technique that damages fronds without severing them. While faster on rough terrain, experiments have shown it to be less effective than cutting or asulam.
Ploughing/Discing: Aimed at breaking up rhizomes, but is limited to flatter, suitable land. Recent attempts showed initial impact but no long-lasting effect. Rotavating has similar terrain limitations.
Herbicidal Control (without asulam): The main remaining option is glyphosate.
Glyphosate is non-selective, meaning it affects almost all other plants, making it less suitable where protecting underlying vegetation is important.
It is not licensed for aerial application.
Most glyphosate applications for bracken are likely to be done using weed wipers, which selectively apply herbicide to taller bracken, leaving shorter plants untouched. However, weed wipers have the same terrain limitations as cutting, needing to be drawn behind vehicles.
Research into alternative herbicides like amidsulfuron has shown some promise, but it provided less long-term control than asulam and affected more non-target species. It's also not licensed for aerial spraying.
Biocontrol Approaches: These include using insects or fungi to control bracken.
Several insect species and a fungus (Ascochyta pteridis) have been investigated, but these methods are still in the laboratory development phase and require significant testing before large-scale use.
Other potential biocontrol includes planting trees to shade out bracken (though this hasn't been widely tested experimentally) and using grazing animals like cattle, horses, or sheep to trample it (though this poses risks to animal health due to bracken's toxicity). Virtual fencing could potentially help manage grazing in specific areas.
Lessons Learned: Long-Term Commitment is Key
Studies during the asulam era have highlighted a crucial point: regardless of the control method, long-term, repeated treatments are necessary for lasting bracken control. Asulam, even at its best, provided good control for only a short to medium term (up to 10 years).
Excellent results with asulam were often achieved through initial application followed by annual spot treatments. The same principle likely applies to other herbicides and control methods.
Follow-up treatments don't necessarily need to be the same as the initial method. Grazing livestock (especially cattle) after primary treatment can be beneficial.
Where mechanical cutting has been successful, it has required many years of consistent effort. Even after successful long-term control, ongoing monitoring and further treatments will likely be needed if bracken starts to recover. An adaptive management strategy combined with long-term monitoring is essential.
Moving Forward: Addressing the Bracken Challenge
The withdrawal of asulam has created a pressing need for a renewed focus on bracken management in the UK. Key actions suggested by experts include:
Better Bracken Mapping: Implementing improved methods to accurately estimate the full extent of bracken, including sparse patches, woodland bracken, and linear features. This could involve using AI with remote sensing data (combining spectral imagery, digital elevation models, and LIDAR), re-doing the Countryside Survey with detailed bracken assessments, and potentially using drones with specialized sensors.
Evaluating Agri-environment Schemes: Assessing the effectiveness of past funding in controlling bracken and determining what can be achieved in the post-asulam era.
Improving Control Techniques: Investing in research and development of more effective and sustainable control methods, including:
Improved cutting machinery for slopes.
Further investigation and experimentation with bruising techniques to determine if and when they can be effective.
Studies on the use of grazing animals for follow-up treatment, alongside thorough animal welfare assessments.
Developing alternative herbicides that are safe and effective.
Techniques for applying herbicides on upland terrain using ground sprayers or drones, now that helicopter spraying is unavailable.
Further development of weed-wiping technology for slopes and uneven ground.
Continued Water Monitoring: Regularly monitoring water bodies for asulam and its derivatives to understand their persistence and identify potential alternative sources if they remain present after asulam use ceases.
National Leadership and Collaboration: Supporting the efforts of the Bracken Control Group to encourage debate and develop new approaches. This requires government agencies and stakeholders to work together and provide necessary research funding.
Conclusion
Bracken is a significant weed problem in Great Britain with far-reaching consequences for biodiversity, agriculture, human and animal health, and water quality. The loss of asulam has highlighted the need for a cohesive and well-funded strategy to manage this resilient fern.
We need better data on bracken's extent, innovative approaches to control, and a long-term commitment to managing its spread. By supporting research, fostering collaboration, and implementing adaptive management strategies, we can work towards mitigating the negative impacts of bracken and ensuring a healthier and more diverse UK landscape for the future.
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