A rapid review was commissioned by the Scottish Government to examine the potential consequences of not controlling bracken growth on biodiversity, rural productivity, and public health in Scotland. The study specifically focuses on the traditional use of asulam, marketed as Asulox, as the primary herbicide for bracken control in rugged and steep terrains since the 1980s.
The review found that not controlling bracken in areas without vehicle access and where asulam was previously used for control has several potential negative consequences, including limited options for controlling bracken in rough or steep terrains, reliance on less effective control methods, disruption of follow-up treatments, implications for woodland and forestry establishment, potential spread of bracken into sensitive biodiversity areas, and continued loss of grazing land.
However, not using asulam eliminates the risk of its impact on non-target species and allows for the release of carbon stores. It may also drive innovation for alternative control methods. However, key data gaps need to be addressed to make informed decisions, including trends in bracken areas, mapping of bracken distribution, long-term effectiveness of control programs, cost-benefit analysis, alternative herbicide options, exposure studies, and tick densities in different habitats.
The Importance and Challenges of Bracken Control in Scotland
In 2012, authorization for the use of Asulox was withdrawn, leading to its recent employment under emergency authorizations on an annual basis. Emergency authorizations enable the controlled use of plant protection products in situations where standard authorizations are not available. Such authorizations are granted under special circumstances where the absence of control poses risks to crops, other plants, the environment, or human and animal health. The decision to grant emergency authorizations is carefully evaluated based on a thorough assessment of potential risks and benefits. The authorization period for emergency use cannot exceed 120 days.
The manufacturer of Asulox is actively seeking approval for asulam through the standard regulatory process, which involves rigorous testing and evaluation. However, completing all the necessary tests and assessments may take several years. If the known data gaps can be filled, further evaluation of asulam will be required to determine whether it meets the regulatory conditions for approval.
The absence of asulam as a control method would result in the loss of the most effective means of eradicating bracken. The current practice involves initial blanket spraying followed by a systematic follow-up program. Its unavailability would render control in steep terrains, where aerial spraying is the only viable option, impossible. Furthermore, the forestry management practice of utilizing asulam for treating bracken-infested areas to facilitate tree establishment would also be compromised. Although cutting has proven to be effective, it is restricted to areas accessible by vehicles, is carbon-intensive, and takes significantly longer to suppress bracken. Recent evidence indicates that rolling/bruising is ineffective compared to cutting and asulox treatment. Whether certain control methods are used and where (aerial) spraying is the only viable option currently lack comprehensive data. Therefore, the updating of bracken control guidance is necessary irrespective of the continued use of asulam.
The report reveals that bracken expansion in Scotland has led to the replacement of other habitats, particularly acid grassland and heathland. However, data on trends since 2007 are unavailable. Bracken's dominance and expansion can be attributed to climate change, especially in regions where its growth was previously restricted due to cold temperatures. Combining information on control effectiveness and the extent of treated land suggests that past use of asulam has successfully rendered several hundred square kilometres bracken-free, resulting in significant benefits for biodiversity and rural productivity.
The encroachment of bracken into semi-natural habitats poses a threat to associated biodiversity, as hill ground taken over by bracken tends to have limited ecological value. While there is limited scientific evidence on the direct effects of bracken as a source of toxic or carcinogenic compounds within a Scottish context, UK-level assessments indicate low risks. Some studies indicate that bracken harbors high densities of ticks, although it remains unclear whether these levels are higher compared to other habitats, as tick numbers are dependent on the presence of host mammals. Further research is required to evaluate whether the presence and expansion of bracken increase the risk of tick-borne diseases such as Lyme disease in animals and humans.
The impact of bracken on rural productivity remains unknown. While partial information on control costs can be obtained through grant support, there is a lack of data concerning the opportunity costs.
This report was funded by the Rural & Environment Science & Analytical Services Division of the Scottish Government Underpinning National Capacity Support to Policy Function.
The content of this report does not reflect the official opinion of the Scottish Government. Responsibility for the information and views expressed therein lies entirely with the author.
The report is funded as part of the Scottish Government’s Environment, Natural Resources and Agriculture Research Portfolio’s research and expertise to ensure scientific evidence helps inform the health, wealth and wellbeing of Scotland.
The research component of the Portfolio is one of the largest, multidisciplinary programmes of strategic research in the UK, worth over £200 million across the five years.
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