Biological invasions by alien species are regarded as one of the top five direct drivers (together with habitat destruction, over-exploitation, climate change and pollution) of recent global biodiversity loss, according to the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services. The impacts of alien species are linked to the declining conservation status of threatened species, and their accelerated spread into protected and biodiverse habitats are a major global concern. Ancient woodlands are the richest and most complex terrestrial habitat in the UK, home to more threatened species than any other. However, encroachment and establishment of invasive alien species pose additional challenges to the unique flora and fauna, as well as ecosystem function, of ancient woodlands.
Ancient woodlands are areas of woodland that have persisted since 1600 in England and Wales, and 1750 in Scotland. Only 2.5% of the UK land is covered in ancient woodland. Ancient woodlands are valued as being among the most complex and diverse of ecosystems, and as having plants and animals that do not occur in woodlands of recent origin. Ancient woodlands are often isolated sites, having a characteristic flora which do not easily migrate between woodlands. Bluebells, Hyacinthoides non-scripta, are an iconic feature of UK ancient woodlands, currently under threat from hybridisation with non-native bluebells, Hyacinthoides hispanica. Other indicator species, including birds such as hawfinch and wood warbler, and understory plants such as herb paris, often feature in ancient woodland sites. As well as monitoring the status of indicator species within ancient woodlands, the regeneration of ancient woodland trees is also important in maintaining longevity of these ecosystems.
Up to 70% of ancient woodland has been lost or damaged due to conifer plantations, overgrazing and the spread of invasive species. Currently, 1207 ancient woodlands remain under threat in Britain. Himalayan balsam, Impatiens glandulifera, is an annual plant native to the western Himalayas, prolific along waterways but now thriving in woodland areas. It is listed under Schedule 9 of the Wildlife and Countryside Act 1981, therefore it is an offence to plant or otherwise cause this species to grow in the wild. Himalayan balsam, by forming tall, dense colonies, shades out and results in the extirpation of native vegetation. This species is notoriously difficult to manage, particularly if an adjacent water way provides a constant input of viable seeds.
Although much research has been done on the impacts of Himalayan balsam within riparian habitats, little is known of its impacts in woodlands, particularly ancient woodland sites. Alongside this, managing this invasive alien species in protected habitats such as ancient woodlands are challenging, as traditional control methods (for e.g. chemical and mechanical), may not be feasible due to indirect impacts on threatened and specialist native species.
In the North East of England, The Woodland Trust have a protected 100Ha ancient woodland site Pontburn Wood situated alongside an invaded river, the River Derwent. Establishment of Himalayan balsam in the adjacent woodland has increased exponentially over the last 5 years, with further encroachment apparent.
The overall aim of this project is to understand the impact of Himalayan balsam on ancient woodland biodiversity and determine an effective management strategy to control and mitigate further invasion.
Specifically, the project objectives are to:
1) Undertake a baseline assessment of the impacts of Himalayan balsam on ancient woodland flora both on above-ground vegetation and below-ground soil seed bank,
2) Determine how Himalayan balsam impacts native tree sapling growth and establishment due to potential preferential herbivory of native vegetation and competition dynamics of early growing Himalayan balsam seedlings,
3) Undertake an assessment of effective management strategies for reducing the abundance of Himalayan balsam in the woodland, and
4) Assess the viability of mitigation methods for reducing further colonisation from the river