The problem with bracken

Bracken (Pteridium aquilinum)  is a contentious species which is almost universally hated on Dartmoor (except in the handful of places where it harbours the nationally threatened butterfly, the High Brown Fritillary (Argynnis adippe).

Bracken was once harvested as an important crop and used for animal bedding and as a roofing material. These practices dropped out of favour as other more modern materials were used instead and as a result bracken was no longer managed and began to spread.

This spread of bracken can be damaging to the historic environment as its dense network of rhizomes can seriously interfere with any sub-surface archaeology. It is also a species which is very unpopular with hill-farmers as it spread reduces the area of palatable grazing for livestock. Likewise conservationists do not like the species as its spread can reduce areas of inherently interesting vegetation communities e.g. heather stands (Marrs and Watts 2006).

Bracken tends to grow on deep well drained soils which do not become waterlogged. As a result it is absent on Dartmoor from the blanket bog and wet heathland communities as these are too wet. These deeper better drained soils on the moor support heathland (NVC H8 Calluna vulgaris-Ulex gallii heath) and upland grassland communities (NVC U4 Festuca ovina-Agrostis capillaris-Galium saxatile grassland and U5 Nardus stricta-Galium saxatile grassland). Bracken can encroach into wet heath communities (M15b Trichophorum cespitosus, Erica tetralix wet heath and M15d Trichophorum cespitosus, Erica tetralix wet heath, Vaccinium myrtillus sub-community if these have become drier as a result of hydrological changes, over grazing or burning (Marrs and Watts 2006).

These National Vegetation Classification (NVC) communities can change depending on the management regime they receive. Averis et al (2004) suggest the following changes to National Vegetation Classification communities which can all lead to an increase in bracken communities. The communities described below follow the NVC (Rodwell 1991 & 1992).

• If H8 Calluna vulgaris-Ulex gallii heath or H12 Calluna vulgaris-Vaccinium myrtillus heath is over grazed or over burnt it can change into M25 Molinia caerulea-Potentilla erecta or U4 Festuca ovina-Agrostis capillaris-Galium saxatile

• If U4 Festuca ovina-Agrostis capillaris-Galium saxatile grassland or U5 Nardus stricta-Galium saxatile grassland are under grazed then can turn to H8 Calluna vulgaris-Ulex gallii heath or H12 Calluna vulgaris-Vaccinium myrtillus heath

• However all four of the communities mentioned above (H8, H12, U4 & U5) can be invaded by bracken and turn into U20 Pteridium aquilinum-Gallium saxatile

Bracken can be controlled either by cutting, rolling or the use of herbicides (asulam). However treatments must be repeated yearly if bracken is to be controlled, complete eradication is usually not possible. All these methods are time consuming and expensive. Rolling is often not possible due to the terrain or rocks and asulam can now only be used under an Emergency Authorisation licence as its use was outlawed in 2012 [1], there are also concerns that stocks of asulam are not in short supply as it is no longer being manufactured.

Bracken rolling on Dartmoor by the National Trust

Pakeman et al (1995) showed that bracken had increased significantly over the past few centuries in Britain but they also showed that between 1970 to 1980 there had been a 3.4% decrease on Dartmoor (by 1.9km² of bracken being gained but 3.8km² being lost).

Pakeman et al. (2000) ‘concluded that the current abundance of Pteridium was less than, or at worst, equivalent to maximum historical records’. It is clear therefore that the abundance of bracken has fallen and risen depending on its harvesting or clearance by humans.

Werkman et al (1996) carried out experiments where bracken and heather and a mix of the two were grown in open topped tents to mimic climate warming and where different plots were treated with additional nitrogen inputs. They found bracken growing in the tents with additional nitrogen grew more vigorously and for a longer growing season than plants not grown in tents where no nitrogen was added. The bracken under the former conditions also encroached into the heather stands.

Werkman et al (2002) in another experiment found that bracken responded positively to increased temperatures but did not respond to increased nitrogen levels. They concluded that in a warmer climate bracken will continue to replace heather. They added a caveat that if climate change led to drier summers then water could be a limiting factor in the spread of bracken.

The implication of both papers by Werkman et al (1996, 2002) is that bracken will also spread into areas of upland grassland as well as areas dominated by heather.

Hill-farmers and other land managers on Dartmoor such as the National Trust spend considerable time and money attempting to control bracken on the moor, it would appear that in the future more effort will be required if bracken is not to spread further.

So rather like Purple Moor Grass Molinia caerulea, bracken does respond to anthropogenic changes to the atmosphere. In the case of bracken to increased temperatures caused by rising CO₂ levels and in the case of Molinia to increased nitrogen levels caused by pollution from vehicles and agriculture (see here).

If a future climate change scenario on Dartmoor led to rising temperatures and reduced rainfall it is not impossible that the current areas of wet heath and those areas dominated by Molinia could become dry enough to allow the encroachment of bracken into these areas too.

 References
Marrs R.H. & Watts A.S. (2006) Biological Flora of the British Isles: Pteridium aquilinum (L.) Kuhn. Journal of Ecology 94, 1272–1321
Pakeman, R.J., Le Duc, M.G. & Marrs, R.H. (2000) Bracken distribution in Great Britain: strategies for its control and the sustainable management of land. Annals of Botany, 85B, 37–46.
Pakeman, R.J., Marrs, R.H., Howard, D.C., Barr, C.J. & Fuller, R.M. (1995) The bracken problem in Great Britain; its present extent and future changes. Applied Geography, 16, 65–86.
Rodwell J.S. (ed) (1991) British Plant Communities. Volume 2. Mires and heaths. Cambridge University Press. Cambridge.
Rodwell J.S. (ed) (1992) British Plant Communities. Volume 3. Grasslands and montane communities. Cambridge University Press. Cambridge.
Werkman B.R. & Callaghan T.V. (2002) Responses of bracken and heather to increased temperature and nitrogen addition, alone and in competition. Basic and Applied Ecology 3: 267-276.
Werkman B.R., Callaghan T.V. & Welker J.M. (1996) Responses of bracken to increased temperature and nitrogen availability. Global Change Biology 2: 59-66.

[1] http://www.brackencontrol.co.uk/asulam