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

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-bruising-003Bracken 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

Dawlish Warren Beach Management Scheme

There are some big plans ahead for Dawlish Warren. The fear is that a strong southerly storm could cut through the Neck of the Warren and split the sand dunes into two. This will then increase storminess in the estuary and create a heightened flood risk in the village of Dawlish Warren.

dw1
This information board at the Warren explains the detail.

dw2Looking across to Exmouth

dw3One of the groynes that needs to be replaced.

dw4This is part of the first phase of the project that was completed a couple of years ago – an internal bund that will hold back the sea during storm surges.

Later in the year a new bund will be built near the Neck made of geotextile bags.

More flooding in Lancashire, Yorkshire and North Wales from a Storm with no name

It’s Boxing Day – I’ve had a lovely relaxing day doing my own thing. It is quite a contrast to what I have been following on my digital media streams regarding the floods in Lancashire, Yorkshire and North Wales. To be honest I feel pretty awful – I’m having fun,  whilst others are having a torrid time and are suffering emotionally and financially. I am several hundred miles away from the unfolding events and am powerless to help.

On Christmas Eve I was speculating over a beer with my friends that Cumbria was going to be flooded for the 4th time this month – but Cumbria (at least it has been as I am writing this late boxing Day eve) has been lucky – the forecast deluge has hit further south and affected Yorkshire, Lancashire and North Wales instead.

Boxing Day floods 1
It is very unusual to see red, amber and yellow rain warnings all on the same map

This has been a very unusual storm event – records have been broken in an exceptional fashion.

Boxing Day floods 2
This is the Environment Agency gauging data for the River Aire in Bingley – the previous highest recorded river levels have been exceeded by 100% – this is unprecedented

Boxing Day floods 4And here is a tweet from Guy Shrubsole – the climate and energy campaigner at Friends of the Earth – every river in Lancashire has broken its previous highest level!

In 2015 the Met Office started naming storms in a similar way that the United States names hurricanes – this storm however was nameless – Storm Eva went through the UK on Christmas Eve and Storm Frank has yet to arrive – somehow odd?

Boxing Day floods 3

The scale of this storm can be seen in this Environment Agency graphic – the North and North Wales is covered in amber and red river warnings – see here for the current situation.

One of the worst places hit by flooding today was Hebden Bridge – I pick out this town simply because I have been there recently when I was visiting Gibson Mill and Hardcastle Crags with work colleagues. We all stayed in the town and it was a lovely place. Hebden Bridge was seriously flooded in 2012 and now it has happened again. These Twitter posts from Boxing Day set the context.

Boxing Day floods 5
I can’t imagine this – I’ve never suffered from flooding

Boxing Day floods 6
I’ve walking down these streets and been in these shops

These floods in Yorkshire, Lancashire and North Wales (and previously in Cumbria) are the result of unprecedented weather events but is there more we could be doing as a society other than calling for better flood defences and increased budgets?

I have written and tweeted extensively recently about land use and flooding (see here and here for example for issues around maize and here about Storm Desmond and land use). The issue of adjacent land use is also not unfamiliar to the residents of Hebden Bridge. Close by is an intensively managed grouse moor which some residents blame for their 2012 flooding problems – see here, here and here. They claim that the burning of the heather moorland makes the land less able to absorb heavy rain which then runs down the hillsides into the River Calder and then into Hebden Bridge.

The importance of the way that upstream catchments are managed is becoming more understood even in political circles.

Screen Shot 2015-12-27 at 00.10.57
This is potentially very good news unless the policy morphs in  a new plan to cover the Uplands with conifers  or ends up destroying some of our finest historical landscapes with trees – see the end of this blog for example.

Aside for the actualities and the politics, what about the science? Why is this weather happening? Two phenomena seem to be at play – climate change and the El Nino in the Pacific.

The modelling around climate change suggest winters here will be milder, wetter and we will see more extreme weather.

The Met Office in October said this about the El Nino effect and the UK – see here

”Most of the global drivers discussed above tend to increase the chances of westerly weather patterns during our November to January outlook period. Our numerical prediction model, being sensitive to these drivers, also predicts a higher-than-normal chance of westerly conditions. This results in an outlook for an increased chance of milder- and wetter-than-usual conditions, and a decreased chance of colder and drier conditions, for the UK. Our outlook also indicates an increase in the risk of windy or even stormy weather.”

Interesting the Met Office predictions goes on to say “Finally, there are hints that the outlook might be rather different in the late winter (Jan-Mar), with an increased risk of cold weather developing.” The last El Nino event in Britain coincided with the huge post Christmas snow falls in 2010. If I was a betting man which I am not ……

 

 

 

 

 

 

 

El Nino

http://blog.metoffice.gov.uk/tag/el-nino-southern-oscillation/

Milder, Wetter stormier and then much colder and drier in 2016

Storm Desmond, climate change, land use and what it means for Dartmoor

My heart goes out to all the people and communities in Cumbria and southern Scotland who have been impacted by the dreadful flooding. In addition the National Trust is a major landowner in the Lake District (we own around 25%) and I personally know many of the Rangers and General Managers up there. I can only imagine how stressful everything must be and how disheartening it will be to see so many places and projects damaged by the flood waters.

Fell Foot floods
The National Trust’s property at Fell Foot on Lake Windermere on Sunday – I was at a meeting in this building 18 months ago (photo from https://www.facebook.com/ntfellfoot/) You can find out more here about the flooding in the Lake District and how it has affected the NT.

Much has been written already about Storm Desmond and no doubt a great deal more is to come. I have heard a number of politicians understandably saying that we must never let this happen again. I fear though that those words will be easier to say than achieve.

The questions therefore that need to be answered are why did it happen and what can we do to better protect places and people?

Why did it happen?
I guess the obvious needs to be stated – there was a huge storm….. in my blog yesterday I set out a few of the startling statistics provided by the Met Office and the Environmental Agency. I also quoted the following ‘279mm rain fell in 1 day in 1955. This UK record lasted for 54 years, till 316mm fell in 2009. Just 6 years later 341mm falls.’ Cumbria has now been hit by huge storms in 2006, 2009 and now in 2015. Each of those events was a 1 in a 100 + flood event – three such events in less than a decade. That is just Cumbria – we mustn’t forget the Somerset Levels events in 2013-14 and those all along the Thames in the same year, then there was Gloucester, York, Boscastle etc etc.

To quote the Guardian “The Met Office has warned that ‘all the evidence’ suggests climate change played a role in the floods which have devastated thousands of homes following Storm Desmond. The Met Office’s chief scientist Dame Julia Slingo said the extreme weather conditions were ‘extraordinary’. She told BBC Radio 4’s World At One: ‘Is it to do with climate change? There can’t yet be a definitive answer but we know that all the evidence from fundamental physics and what we understand about our weather patterns, that there is potentially a role.’”

Yesterday in Parliament Liz Truss, the Environmental Secretary said “The government’s view is that these floods are consistent with what we know about climate change. The government’s flood defence plans contain assumptions about climate change. But that modelling will have to be reviewed in the light of the current floods.”

Again according to the Guardian a spokesperson for the Environment Agency said “Climate change is happening now and we must build resilience and adapt to the changes that are unavoidable. Climate change adaptation has been going on for some time: sea level rise has been factored into coastal decisions since 1998.”

Lord Deben, (the former Conservation Environment Minister John Gummer), and now Chair of the Committee on Climate Change said on Twitter “People with real concern for flood victims should be constantly reminded of the consequences of climate change.”

This is what the Hadley Centre for Climate Prediction and Research (part of the Met Office) have modelled regarding climate change predictions in the UK.

Forecast changeable projections
Climate change’s fingerprints are all over this

Ironic perhaps that Storm Desmond should occur whilst the International Climate Change Talks in Paris (COP21) are in full flow. Let us hope that these negotiations are successful and effective. We already have seen 1 degree of global warming since the Industrial Revolution – all the science says that we must keep future changes below 2 degrees (i.e. 1 degree more than where we are now).

What can we do to better protect places and people?
The National Trust has set out its concerns along with some of the adaptation solutions we are trialling in its recent document Forecast ChangeableClick here to download the report. For example there is a short summary of the work that has been carried out on the Holnicote Estate on Exmoor – which aims to demonstrate that by working at a catchment scale and changing some land management practices, a natural approach to flood management can be achieved while producing wider environmental and community benefits. See here, here and here for more details of the project.

WWF has also produced a report Slowing the Flow – a natural solution to flooding problems – download it here.

Professor Steve Ormerod (who is also the Chair of RSPB) and Edward Maltby have produced a useful review of the benefits of rivers and wetlands in managing floods and other environmental services.  You can download the chapter here.

So here are two projects and a review document which advocate ‘upstream’ management to try and reduce / eliminate flooding. So why have I picked on these and not talked about building flood defences and dredging rivers?

There is increasing concern in many circles now that the way that we are managing our  catchments and rivers is actually aggravating the problems we face from rain storms. Many catchments are now either denuded of vegetation (are in arable cultivation), have compacted soils (either from stock or agricultural machinery) or have short uniform vegetation (e.g. the uplands of much of Britain). These three factors ensure that any storm water which hits them travels very quickly into water courses (i.e. they are not absorbed by the soil) and then into rivers which have been straightened, deepened and dredged. The water then travels very quickly until it reaches an obstacle such as a bridge or a town – at which point the banks breach and flood defences are overtopped and flooding such as we have just witnessed in Carlisle occurs.

Projects such as the Holnicote one on Exmoor aims to achieve the opposite – that is hold and absorb as much of the storm water in the upper catchment for as long as possible.

Interestingly some habitat types are much better at absorbing water than others, for example maize fields (especially those on slopes) are terrible at it and I have written about this before – see here and here. Other habitats such as woodland on the other hand are very good at it. Research carried out in Upland Wales found that areas planted up with trees were 67 times better at absorbing water when compared to grazed pastures. See here and here.

Encouraging therefore to report that during the Commons debate on the flooding in northern Britain and Southern Scotland earlier in the week that Anne-Marie Trevelyan, a Conservative MP who represents Berwick-upon-Tweed, said that more trees should be planted on high land to reduce the risk of flooding. Liz Truss, the Environment Secretary responded by saying that this is an issue the government is considering.

Depressing though to hear Meurig Raymond, President of the National Farmers Union on the Today programme yesterday, basically saying that more money was need to dredge rivers, channel maintenance and hard infrastructure. He was completely unprepared to countenance upstream management solutions. He seemed to be arguing that farming was so important in the uplands that it couldn’t be altered to incorporate upstream management considerations. You can listen to the interview with Meurig Raymond and George Monbiot here and then draw your own conclusions.

This debate has got a long way to run and to be honest it is not an ‘either / or’ situation, however the status quo simply won’t work and has been shown not to work.

What does all this mean for Dartmoor?
This debate isn’t of course just about the Lake District – this could have happened on Dartmoor. At some point in the future a big storm will hit the West Country – maybe the rainfall won’t be as huge as in the Lake District because the Tors aren’t as high as the mountains but who knows.

Land use on Dartmoor is pretty much identical to the Lakes and the woodlands that exist are largely restricted to the river valleys. The debate therefore about upstream solutions to flooding are very relevant to Dartmoor too. My blog about changes in the vegetation in the Upper Plym (see here) shows that in recent years the moor has become less resilient and less able to hold back the water. There has been some upstream work on Dartmoor via the Dartmoor Mires Project but at this stage it has only covered a tiny area of the moor, was considered controversial by some, is currently being reviewed and any future works are not funded. This type of project needs to shoot up the agenda on Dartmoor.

We shouldn’t also forget that there are a lot of towns around the fringes of the moor located next to moorland rivers. As in the Lake District the first major pinch point for rivers in spate are these towns.

Lots to thinks about and lots to do.