Controlling bracken on Bryn Arw
Where do we plant a million trees?
We all know why we need to plant more trees, but where do we actually plant a million trees in and around the Brecon Beacons National Park?
At Stump Up For Trees, our goal is to plant trees in places where they will grow well, sequester carbon, reduce flooding, increase bio-diversity and improve the landscape, without detracting from existing biodiversity in any way. We also believe it would be madness to plant on prime agricultural land, as this would undermine the raison d'être of our farming community. Where to plant, then?
Our tree plant on Bryn Arw Common (in late winter 2021) is targeting steep bracken banks, which we think offer a potential solution to this question: where to safely and successfully plant more trees?
Soil
Both bracken and trees need good soil. Thus, if bracken is growing in a particular area, it is generally a good indication that trees will do well there too. I planted new trees at the top of our drive in the Black Mountains four years ago, on an area which crosses the natural bracken line. While the trees planted in the bracken are now 2-3m high, the trees planted just 5m away where the soil is thinner and the bracken could not get established, and are only 1m high.
Biodiversity
Bracken stands are often ecologically poor, generally because the litter is so thick. You do find certain mosses and some important species of Fritillary butterflies in bracken. Also, I have noticed that bluebells and other wild flowers can survive on bracken-infested sites that were once ancient woodland, and if the bracken is replaced by trees, these wild flowers can again thrive. However, dense stands of bracken offer few opportunities for other plants. Bracken is also home to the sheep tick (Ixodes Ricinus, the vector for lyme disease).
Land Use
Bracken infested land is agriculturally unproductive and of little economic value; steep bracken-infested banks even more so, as the gradient precludes bracken control with tractor-mounted flails. Many hill farms have large of bracken. In many instances, we believe trees offer landowners a better economic return on these bracken banks. In addition, trees also deliver ‘public goods’ in the form of carbon capture, biodiversity creation, flood prevention and landscape enhancement.
Tree planting potential on Bracken banks
Conservative estimates put the area of bracken-covered land in Wales at around 100,000 hectares. That’s enough to plant a quarter of a billion trees. Obviously, not all of it will be suitable for tree planting, but the point is bracken banks are plentiful; they grow good trees, are frequently deserts for biodiversity, while they are of low economic and food production value. The significant problem is—bracken banks are covered in bracken.
The Perfect Weed
Bracken is a formidable plant. Whatever you do to control it, it grows back. In evolutionarily terms, bracken may be considered one of the most successful ferns. It is also one of the oldest, with fossil records going back over the last 55 million years. It is a highly-adaptable pioneer plant. Left to its own devices, bracken spreads by about 3% by area per year across our landscape, representing a real threat to biodiversity. It shades out other plants producing a deep litter of its own dead, rusty brown leaves, which further prevent other plants establishing themselves. And, to make sure that it triumphs, bracken produces allelopathic chemicals, which prevent other seeds of other species from germinating. If the young leaves are nibbled by insects, bracken releases hydrogen cyanide which causes uncontrollable, repeated moulting and kills them. Over the last 2,000 years, peat core samples show that bracken spore peaks coincide with the loss of woodland. When trees fail, bracken gains, and vice versa.
The Sheep Exclusion Experiment
It has been suggested that excluding sheep from steep bracken banks might allow natural regeneration of scrub and woodland, a form of ‘rewilding’, if you like. Of course, a management approach such as this is inexpensive and it guarantees local provenance of trees—two desirable things. To test this theory 5 years ago, we excluded all stock from an acre of bracken along with 20 acres of woodland. It was a sheep or bracken test. The results in the woodland are amazing, but in the bracken not a single sapling has emerged (photo 3). It seems the bracken, which can grow to 2.5m or more, physically smothers and flattens small saplings, by falling on them in autumn. Interestingly, the only trees to emerge from the area of bracken grew up through the broom plants (photos 1 & 2). The broom holds the falling bracken off the saplings long enough for them to become established. My best guess is, if we excluded stock from the bracken banks for a long time, perhaps century or two, the land would eventually revert to woodland, but we are in a climate and biodiversity emergency. We do not have the luxury of time.
We further tested this theory by planting 400 sweet chestnuts, in another acre of stock excluded bracken, and then left them alone for 4 years, with no effort to manage the bracken. Around 20 of the 400 trees emerged: the rest died.
Our conclusion is that the only way to establish tree cover on these bracken banks is to plant, and then vigorously control the bracken.
Bracken Control
There are two ways to approach bracken control: spraying herbicide; and mechanical cutting or bruising. Asulox, the herbicide of choice for bracken, is extremely toxic to aquatic life. It is also, at best, detrimental to many tree species including beech, when the trees are young; it kills willows of all ages. As there a large number of water courses on the Bryn Arw, spraying Asulox is out of the question.
We are then left with mechanical cutting. A hectare is 100m x 100m. If we plant our trees, as we propose to do, at 2m spacings, we have 50 x 100m or 5km of tree lines per hectare; 70 hectares, therefore, is a line of trees 350km long—a lot of cutting. And it needs to be done twice a year.
The Plan for Bryn Arw
After a huge amount of research into machines, we have purchased a Brielmaier Slope Mower (picture 4). I plan to cut the bracken twice this summer before planting, then twice a year for the next 4 years, between the lines of trees, until they become established. Yes, this is a vast amount of work, but given the 100,000 Ha of bracken banks in Wales, and the tree planting potential of these areas, it has to be a success.
Thanks for reading.
Keith Powell