Woodland management: Analysing the pros of continuous-cover forestry

TWENTY or even 10 years ago, you could not pick up a forestry magazine without an article on continuous-cover forestry (CCF) staring you in the face and pushing the perceived advantages, because to suggest otherwise was tantamount to heresy. The most vocal proponents of CCF were fearsome individuals and not to be messed with. Yet CCF subsequently disappeared from the radar as its highest-profile supporters, relatively few in number but compensating in verbosity and volume, retired or sadly passed away.

Having heard relatively little about CCF for a good while, I was pleasantly surprised to see a report by FJ’s deputy editor Jack Haugh on a conference where CCF was clearly centre-stage. ‘Better Forestry – Is Scotland Ready?’ took place on 13 March in Perthshire, hosted by the Forest Policy Group (FPG), an independent forum which advocates for better forestry in Scotland. What particularly caught my eye was the article’s opening line: “Is this what better forestry looks like?”

CCF has clearly lost none of its persuasive push because ‘better forestry’, by implication, suggests woodland creation and sustainability by regeneration is superior to traditional clear-felling and re-planting. CCF has its positives, but there are negatives too, so perhaps it is time to square things up to get a clearer picture and a more balanced view.

So what is continuous-cover forestry? You could fill a book, let alone a couple of pages in Forestry Journal, trying to define CCF. Go to the hordes of academic websites and you will get multiple variations on the theme, presented boffin style, interest numbing before you even start. The following is a short summary made up of ‘the best of the rest’.

• READ MORE: Better forestry for a better future? The FPG Conference in full

CCF requires the selective thinning of trees to create a diverse forest – in age, structure and species – in order to harvest timber while keeping canopy cover. The over-arching aim is to work in harmony with the site, respecting ongoing and evolving ecological processes, rather than attempting to impose artificial uniformity. In practice, this leads to a presumption towards the use of natural regeneration of trees and the development of mixed-species and mixed-age stands. Positive presumptions around CCF are stability and sustainability, variation in tree age and diversity in species, nature-friendliness and benefits for biodiversity.

Proponents of CCF would have you believe the system is a bed of roses or, perhaps more appropriately, a bed of leaf litter, but the claimed benefits can be questioned on many counts.

RIGHT TREE IN THE RIGHT PLACE

Clear-felling mature trees and cutting of coppice stools has things in common. Tree growth is cut completely, right down to ground level when it reaches the desired stage of maturity, allowing spring-flowering plants to flourish. (Image: FJ)

Relying on regeneration means the offspring will take over as old trees pass their sell-by date and fade away. As such, the newcomers should be well suited to the site based on an assumption of natural succession and possessing the characteristics of parent trees which thrived there before, but this does not always follow.

Go to sandy, heathland-like sites in southern England supporting a mixture of broadleaves (e.g. oak) and conifers (Scots pine) and you will invariably see scruffy oak, having struggled for 100 years or more, alongside tall, majestic pines, having had the time of their lives. Operate a hands-off policy and leave the sites to regenerate and you will end up with virtually pure stands of pine. You won’t even get the next generation of scruffy oak, because English oak does not regenerate under its own shade. But the pure pine scenario goes right against well-established thinking within the Forestry Commission, which dictates only native broadleaves for these sites and no more conifers whether through planting or regeneration – even though Scots pine is native to the British Isles.

And what about relatively recent invaders of semi-natural broadleaf woodland in large areas of England, including mountain ash and bird cherry? Both species are native to northernmost England and Scotland, but regarded by purists (many of whom are adherents to CCF) as alien to the rest of England. Should they be allowed to regenerate?

There are trees that you will want to sustain on your CCF site and trees you really don’t want. I recall an ancient woodland, one of relatively few (or perhaps the only one in existence) with hazel continuously coppiced since the 16th century and shaded by oak and ash standards. As the much shorter-lived ash expired, the owners tried to supplement the remaining standards by planting up with oak, but there was still sufficient shade remaining to prevent the establishment of these new oak trees. The owners were forced to consider classic shade-tolerant trees like hornbeam. It follows that oak regeneration will be nigh-on impossible within well-managed CCF sites with forest canopy in place. If you want to sustain oak as the most important native high-forest tree, then you will be forced to plant the tree and reduce the canopy cover and accompanying shade thus cast. 

Then there’s sweet chestnut, for which production of viable seed in Britain is patchy to say the least. Successful sweet chestnut regeneration depends on viable seed and a relatively high light regime. Viable seed will regenerate where light is sufficient, at the woodland edge and within sizeable gaps inside the woodland.

This woodland has all the makings of a CCF site. (Image: FJ)

Research in Italy showed the presence of standards did not benefit the regeneration of sweet chestnut seedlings. Rather, it displayed a marked depressive effect on the growth rate of young sweet chestnut seedlings and, under significant shading conditions, enhanced the establishment of shade-tolerant trees at the expense of sweet chestnut. CCF enthusiasts, by their own doing, are between a rock and a hard place. CCF has done the job in maintaining shade and sustaining wildlife, but has left insufficient light for the fewer sweet chestnut seeds escaping predation.

Back to those alien invaders – mountain ash, bird cherry and even ornamental (pink-flowered) cherry plum, which I have seen growing in an ancient woodland in South Essex. They are joined by an unlikely suspect in the form of Norway maple, said to be comparatively shade tolerant at the seedling stage, but requiring high light for good growth thereafter. Most Norway maples in Britain are single amenity trees planted growing in high-light conditions. Despite volumes of seed, I have never found Norway maple regenerating in its high-light amenity environment like its fellow ‘maple’, sycamore. However, I did find woodland where Norway maple had been part of the planting mixture and 40 years later, during which time no thinning had taken place, there were masses of Norway maple seedlings across the heavily-shaded woodland floor.

So what to do about all these ‘alien invaders’? Do you leave them in situ to push species diversity to its limit or take them out because, according to the lore, they should not be there? At face value, their removal would appear to go against the CCF grain, which is essentially a hands-off, minimal-disturbance regime with maximum diversity of tree species.

By its very nature, CCF will favour the regeneration of shade-tolerant tree species like common beech and hornbeam at the expense of those like oak and sweet chestnut with relatively high light requirements, therefore not necessarily maximising species diversity to the degree anticipated by the woodland owner. The only way to ensure the right tree is in the right place is to plant it yourself.

A TOO STABLE ENVIRONMENT? 

Ecological and habitat stability is at the very heart of CCF, but can you have a situation where things are too stable with regard to wildlife? Many species of butterflies are known to respond positively to controlled disturbance, with coppice rotation the prime example. Coppicing and clear felling have a lot in common, with both systems of silviculture requiring the cutting and clearing of the site when growth reaches the desired level of maturity. The essential difference is that you don’t have to replant a coppice woodland site, although supplementary planting can be carried out by layering selected rods on the stool at the time of cutting. 

No more re-planting of clear-felled woodland? (Image: FJ)

Richness in wildlife is one of the positive collaterals of sequential, rotational coppicing, but it is only maintained if compartments (coupes) are completely cut (coppiced) down to stool level after the period of growth proscribed for a particular species, which is eight years, in the case of hazel (Coryllus avellana). What happens in the following several seasons is truly amazing and all down to the light allowed into the stand. Spring and early summer flowering plants appear from literally nowhere, like English bluebells and wood anemones which persisted, respectively, as bulbs and rhizomes (underground stems); and foxgloves, which will regenerate from dormant seed.

HOW ABOUT THE HARVEST?

Let the light in and foxgloves will follow from erstwhile dormant seed in the soil. (Image: FJ)

CCF cheerleaders say timber harvest is part and parcel of the system, but to ‘take out the right tree at the right time and in the right way’ makes ‘planting the right tree in the right place’ look easy. Imagine the effort required and difficulty involved in getting heavy machinery into a very mixed-species and mixed-age stand. And then felling and extracting the trees you want to sell for timber. This must be achieved without causing huge amounts of collateral damage to the ecosystems and wildlife you have carefully encouraged by operating a system of CCF. 

My experience in mature stands of rubber trees in West Africa may be a million miles away from a CCF site in West Devon, but is pertinent nevertheless. Faced with rampant root disease, we had the choice of taking down a huge diseased rubber tree to prevent several nearby trees becoming infected. But this caused terminal damage to even more as the cut tree, about same height and size as a mature common ash, came crashing down through the canopy.

It is difficult to see how one set of kit can cover all felling and extraction requirements, meaning multiple excursions into the forest to take down specific species. If CCF achieves the stated aims of maximum species diversity and mixed age/size of trees, and in no discernible pattern of distribution, it is difficult to see anything other than multiple forays by men and women with chainsaws, followed by heavy horses. Idyllic and romantic, but does it pay the bills?

Talking about paying the bills, what about the cost-effectiveness of the whole operation, from felling, extraction, transport right through to marketing? Clear felling is custom-made for a smooth operation. Having a single species of uniform size, with trees equidistant, they can be easily cut and extracted using one set of mechanised kit. And likewise easy to load onto the forwarder, stack roadside and convey by lorry to the mill, while getting the best price according to the volume and uniformity of sawlogs supplied. 

TREE DISEASES – THE NEW DANGER FOR CCF

Tree disease has always been with us and foresters have coped irrespective of the silviculture system in use. In the last 15 years we have been visited by two tree pathogens with significant implications for forestry in general but especially for CCF due to the epidemiology. 
The two diseases are Chalara ash dieback, with common ash as the primary host, and Phytophthora pluvialis, which counts western hemlock among its favoured host conifer trees. By the same token, common ash and western hemlock are just the job for CCF. Common ash is a prime pioneer and coloniser, exploiting gaps in woodland by forming so-called ash cores.

With its high capacity for regen, western hemlock was custom-made for conifer-based continuous-cover forestry but it is also a prime target for Phytophthora pluvialis. (Image: FJ)

Western hemlock is an amazing tree. Go in a clear-felled 50 year old stand of western hemlock, and across the otherwise bare ground is western hemlock seedling regen up to 2 metres high. 
Chalara ascospores released from apothecia on the previous year’s leaf stalks (rachises) littered across the forest floor rise only a relatively short distance, but high enough to infect and kill the seedling ash trees. Likewise Forest Research and the Forestry Commission have released pictures showing swathes of western hemlock regen killed by Phytophthora pluvialis.

At the beginning of the Chalara pandemic, around 2013/14, I recall giving some woodland owners advice on what I thought was the best way to preserve their mature ash trees. That was to pull up and destroy all ash seedlings and saplings in the woodland and its vicinity and remove all the lower branches on mature ash trees up to a height of 10 metres. Did I hear someone say ‘sycamore for CCF’? Good luck with that.

SPARE A THOUGHT FOR THE FOREST NURSERIES

So what is the UK’s forest nursery sector supposed to do if we suddenly stop clear-felling and re-planting and go over to strict CCF?  No-one has said whether the creation of new woodland will be achieved by planting, or simply allowing a designated grassland site to tumble down into woodland over the centuries. Has anyone calculated the almost certain loss of jobs in forestry by adopting CCF over clear-felling and re-planting? I doubt whether the manufacturers of tree shelters and other tree-planting accessories will be too pleased either.