RESTOCKING woodlands whether ancient or otherwise involves planting young trees brought in from outside or taken from another part of the woodland. Dr Terry Mabbett takes a look at hazel coppice – an exception because it can be replenished by intervention, entirely from within and without the use of a tree-planting spade.

Hazel coppice can be regenerated by ‘simple’ or ‘ground’ layering, whereby stems are placed in contact with the ground to stimulate the growth of adventitious roots, new upright shoots and eventually new independent plants (‘adventitious’ describes roots which arise from buds at the internodes on stems rather than ‘classical or true’ roots which arise from dedicated root tissue).
Hazel is layered when stools are coppiced (cut and harvested) at the end of the normal 7–8-year rotation cycle. Several of the longest rods are cut three-quarters through to leave a ‘tongue’ of wood covered with living bark. These partly cut rods are lowered until they are lying on the ground. Failure of these rods to make close contact with the ground, the tendency for them to spring back, or risk of dislodgment by human or animal activity is prevented by pinning them down with stout wooden pegs cut from hazel. Rods are ideally pegged at points where you want the new plants to establish. 
Layering hazel is a well-established art, but with little in the way of supportive, definitive research. Hazel coppice is relatively rare in my neck of the woods (south Hertfordshire), where hornbeam coppice under oak standards was the traditional woodland setup. However, as luck would have it, Hertsmere Borough Council has been rehabilitating hazel coppice at Furzefield Wood in Potters Bar (Hertfordshire) which allowed me to carry out some basic research into hazel coppice re-growth (‘Hazel coppice and a trip back in time’, Forestry Journal, January 2019). The craftsman who carries out the work uses layering, thus providing opportunities for even more research, this time to surmise the science behind layering as a natural vegetative propagation procedure for hazel coppice.

Laying down the ground rules 
Hazel coppice manuals recommend selection of the longest rods on the stool, cut three-quarters through and as close to ground level as possible. The closer the cut to soil level then the shorter the distance before the laid rod comes into contact with the ground.
Cutting and laying the longest rods clearly offers most versatility in exactly where you want the rod to root and establish a new plant for future coppicing. Longer rods should, in theory, be sturdier, with bigger ‘butt-end’ circumferences. Larger-girth rods clearly provide more substantial tongues of wood and therefore more effective conduits for water, nutrients and soluble food from stools into laid rods when the sap rises in spring. 
However, in this instance sap does not ‘rise’ in the strict sense of the word because the partly cut rods are horizontal across the ground. This is where positioning of the hazel pegs is crucially important, because the rod may not be in close contact with the ground along its entire length. Pegs should be placed and secured firmly over the rod at a point(s) where adventitious roots and a new plant are required.  
Layering is undertaken to fulfil one of two requirements. The first is replacement of dead or dying stools. Hazel coppice stools cut on a regular, recommended rotation can live for several hundred years, although when neglected they may become unstable and only last for 70 to 100 years. Layering enables the forester/landowner to site new stools in gaps created by the deterioration of old stools, without the expense of planting new material with all its associated costs. Extra labour costs incurred in layering are minimal since the procedure is carried out when the coupe is cut at the end of the rotation cycle.  
The second reason could be to increase stool density in response to permanent changes in light intensity caused by a loss of standard, shade-tree cover. Hazel coppice in England was traditionally cultivated under standard tree cover, usually provided by English oak and English ash. Older texts say standard tree cover and the shade thus provided are essential for hazel coppice but others now argue that growing hazel coppice under oak and ash standards was socio-economic rather than agronomic in nature, and peculiarly English. It was also due to pressure on land use and the needs of landowners and tenants for construction timber as well as hazel rods, the latter for making hurdles and thatching spars and faggots (bundles of brushwood) for burning. They point to France, which did not have the same pressures on land use and where hazel coppice was cultivated without standard tree cover. 
Given the ability of well-managed hazel coppice to survive and yield for several centuries at least, it is entirely possible for stools to outlive standard trees, especially woodland ash trees with a longevity of 200 years at best. With less shade from fewer standard trees, a compartment should be able to support a higher density of hazel coppice stools without any negative impact on yield per stool.

Furzefield Wood
Initial observations were made in late April 2018 inside a compartment coppiced in autumn/winter 2017/2018, with selected rods layered at the same time. The ‘Beast from the East’ had departed but had left an exceptionally late spring in its wake, with hazel buds only just breaking. However, wildlife was already responding to increased light levels post coppicing with comma butterflies drinking nectar from resurgent English bluebells, and a speckled wood butterfly warming itself on the sunlit leaf litter. 
There were signs of shoot growth along the entire length of layered rods but differential growth rates would clearly unfold at various points as the growing season proceeded. I returned in early autumn, when growth had ceased, to take measurements of shoot growth on the layered rods after one season of growth. I selected 10 layered rods, taking shoot measurements for each and comparing those from the three longest and three shortest rods. The range of measurements taken were:
Length (cm) of layered rod
Butt-end girth (circumference) of the rod just above the cut (cm)
Number of shoots at least 100 cm in height
Cumulative height of these shoots (cm)
Cumulative butt-end girth (cm) of these shoots, taken as a measure of their sturdiness
The three longest layered rods were 590, 550 and 500 cm long, had butt-end girths of 23, 24 and 25 cm and carried six, four and five shoots with a minimum height of 100 cm. Respective cumulative shoot length was 700, 580 and 765 cm with a cumulative butt-end girth of 19.5, 14.0 and 19.5 cm.
The three shortest layered rods were 270, 260 and 240 cm long, had butt-end girths of 12, 12 and 11 cm and carried two, two and one shoots with a minimum height of 100 cm. Respective cumulative shoot length was 270, 260 and 240 cm with a cumulative butt-end girth of 5.0, 5.5 and 2.5 cm. 
Even without conducting statistical analysis on the whole 10-rod sample, the following facts were abundantly clear: 
Longer rods have correspondingly bigger butt-end girths
Longer layered rods produce correspondingly more tall (100 cm+) shoots  
Longer layered rods generate correspondingly taller and sturdier extension growth

Practical tips for hazel layering
The longest and fattest layered rods produce a greater amount of sturdier re-growth while increasing the options on where to strike a new stool. However, the practicalities are not always as straightforward as laying a partly cut rod on the leaf litter to form adventitious roots exactly where required. 
A significant problem arises with old coppice stools which are high off the ground. This means the lowest point at which rods can be cut is one foot or more above soil level. The angle thus created when the cut rod is layered means the first point of contact with the ground is way out along the rod – at three metres or more – thus reducing the advantage of using the longest rods on the coppice stool. Rods generate shoots along their entire length but those at points along the first three metres or more cannot root and produce new plants because the rod is in not in contact with the soil.
This problem was evident in a compartment of old, high-off-the-ground stools coppiced five years earlier. The first point of contact with the ground was typically 3+ metres out from the stool. The strongest shoots had generated close to the stool but, with no option for rooting, had subsequently died. Much smaller shoots further out along the rod, clearly deprived of water and nutrients in the initial stages of growth, had failed to root causing the whole layering exercise to fail. 
Potential layering (rooting) points closest to the stool will clearly benefit first from initial flow of sap in spring. So it is entirely logical that the fastest-forming, quickest-growing growth, and in the end the tallest and sturdiest shoots, should occur at these points on the rods. 
However, they are of no practical benefit because these shoots have formed at points where the rod is not in contact with the ground and therefore cannot root; if they do root, new established plants will be too close to the ‘mother’ stool. Perhaps such redundant shoots should be removed during the first season of growth, thus allowing potential rooting/layering sites further out along the hazel rods to gain the full benefit of the sap, which starts to flow into the rods from the ‘mother’ stool in spring.

A good move for Furzefield 
Layering to replenish hazel coppice is a good move for Furzefield Wood. Many stools are very old, indeed past their ‘sell-by date’. Rehabilitation of all compartments is long overdue. Standard tree cover at the southern end of the wood, where these studies were conducted, is (or rather was) mostly common ash. Common ash as a standard woodland tree is not long-lived compared to English oak. Many ash trees have already failed leaving large areas of the woodland bereft of shade tree cover. 
The ongoing ban on the movement of Fraxinus (ash) material means ash tree planting material cannot be brought in from outside. There is seed-bearing female English ash in Furzefield Wood, and parts of the recently coppiced compartment where I carried out this work were covered with tiny ash seedlings. However, there was scant evidence to suggest that ash seedlings are making it much beyond this post-germination stage to successfully regenerate anywhere in the wood. This is almost certainly due to prompt invasion by bramble soon after a compartment had been coppiced.
Attempts to restock with English oak in other parts of Furzefield Wood have failed. This is mainly due to the uncontrolled growth of bramble in the years immediately following coppicing, to such an extent that the full benefits of varied ground-cover species and associated invertebrate animals, which are reasonably expected with appropriate shade levels cast by standard trees, are not being realised.
Layering for Furzefield Wood appears to be the logical way forward to increase stool density and thereby:
harness and exploit higher light levels due to loss of standard tree cover, thereby maximising coppice growth and yield
allow hazel to create its own mutual shade to increase its competitiveness with invasive bramble, thereby creating and maintaining improved conditions of space and light for more varied ground plant cover and associated wildlife.

How did it all begin?
On the way home I pondered how it all began – what gave our ancestors the idea and inspiration to layer hazel rods? The next day I happened to be speaking with Mick Dunn (MH Dunn Forestry Contractors of Witham in Essex), a forester who knows his way around hazel coppice and who I think has the answer. He said: “I have seen hazel coppice stools smashed to bits by failing standard trees, only for new plants to ‘spring up’ by natural layering where the rods were pinned to the ground by fallen oak and ash branches.”