Hazel dynamics

Note: For more information, explanation and illustrations, take a look at chapter 3    of the free Scottish Natural Heritage publication 'Atlantic Hazel'.

Old-growth Atlantic hazel, the 'core' stands

The 'special' crustose lichens of the Graphidion (the 'script' lichens), and the foliose (leafy-lobed) lichens of the Lobarion found on Atlantic hazel, are all considered species which require 'old-growth' woodland, or long periods of ecological continuity.

These lichens are not present in hazel that has been coppiced. Nor are they present in hazel stands that have recently expanded.

 

So, how does Atlantic hazel maintain 'old-growth' habitat?

A typical hazel stool has a cluster of thin, medium-sized and thick stems. The smooth-barked young stems are colonized by the crustose lichens of the Graphidion. As these stems become older and thicker the bark roughens and crustose lichens give way to bryophytes and foliose lichens of the Lobarion. The ageing stems tend generally to lean outwards, probably from the weight of the canopy they support. This creates a gap in the overall canopy, which enables new young stems to arise and fill the space. Winter storms will damage the canopy, breaking off twigs; abrasion from stems rubbing together in windy weather allows fungal pathogens (such as the Glue fungus Hymenochaete corrugata) to attack and gradually kill off individual stems. This all leads to a considerable turnover of stems within the stool.

New hazel stems (whips) appear to be produced from the base of the stool every year, and this is a critical feature. Initiation of new stem production may be triggered by short periods in the spring when light conditions are favourable. When existing stems are in full leaf, forming a closed canopy, new stems will abort by late summer owing to heavy shade. If a gap persists, however, then new spring whips are able to grow rapidly and fill the space. The annual production of new stems appears to act as a 'fail-safe' strategy to ensure the continued viability of the stool.

In the right conditions therefore, each stool is a self-perpetuating ecological unit, always with some young, smooth-barked stems (supporting crustose lichens) and generally some older stems (supporting the foliose lichens). There is an unbroken cycle of replenishment, which is crucial for the ecological continuity required by the more specialized and niche-demanding 'old-woodland' lichens.

Hazel stools in different situations behave in slightly different ways

On deep soils in sheltered conditions, the stools tend to be large, well-grown and widely spaced with the canopies of adjacent stools often meeting. In these situations:

• canopy spread can reach 6.5m in diameter, and canopy height can be more than 6 m.
• stools tend to support mostly shade-tolerant 'old woodland' lichens of the Lobarion, with the more light-demanding species of the Graphidion only poorly represented.
• there are large numbers of stems per stool, ranging from thin and spindly, to very thick and woody, the latter becoming almost horizontal. The canopy twigs of these collapsing stems gradually adjust to vertical as the main stem assumes a horizontal position. Hence, when the old stems eventually collapse, the canopy twigs are often well above browsing height.
• In some cases, layering of collapsed stems takes place and new stools can be formed, but this is successful only if there is a 'space' as competition for light is a prime factor controlling successful hazel establishment.
• Within a closed canopy stand of hazel, the shade created by summer canopies in full leaf is extremely dark, which perhaps partly explains why tree seedlings are unable to establish and are rarely encountered.

On thin soils in exposed situations, stools tend to be closer together, with few thick woody stems.

In these situations:

• the Lobarion is often poorly represented, with the Graphidion dominant on the smooth-barked young stems.
• there is a rigidity within the stand, with a tight, interlaced network of small twigs, which means that internally, the stand is very sheltered, although the outer twigs are often wind-clipped and distorted.
• turnover of stems appears to be far more frequent, with the oldest stems often being no more than 12-15 years old (as a very rough rule of thumb - 1cm girth approximates to one year of growth).

But, despite appearances, stools composed of young stems can be self-perpetuating 'old woodland' stands that may have persisted for thousands of years.

Do individual hazel stools eventually die, or do they just get progressively bigger?

Hazel has two strategies for reproduction:

• sexually by wind-pollinated catkins; resulting in the production of seeds encased in hard, woody nuts. Nuts require a mammal or bird to distribute them; but they also float, so can be carried along waterways.
• by cloning, in which new shoots arise from the underground stool; the term 'self-coppicing' is sometimes used to describe this mechanism.

How can you age a hazel stool?

How long the stool will live (given ideal conditions), is not known.

In the absence of browsing, basal shoots will be regularly produced from the outer perimeter of the stool. Over time this will lead to an increase in circumference of the hazel stool until, over hundreds of years, it could be enormous. However, no hazel stools are ever encountered with gigantic basal circumference.

Based on casual observations from many Western Scottish hazelwoods over several decades, some observers began to see what they interpreted as 'hazel rings' - rather like the fairy rings formed by fungi in old meadows: there would be a series of discrete hazel stools ('satellite stools'), seemingly arranged around open space. Examples have been seen from Ballachuan and from Mull, where satellite stools were observed around an empty circular space measuring 1.1m to 2.3m in diameter.

The ages of these rings have been estimated at over 1000 years old! More research into this fascinating area is clearly still needed, but the work so far puts a whole new perspective on the way we should perhaps consider the ecological importance of Atlantic hazelwoods, as potentially one of the longest surviving, relict habitats in Scotland.