Information and Advisory Note Number 35 Back to menu

Heather layering and its management implications

1. Introduction

1.1 Heather moorland is a semi-natural vegetation type of considerable European conservation importance, supporting characteristic assemblages of plants and animals and representing a distinctive but disappearing cultural landscape, which is particularly well represented in the Scottish uplands. Heather or ling (Calluna vulgaris) is an important food for red grouse (and to a lesser extent for black grouse), and important winter forage for mountain hares, red deer and sheep. As nesting habitat and hunting ground heather moorland is important for a number of rare raptors such as merlin and hen harrier. It also supports a considerable diversity of invertebrates. Heather is one of the most important components of open moorland vegetation communities and habitats, being dominant over about 17% of Scotland (approximately 1.35 million ha) and present over about 30% of the land. Appropriate management of heather is of both conservation and economic significance.

1.2 Heather moorland has long been managed by burning. In managing for sheep and red grouse the traditional ideal has been to bum on a 10 to 15 year rotation, sheep management generally favouring shorter rotations. However, over large parts of the uplands rotations have been many times longer than this in the period since 1945. Burning on red deer range tends to be even more sporadic.

1.3 The purpose of burning is principally to restructure the vegetation so that feeding sites and nesting sites of appropriate height are provided for grouse or sheep. Red grouse need an intimate mixture of short heather (10 cm to 20 cm tall) for feeding and taller heather (20 cm to 30 cm) for nesting. Red grouse tend to avoid heather which is taller than 35 cm. For sheep, the principal management aim is to maintain most of the heather less than 20 cm tall with some limited taller areas to provide accessible food during snow. Structural variation in the vegetation over a range of patch sizes and frequencies of disturbance (including some undisturbed areas) is likely to favour the widest range of associated fauna and flora.

1.4 It is a common misconception that the main purpose of burning is to regenerate the heather. The implication is that the heather will disappear if it is not burnt but this only happens in old, even-aged heather stands which are heavily grazed.

1.5 As a heather plant grows larger and more woody its growth rate and form changes. The changes are described classically according to four developmental phases: pioneer, building, mature and degenerate. The vigour of heather plants is greatest during the first two phases while stand productivity is greatest when all the heather plants are in the building phase, when the heather canopy attains maximum coverage and density. In dense, even-aged stands each phase typically lasts from 5 to 10 years.

1.6 After burning (or cutting) heather regenerates by resprouting from the bases of the stems (if these survive and are not too old and woody), or by the exposure and germination of seeds which have lain dormant in the upper few centimetres of the soil. In both cases the new shoots produced grow more vigorously than on the bushes prior to burning /.e. the plants are rejuvenated. The effect is similar to that produced by coppicing of trees. The fertilising effect of ash may also contribute: concentrations of nitrogen and phosphorus in the shoots can double, though this effect disappears over three to five years.

1.7 However, in common with many other plants, heather can also regenerate by layering of stems. Burning, or other kinds of disturbance, are not required for this to occur.

2. What is layering ?

2.1 Layering is the production of adventitious roots from along the stem of a plant. "Adventitious" roots are roots other than those which develop from the primary root system of the seedling. The capacity of heather to layer has long been recognised and it is a common horticultural method for propagating heathers and other woody plants. Under appropriate conditions adventitious roots can be produced from almost any part of the stem from near the root collar to the most recent shoots. Rooting occurs more readily in younger plants and younger plant parts. Recently formed adventitious roots appear as white, unbranched hair-like structures, several millimetres to several centimetres long and less than half a millimetre in diameter.

Their density on stems is very variable but when conditions are suitable they can form a dense "fuzz" which almost obscures the stem. If conditions remain suitable these young, delicate adventitious roots may form more substantial, dark brown, branched structures with branches one or two millimetres in diameter, especially if they manage to penetrate the soil. Such well-developed adventitious roots may replace the functions of the primary root system. The latter may then die with little ill effect on the plant. Adventitious roots will develop most vigorously if they are close to a section of the shoot system growing in unshaded conditions, such as in a gap in the vegetation canopy.

2.2 Layering, like burning or cutting, rejuvenates heather plants. A heather stand in which vigorous layering is maintained will not become degenerate (although occasional stems may die) and high levels of productivity can be maintained for many decades.

In this type of stand over 90% of stems will show some evidence of adventitious root production and over 50% of the stems will have strongly developed adventitious roots which penetrate the soil from one or more points along the stem.

2.3 Heather is not the only moorland plant capable of layering. It can also occur in most other moorland dwarf-shrubs. Layering occurs in prostrate species, like bearberry {Arctostaphylos uva-ursi) and prostrate types of juniper; and in rhizomatous species, like blaeberry (Vaccinium myrtillus) and cowberry (Vaccinium vitis-idaea), the rhizomes are simply underground, root-producing stems and layering is effectively an obligate part of plant development.

3. Ecological conditions associated with layering

3.1 Layering of heather has been found in dry upland heaths, wet heaths and bogs, prostrate mats of heather in exposed or heavily grazed areas, acid dune heaths and in heather plants growing among tussocky grassland.

3.2 Layering is most likely to occur extensively when two main conditions prevail.

First, a substantial proportion of the live stems should be prostrate, at least along part of their length.
Second, the prostrate parts of the stems should experience darkness, continuous high humidity, and a lack of mechanical disturbance.

3.3 Stem prostrateness may be due to plant genotype. Heather shows considerable genetic variation in growth form. (Some other types of dwarf-shrub are obligately prostrate.) Prostrate genotypes are„commoner among plants from high altitudes and exposed locations. More upright genotypes, when young, can adopt a more prostrate habitat when subjected to impacts such as browsing, or wind-clipping, which kill or remove the taller and more apically dominant shoots.

3.4 The density of stems in a stand also influences whether a population of prostrate stems will be present Heather seedlings have both a vertically growing main shoot and horizontally growing lateral branches. In relatively open stands the lateral branches can continue to grow outwards, are very likely to layer, and may persist more or less indefinitely. It is possible to find layering heather stems which are over 2 m long which have presumably formed in this way. However, in very dense stands these lateral branches are likely to be suppressed by shading and may be completely eliminated. Such conditions are produced when post-fire regeneration has been very successful in stands of heather composed mainly of erect-growing genotypes. In fact, under these conditions there will be selection for erect forms and this will tend to reduce the future layering capacity of the stand.

3.5 Certain environmental conditions may also influence prostrateness. Wind-clipping is one which has already been mentioned. Another is the flattening of stems by snow accumulation. However, it is uncertain if this significantly encourages layering: snow-mould will also be encouraged, and may kill flattened stems, and old heather stems either may break or be too stiff to make contact with the ground.

3.6 Dark, undisturbed, humid conditions around portions of the stem can occur for a variety of reasons Deposition of soil materials as a result of erosion elsewhere is one possibility. This can occur beside streams, eroding footpaths, and in sand dune situations Suitable conditions may also occur where there is a high deposition rate of plant litter, either from heather itself or from associated species like wavy hair-grass (Deschampsia flexuosa), purple moor-grass (Molinia caerulea) and hare's-tail cottongrass (Eriophorum vaginatum). Envelopment of stems by cushions of Sphagnum bog moss or bulky mats of pleurocarpous mosses, like Hylocomium splendens and Pleurozium schreberi (but not Hypnum species), also will encourage layering. The presence of a humus-rich soil and a humid microclimate will enhance layering in ail these situations.

3.7 Even if these conditions apply, layering may not develop fully if stems are mechanically disturbed. Young adventitious roots are fragile and are liable to be damaged by stem movement. Disturbance can be caused by the wind or livestock traffic in stands of taller than about 20 cm. There is some positive correlation between the amount of layering and the degree to which heather stems are growing in sheltered conditions.

4. Geographical distribution

4.1 Layering heather has been found throughout Scotland where the ecological conditions are conducive to its development. Such conditions are likely to occur most frequently in the west and north, but vigorously layering heather stands can be found even in drier eastern areas.

5. Implications for heather management

5.1 Heather cover can be maintained by layering for many decades in the uplands without burning, even in drier eastern areas.

5.2 Deep, dense mats of heather mixed with bulky mosses and/or grasses can sometimes form in vigorously layering stands. These often support relatively few plant species and are likely to provide habitat for a narrow range of animals since structural variation is limited and the variety of plant food sources is small. Bryophytes diversity may be less affected and the diversity of decomposer organisms may be relatively high, though information on this is very limited. Therefore there may still be reason to consider managing some stands by burning even when layering is occurring.

5..2 However, regular burning (or cutting) is likely to reduce the capacity of stands to maintain themselves by layering should burning (or cutting) cease This may lock management to regular burning cycles which may be difficult to maintain because of the declining availability of labour for proper control of fires.

5.3 When burning ceases, after a period when a stand has been regularly burnt and has regenerated densely, the amount of heather present may undergo pronounced cycles due to the stand passing through successive degenerate phases while conditions conducive to layering slowly become established. If this occurs each cycle is likely to last 25 to 40 years. In the absence of succession to woodland, heather cover is likely to stabilise eventually with the formation of an uneven-aged, or more precisely an unevenly structured, stand of heather plants. This may occur more quickly, and without cycling, if the heather stand is initially fairly open rather than dense and uniform.

5.4 Some stable, layering stands have experienced prolonged periods without any form of major disturbance. These stands provide a habitat favourable to certain liverworts and invertebrates which need high humidity and shelter. Many of these species are uncommon and/or have poor powers of dispersal and recolonisation The presence of juniper or mixed cushions of liverworts such as Anastrepta orcadensis, Bazzania tricrenata, Mylia taylorii and Scapania gracilis could be used as possible field indicators for such undisturbed sites In areas with frequent aerial photograph coverage it may be possible to directly identify long unburnt areas. Such stands should not be burnt or cut.

5.5 Where resources for controlled muirburn (or heather cutting) are limited - a common situation - then managers should

not bum any stands which occur in conditions which are likely to be conducive to well developed heather layering
only bum stands of dense, tall heather on dry substrates

Where resources are not limited managers should

exclude from active management any stands which have not been burnt for long periods (more than 40 years) and which have well developed heather layering
avoid burning heather in wet, shaded or humid situations (e g steep northerly slopes, bogs) where layering is likely and where species sensitive to burning are likely to be found
concentrate muirburn activity where heather forms dense, continuous, evenly-structured stands

Author

Angus MacDonald
Uplands and Peatlands Branch,
Research and Advisory Services Directorate
Scottish Natural Heritage
2 Anderson Place,
Edinburgh EH6 5NP
Tel 0131-447 4784

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