Plant of the Week – 6th September 2021 – Heather (Calluna vulgaris (L.) Hull )

This plant clothes much of Scotland’s high ground. It turns the landscape purple in late summer and brown in winter. It has been important in the Scottish economy, beloved of visitors (being one of the iconic sites that gives Scotland that ‘altogether different’ reputation), and cherished by the big estates who derive income from grouse and deer hunting on heather moors.

View of the Pentland Hills from the East, late August with heather still in bloom, showing heather-clad hills (left, Scald Law; centre, Black Hill; right, Carnethy). Foreground: Gorse and Deschampsia cespitosa Photo: John Grace.

The species belongs to the Ericaceae, and it is quite similar to the various species of Erica. It often grows with Erica cinerea or Erica tetralix but those species never become dominant in the vegetation like Calluna can be. In botanical keys, it separates from Erica by having leaves that are opposite rather than in whorls; the flower structure is different too (Beijerink 1940). Reflecting its iconic status and its economic and conservation importance, it has long been one of the most-studied species in the entire British flora (see Gimingham 1960). These days, it has become centre-stage in discussions about how to manage Scotland’s landscape as a store and sink of CO2 and a reservoir of biodiversity (Thompson and MacDonald 1995, Garnet et al. 2000, Glaves et al. 2013).

Close-up of Calluna flowers and also see the groove on the underside of the leaf where the stomata are situated. Photo: Chris Jeffree.

It is a woody evergreen shrub, a native, growing (usually) no taller than 60 cm and rather shallow rooted. Its leaves are among the smallest of any native shrub (3 mm long) and curiously bent back at the margins (‘revolute’). It grows on nutrient-poor acid soils (pH 4-6). In drier parts of the British Isles Calluna-dominated vegetation is termed ‘heathland’ – here the soil is rather shallow and often strongly podsolized. In wetter regions (much of Scotland, especially the west, and in the Pennines of England) it grows on deep peat where its young shoots are in competition with bryophytes (Sphagnum especially). There, when its stems are surrounded by damp moss, it sprouts roots. One plant can therefore spread itself out over several square metres by a process termed ‘layering’.

The peculiar leaf structure looks like a xeromorphic adaptation. The stomata are in a deep groove that contains a forest of hairs (see, image above, the white line on the underside of the leaf – that’s the groove). But why should these plants of wet places need to restrict water loss as if they were a desert species? During frost the surface layers of soil become frozen and so water is unavailable. In certain winter and spring conditions (wind from the north, low humidity and bright sunshine) the plant is therefore exposed to ‘physiological drought’. It needs to restrict its water loss. Even so, sometimes desiccation and ‘winter browning’ occur, with photo-oxidation of chlorophyll or death of the terminal shoots. The same phenomenon occurs in some conifers, Norway Spruce is well-known for it.

Calluna is common, and is found in many other types of vegetation and habitat but there it is not usually dominant. It is part of the ground flora of native pinewood and birchwood, where it grows rather tall and slender (‘leggy’ ) and produces fewer flowers. It is found in many kinds of open woodland and it persists even in dense plantation forests, along the forest tracks. It crops up even in towns and cities occasionally, wherever an acidic substrate is available.

In Scotland it is the food of grouse and sheep as well as Red Deer (although the latter prefer less fibrous material). To maintain a crop of tender young shoots for these grazing animals, the vegetation is periodically burned (Elliot 1953, Gimingham 1973, Welch 2016). If the plants are not too old when this occurs, they sprout new shoots from the base of the stem. For older plants, regeneration is from seed. The soil contains a large reservoir of seeds (up to one million seeds per square metre), and seeds remain viable for about five years unless they are killed by very hot fires.

The practice of controlled burning of small patches (‘muirburn’) has been going on for 150-200 years (according to Dodgshon and Olsson 2006, burning started much longer ago). Burning destroys tree seedlings and creates a mosaic of plant height, providing shelter and nesting ground for birds and fresh growth for grazing animals. Also tall heather provides cover for the shooters.

Patchwork formed as a result of controlled burning, Pentland Hills. Photo: John Grace

Burning is controlled by law. Burns are allowed between October 1st to 15th April. Burning strips should be about 2 hectares and re-burning should be every 10–25 years. Often, April is a dry month and plumes of smoke may be seen on fine days in the Spring. They carry away significant quantities of nutrients and especially nitrogen (as NOx). Also CO2 and CO, in the same way as burning of the Amazon forest does. If the fire is very hot, the organic layers of the soil are charred and when the rains come the rivers run brown with organic matter. More carbon and more nutrients are then lost. The management of heather was investigated rather thoroughly by Charles Gimingham and his students at the University of Aberdeen (Gimingham 1972) and based on this work guidelines on heather burning have been introduced. Burning has currently become controversial, as peatlands are these days considered as an important carbon stock. Douglas et al. (2015) have asserted that burning is increasing, often on conservation-sensitive sites, and too much carbon is being lost.

See Helen Armstrong’s brilliant clip about the Scottish uplands; she talks about muirburn about 18 minutes in: (8) The Scottish Uplands: how to revive a degraded landscape – Dr Helen Armstrong – YouTube .     

Should burning be stopped to allow more carbon to accumulate? It’s debatable (Glavces et al. 2012, Welch 2016). Rather like the Eucalyptus forests of Australia, there is a belief that too much build-up of carbon will lead to catastrophic wild fires from which the ecosystem may not properly recover. Heather might then revert to grassland. An example of the effect of severe fire is provided by the work of Colin Legg and colleagues. Following the Yorkshire fires in the very dry year of 1976 the seed bank was largely destroyed (Legg et al. 1992). Careful investigation of the patchwork of supposedly controlled burns in the Pentland Hills (now a Regional Park) show that heather has often been replaced by grassland, graphically illustrating this point (see the image above, taken a few days ago).

Calluna in full flower. Photo: John Grace

Calluna-dominated vegetation occupies by far the largest area of all semi-natural vegetation in the UK. Similar vegetation occurs in several parts of Europe, particularly Nordic countries including Denmark. It is found too in Spain, Portugal and Italy but Britain has by far the largest share of Calluna. We must turn to the pollen record and the history books to identify the reasons for this. In Britain, Calluna appears in the pollen record at the same time as a decline in woodland and the appearance of weed species such as Plantago lanceolata – about 4000 to 3000 BC. The clearance of forest by humans is thought to be the most likely explanation of this transition, although climate change would probably have played a part too. There are some areas where Calluna may be the natural vegetation, for example in a zone just above the altitudinal treeline and on some exposed coasts. Dodgshon and Olsson (2006) have discussed the rise and fall of heather according to the increase in grazing intensity after the Highland Clearances and the impact of harvesting young trees for building. Evidently, many factors are in play.

Regeneration via seedlings. Photo: John Grace

What of the future? If the intention is to reforest Scotland in the interest of capturing carbon, a rethink is required: in many places moorland and heathland will need to be replaced by some something quite different. The very wettest sites in Sutherland are unsuitable for planting trees and the carbon stocks of many moorlands are huge and should not be put at jeopardy by draining. Native trees (birch, rowan and pine) will thrive if seedlings are protected from deer and sheep, and a forested landscape, seen as a contribution to ‘rewilding’ could also have value for huntsmen. As tree-planting targets become more and more ambitious in an effort to remove carbon dioxide from the atmosphere, landowners will be offered incentives to encourage trees. The uplands including moorlands and heathlands are the most obvious place for afforestation (or reforestation). A plantation of Sitka Spruce can absorb up to 7 tons of carbon per hectare per year, much more than Calluna. Native trees might manage half of this. As for rising temperature, Calluna currently grows in some very warm locations (Salamanca, Spain, July temperature 21 degrees, contrasting with Braemar, Scotland, July temperature 13.5 degrees). In my undergraduate days I spend many happy days cutting thin sections of Calluna stems at a remote field station in the Pennines (Moor House, then a flagship site of the Nature Conservancy). The growth rings were larger in warm years (we even published this result, a few years later, Grace and Woolhouse 1974). As for rising CO2, the tallest heather I ever saw was chest-height, growing near a CO2 spring in Italy.

Calluna is a remarkable species, much-loved and much-used. Its global distribution (below) shows how well it has adapted. In New Zealand, where it was introduced in 1912 it has become invasive. It out-competes native species.

Global distribution of Calluna vulgaris. From GBIF.

References

Beijerinck W (1940) Calluna. A monograph on the Scotch Heather. Verhandelingen Akad. Wet.Amst. 38, 1-180.

Dodgshon, RA Olsson GA (2006) Heather moorland in the Scottish Highlands- a history of the cultural landscape 1600-1880. Journal of Historical Geography 32 (2006) 21e37

Elliot RJ (1953) effect of burning in the heather moors of the South Pennines. Sheffield PhD thesis. https://etheses.whiterose.ac.uk/3638/1/488807.pdf

Garnett MH, Ineson P and Stevenson AC (2000) Effects of burning and grazing on
carbon sequestration in a Pennine blanket bog, UK. The Holocene 10, 729-736.

Gimingham CH (1960) Calluna. Journal of Ecology, 48, 455-483.

Gimingham CH (1972) Ecology of Heathlands. Chapman and Hall.

Glaves DJ, Morecroft M, Fitzgibbon C et al. (2013) Natural England Review of Upland Evidence 2012 – The effects of managed burning on upland peatland biodiversity, carbon and water. Natural England Evidence Review, Number 004. http://publications.naturalengland.org.uk/publication/5978072?category=5968803.

Grace J and Woolhouse HW (1974) A physiological and mathematical study of growth and productivity of a Calluna-Sphagnum community. IV. A model of growing Calluna. Journal of Ecology 11, 281-295. https://doi.org/10.2307/2402021

Legg CJ, Maltby E and Proctor MCF (1992) The ecology of severe moorland fire on the North York Moors: seed distribution and seedling establishment of Calluna vulgaris. Journal of Ecology 80, 737-752.

Thompson DBA and MacDonald AJ (1995) Upland heather moorland in Great Britain: a review of international importance, vegetation change and some objectives for nature conservation. Biological Conservation 71, 163-178.

Welch D (2016) The floristic changes of Scottish moorland dominated by heather ( Calluna vulgaris , Ericaceae) but unburnt for 50 years and kept checked by moderate grazing. New Journal of Botany 6, 31-42. DOI:10.1080/20423489.2016.1178061

John Grace

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

%d bloggers like this: