Clones often get a bad press. Multiple identical copies of the same individual are seen as undesirable, vulnerable to diseases, unlikely to survive. Yet our woods are full of them (and so are many gardens).
Clones are simply groups of individuals that are all identical genetically. When a daffodil or bluebell forms a new bulb that is a clone of the original plant.
Daffodil clones, spreading through bulbs; dog’s mercury via underground shoots.
Another way that plants clone themselves is along underground shoots and roots. Dog’s mercury spreads via underground stems, from which periodically a new above-ground shoot will develop. What appear to be lots of individual separate stems may be all connected, at least for a while. As any one dog’s mercury plant produces either only male or only female flowers a patch that is all one sex is quite likely to be a single clone. If the underground connections between different parts of the patch break, there become two functionally-separate, but genetically-identical, blocks of mercury. Nettles do a similar thing, but their underground connectors are the dense root system. On a much bigger scale, Aspen, Wild Cherry and Wild Service Tree all throw suckers from their roots so that a single-species grove of trees develops – all clones. Once established the individual stems may behave as separate trees.
Nettle clones spread via roots.
Other plants spread through above-ground stems. The Ancient Greek Tyrtamus (usually known as Theophrastus, born c.370 B.C) wrote about the herbal properties of many plants, but also other aspects of their growth, for example that brambles spread by stems growing down and rooting at their tips, something I studied at Wytham. A new plant starts to form from the rooted part and initially this growth is supported by the old stem: if you cut the old stem the new plant does not grow much. However, the new plant soon develops its own leaves and becomes independent.
A statue of Theophrastus (not from life!) and a bramble tip pulled up to show the roots.
Mike Hutchings and colleagues at the University of Sussex looked in detail, at the relationship between the parent and daughter plants of ground-ivy (also known in the past as Blue Runner or Gill-creep-by-the-hedge) and have shown that there is movement of material along the runners towards the plants at the tip. Where a plant grows in a variable environment with both high and low nutrient conditions there is a greater concentration of rooting by the daughter plants in the rich patches, so that the necessary minerals are taken up more efficiently. Daughter plants in high light patches may specialise more in carbon assimilation. This division of labour allows the whole runner to grow more vigorously, but if the connections break individual sections grow on on their own.
Clonal reproduction is a good approach for local spread and holding ground. After all, if the parent plant is doing well (and it will need to have built up food resources in order to produce a new plant) then its clone should also find the conditions suitable. Why risk changing a winning formula? However the converse also applies: if conditions become unfavourable the whole clone is at risk. Many English elms spread by suckering so that a whole hedgerow, or patch of woodland, might be just the one clone. This meant that the English elm population was impacted more by Dutch Elm Disease in the 1970s, than wych elm stands which are more variable because they reproduce more from seed.
The northern pinewood specialist – twinflower – is another clonal plant, spreading within sites through shoots creeping through the moss carpet. The shoot connections eventually break down and as with dog’s mercury in southern woods, many apparently separate patches may be the same clone. While pollen may be spread from one patch to a flower on another, that is still effectively self-fertilisation, which is rarely successful. Poor seed production reduces the chances that the plant can colonise new sites.
Twinflower on a Swedish banknote.
To try to overcome this problem plants from different sites which are likely to be genetically different have been planted around an existing twinflower patch as part of conservation projects in the Cairngorm National Park. This should increase the chances of cross-fertilisation and seed set. Plants of eleven separate clones are being grown-on and four new mixed populations have been planted out at one site, with each population having a minimum of five clones to improve the chances that flowers will be cross-fertilised. There are plans to repeat this at other sites.
So, like any life strategy, cloning has strengths and weaknesses, but given the frequency with which it occurs within woodland plants, it clearly can be a useful trick to have.