This is an episode from the predecessor of the regular column ‘5 Questions’: QED. Each instalment centred on a single question.
Many Christmas trees end up as green waste in the first week of the new year. Can a new tree grow from an old branch, and can that process be repeated indefinitely?
Whoever was planning to snip branches from the old Christmas tree after the holidays and root them in water is in for some bad news from Hans van Roon, curator of the conifer garden at Stichting Pinetum Blijdenstein in Hilversum. ‘Taking cuttings from conifers is genuinely very tricky’, he says: ‘That’s because root formation is generally more difficult in conifers than in broadleaved trees.’
A great deal of patience
Despite the well-known Christmas carol, it is usually a spruce – not a pine – that stands decorated in the living room in December. Professional tree growers therefore cultivate the species most commonly used as Christmas trees – Nordmann fir (Abies nordmanniana), Serbian spruce (Picea omorika) and Norway spruce (Picea abies) – almost exclusively from seed, according to Van Roon. Grafting, in which the propagator attaches a piece of one plant to another, is also done occasionally. ‘Taking spruce cuttings is only for true enthusiasts with a great deal of patience. The same goes for pines, incidentally. Because a tree without a root ball does not last very long, the chances of success with a Christmas tree are virtually nil’, says Van Roon.
It is therefore not possible to pass the same Christmas tree down through the generations as a family heirloom – by repeatedly allowing a small twig to grow into a new winter centrepiece. A great pity, but what about tree species that are more amenable to propagation by cuttings?
Wounding
Many plants naturally regenerate from small fragments, such as tubers, rhizomes or broken-off branches. This is possible because plant cells are capable of reverting to an undifferentiated state and subsequently forming different types of tissue anew. Taking a cutting wounds the plant, causing undifferentiated cells to form at the cut surface. Following wounding, a plant – driven largely by interactions between various plant hormones – forms new roots or shoots. The hormonal balance also determines that new organs sometimes arise in unusual locations, such as roots on a stem in cuttings, after which a single small branch can grow into a complete tree (Plants, 2021).
Reprogramming
In this sense, a cutting functions as a kind of reprogramming of physiological age: the cells of the cutting find themselves once again in a ‘juvenile’ developmental stage, even if the original branch came from an old mother tree. The genetic age – the number of years the genome has existed since the plant germinated from seed – does, however, continue to accumulate with cuttings. Sexual reproduction largely erases the traces of genetic ageing from the parent plants, but asexual propagation does so less completely. Mutations in the DNA of somatic cells that a tree accumulates over the years are partly preserved through cuttings (Evolutionary Applications, 2022). Prolonged asexual propagation of, for example, fruit trees can therefore be associated with reduced vigour or limited growth, but equally with increased yield or exceptionally juicy fruit.
‘A tree can be propagated by cuttings time and again for a very long time’, says Van Roon. ‘I see no reason why that could not continue indefinitely. Some plant species manage to sustain themselves asexually for millennia.’ Not every twig is equally suitable as a cutting, he warns: ‘In the pinetum there is a rare conifer that grew years ago from a low-hanging branch of an ancient forest tree. At the time, picking a leading shoot without a ladder was impossible. This tree barely grows vertically and simply wants to creep along the ground – that is just how it is genetically.’
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