MistleGO! a year on the UK mistletoe trail

The hunt for the hemiparasite returns...

A National Tree Week update from Ollie Spacey, The Tree Council supported University of Oxford PhD candidate. Read Ollie’s Tree Talk from last season: Mistletoe: Tracking the festive flora that divides opinion

Ollie’s mistletoe research begins its second season of data collection. The hemiparastic plant is much easier to spot in Winter. Pictured here in the Cotswolds.

When I think of iconic Christmas plants, one reigns supreme above ivy, holly and poinsettias—mistletoe. Since ancient times, mistletoe has been a symbol of fertility, associated with magic, used for its medicinal properties and revered in druidic rituals. By the Victorian era, people had established a tradition of kissing under a sprig of mistletoe, plucking a berry for each kiss stolen. Romantic connotations of mistletoe continue today, featuring in the lyrics of most (good) Christmas love songs.

I would argue, however, that the biology of mistletoe and its impacts on our ecosystems are far more interesting. As the distribution of mistletoe in the UK changes, my research is showing how this change will occur and what the impacts might be.

Mistletoe is a parasitic plant, requiring a host tree from which it steals water and nutrients to survive and grow. The only mistletoe in the UK is European mistletoe (Viscum album), a native plant which can grow on over 230 tree species. Mistletoe can grow extensively on its favourite hosts (apple, poplar and lime) leading to decreased tree growth, and sometimes contributing to tree death. Orchards may be overrun by mistletoe if it is not managed sustainably, making mistletoe an understated pest for cider farmers.

Mistletoe forming on the lichen covered branches of an apple tree in Devon

Nonetheless, mistletoe is often harmless in small amounts and is beneficial for the wider ecological community. Mistletoe provides food for winter birds such as mistle thrushes (Turdus viscivorus) which spread its seed. Moreover, mistletoe in the UK houses 6 invertebrate species found only on mistletoe clumps. As old nutrient-rich leaves of mistletoes fall after new leaves have developed, this promotes nutrient cycling, solidifying mistletoe’s title in the ecosystem as a “keystone species”.

Whether you view mistletoe positively or negatively, its impacts on our environment are important. With a changing climate and shifting treescape, mistletoe is expected to be on the move northward, and spreading extensively in local hotspots. Quantifying this change is difficult, however. Like many parasites, potential for transmission depends on local abundance, yet data on local mistletoe density has never been collected at a national scale (only ever presence/absence records). Also, data on host tree ID was poor in previous surveys, and the role of bird vectors which spread the mistletoe neglected.

Ollie on the mistletoe hunt

That is why last year we launched the first national-level mistletoe survey since the 1990s: MistleGO!. Our app-based mistletoe abundance survey developed and operated by me at the University of Oxford, and colleagues in the Science & Research team at The Tree Council.

Recorders are encouraged to submit observations of mistletoe they see via the MistleGO! survey which they can install onto the Survey123 app on their smartphones. Quite simply, when you identify one or more trees with some mistletoe, open the app and the location will be automatically recorded. Then, take a photo of the tree (or trees) and record roughly how many clumps you can see (e.g., 1, 2-5, 6-10, etc).

The final part of the survey records the habitat the mistletoe is found in as knowing whether the mistletoe is found in an orchard, park or garden will be crucial to predicting future mistletoe distribution. The recorder can add in extra info such as host ID (which we appreciate is difficult in winter) if they wish, and otherwise, submit!

Don’t worry if there is no mistletoe near you—you can even record areas where mistletoe is absent! It is as important to know where mistletoe doesn’t grow as where it does, so absence records are greatly appreciated. And if there is absolutely loads of mistletoe near you, you can record multiple trees at once, and record the presence of nearby trees which also contain mistletoe without submitting an extra record.

Last year, we received more than 1250 records from more than 140 citizen scientists, and found mistletoe across all nations of the UK, some in surprising areas. Large infestations on poplars, limes and apples were recorded as well as on some rarer host species. Mistletoe was found in high abundances in parks and gardens, including in areas it was not reported in before. It is too early to confidently predict how mistletoe distribution may be changing, but an intriguing picture is emerging.

The recognisable silhouette of mistletoe among the branches at dawn in Cambridgeshire

We want to build on this brilliant start to our project with more records this winter (November - March), allowing us to confidently map the UK’s mistletoe. Only with more data can we attempt to predict where mistletoe will likely end up in future. With this information, we can estimate what new impacts might be, and where management may be helpful.

Whether you kiss under mistletoe, see it as a pest, or are fascinated by its biology, you can see that mistletoe is an important member of our British flora. Help us understand its present and future by submitting a record to MistleGO! today!

Ollie Spacey is a PhD Candidate at the University of Oxford. His research is supported by The Tree Council and Fera Science. Find our more here.

It’s National Tree Week 2024 (November 23 - December 1), get involved at www.nationaltreeweek.org.uk

MORE: Mistletoe: Tracking the festive flora that divides opinion