Seasonal Update (December 2022)

Robins are the quintessential bird of winter, cameoing on many a Christmas card. - Credit: Marc Baldwin

Welcome to winter and, for the first time in a few years, a winter that feels wintry—so far, at least. I confess to being a fan of cold weather in winter, and it's important for regulating the population of many “pest” and parasite species, but I do also appreciate that, with the cost of living issues in much of Europe at the moment, cold is not welcomed by all. With that in mind, if you're struggling at the moment, the Citizen's Advice Bureau has some suggestions.

Weather update

October was unseasonably warm across the UK and most of Europe owing to the jet stream looping around an enormous low-pressure gyre in the North Atlantic. Temperatures were more akin to those expected in late August or early September than late October. The October mildness came as relief to those suffering in the energy crisis in Europe, and multiple countries recorded their warmest October on record, some by a large margin.

Temperature anomaly map on 12th November. November was a very warm month across the UK and most of Europe. - Credit: Karsten Haustein

The first half of November was very unsettled; successive low-pressure systems brought torrential rain and strong to gale force winds, gusting above 70mph along the south coast. Many of the southern counties that bore the brunt of the summer's drought saw significant flooding, although to illustrate the depth of the drought, river and reservoir levels remain low as we move into December, despite more than two weeks of heavy rain. Temperatures remained mild across much of the country, with all but north-east Scotland seeing above average warmth. Daytime highs ranged from 11C (52F) in the north to 16C (61C) in the south-east during the first two weeks, with night-time lows falling by only a couple of degrees Celsius from the daytime temperatures. The seasonal average is around 10-12C, north to south.

Most of the UK saw the warmest Remembrance Sunday on record, with Aviemore in Scotland recording 17.2C (63F), Bridgefoot in north England reaching 19.2C (66.6F), and Porthmadog in Wales peaking at 21.2C (70.2F), all of which were new country records. Northern Ireland saw 16.5C (61.7F), which was 0.2C below the current record. The middle week saw temperatures drop down towards average as the week drew on, with periods of wet and at times very windy weather for the whole of the UK. Indeed, the 16th saw severe gales along the south coast and up in the northern islands accompanying heavy rain, which caused widespread flooding across much of England and Wales. The penultimate weekend was cooler, with some overnight frosts in the north and east, before things turned milder again as more low-pressure systems brought the rain and wind back. By the final weekend, here on the south coast, temperatures were back up to 14C (57F).

Outside of the UK, October ended and November began on a historic warm spell in Europe. The final weekend in October saw 33.3C (91.9F) at Lomnè in France, Germany reached 28.7C (83.7F) at Mullheim and 27.8C (82F) at Freiburg, and Brussels hit 25C (77F). Some monthly records were broken including 21.8C (71.2F) at Zinnwald-Georgenfeld. There were also some staggering temperatures in Switzerland, with 27.4C at Delemont, while Slovenia recorded their hottest November day on 1st at 26.2C (79F), and Poland nudged just over 24C (75F). Sweden saw 15.1C (59.2F) in Stockholm, and Uppsala recorded a new November record of 15.6C (60.1F). In Finland and Estonia several November records also fell, including the Finnish capital Helsinki with 14.3C. Trois-Villes and Samadet in France each recorded a staggering 26.2C (79.1F) last month.

The middle weekend of November saw a powerful storm dump several foot of snow on New York. - Credit: Anthony Quintano (CC BY 2.0)

November produced an exceptional heat wave in China, temperatures rising to 34.3C (93.7F) in Yunnan Province on the final weekend and above 32C (90F) in five provinces. Similarly, Luodian in Shanghai broke the November provincial record for Guizhou when the mercury hit 34.2C (93.6F) at the end of the month. Also in the east, Jeddah in Saudi Arabia recorded 180mm (7 inches) of rain in only eight hours, causing widespread coastal flooding.

After a warm start across much of North America, Southern Ontario reaching 27C (81F) during the first weekend of last month, well above the seasonal average of 9C (48F), most states ended the month in extreme cold and the Great Lakes reported record-breaking snowfall. More than five feet of snow was dumped on New York by a powerful storm that struck over the middle weekend of November, tragically killing two people. We also saw significant and widespread cool weather in South America and Australia to kick off last month, courtesy of La Niña. Indeed, last month was the coldest November recorded in the tropics. Paraguay set a new November national record low with 2.8C (37F) at Pozo Hondo, while Asuncion fell to 8C (46F), also a new record.

News and discoveries

Boiling bats. Bat researchers working on Jersey have reported that a whole generation of endangered grey long-eared bats may have been killed off by June's exceptional heatwave. It seems that while adults were able to abandon the roost, the juveniles couldn't follow and perished. It's hoped that the population can withstand this year's breeding failure, but the findings raise questions of how bat populations will cope as hot dry summers become more common.

Cheetah scent-marking posts seem to be important information boards for other animals sharing the cats' home range. - Credit: Marc Baldwin

Serene spiders. A study of orb-weaver spiders by undergrads at University of California, Los Angeles has found that, when kept in a colony, males of these normally solitary arachnids fought less and cooperated more when there were more females present. As the number of females increased, the students recorded more instances of spiders cooperating to construct webs and even to wrap up prey, although they didn't share the food.

Cheetah communication. Cheetahs, like most predators, maintain territories that they mark with scent. Trees are particularly important “sign posts” for these medium-sized felids and a recent study by scientists from the Leibniz-IZW Cheetah Research Project has established that other species read these messages, too. The researchers found that African wild cats, black-backed jackals and warthogs, three species rarely on the cheetah's menu, visited and sniffed at the trees marked by the cats more frequently than similar trees in the area. Duikers, however, actively avoided the area, presumably because these are a common prey item of cheetah in this part of Namibia. The suggestion is that other carnivores and omnivores/scavengers may use the trees to help follow the cheetah and perhaps score themselves some leftovers.

Mussel wastage. University of Cambridge biologists have repeated a survey of freshwater mussels in the River Thames that was first conducted in 1964, and uncovered an alarming decline. Not only had populations of native mussels severely declined, but those remaining were also much smaller for their age, indicating that they're now growing more slowly. The study also recorded two new invasive species now present in high numbers: the zebra mussel (Dreissena polymorpha), and Asian clam (Corbicula fluminea).

Seasonal highlight – Christmas kisses

For many of us no house is properly decorated, no office party complete, without a sprig of mistletoe hanging from a doorway or ceiling. In the Middle Ages this poisonous parasitic plant was hung above doors to prevent witches and ghosts from entering. More widely, however, and largely as a result of Celtic influence, mistletoe is considered a symbol of fertility and a meeting point under which couples kiss at Christmas. This plant is more than just a festive decoration, though—it has an interesting biology and has relatively recently shown promise as an anti-cancer drug in medical trials.

A white mistletoe plant growing from a branch. - Credit: Marc Baldwin

Here in the UK white mistletoe—Viscum album, from the Latin visco, meaning “sticky”, which seems either to be a reference to their sticky seeds or their way of life (“sticking” to trees), and albus, meaning “white”—is found from Devon in the southwest to about Yorkshire in the north, being particularly common in central and southern England and around London. It is the album subspecies that can be found growing wild in Britain and it is found mostly on apple, lime and poplar trees, although it will also grow on blackthorn, hawthorn, rowan, willow, cherry, ash, sycamore, pear, whitebeam, crab apple and occasionally on oak. More widely, white mistletoe is found from North America (where it was introduced to California around 1900) east to southern Scandinavia and across central Europe to southwest and east Asia and Japan.

White mistletoe (henceforth just “mistletoe”) is typically found in roughly spherical branched clusters in the branches of host trees that may exceed a metre (3 ft) in diameter and, owing to the absence of leaves on the host, it is most visible at this time of year. On closer inspection, mistletoe has smooth-edged leathery lanceolate (narrow oval-shaped) evergreen leaves that sprout in pairs along a woody stem and, during the winter, waxy white berries borne in clusters of between two and six. Ultimately, if you spot a tree devoid of leaves in all but a few isolated clumps among its branches, you are almost certainly looking at mistletoe.

Winging it

The white berries of mistletoe are attractive to birds, particularly during the winter, and is the main method by which the plant is spread. - Credit: Marc Baldwin

Excluding instances where humans have deliberately or inadvertently seeded mistletoe, the plant is reliant upon birds to spread. Mistletoe is poisonous to humans, consumption of the fruit leading to gastrointestinal problems because it deactivates part of our cells' protein-making machinery. Many species of animal have, nonetheless, evolved mechanisms to cope with its toxic effect and mistle thrushes, fieldfares and blackcaps readily consume it, thus being the main means by which it's spread in the UK. Waxwings will also feed on the berries.

Birds are attracted to the white berries during late autumn and early winter when many other fruits have either already been eaten or are past their best. The berry may be picked, the flesh eaten, and the sticky seed wiped on to a nearby branch or, more commonly, the fruit is swallowed whole and the still-sticky seed is voided a few hours later, either in the faeces or by regurgitation depending on the bird species. The adaptation of the mistletoe to get seeds on to new hosts is fascinating. The berry is composed of a small seed covered in an adhesive, a complex mix of cellulosic strands and mucopolysaccharides for the biochemists amongst you, called viscin that survives passage through the bird's digestive system. When the seed hits a branch the viscin fixes it firmly as it dries.

Germination is triggered by temperature and, consequently, seeds will generally lie dormant on the branch until the following spring. Come April the seed starts to germinate by putting out a structure called a hypocotyl. The hypocotyl is the part of the initial root that seeds put out to secure themselves in substratum (soil, ordinarily) and that will eventually be part of the stem leading from the substratum to the first pair of leaves. The hypocotyl of mistletoe seeds only has a rudimentary scrap of radicle (root tissue) but is nonetheless able to penetrate the bark of its host, although the process may take a year or more to complete. The hypocotyl is used to tap into the host's circulatory system (xylem in most mistletoes, although some species have evolved to penetrate phloem tissues too) from which it can extract nutrients and minerals. Until the connection can be completed, the mistletoe seedling must produce its own carbohydrates; mistletoe leaves contain chlorophylls a and b as well as the carotenoids necessary for photosynthesis, and this sustains the seedling until it can properly connect to the host. The adult plant's dependency on the host species for water and minerals while still maintaining some low-level photosynthetic capability makes mistletoe a hemiparasitic plant.

In common with two other plants often associated with the festive season, holly and ivy, mistletoe is dioecious - in other words, rather than having male and female organs on the same organism, it produces male and female flowers on different plants. Ergo, a mistletoe bush is either male or female, and survey data suggest that female bushes are about four times more common than males. Flowering varies according to location and weather conditions but generally occurs between February and April, although in warm locations and unseasonably mild years mistletoe may flower as early as December or January. The flowers themselves are small and inconspicuous; those of the male are larger and have a simpler configuration than those of the female. Male flowers consist of four simple leaves surrounding the stamens in which pollen grains are embedded. Female flowers consist of a spherical lump of green fruit-producing tissue, as well as the four simple paragon leaves that surround the nipple-like stigma. Once open the flowers produce sweet-smelling nectar that attracts insects, including ants and small flies, to carry out the pollination. Following successful pollination, females produce white berries from about late October until May, each containing a single green seed surrounded by a sticky pulp.

A sectioned mistletoe berry, showing the seed surrounded by its viscous coating. - Credit: Marc Baldwin

Live and let leech

Mistletoes sequester minerals, carbohydrates and nitrogen from the trees on which they grow. Consequently, all mistletoes affect their host, although the impact is dependent upon the health of the tree and the prevailing conditions as much as the intensity of infection. Mistletoe infection has been linked with reduced trunk/branch/leaf/needle growth, increased leaf/needle shedding, lower vigour, reduced fruiting, lower wood quality and increased susceptibility to parasite/pest infection of host species. The result can be a higher-than-expected mortality among infected trees.

With the significant potential for mistletoe to cause damage to both commercial and ornamental plantations, a great deal of research has been directed at trying to find ways to control the plant. The close association with its host means that mistletoe is very tough to control by traditional methods; chemical treatments can damage the host too. Pruning mistletoe off infected branches can be effective but is generally only applicable on a tree by tree basis, making it expensive. Clear felling is also an effective method of eradicating mistletoe from a heavily infested stand, while selective thinning of infected trees and favouring or introducing non-host species are management options that can reduce mistletoe abundance. There is also work underway looking for bacterial and fungal parasites to attack the mistletoe specifically.

Leaves and buds of mistletoe. - Credit: Marc Baldwin

It's not all bad news, though. In a 2001 paper to the Annual Review of Ecology and Systematics, David Wilson at Charles Sturt University in Australia reviewed the role played by mistletoes in forests and woodlands across the globe. Wilson concluded that mistletoe has a key role in helping some tree species to spread. It transpires that more juniper berries sprout in stands where mistletoe is present than in those devoid of the parasite, because mistletoe attracts berry-eating birds which also eat juniper berries. Interactions such as this can have a dramatic influence on local diversity, and areas with greater mistletoe densities generally support a greater diversity of animals. Thus, despite being a pest to forestry in some circumstances, mistletoe can be beneficial in providing food and habitat for a range of forest species and having an overall positive impact on the biodiversity of woodlands.

A human connection

Why mistletoe has such cultural significance, particularly around Christmas time, remains unclear. There is, however, no shortage of stories about how kissing under the mistletoe came to be a favoured festive pastime. Legend has it that mistletoe was considered sacred by Celtic Druids because it was leafy and berry-laden even during the depths of winter when the host trees were bare. Indeed, in his Historia naturalis, Pliny the Elder wrote of how mistletoe was especially venerated by the Druids of Gaul as a symbol of eternal life. There are records of various early cultures using mistletoe together with aromatic substances, such as an incense, to scent houses, animals and men in a bid to protect against lightning, spells and bad dreams, or to get in contact with the “elementary power of nature” and find “inner stability”. Subsequent authors point to a particular association between mistletoe and romance appearing in ancient Norse mythology and note that, by the eighteenth century, it was apparently common practice for servants in British houses to steal kisses under bunches of mistletoe hung in doorways at Christmas. In an article for Fox News in 2014, Adam Verwymeren said that it was actually bad luck to kiss under mistletoe and stated that a berry was taken from the bunch after each kiss - when the berries ran out, the bough no longer had the power to command kisses.

Part of the evolution of ancient beliefs that mistletoe offered protective power may well stem from early observations that the plant had medicinal applications. Hippocrates, for example, recorded his use of mistletoe to treat spleen complaints and problems associated with menstruation. Pliny the Elder subsequently wrote of how mistletoe from oak trees could help treat epilepsy, infertility and ulcers, when applied as a chewed pulp. A widely held belief of the Middle Ages was that wearing oak mistletoe could ward off epilepsy. Indeed, even a cursory look at the literature suggests that mistletoe has, in one form or another, been used in the treatment of a variety of conditions from high blood pressure and rheumatism to various types of cancer.

Mistletoe is still widely used as a Christmas decoration, commanding the power of a kiss. - Credit: It's No Game (CC BY 2.0)

The advent of mistletoe as an anti-cancer treatment appears to have started in 1916 when Austrian-born Rudolf Steiner created a philosophy called anthroposophy. In essence, Steiner believed that the semi-parasitic lifestyle of the mistletoe might provide an antidote to the parasitic cancers known to infect human tissue. In a paper published in 1920, Steiner recommended a drug extract produced in a complicated manufacturing process involving sap from mistletoe harvested during the winter and summer. A flurry of tests and clinical trials ensued that appeared to generate some very positive results, suggesting that mistletoe might be useful in the stimulation of the immune system, boosting of antioxidants and regulation of blood glucose, as well as treatment of cancer and dementia. Some studies indicated that the antioxidant properties of the mistletoe varied according to the species of host tree on which it grew. Many of these studies were, unfortunately, of questionable scientific rigour.

Mistletoe contains several active compounds with therapeutic and physiological benefits. The most widely studied of these are sugar-binding proteins called lectins, which appear to be able to stimulate the human immune system and attack some cancer cells. It does appear that these lectins give a dose-dependent response; some concentrations speed up the death of cells in cancer patients, while others slow it down. Similarly, some studies have found few or no side effects to treatment, while others have complained of fever, allergic reactions and even kidney failure. Ultimately, mistletoe extract shows some potential as an anti-cancer drug, but the jury remains out and more data from appropriately moderated clinical trials are required.

(Note: Any attempt to consume mistletoe or mistletoe extract that isn't sanctioned by a medical professional is strongly discouraged. Mistletoe leaves, stems and berries can be poisonous to humans if ingested, with even three of four berries producing stomach aches and consumption of larger quantities resulting in gastroenteritis and diarrhoea.)

For a round-up of Britain's seasonal wildlife highlights for December, check out my Wildlife Watching—December blog. As pannage season draws to a close this month, a reminder that you can also find out everything you ever wanted to know about the ancient tradition in my pigging out on the forest blog.

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