2020

Common Yew: Poisonous and Protected

Nature researcher Ilmārs Tīrmanis continues to introduce us to the planet`s oldest trees, and this time all attention is on the yew. The yew or the common yew (Taxus baccata), just like the juniper, is a tree or, more often, a shrub belonging to the class of conifers or pines. Just like the juniper, the yew has been on our planet for tens of millions of years.

Protected and rare

The common yew is a very rare, protected plant growing in the wild. In order to ensure the protection of practically all yew findings, micro-reserves can be established in their natural growing areas in Latvia. This species is also listed in the Estonian Red Book (Category II) and the Baltic Sea Region Red Book.

The largest and thickest yew in Latvia (the circumference of the trunk exceeds 2.5 metres, the height – 12.5 metres) grows (and is taken care of) in Tukums Municipality Zentene Manor Park. But it is unlikely that this mighty specimen appeared there naturally.  

Natural yews in the Baltics grow mainly in Latvian coastal lowland forests and also in Estonia – mainly in coastal forests. The mild climate that is provided by the sea, ensures the survival and development of yews.

The largest share of Latvia's natural wild yews can be found in Slītere National Park, and there are even a few large specimens among them.

In Lithuania, the common yew has already disappeared. However, near the Lithuanian border, in Nīca Rural Territory, JSC “Latvia’s State Forests” (LVM) and State Forest Service`s Forest Research Station manage Natura2000 territory nature reserve “Rucavas īvju audze” (Rucava Yew Grove), which ensures undisturbed development for one of the few natural findings of the common yew that is known in the country. LVM GEO database contains 23 findings of these species, but the nature data management system “Ozols” contains information about 75 findings of common yew in LVM-managed areas.

Favourable growth conditions

Unlike the light-loving juniper, the common yew is a tree species, which develops best in partial shading. It is threatened by cold in open areas – by winter and spring frosts. This plant is especially suitable for living “on the second floor” of the forest. However, if it is overshadowed too much, it will wither. Excessively shaded yews tend not to form generative or reproductive organs.

The yew is characterized by a relatively modest demand for soil conditions. However, the yew prefers to grow in mineral soils (both dry and wet) as well as in loose, calcareous clay soils.

In a favourable environment, a tree may develop; it is relatively small in size (usually only up to 2.5 metres tall), but it is still a tree, which forms a crown in pyramidal form. The crown of a yew tree is characterized by very thick branches. In turn, under the influence of unfavourable factors (for example, in clearly exposed spruce shading, in conditions of high groundwater level, etc.) the yew develops a bushy shape. Also, when exposed to direct sunlight, it does not tend to form a classic shape of a tree.

The yew, just like the juniper, grows relatively slowly, but lives long; in a favourable situation its life can reach even thousands of years.

The wood of this plant has great flexibility, strength, toughness; it should also be mentioned that it has fine fibres and it does not contain resin. The surface of the yew trunk and branches is grooved, the surface of its bark is flaky, grey (for old specimens) or reddish-brown (for young plants). Yew needles on new shoots are arranged in a spiral, on older twigs mostly curved parallel to each other and placed in one plane. Needles are 3 (3.5) cm long, up to 2 mm wide, soft, green (dark green at the top, shiny, yellow-green at the bottom, matte), flat, with downward-sloping edges, marked with a longitudinal rib on the surface. They remain on the branches for (3) 5 – 8 (10) years.

From pollination to yew cones

Yew pollination period in Latvia lasts from the end of March till the beginning of May.

Common yews, like junipers, are dioecious plants, which means that they can be differentiated into male and female specimens. However, the sex can be determined only for plants that are at least thirty years old, because they usually do not develop reproductive organs earlier.

Male reproductive organs or male strobes (microstrobils) develop in the armpits of needles, several of them holding tightly together on a short stalk. They are disk-like, scaly and yellow. Female reproductive organs or female strobes (macrostrobils) also form in the armpits of needles, but develop one by one. They are significantly smaller than male organs, scaly, greenish. Each female strobil contains one cotyledon.

Growing in the forest, it is desirable for individuals of the opposite sex to be as close to each other as possible, because even if both male and female yews successfully form genitals, high densities of other trees can make pollination difficult or even impossible.

If pollination has taken place, green, egg-shaped seeds develop. A peculiar seed shell is formed around each seed, and it is called aril. This process generates yew cones. Seeds ripen in August, September. For immature seeds the seed shell is thin, green, but when the aril ripens, it swells up to 8 mm in diameter, becomes juicy (80% of water), smooth and sticky, sweet, turns bright red, its surface becomes lightly scaly, the tip becomes cup-shaped. In autumn, cones begin to fall off.

Cones, thanks to their purple, “fleshy” seed shell, deceptively resemble fruit – berries. Due to this similarity, the common yew name “berry yew” originated some time ago and is still very common. This is a botanically incorrect name, because gymnosperms, which also include yews, are known to have no flowers (gynoecium); therefore, fruit, including berries, cannot form.

The older the tree, the more poisonous it gets

The yew is relatively sensitive to various adverse environmental factors, but resistant to fungal and bacterial damage, insect attacks. Yew's ability to successfully resist these potential offenders is most likely related to its toxicity.

Absolutely all parts of the plant, with the exception of the aril, contain a very toxic mixture of alkaloids called taxis. It was in honour of the taxis that the yew got its Latin name Taxus.

Yew needles, wood, bark and seeds all contain poison. Moreover, the older the tree, the more poisonous it becomes. Dead yew is also toxic. Even when dried, it does not lose its toxicity much.

Toxicity of the common yew is one of the reasons why in some parts of Europe plants of these species were once deliberately removed by humans to take care of their own health and the health of animals. The second most important reason is the high practical value of yew timber. Yew has a resilient timber that has often been used in the past to make tools and weapons. Yew is mahogany – high-quality decorative wood for further processing. 

Yew poison is dangerous to both humans and many animals. To many, but not to all animals! Among herbivorous mammals there are species that look for yews in order to eat the needles and bark of these plants without harming themselves. Why!? Maybe because of the strong bactericidal properties of the yew? Maybe to get rid of parasites in the digestive tract? Maybe for both reasons? But maybe there is another explanation.

Of course, there are many of those who want to eat the only non-toxic part of the yew – the aril. The sweet, seductive ruby-red seed shell attracts birds in late summer and autumn. If it was not for aril-hungry birds, there would be no yews in the wild. Birds are the most important natural distributors of yew seeds. Their digestive system, which digests the seed shell, quickly gets rid of the poisonous seeds. Seeds that were not harmed by bird`s digestive system and did not harm bird`s health, fall to the ground somewhere nearby. It is possible that a new sparrow will sprout in one of these places in a few years.

Some sweet loving martes (martens, badgers) also like to snack on yew cones. Delicious, healthy seed shell remains inside the animals, but the poisonous seeds remain in their faeces.