The Swiss National Forest Inventory (NFI) provides statistically significant statements about the state and development of Swiss forests in five regions: the Jura, the Plateau, the Pre-Alps, the Alps and the Southern Alps. It was conceived as a sampling inventory with permanent sampling plots within a square grid. During the first NFI (1983-1985) a total of 128,450 sample trees, from 12 cm DBH upwards, were recorded. Around 55,000 were spruces, but only 145 yews. The thickest yew had a DBH of 54 cm, the tallest was 19 m.

The NFI was not brought into being to record relatively rare species such as the yew tree, however, on a small scale quantitative details such as stem numbers and changes in them are statistically meaningful for the whole of Switzerland and are of a quality which could not be achieved with other surveys.

Geographical and habitat distribution

Switzerland lies in the centre of the European yew tree (Taxus baccata) area. The major distribution zones are in the eastern Plateau, the Jura, along the edge of the Alps between Lake Constance and Lake Thun and in the lower Rhone Valley. Accumulations of NFI sample plots with yew tree presence (Fig. 3 – black dots) are indicators of actual distribution focus points. According to the NFI1 the largest accumulation of yew trees in eastern Switzerland is in the area around the Hörnli region.

Within the Alps themselves, and on the southern slopes of the Alps, yews are only to be found sporadically. The areas which were mapped in 1970 as being rich in yew trees in the regions of Sargans, Prättigau and the Bündner Rhine valley have not been confirmed by the NFI1. Using data from following inventories (NFI2 (1993-1995) and NFI3 (2004-2007)) no migration or change in the main distribution zones could be proved.

According to the NFI the yew tree, due to its sensitivity to frost, grows mainly at colline/submontane (48%) and lower montane (44%) altitudes only. More than half of the yew trees grow between 600 and 800 m above sea level (Fig. 4). The median (of tree distribution in altitude) is around 720m above sea level, only slightly below that of the beech tree. In Switzerland the yew can grow up to altitudes of 1400m. In the NFI the highest plot with yew trees was registered in the Canton of Glarus at an altitude of 1250m. This typical ancillary tree species grows well in Switzerland with around 60% of trees growing in pure or mixed hardwood stands where the following tree species are most predominant: beech 34%, spruce 18% and fir 16%.

Present occurrences are restricted to places where beech trees are less competitive (dry or damp sites) and which are optimal yew tree locations such as on slopes where air humidity is higher and lateral lighting restricted. Accordingly in the NFI yews are often to be found on steep slopes and along crests. They are most frequent in northeast to southeast locations on 30 to 40 degree slopes. Characteristic for the yew is its preference for alkaline soils: three quarters of all individual trees were growing in soils with a pH value of over 6.2.

Frequency, timber stock and yield

According to the NFI3 there are 1.17 million (+/-20%) yews growing in Swiss forests with a diameter at breast-height (DBH) of 12 or more cm. (Table 1). This means that amongst domestic tree species the yew takes place 30 ahead of the field maple and is nearly as frequent as the Norwegian maple, the aspen or the broad leafed lime. Around 2 of every 1000 forest trees are yews. In the Plateau and the Jura it can be 4 or even 5 of every 1000. The density is highest in Canton Zurich where nearly every hundredth forest tree is a yew.

Table 1 - Distribution of yews from 12 cm DBH grouped geographically according to the Swiss National Forest Inventory. *Yew occurrence on sample plots, which were not visited in NFI2 and NFI3.

Although the majority of Swiss yew trees are still relatively young there is a large quantity of them in relation to other European regions. The Swiss yew population is therefore very important for the maintenance of the species in western and central Europe. A similar number of trees are only to be found in the Caucasus. This could be due to the fact that the over-exploitation of the yew in the late middle-ages was less intensive in areas which are now Switzerland than in those in Germany and Austria. In any case none of the 16th century yew transport routes ran through Switzerland, even though yew timber has been exported from the Albis area to England since the 15th century.

Within 20 years the number of yews with a DBH of over 12 cm has increased from 0,74 to 1,17 million in number. The most noticeable increase is in the Jura and the pre-Alps in western Switzerland, as well as in the eastern Plateau. The yew timber stocks have increased even more: from 100,000 m³ (1983/85) to 139,000m³ (1993/95) to 354,000m³ today. Bole wood with bark has more than trebled, even though yew is only rarely used.

The figures for yew growth and its use in cubic meters from the NFI are not exceptionally useful due to estimation errors. However, the sample trees show how small the percentage of used trees above 12 cm DBH is. On the joint sample plots of the NFI2 and NFI3 inventories the number of used yews is ten times smaller than those which have become ingrown This shows a very moderate and responsible usage.

Stand development and browsing

The large increase in the number of yews both growing and in stock with a DBH larger than 12cm should not hide the fact that new yew growth in Switzerland is not secure. Only plants above the height of 1.3m (that is to say with a DBH above 0.1cm) are big enough to escape terminal shoot browsing. In this case we talk about “secure new growth” or “secure regeneration”.

If we look at the number of yews with a DBH above 0.1cm the total number of individuals has increased only negligibly from 2.6 to 2.7 million examples in the last 20 years. What is noticeable however is the increase in examples with wider girths (Fig. 6): there are noticeably more thick yew trees. Around 150,000 individuals have a DBH of 24cm and more. On the other hand the number of individuals in the diameter class of 0-3cm (DBH 0.1 to 3.9cm) has decreased and these are, with only 170,000 examples (NFI3), extremely under-represented. The yew population has therefore just become thicker in girth and the amount of new yew growth had tendendencially decreased. The present stands are not sustainable.

This problem of the lack of yew tree rejuvenation can be explained by looking at the first NFI from 1985. The distribution of trees in the NFI1 showed a development, as for a normal population, for classes with a DBH of 8cm: an exponential reduction with an increase in diameter (Fig. 6). The 4-7cm class does not follow this pattern it is “under-represented”.

The question arises as to when this drop could have occurred. From when did the well represented trees of the class 8-11cm seen in the NFI1 originate? Using the diameter growth figures from the period between the NFI2 and the NFI3 (11 years) it can be seen that Swiss yews increase in diameter 1-2mm per year according to their present diameter. Assuming a regular growth rate of 1-2mm per year then the yews of the 8-11cm class would have passed the threshold of 0cm around 40-110 years before the NLFI1. That is to say between 1875 and 1945. After this period of time less yews grew into the stage (over 1.3m) where they are no longer endangered by browsing.

Parallels can be seen between these figures and the development of the game population. Around 1800 the game population was at its lowest: as a consequence of the political upheaval following the French Revolution. Thanks to wide spread poverty and a lack of food four out of five game species were nearly wiped out. In 1875 the first Federal Laws on Hunting and Bird protection were introduced nation wide regulating hunting and paving the way for an increase in the hoofed game population.

Since 1900 the roe deer population has recovered and rapidly increased. In the NFI1 already 40% of yews with a height of 0.3-1.29m showed signs of browsing on terminal shoots. The amount of browsing is greater on yews than on any other species and three times higher than that of the silver fir, which is also an endangered species and this is way above the acceptable level.

In many places hardly have the yews emerged from the ground cover then they are attacked by game to such an extent that they rarely reach a height of more than 50 cm. In Switzerland therefore the greatest challenge in promoting yews is the protection of new growth from damage by game.

Niches for yew promotion

Although the yew is relatively common in Switzerland in comparison to Europe, the lack of new growth needs active promotion and protection measures where possible and appropriate. Major successes should be expected in the main, typical yew sites. Based on data from the NFI and other surveys a first provisional map of potential yew spread areas in Switzerland, under present day competition and climate conditions, has been produced. (Map excerpt Fig. 8).

The modelled distribution corresponds to spatial predictions of present day achieved niches. These are much smaller than potential niches, especially with non-dominants species. Site data from 302 yew areas were used for the model. The calculations are based on the “Boosted Regression Trees” method.

The calculated yew occurrence probability map shows for practically all of the present occurrences a high or in some cases an average probability. These modelled environment niches therefore cover present occurrences well. If the definitive model for Switzerland produces the same spread pattern, then we can deduct that measures taken to encourage the yew in the Pre-Alps and along the northern edge of the Alps in central and eastern Switzerland could be most effective, because here are the majority of areas where the probability of yew occurrences were predicted as being at their highest.

Yew friends ....

….are a group of yew enthusiasts from central Europe. Established in 1994, the year the yew was made Tree of the Year the group holds an annual international conference and excursions which last for several days. 3 to 4 dozen of the 300 yew friends, from 15 different countries, regularly take part.

Translation: Dawn Meister (Affoltern a. A.)