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Prof. Dr. Ulrich Kohnle

PD Dr. Ulrich Kohnle

Forest Research Institut Baden-Württemberg (FVA)
Dept. of Forest Growth

Wonnhalde 4
D-79100 Freiburg i. Breisgau

Tel. +49 761 4018 251
Fax +49 761 4018 333


Author(s): Ulrich Kohnle
Editorial office: FVA, Germany
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Spruce and climate change – what is to be done?

Climate change: yes or no? This is a topic where there seem to be fundamental differences of opinion. At a second glance, however, it becomes clear that there is also far-reaching agreement. There is the finest consensus in the assessment that climate fundamentally underlies change and that in the past this has been the rule, rather than the exception. Conversely, the arguments principally centre on the issue of whether and to what extent the climate is currently changing, what course this may take and what might ultimately be the causes for this


Mischbestand: Die Risiken auf verschiedene Baumarten zu verteilen, ist unbestritten eine der wirkungsvollsten forstlichen Maßnahme im Hinblick auf den Klimawandel.
Fig. 1: Mixed stand: The policy of distributing the risks across various tree species is without argument one of the most effective forestry techniques with respect to climate change.

At all events it can be proved for Baden-Württemberg that climate change has accelerated in the last century. Indeed, the weather conditions fluctuate from year to year. On average, however, since the mid-20th century there have been clear indications of a rising trend in temperature, which has markedly intensified since the 1980s. Conversely, the changes in precipitation have been smaller. Annual precipitation levels have remained more or less the same; however the duration of the (summer) dry season has tended to increase.

Tree species suitability maps show the risks for cultivation

Rising temperatures with consistent to slightly reducing precipitation levels have the following consequences for plants: Conditions for water supply are more sharply under stress. The nub of the matter for this issue is whether and for how long this trend will progress in the future. This in turn cannot be predicted with any certainty because of the many and varied effect relationships and influence quantities. For according to a quotation, which is attributed to Kurt Tucholsky "Forecasts are difficult – especially when they concern the future." Despite these real uncertainties there are perfectly workable tests, which can assess how climate change affects the cultivation of spruce, which is currently our most important timber species, in Baden-Württemberg. In principle this is done according to the following method:

Klima-Risiko-Karte für Fichte (Baden-Württemberg). Die linke Karte enthält das ausschließlich aufgrund klimatischer Faktoren berechnete Anbaurisiko für die im Jahr 2010 geltenden klimatischen Verhältnisse, die rechte Karte die Verhältnisse unter den Bedingungen einer Klimaprojektion für das Jahr 2050 (Farbgebung der Klimaeignung: grün: beste Voraussetzungen [Optimum]; rot/schwarz: ungeeignete Voraussetzungen [Arealrand]).
Fig. 2: Climate risk map for spruce (Baden-Württemberg). The map on the left contains the risks for cultivation calculated solely on the basis of climatic factors for the conditions relevant for 2010; the map on the right shows the situation under the conditions of a climate projection for the year 2050. (Colour coding for climatic suitability: green: the best conditions [optimum]; red/black: unsuitable conditions [edge of the area]).
  • In the first stage, based on a Europe-wide observation network, the functional relationships are identified under which climatic conditions spruce is able to grow in Europe – and those under which it cannot grow. These functions are then fed with the present climatic specific values in Baden-Württemberg. The result is a map which shows where the cultivation of spruce is already exposed to considerable risks under present climate conditions (Fig. 1, map on left).
  • Subsequently the functions are applied to a climate-change scenario projected towards the future. The scenario used at the FVA presumes a temperature increase of just below + 2°C until the middle of the century and a slight reduction in annual precipitation. The result is a climate risk map for the conditions anticipated in the future (Fig. 1, map on right).

These tree species suitability maps provide the estimation of the risks for cultivation required to evaluate the risks for silviculture. The climate risk maps are indeed the preliminary step, but they are still not nearly finely tuned enough in terms of area to assess the risks for silviculture.

Balance sheet of the tree species aptitude of spruce in Baden-Württemberg
Fig. 3: Balance sheet of the tree species aptitude of spruce in Baden-Württemberg under present climate conditions (2010) in comparison with a projected climate scenario for the middle of the century (2050)

Even though the selected climate scenario is markedly moderate, it becomes evident that the risks for the cultivation of spruce are likely to become significantly higher in the next decades. The balance sheet illustrates this tendency: Whereas currently some 24% of the forest area is suitable without restriction for the cultivation of spruce, this proportion is likely to reduce to only 4%. Conversely, the proportion of the area unsuitable for the cultivation of spruce is likely to increase from 32% to 59% (Fig. 3). In the long term we can expect only localities in the higher mountain areas to be suitable for the meaningful cultivation of spruce.

What is to be done? Recommendations for action in silviculture

First and foremost it must be said: No need for panic, stay cool! It really cannot be inferred that stands of spruce tree will suddenly and abruptly disappear from large areas throughout the country. Rather, the process is expected to creep stealthily along. It has been possible to observe this trend for some time now in the warmer, more low-lying areas of the country. As a consequence of drought, bark beetle infestation, storms etc. the spruce, which was formerly widespread in the hilly areas with a climate suitable for wine-growing and the more low-lying sub-mountainous areas, has in the meantime taken its leave of those areas to the greatest possible extent.

Stands of spruce, which (still?) show no greater signs of disintegration, are therefore basically further routinely farmed, irrespective of where they grow. This also applies if such stands are in areas where the climatic suitability forecast is unfavourable.

Routinely farming means that you begin early and intensively to thin out the young trees (stand height about 15 m). This promotes vitality and the growth of girth. Thus, it is hoped, the spruce trees become on the one hand more resistant and on the other the trunks will have reached thicker dimensions in the event of felling (in the case of premature felling) and commercial losses will be kept within narrow bounds. Holding back on thinning out in stands of young spruce in view of climate change is not the correct thing to do; it is exactly the wrong thing!

It would, however, also be wrong to continue intensively thinning out until the trees are older or even to try to make good neglected thinning out of young trees in older stands. If the stands reach heights of about 25 m, it becomes increasingly important to put the brake on thinning out. There are two reasons for this. Firstly, the ability of the spruce to react reduces significantly and secondly, major interference in taller stands considerably increases the wind storm risk. New insights from research into storm damage show clearly that – despite rumours to the contrary – subsequent stabilisation of taller (older) stands of spruce is not possible.

If stands of spruce are located close to areas with unfavourable climate projections for scheduled rejuvenation or if they have even begun to disappear prematurely, the following aspects should be borne in mind for rejuvenation:

(1) Principle of mixed stands

The distribution of risks to different tree species is without doubt one of the most effective forestry techniques for the prevention of risk. The mixed stand principle therefore becomes very significant with the prospect of the uncertainties of climate change. This means that the more uncertain the forecast is for a tree species, the more important it is to create mixed stands with potentially more climate-resilient tree species. The possibilities illustrated in the following therefore relate without exception to the cultivation of mixed stands. This applies not only to spruce and possible conifer alternatives, such as fir or Douglas fir, but also, of course, to deciduous trees.

(2) Should we continue with spruce in spite of the problems of climate risk?

If at all, then only in mixed stands with climate-resilient tree species and for logical reasons only in those places where the projections for the spruce are not totally pessimistic. Therefore, never in lower-lying localities (< 300 m above NN) or on soils with poor water and air supplies (low precipitation, sandy soils, heavy soils).

Furthermore, in the case of the spruce ratios it is urgently recommended that you work with weaker target assortments achievable in shorter production times (diameter at breast height of about 45 cm), in order to reduce the absolutely certain increased climate risks. This means widely spaced planting and if required, vigorous reduction of dense natural rejuvenation and the early and consistent undertaking of thinning out.

(3) Or perhaps it would be better to change to a more climate-resilient tree species?
  • Deciduous trees

Beech works in almost every locality as a climate-resilient alternative to spruce. However, because beech does not like open spaces, it should be pre-planted under a lighted screen of the spruce stand.

If it is necessary to plant on open spaces, sycamore is the species recommended. Until recently ash would have been an alternative, however, in the present state of ash dieback, it is currently not an option

  • Silver fir

In principle a somewhat greater tolerance to heat can be attributed to the native silver fir than to the spruce. However, we cannot expect miracles from it either; fir is as little suited to warm low-lying areas (< 300 m) as spruce! On the other hand, the intensified cultivation of fir in mixed stands in mid-range and higher areas of the natural fir habitat (the Black Forest, the Swabian Franconian Forest, the south-western part of the Alb, the foothills of the Alps) definitely makes a good contribution to stabilising climate-unstable spruce forests.

However, the fir must be pre-planted under a protective screen. In addition, unless the stands of hoofed game are adjusted, fir will not succeed.

  • Douglas fir

A top performer with a migration background, this species exhibits the greatest tolerance to heat and drought of all the productive conifer species. In principle in mixed stands it is an excellent alternative to spruce in all localities (even in more low-lying, warmer areas, such as in a wine-growing climate). In fact, outside the natural habitat for fir, the Douglas fir is the only seriously productive conifer alternative to climate-unstable spruce. In fact only heavy, poorly aerated soils, waterlogged localities or soils with free lime in the top soil are unsuitable.

It is often not realised that in the regeneration phase Douglas fir passes itself off as true mimosa. Fresh, high-quality plant material is an absolute must. It should by now be common knowledge that planting techniques must adapt to the plants – and not the other way round.

Planting is best done on a protected site on one edge of a stand. Relatively large open spaces can pose problems. Planting under a screen is not really the best solution either. For this latter case the following applies. As soon as the Douglas fir trees have grown, the screen over the areas planted with Douglas fir should rapidly disappear. Otherwise root development suffers.

It is particularly important for Douglas fir that thinning out is undertaken early and consistently. If you want to produce high grade wood, you must wield the pruning saw at the right moment.

A commercially interesting possibility could be to replace for example a spruce plantation or to naturally rejuvenate it with a few, but costly, Douglas firs on partial areas on which a certain proportion of Douglas firs is to be achieved permanently. The (significantly cheaper) spruce core collection can be harvested from these areas in the thinning phase as a lucrative time mixture to the advantage of the Douglas firs.

But beware! If considerations of principle mean that you must refrain from increased planting of non-native tree species, as is the case for example in FSC-certified areas, you must ensure that no further stands are freshly planted with Douglas fir as the main species. Douglas fir is used in this instance solely as an additional tree species for mixed stands. In order not to put the ratio of the mixed tree species at risk, it is essential to keep the mixed tree species in the stands sufficiently free from competition from the taller Douglas firs.

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