Non-native tree species

Trees are more susceptible to the negative consequences of climate change when they are planted on the edge of their warm/dry ecological niche. As a last resort, this can countered with a change in species (e.g. from spruce to beech). The switch in tree species utilized and the conversion of the species composition in forest stands in Bavaria is a common practice in order to adapt the forest to the coming changes from a warming climate. This conversion will be incurred at a considerable cost.

The spectrum of eligible central European tree species that are appropriate for this conversion is limited to less than three dozen species. Many of them have a low economic value due to their rareness and are primarily deciduous trees. Presently, conifers dominate the forest in central Europe, which is in contrast to the potential natural vegetation. Clearly, the timber industry in central Europe is specialized on conifers. As a result, the forestry sector does not want to limit itself to central European broad-leaved species in its adaptation to climate change and considers conifers from other regions of the world.

In comparison to the life of a tree, the practice of planting non-native tree species has not been around for a long time. In most cases, the length of the experiences is not more than 150 years and not longer than two rotations.

The interest for alternatives to be used in forest conversion has grown immensely with the change in climate. Naturally the desire for higher yields in growth, accompanied by a good suitability to a warmer and dryer climate, also play an important role. A search for alternate species in order to transform the forest to better face climate change has been underway for a long time. It is now imperative to examine the alternatives based on clear principles, in an emotionless manner and without stereotypes.

In the past, the forestry sector in Germany has almost exclusively cultivated non-European tree species from points originating in the Pacific Northwest of North America, eastern North America and from the Japanese islands of Honshu (Table 1 and Figure 2). The success of cultivation depends on various factors and should definitely not be taken for granted. The general risk of cultivating non-native tree species will not become less with the new, and more importantly, strong variations in climate.

The prerequisite for the cultivation of non-native tree species is a strong similarity of the climate in its native range. Accordingly, a comparison of the climatic conditions between the areas of origin and cultivation for non-native species has to be undertaken as a first step.

An overview of the worldwide distribution of the climate type Cfb, the predominate climate type in Europe, shows that this warm to moderately humid climate with warm summers is only found in a few locations outside Europe (Figure 3). The requirement for similarity can only be partially fulfilled. Additionally, the classification is based solely on the present conditions and will not remain constant during climate change.

More promising is the utilization of climate parameters. Through a comparison of dominant average temperatures and their corresponding standard deviations (Figure 4) a similarity between areas of origin and cultivation can be estimated. We use the typical areas of origin for Douglas fir, Red oak and Japanese larch (Figure 2). The area of cultivation spans the extent of Germany and the range of European beech typically covers all of central Europe.

The average yearly temperatures in the areas to be compared differ only slightly (Figure 4.1). In this regard, it seems that the similarity prerequisite has been fulfilled. Solely Douglas fir exhibits considerably lower average yearly temperatures in its entire range. More can be revealed if seasonal temperatures are considered.

In the winter months from December till February, the temperatures in the ranges of the Japanese larch, Douglas fir and red oak vary from those in the areas of cultivation (Figure 4.3) and the winters in the areas of origin are much harsher than in Germany. In contrast, the area from where coast Douglas fir is imported has warmer winter temperatures than the areas of cultivation in central Europe.

Another view arises with a glimpse at the summer temperatures (Figure 4.5). From July to August the Japanese larch and red oak endure higher temperatures as in the areas of cultivation. In the areas from where Douglas fir is imported (and its whole native range for that matter) the summer temperatures are predominately lower than in the areas of cultivation in central Europe.

Additionally, if the standard deviations in figure 4.2, 4.4 and 4.6 are taken into consideration, particularly lower values for temperature can be seen in the areas from where coast Douglas fir is imported. The yearly, winter, and summer values exhibit a well-balanced oceanic climate. Also, there is little variation in winter temperatures throughout the range of Japanese larch. Similar temperature variations as in central Europe prevail in the native range of red oak. This makes it clear that there are limitations when comparing climate similarities with simple climate parameters. Failures in cultivation can be traced back to a lack of conformance between the climate in the area of origin and the area of cultivation.

Decisive for success is the intended main effect. Economic motives clearly stand in the forefront with regard to the cultivation of non-native tree species. Non-natives should only be cultivated when the financial yield exceeds that of natives and the risk of cultivation from the landowner is kept within reasonable limits.

Unintended side effects should additionally be kept in mind, especially with regard to the problem of invasiveness. What begins as cultivation can, under the right conditions, end in an unwanted "naturalization". Considerations for nature conservation or the suppression of native species are not the only characteristics used to judge invasiveness. Undesired consequences can also arise from invasive plants in forestry.