Exotic tree species in Norwegian forestry – experiences with new method for risk assessment

2012.10.20

Introduced species are regarded one of the most serious threats to biodiversity worldwide, and the most severe effects are connected to predators and pathogens, particularly in isolated or semi-isolated ecosystems such as islands and lakes. Introduced plants may seriously affect abundance and occurrence of other species, as well as important ecological processes, but examples of eradications are very scarce. Northern Europe is in fact species poor due to repeated ice ages over the last million years. The 2012 Black list is based on quantitative criteria to facilitate reproducibility, and the criteria are developed for all introduced species, irrespective of being an insect or a tree species. Only introductions that have caused reproducing populations after 1750 are included. The effects should be evaluated in a 300 year/ 5 generation perspective. An important premise is that the effects should only be assessed in wild populations. Thus, in trees the effects refer to the spread areas, and not the planted stands.

A basic principle in the method is that ecological risk is a function of spread (x-axis) and ecological effect (y-axis), as shown in Figure 1.  Thus, the more spread and the higher ecological impact of a species, the higher ecological risk is assumed. Both axes have four steps, and a given species can be assigned to 16 positions in the matrix, from rapid spread of a species with serious ecological impact (A), to a species with no spread and no (known) ecological effect (E). Category A - consist of species representing “Very high ecological risk”, with the potential of establishing over wide areas with actual (or potential) harmful effects. Included on the black list. Category B - consist of “High risk” species, characterized by a combination of high score on one axis and low/intermediate on the other axis. Included on the black list. Category C species have “Potentially high risk”, either as a result of large spread and small ecological effects, or the opposite combination of traits. Category D species have “Low risk” as no substantial spread or ecological effects have been documented. Category E consists of species with no known risk.

According to the method, expected longevity of > 1000 years of most introduced tree species put them in the second highest spreading class (moderate). In many trees there is a spread between introduced populations, which assign such species to the highest spreading class (large). Thus, longevity and subsequent spread between founder populations imply that many tree species are categorized as Potential risk species, even before ecological effects are considered.

In closed forests introduced tree species are rarely capable of displacing native species to a great extent. Of the ecological effect criteria, it was primarily spread into endangered nature types and ability of being host for pathogenic fungi that caused black listing.

Using the present quantitative criteria, 9 out of 28 forest tree species (31%) were black listed (category A and B), whereas the other species ended up as potential risk species (C) or low risk species (D) (Table 1).

Colonisation of threatened nature types was crucial for black listing several species, such as sitka spruce (

), European larch (

) and subalpine pine (

). This deserves a comment. After cessation of traditional use (grazing and regular burning), natural reforestation is beyond doubt the greatest threat to coastal heathland. Spread of exotic species into coastal heathlands is thus an alternative to natural reforestation by native species, such as birch and pine. Some argue that the main focus should be whether the effects of e.g. European larch deviate substantially from birch, not whether a species taking part in the reforestation is native or not. This touches on the question of which nature we wish, which is a normative issue. Thus, right and wrong is discussed in parallel with the ecological assessments. Hemlock (

 below) is very shade tolerant and can spread and establish also in closed forests. The main reason for black listing hemlock is thus the ability to locally displace other species. The same is true for European silver fir (

), but to a lesser extent. European silver fir may regenerate very densely, but in most cases only a modest part actually reaches the canopy.  Black listing of Eastern white pine (

) and Macedonian pine (

) refers to the fact that they may act as hosts for a harmful fungus (

), that attacks red and black currant. Among the remaining species there are several of which have good dispersal, but limited ecological effect (e.g. logde pole pine

 and Swiss pine

), but the largest category are the low risk species, having limited to moderate spread and no/small ecological effect under Norwegian conditions. This group is exemplified by balsam fir (

), blue spruce (

) and noble fir (

). Some of these have limited seed production and may rely on regular planting for survival in Norway in the long term. The present method using primarily quantitative criteria represents a leap compared to the previous qualitatively based black list published in 2007. Nevertheless, ecological risk assessment is a difficult exercise due to the combination of the long time perspective of the assessments (300 years) and inadequate data on demography and spread. The uncertainty advocates careful use of the assessments. In Norway new tree species were introduced in forestry from the 1780ties onwards, and the era started with European larch. But the boost did not occur until after World War II. Accordingly, the history of introduced species in Norwegian forestry is short, and considering also the long generation time in most forest tree species, deduction of ecological effects several hundred years ahead is complicated. No method has sufficient robustness to compensate for poor data, and one way of dealing with this is inclusion of a Data Deficiency (DD) option, as in the Red list, by which the assessment can be concluded at an early stage when data are insufficient. Alternatively, the certainty of the assessments could be given a score (e.g. in three classes) depending on the data available. A complete overview of the risk assessment of trees and other vascular plants in 2012 can be found at