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

Date: 2012.10.20

In June 2012 the Norwegian Biodiversity Information Centre launched the Black list which is a risk assessment of introduced species. We will here briefly review the method used in the assessment and some experiences with risk assessments of introduced forest trees. The article is based on a report financed by the Nordic Forest Research Cooperation Committee (SNS).

Sitka spruce is the most widely used introduced tree species in Norway, covering about 500 km2 in coastal areas. Photo: Tor Myking

Introduction
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.

The method
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).

Fig. 1. Ecological risk is the product of spread and ecological effect. Only species assigned to category A and B are included on the black list.

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.

Invasion potential/ spread (x-axis)
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.

Ecological effect (y-axis)
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.

Results and discussion
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).

Table 1. Distribution of the individual species in the different risk categories. None of the present species were assigned to the No known risk category. The numbers in parenthesis represent the position on the x- and y-axis (Fig. 1), respectively.

Very high risk (n=6)	High risk (n=3)	Potentially high risk (n=5)	Low risk (n=15)
Acer pseudoplatanus (4,3) Larix decidua (4,3) Picea sitchensis (4,3) Pinus mugo (4,3) Pinus strobus (3,4) * Tsuga heterophylla (4,3)	Abies alba (4,2) Picea glauca (4,2) Pinus peuce (2,4) *	Abies concolor (4,1) Abies grandis (4,1) Abies sibirica (4,1) Pinus cembra (4,1) Pinus contorta (4,1)	Abies balsamea (2,1) Abies lasiocarpa) (3,1) Abies procera (3,1) Larix kaempferi (3,1) Larix x maschlincii (3,1) Larix sibirica (3,1) Picea x lutzii (3,2) Picea engelmannii (3,2) Picea pungens (3,1) Pinus mugo uncinata (3,1) Pinus nigra (3,1)

*High score on the effect axis is due to being intermediate host for Cronartium ribicola (criterion 6, Tab. 2).

Previous outlying pasture being naturally forested by birch and juniper. Would the effects have been different with European larch (Larix decidua)? Photo: Tor Myking

Colonisation of threatened nature types was crucial for black listing several species, such as sitka spruce (Picea sitchensis), European larch (Larix decidua) and subalpine pine (Pinus mugo). 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 (Tsuga heterphylla, 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 (Abies alba), 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 (Pinus strobus) and Macedonian pine (Pinus peuce) refers to the fact that they may act as hosts for a harmful fungus (Cronartium ribicola), that attacks red and black currant.

Hemlock (Tsuga heterphylla) spreads effectively and may displace other species also in closed forests. Photo: Tor Myking

Among the remaining species there are several of which have good dispersal, but limited ecological effect (e.g. logde pole pine Pinus contorta and Swiss pine Pinus cembra), 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 (Abies balsamea), blue spruce (Picea pungens) and noble fir (Abies procera). 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 http://databank.artsdatabanken.no/FremmedArt2012?SpeciesGroup=karplanter