Spread of the Introduced Sitka Spruce (Picea sitchensis) in Coastal Norway

Per Nygaard,Bernt-Håvard Øyen

doi:10.3390/f8010024

Abstract

Positive and negative effects on ecosystem services from plantation forestry in Europe have led to conflicts regarding non-native tree species. Sitka spruce (Picea sitchensis (Bong.) Carr.) is the most common plantation species in northwest Europe, covering 1.3 Mha. In costal Norway, Sitka spruce was intentionally introduced and is currently occupying about 50,000 ha. Sitka spruce was blacklisted in Norway in 2012, mainly based on the risk for invasive spreading, but little quantitative documentation exists on spread. Here we quantify spread from plantations into abandoned heathland and pastures in thirteen sites where natural regeneration occurs. Spread distances and zero-square distributions related to the nearest edge of the parent stand were fitted by use of Weibull. The median expansion rate was 0.8 m·year−1 in north Norway and 4.4 m·year−1 in west Norway. The maximum establishment distance measured was 996 m. A peak in sapling density occurred within 50 m from the edge, and there was a general decrease in saplings with increasing distance. Conversely, increase in zero-squares percentages occurred with increasing distance. We argue that inclusion of abundance in assessing spread is necessary to define invasiveness. Based on spread models and prevailing forestry practices we recommend that the establishment of new Sitka spruce plantations within 200 m of protected areas should be avoided.

Highlights

There is considerable debate on the effects of introduced tree species on ecosystem services and landscapes [1,2,3,4]
We mapped 3461 established Sitka spruce saplings distributed over a recipient area of 206ofhectares
The establishment pattern revealed that the spread of Sitka spruce into abandoned heathland and pastures in coastal Norway (Figure 2) seems to follow the model of “stratified diffusion” [40]

Summary

Introduction

There is considerable debate on the effects of introduced tree species on ecosystem services and landscapes [1,2,3,4]. Increased afforestation during the 20th century has resulted in the establishment of non-native plantation forests in many parts of the world [5]. Invasions of tree species like Lodgepole pine (Pinus contorta Dougl.) in New Zealand and Monterey pine Don) in Australia and southern Africa have been reported [6,7,8]. Most of the non-native conifer invasions have so far been reported from the southern hemisphere, while there is limited evidence from the northern hemisphere, and in particular from Europe [9]. The introduction of non-native tree species within

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