Barbara Moser1, Christoph Bachofen1,2, Esther R. Frei1 and Thomas Wohlgemuth1
1Swiss Federal Research Institute WSL, Birmensdorf, Switzerland
2Environmental Systems Science, ETH Zürich, Switzerland
Douglas fir (Pseudotsuga menziesii) is the second most common non-native tree species in European forests and is regarded as a promising commercial alternative to spruce (Picea abies) due to its drought resistance. The expansion of Douglas fir in Central European forests is, however, critically appraised by nature conservationists, who fear negative effects on biodiversity and forest functions. In order to evaluate the inherent risk of uncontrolled spread of Douglas fir from stands with old, seed producing trees, we experimentally studied seedling heights and relative growth rates of 1–3 yr-old Douglas fir seedlings and compared their performance under different combinations of water, nutrient and light availability with that of seven native tree species: beech (Fagus sylvatica), sycamore (Acer pseudoplatanus), common oak (Quercus robur), sessile oak (Quercus petraea), silver fir (Abies alba), spruce (Picea abies), and Scots pine (Pinus sylvestris). The plants were grown in mesocosms subjected to full factorial treatments of water (P: ambient, i.e. 100%, vs. 50% precipitation), nutrients (N: 48/12/19.2 vs. 24/6/9.6 vs 12/3/4.8 mg N/P/K m-2 y-1) and light (L: 80% vs. 60% sunlight).
In the first year of establishment, height growth of Douglas fir seedlings was larger than that of native conifers and lower than all broadleaved species independent of light, nutrient and water supply (Fig. 1). By the end of the third growing season, Douglas fir was still universally taller than silver fir and spruce (except under conditions of P100 x Nlow x L60), and smaller than sycamore and beech. Surprisingly, Douglas fir seedlings were gaining on common oak throughout the experiment and finally equalled their size under conditions of low nutrients combined with low precipitation and high light availability. At the same time, it outcompeted Scots pine under conditions of low nutrients as well as sessile oak under low nutrients combined with low precipitation. Results on relative growth rates in terms of above ground biomass will follow. Our results suggest that the competitive ability of Douglas fir and the concomitant risk of uncontrolled invasion depends on environmental conditions.