Douglas fir (Pseudotsuga menziesii) was introduced to New Zealand in 1859 for timber, and in the last few decades has been recognised as an invasive species, particularly into grassland. However, its potential to invade native forests is still poorly understood. I investigated the invasion of Douglas fir into mountain beech (Nothofagus solandri var. cliffortioides) forest, particularly the factors limiting the spread, at Cora Lynn, near Arthur’s Pass. Adjacent to the beech forest is an 80 ha Douglas fir and Corsican pine (Pinus nigra) plantation, whose invasive potential started to raise concerns in the late 1980s. The study was divided into three parts. The first consisted of resampling plots established on the site in 1989 to count Douglas fir seedlings spreading into Nothofagus. In the second part, I investigated the factors limiting the establishment of Douglas fir seedlings in the beech forest. To do this I established 400 points in the native forest, and at each point I assessed the light environment (via hemispherical photography), measured altitude, and distance to the nearest seedling. Lastly, I conducted a root competition and fertiliser-addition experiment to investigate the factors limiting the growth of the Douglas fir seedlings. I selected 544 naturally regenerating seedlings (30 to 70 cm tall) in the beech forest, and applied one of four treatments: fertiliser addition, root trenching, fertiliser addition plus root trenching, and control. Light environment and altitude at each seedling were measured.

The mean density of seedlings in the plots has increased 13-fold since the first measurement in 1989, from 11,267 seedlings/ha to 150,333 seedlings/ha in 2016. There is a widespread Douglas fir invasion of the mountain beech forest in progress – in only a single point out of 400 did I fail to find a seedling within a 10-m radius. Altitude had the strongest effect on the distance to the nearest seedling, with lower seedling density at higher altitudes. Although distance to the nearest seedling decreased with light, the seedlings were not restricted to light-wells or canopy gaps as generally presumed, but present throughout the native forest. Light had the strongest effect on seedling growth. At the experimental seedlings, light ranged from 3.01 to 10.29 mol m-2 d-1, that is 8.12% and 27.8% respectively of full sunlight. Altitude had a negative effect on seedling growth. Nutrient availability was second to light as a growth limiting factor. Fertiliser addition had the largest effect on seedling growth across treatments, increasing it 18.3% above that of the control. Root trenching had a small negative effect on growth, while fertilizer plus trenching had a positive effect, but still smaller than expected. I have demonstrated that Douglas fir is well able to invade Nothofagus forest, albeit slowly, and that the spread was affected by a complex relationship between light, nutrients, root competition, distance to the seed source, and altitude. In New Zealand, poor control of conifer invasions into grasslands and shrublands in the past has led to large environmental and economic impacts. The potential negative effects of the Douglas fir spread into native forest could be minimized by early control. I hope that my work will contribute to a better understanding of the Douglas fir’s invasive potential, as well as draw attention to the need for managing the spread in progress.