How are monospecific stands of invasive trees formed? Spatio-temporal evidence from Douglas fir invasions

Invasive plant species can have many, often detrimental, impacts on native plant communities, especially when they reach high densities and form monospecific stands. Monospecific stands are common among invasive plant species and have been documented in many different ecosystems, including woodlands and grasslands. Once an invasive species has formed a monodominant stand, it becomes extremely difficult to restore the area. Despite the impact they have on native ecosystems, very little is known about how these stands are formed.

A recent study by Nuñez & Paritsis published in AoBP aimed to investigate the processes involved in the formation of non-native tree stands. The authors proposed three pathways for formation; gradually via vegetative growth or short-distance seed dispersal, synchronous establishment owing to adequate growing conditions across a large area, or a combination of these two with pulses of synchronous establishment over time. The study documented how monospecific stands of non-native Douglas fir (Pseudotsuga menziesii) were formed in Patagonia, Argentina. The authors collected data on tree density, tree age, tree height and other related measurements for this tree species along transects from the original seed source (an 80-year-old plantation). They found that the monospecific stands arose in a more complex way than expected. Whilst on average establishment occurred at roughly the same time at all distances from the seed source, there was large variation in tree age at all distances. In other words, stand establishment occurred across a long time period but the establishment pattern did not vary spatially. Tree density was much greater closer to the original plantation, decreasing with distance from this source. To return to the original hypothesized pathways of establishment, it is most likely that formation of this stand occurred through pulses of synchronous establishment over time. Different factors could account for the observed pattern of tree establishment, including seed dispersal, mycorrhizal facilitation and herbivory. The results of this study elucidate the mechanisms of single species stand formation and may help to inform future management strategies.