Recruitment curves of three non-native conifers in European temperate forests: implications for invasions

Few conifers are considered invasive in Europe, yet recent studies indicate that several species used for forestry display abundant regeneration and spread into surrounding natural habitats. Three species were identified as being particularly at risk in Belgium, but data is lacking regarding their dispersal. We characterized the recruitment curves of Tsuga heterophylla, Abies grandis and Thuja plicata. Isolated plantations were monitored and realized dispersal (i.e. seedlings and recruited regeneration) was recorded and measured over 750 m in different directions. We calculated the wave expansion rate and frontier expansion rate for each planting site and fitted dispersal kernels for each site and species. Regeneration was classified in three size categories (seedlings, saplings and trees above 1.5 m), and the recruitment distances were analyzed for each size class. The effect of the forest type (deciduous, coniferous, open or mixed) on the density of regeneration was also investigated with regression models. The recruitment curves varied greatly across sites, showing heterogeneous habitat suitability and uneven post-germination processes. Considering the frontier expansion rate, the three conifers appear to spread beyond documented threshold rate of invasiveness. Regeneration density was higher in coniferous forest type, as well as open areas for Tsuga heterophylla. An escape effect was noticed as mean and maximal dispersal distances of saplings and taller trees were greater than those of seedlings. Our study indicates that Tsuga heterophylla displays the highest risk of rapid spread into adjacent natural habitats, followed by Abies grandis. Thuja plicata faces more recruitment limitations.