Identifying emerging invasive species is a priority to implement early preventive and control actions. In terms of the number of invasive tree species, forestry represents the second largest pathway of introduction, with an invasive debt likely existing for alien conifers in Europe. In the early 1900s, a network of arboreta was established in southern Belgium to assess the wood production potential of prospective conifer and broadleaved species. Here, we use eight arboreta as natural experiments to identify alien conifers presenting invasive behavior. Through systematic sampling, we quantified the natural regeneration of alien conifers and recorded local environmental variables. For each species, regeneration density, dispersal distances, and age structure were analyzed. Generalized mixed effects models were fitted to test the effect of planted area and tree-stand type on regeneration. The environmental space occupied by regenerating alien conifers was evaluated using principal component analysis. Out of 31 planted alien species, 15 (48%) were identified in natural regeneration, of which eight (26%) exhibited important regeneration density and dispersal distances. The most invasive species were Tsuga heterophylla and Abies grandis, confirming earlier field observations. Both large planted areas and areas planted with alien conifer species increased the density of regeneration. Species that had the highest regeneration density tolerated a wide range of environmental conditions, including shaded understory, which could lead to the invasion of mature, undisturbed forests. This study showed that 17% of the studied alien conifers are potentially invasive because they show important regeneration, long-distance dispersal, and, of importance, have already produced offspring that have matured and are capable of creating new satellite populations. In conclusion, our results provide a guideline for future planting operations, recommending extreme caution when planting these species in the temperate forests of Western Europe.

Arboretum, dispersal, gymnosperm, invasiveness, non native trees, propagule pressure, regeneration

Early identification of emerging invasive species remains one of the most challenging issues in invasion science. Following numerous introductions worldwide for ornamental or production purposes, many tree species have since been recognized as invasive (Dodet and Collet 2012; Richardson et al. 2014). Rejmánek (2014) identified 76 tree species exhibiting invasive behavior in Europe. Alien woody species have the capacity to modify the structure of invaded ecosystems substantially and cause extensive ecological and economical damage (Lamarque et al. 2011; Pyšek 2016). In addition, management actions are often taken too late, when the species are already widespread and when the management costs of mitigation are prohibitive (Rejmánek and Pitcairn 2002; van Wilgen and Richardson 2014).

Most problematic tree species in Europe were introduced decades or centuries ago (Nyssen et al. 2016) and actively spread by human. In parallel, some dispersed outside their cultivation areas and spread via small satellite populations over kilometers (Mack 2005; Pyšek and Richardson 2012). Delays between the installation of these satellite populations and their capacity to reproduce create an important lag phase between the introduction of a species in a new area and its invasion of natural habitats (Wangen and Webster 2006). In Germany, this lag phase has been estimated to last 170 years on average for trees (Kowarik 1995). Future invasive species might, therefore, already have been introduced but might not have completed the naturalization–invasion continuum, yet. This time-delayed invasion is referred to as the invasion debt (Rouget et al. 2016). Because introduction events increased during the second half of the 20^th century, an invasion debt, without doubt, exists in Europe for trees (Essl et al. 2011) and must be evaluated to anticipate new invasions (Richardson and Rejmánek 2011).

When the number of introduction events increases, so does the probability of naturalization (Heger 2016). Along with the propagule pressure, several functional traits can help predict the invasiveness of plant species such as an important SLA, growth rate, height, germination rate and fitness (van Kleunen et al. 2010; Lamarque et al. 2011; Kutlvašr et al. 2019). For conifers specifically, Richardson and Rejmánek (2004) identified a small seed mass, short juvenile period and short intervals between large crops as traits associated with invasiveness.

The forestry sector has been introducing alien tree species for centuries in Europe for timber production, including many conifers from Asia and north America (Krumm and Vítková 2016; Gil-Moreno 2018). Species selected for forest plantations often originate from regions with a similar climate and present high growth rates, two factors contributing to enhanced invasiveness (Richardson and Rejmánek 2004). Introduced species are also grown in large-scale plantations using cultivation techniques that enhance survival rates, which lowers the probability of local extinctions and creates a massive propagule pressure, increasing the probability of them escaping from cultivation (Mack 2005; Křivánek et al. 2006; Pyšek et al. 2014). For example, in the Czech Republic, 25% of tree species introduced for forestry have become invasive (Pyšek 2016). Most invasive trees in Europe are light-demanding and have better invasion success in disturbed habitats (Richardson and Rejmánek 2004; Meloni et al. 2016). However, shade-tolerant species also exhibit invasive behavior once introduced to mature forests with low disturbance. For instance, Prunus serotina invaded the undergrowth of forests in western Europe (Hernandez et al. 2016).

Conifers in particular have been introduced to many areas and were widely planted for timber production, providing substantial opportunity for invasion (Richardson and Rejmánek 2004; Broncano et al. 2005). Globally, 36 species of conifers are already considered invasive (Richardson and Rejmánek 2004) with alien conifers used in commercial forestry having a significantly higher probability of escaping cultivation compared to species not used for timber production (Essl et al. 2010). Conifers are still not often perceived as problematic in Europe because their invasions have been primarily documented in the southern hemisphere so far. Yet, studies indicate that alien conifers are showing invasive behaviors in Europe, too (Carrillo-Gavilán and Vilà 2010; Essl et al. 2010). If the planting of alien species in European forests continues, which is likely to happen with the dieback of several native species, it is necessary to distinguish high risk species from those that are unlikely to become invasive (Dodet and Collet 2012; Heger 2016).

Forest trials and arboreta offer the opportunity to monitor the regeneration dynamic of exotic species, acting as sentinel sites of which careful observations could facilitate the detection of new invasions (Carrillo-Gavilán and Vilà 2010). These areas are also likely to act as sources of propagules and sites of entry for new invasions (Richardson and Rejmánek 2004; Brundu and Richardson 2016). During the 19^th century, the Belgian Forest Department started to diversify forests plantations with exotic species. A network of 23 forest arboreta was set up between 1890 and 1914 throughout the country to monitor the growth and wood production potential of both native and alien species, especially ones from Japan and the west coast of North America (Nyssen et al. 2016). The arboreta were implemented in various ecological regions to cover the diversity of climates and soil types of the country. Every arboretum differed in its setup and list of species. Unfortunately, the geographical origin of the seeds remains unknown. Recently, a new interest in these arboreta emerged and new inventories were performed in 2016 to study the production potential of exotic species in the light of climate change (Lhoir and Scholzen 2017). In several of these arboreta, no management actions of the understory were implemented over the last 15 years except for clearing the pathways. The natural regeneration is therefore mostly untouched.

In this study, we aimed to identify alien conifer species presenting invasive potential. To do so, we systematically quantified the natural regeneration of alien species in and around eight selected arboreta. Richardson et al. (2000) defined invasive plant species as species producing reproductive offspring in very large numbers and at considerable distances from parent plants. By combining information on tree density, realized dispersal, and the size structure of the natural regeneration of alien conifers, we assessed their invasive potential. Specifically, we evaluated i) the density of natural regeneration and the realized dispersal distances from nearest parent trees; ii) whether a diverse size structure exists in the natural regeneration of species that regenerated; and iii) the influence of tree-stand type and environmental conditions on the regeneration density of species of highest concern. The correlation between the regeneration density and traits linked to invasiveness in previous studies was also assessed.

In total, 1109 plots were surveyed and 4148 individuals recorded, from small seedlings to mature trees over 60 cm of DBH. Due to the size of the sampling plots, we never found more than one non-planted tree with a DBH > 20 cm in one plot. These individuals belonged to 31 alien conifer species planted between 1898 and 1916 in eight arboreta across the Walloon Region (Table 1). For 15 of the planted species, no regeneration was detected, while six species had less than 10 individuals recorded across all sites. In contrast, some species presented abundant regeneration. The most frequent seedlings encountered were Tsuga heterophylla and Abies grandis. The planted area, time since planting and distance to the nearest parent trees significantly affected the density of regeneration (Table 2). We further used the Weighted Regeneration Density (WRD) for between-species comparison.

Tsuga heterophylla was the most represented alien conifer in natural regeneration with a WRD of 2794.0 indiv.ha^-1.ha_planted^-1. This species was followed by Abies grandis (WRD = 1493.8 indiv.ha^-1.ha_planted^-1), Abies nordmanniana (688.3 indiv.ha^-1.ha_planted^-1) and Thuja plicata (637.8 indiv.ha^-1.ha_planted^-1).

Ten species had at least 10 seedlings recorded in the natural regeneration. They tended to be found close to parent trees (Fig. 2A). However, the seedlings of nine species were sometimes detected at >100 m distance from possible parent trees. Four species had a 95^th percentile for dispersal distance distribution exceeding 100 m. Only Abies nordmanniana displayed very low dispersal distances, with all recorded individuals occurring within 6 m of planted parent trees. The maximal DD exceeded 200 m for the seedlings of Tsuga heterophylla, Pseudotsuga menziesii, and Thuja plicata, and even 300 m for Abies grandis.

For the same 10 species with 10 recorded individuals, size structure was used to investigate the survival of the regeneration. Ninety-three percent of recorded trees in natural regeneration were <1.3 m high. All individuals of P. sitchensis and P. strobus were seedlings <0.3 m high (Table 3). However, older trees with a DBH >20 cm were detected for C. lawsoniana, P. menziesii, T. plicata, L. kaempferi, and T. heterophylla.

Though conifers and broadleaved stands were almost equally represented in the plot data, alien conifers mainly regenerated under coniferous stands. Specifically, 69% of individuals were found under exotic conifers, 18 % under European conifers, 7% in clear-cut areas, and only 6% under broadleaved species. Open areas and exotic coniferous stands significantly increased the regeneration count of alien conifers (Table 2).

From the principal component analysis (Fig. 3), two environmental gradients were identified and were regulated by soil pH (first Principal Component, PC1) and canopy openness (PC2). Wet soil tended to be more acidic. Litter was thicker on acidic plots with low light availability. Along these gradients, we projected the presence of six species showing a combination of important WRD (>100 indiv.ha^-1.ha_planted^-1), high dispersal distance (perc. 95 > 50 m), and developed size structure (individuals >10 cm DBH): Tsuga heterophylla, Abies grandis, Thuja plicata, Chamaecyparis lawsoniana, Larix kaempferi, and Pseudotsuga menziesii. We found that these species occupied a wide range of environmental conditions of the arboreta, including areas with low canopy openness. Ecological niches and optimums of presence were rather similar for the six species. Abies grandis also occurred on dry and basic soils.

Figure 3.

Distribution of alien conifers in the environmental space. Regeneration of six conifers in the environmental space made by the two first axes of the PCA. The circle of correlation of four environmental variables was projected on the graph: pH, litter thickness, canopy openness (referred to as “Light”), and soil drainage class (referred to as “Humidity”). The percentage of explained variance for each Principal Component is indicated. Dots represent all plots of the eight arboreta. Black dots are those in which at least one of the six species is regenerating. Density lines are drawn for each species along the two axes of the PCA.

Kendall’s correlation highlighted a significant positive relationship between the height of species and their WRD (tau=0.459, z=3.096, p value = 0.002). On the other hand, the correlation was not significant for the seed mass (tau = -0.064, z=-0.411, p value = 0.681).

This study demonstrated that alien conifers naturally regenerated in each arboretum that was visited, sometimes in dense patches of seedlings. Of the 31 alien species considered, 16 were detected regenerating. Eleven species (35%) had a Weighted Regeneration Density of more than 100 indiv.ha^-1.ha_planted^-1. The planted area and the time since plantation both had a positive significant effect on the count of regeneration, confirming the important influence of the propagule pressure on the regeneration of alien species (Lockwood et al. 2009; Pyšek et al. 2009).

Most species primarily regenerated close to parent trees. Long dispersal events of over 100 m were detected for nine species. For the prolific species Tsuga heterophylla, five percent of regeneration occurred past 124 m, and some even reached 300 m one century after planting. Thus, long-distance dispersal events are frequent for this species. The 95^th percentile of dispersal distance also exceeded 100 m for P. strobus, C. lawsoniana, and Abies veitchii. However, the prospected area was limited, with even longer distances from the closest parent trees being possible. Our estimates of long-dispersal distances can therefore be considered conservative. Given the importance of long-distance dispersal events in the invasion process, more exhaustive inventories of the dispersal potential of these species along transects are required until no individual is found for a given distance lapse (Higgins and Richardson 1999).

The weighted regeneration density and the dispersal distance are useful tools for monitoring the invasive behavior of alien conifers. However, as invasive species must maintain viable populations, the age structure of natural regeneration must be incorporated (Wilson et al. 2014). For A. nordmanniana, the high number of individuals was attributed to a single large germination event resulting in hundreds of seedlings of less than one-year-old being detected in one plot; 142 out of the 145 individuals recorded were young seedlings beneath a parent tree, indicating that most regeneration is not viable. Pinus strobus and P. sitchensis seedlings were recorded at further distances, but only seedlings smaller than 0.3 m were found. In comparison, T. heterophylla, P. menziesii, A. grandis, C. lawsoniana, L. kaempferi, and T. plicata also tended to exhibit large germination events beneath parent trees but older trees were also recorded (see Table 3), including mature ones. Thus, these species likely have the capacity to create new satellite populations.

The question of whether some species cross the benchmark of 100 m dispersal distance over 50 years was evaluated in this study. Richardson et al.(2000) stated that, for a species to be invasive, there must be “clear evidence that it regenerated naturally and recruited seedlings more than 100 m from parent plants”. This distance is associated with a time-lapse of 50 years since introduction, and reproductive offspring must be found beyond 100 m. These events involve the tail of the dispersal curve, as only a small number of long-distance dispersal events producing reproductive offspring is necessary to create a new population at a far distance. Individuals of A. grandis, T. heterophylla, T. plicata, C. lawsoniana, P. sitchensis, and A. veitchii occurred over a DD50 of 100 m. If no mature individuals were measured, individuals taller than 1.3 m were recorded for A. grandis and T. heterophylla over the specified distance. It means they survived the most vulnerable seedling stage, and could potentially grow to sexual maturity.

Six species exhibited high invasive potential based on the three studied factors: T. heterophylla, A. grandis, T. plicata, C. lawsoniana, L. kaempferi, and P. menziesii. They were selected for the environmental analysis. Once projected on the PCA, these six species occupied a large proportion of the environmental space encountered at the surveyed sites, and displayed generalist behavior across common environmental conditions. Of note, T. heterophylla preferentially regenerated on acidic soils, supporting existing knowledge on the ecological preferences of this species (Rooney et al. 2000). In comparison, A. grandis tolerated drier soils, which might be beneficial under climate change as water stress is likely to become more frequent in the near future (Campioli et al. 2009). An important regeneration was recorded in open areas resulting from clear cuts. Interestingly, these six species also exhibited shade tolerance during regeneration with many individuals occurring in plots with low canopy openness, allowing them to spread in closed forest ecosystems. Martin and Marks (2006) demonstrated that invasions of undisturbed forests by shade-tolerant alien species frequently occur but require a longer time span than invasions in disturbed habitats, resulting in their invasiveness often being underestimated. The combination of a generalist behavior across soil types and shade-tolerance could allow these alien conifers to invade mature, undisturbed forests. This phenomenon is likely to be facilitated by areas planted with conifers. Consequently, because of the capacity of conifers to transform habitat, increasing the proportion of coniferous stands in wood production forests might accelerate the invasion rate of alien conifers (Jagodziński et al. 2015). In 2011, 48 % of the southern Belgium forest was planted with conifers (Lecomte 2017). Pseudotsuga menziesii represented 6 % of the surface inventoried by the Belgian permanent forest inventory in 2011, far more than the other species highlighted in our study, and its proportion has increased by 52% since 2001. Larix sp., A. grandis, T. plicata, T. heterophylla and C. lawsoniana together represent a marginal section inventoried surface in 2011, and it is difficult to know the real extent of these species in public and private forests (Bauwens 2020). However, trials with alien species are becoming more frequent (Richardson et al. 2014), and exotic conifers are more and more considered as replacement species to compensate for the die-back of native productive species. For example, C. lawsoniana and T. plicata are selected for the REINFFORCE arboreta network, aiming to collect data on the growth of alien species in view of the diversification of European Atlantic temperate forests in light of climate change (Orazio et al. 2013).

Our sampling covered a large diversity of environmental conditions met in southern Belgium forests, from calcareous to acidic soils, from forests dominated by native broadleaves to spruce plantations. These species can potentially invade a large proportion of forest lands, especially productive lands planted with conifers and managed with clear-cut regimes. However, this study did not cover the full diversity of temperate forests in Western Europe, with wider gradients potentially generating greater differences in the environmental space occupied by each species.

These six highlighted species also exhibit invasive behavior in other European countries (Rejmánek and Richardson 2003; Richardson and Rejmánek 2004; Broncano et al. 2005; Orellana and Raffaele 2010; Forestry Commission Scotland 2015). In western Norway, T. heterophylla is considered to be a very invasive conifer due to its high potential for spreading into neighboring stands and clear cuts (Oyen 2001). Plantations of T. heterophylla generate intense shade with few plants being able to live beneath them (Harmer et al. 2011). Galoux (1951) demonstrated the high regeneration capacity of T. heterophylla in Belgian arboreta, mentioning dense regeneration patches that occur beneath seed-bearers and in the neighboring plantations. The same author also stressed the abundant seed production and regeneration potential of C. lawsoniana, P. menziesii, and A. grandis. As the report was written in the middle of the 20^th century, we know that the natural regeneration of these species has been ongoing for at least 70 years in the arboreta.

A small seed mass and an important maximal height have been linked to a better invasion success of plants in previous studies (Richardson and Rejmánek 2004, van Kleunen et al. 2010; Kutlvašr et al. 2019). Both traits are linked with the capacity of species to spread at long distances. We did find a positive correlation between the maximal height and the Weighted Regeneration Density, but not with the seed mass. Dawson et al. (2011) surveyed exotic plants escaping from a tropical botanical garden. They concluded that propagule pressure was of greater significance than the functional traits in the establishment of alien plants in natural habitats. In a study conducted in North America, Pyšek et al. (2015) concluded that the importance of biological traits is highly dependent on the invasion stage, and often over-estimated. Further investigation on the role of functional traits on the invasiveness should be led in the local conditions of the Belgian arboreta, including the relative growth rate and specific leaf area, to test whether similar conclusions can be drawn.

The species exhibiting an important invasive potential in our study could be part of the invasion debt sensu Rouget et al. (2016) in Belgian forests. The important lag phase might be misleading concerning the potential impact of alien conifers, especially T. heterophylla. Twenty percent of the studied species exhibited invasive tendencies and they will certainly continue to expand in the future, especially if planting effort increases. In comparison, we did not detect any regeneration for half of the studied species. If foresters want to diversify forest plantations, they should avoid introducing species with high invasiveness and prefer native species or low-risk alien species (Brundu et al. 2020).

We identified species that were likely to become invasive based on small forest trials. The effect of mass plantings was not addressed. However, we demonstrated that the size of planted areas positively impacts regeneration density. Previous studies showed that propagule pressure has the potential to overwhelm ecological resistance of ecosystems to invasions (Von Holle and Simberloff 2005). Even species considered to be dispersal-limited but with strong potential for wood production might cross a propagule pressure threshold and become invasive in the future due to high planting intensity (Richardson et al. 2004; Jagodziński et al. 2018).

Ennos et al. (2018) demonstrated that using non-native species for wood production and the diversification of forests presents great ecological and economic risks, potentially to the detriment of native tree species and associated biodiversity. Based on experience in countries with longer histories of using alien conifers, along with objectives to prevent further ecological damage, risk analyses of introduced alien conifers must be performed by monitoring old forest trials and arboreta (Richardson and Rejmánek 2004).

Given the observed natural regeneration and dispersal of alien conifers in the old forest arboreta of southern Belgium, we recommend exercising caution when planting them in western temperate Europe. Half of the studied species regenerated, with almost 20% of these exhibiting an invasive behavior. Species showing the highest risk of being invasive were T. heterophylla and A. grandis, and to a lesser extent C. lawsoniana, T. plicata, L. kaempferi, and P. menziesii. Species with more limited dispersal capacities or a lesser proportion of mature trees, such as A. nordmanniana, P. strobus, P. sitchensis, and A. veitchii, could become of concern if planted at large scales. The results show that forest arboreta act as entry points for invasive species, especially now that more forestry trials are being set up to compensate for the die-off of native productive species. Thorough monitoring of alien conifers introduced for wood production is therefore needed to take early action for control and avoidance of larger introductions.

Maps of the arboreta were provided by the “Arboreta” project run by the Earth and Life Institute of UCLouvain (Scholzen and Lhoir 2018). We thank Moran Durtschi-Guillemot for providing field assistance. We would also like to thank the “Division de la Nature et des Forêts”, more precisely the Forest Sections of Profondeville, Viroinval, Wellin, Saint-Michel, Spa, Seraing, Virton and Neufchâteau.