Le potentiel invasif d'arbres exotiques introduits en forêts tempérées européennes: vers un syndrome d'invasion forestier ?

Many exotic tree species have been introduced worldwide for ornamental or wood production purposes, and some have become invasive in the introduction area, representing a substantial threat to native biodiversity. Forestry is a major pathway of introduction of non-native trees (NNT), and this trend is increasing in Europe due to current afforestation programs promoting the use of NNTs to diversify and improve resilience of planted forests in the face of climate change. However, species selected for wood production are often fast-growing species massively planted on suitable sites, with silvicultural practices enhancing survival rates. These factors increase the probability of these species escaping from cultivation and invading natural habitats. We can therefore assume that an invasion debt exists in Europe regarding NNTs, concealed by the long lag-phase needed by trees to go through the introduction-invasion continuum. Time since introduction and propagule pressure have been proven to increase the probability of naturalization of an NNT. Functional traits and dispersal capacities play a role in later stages of the invasion process. The process of invasion by NNTs has been extensively studied in the Southern hemisphere and with pioneer, light-demanding species such as pines, eucalyptus or acacias, yet there is a lack of understanding of the invasion process of NNTs in dense temperate forests. If these ecosystems have long been considered more resistant to invasions, there is an increasing number of records of shade-tolerant species invading native natural or semi-natural forests. It is therefore crucial to unveil the mechanisms underlying forest invasion by NNTs and to identify potentially invasive species before they become widely planted. I used a network of eight old forest arboreta in Southern Belgium as sentinel sites to detect potentially invasive tree species. A systematic monitoring of the sites allowed me to gather data on the density, distance and size structure of the natural regeneration of NNTs. Abiotic characteristics of the habitat were also measured. Several NNTs displayed an abundant natural regeneration in the arboreta, which was further enhanced by planting intensity. Some of these species were already known to be invasive, such as Quercus rubra, Prunus serotina and Robinia pseudoacacia. Maple species were also found in dense regeneration patches, especially Acer rufinerve, which is already listed as invasive in Belgium. Most importantly, almost 20% of the frequently planted conifers displayed important regeneration and dispersal potential, and tolerated a wide range of environmental conditions, including shaded understorey, which could lead to the invasion of mature forests. Tsuga heterophylla was particularly prolific, and created dense, impenetrable stands. These maple and conifer species could be part of the invasion debt threatening European forests. To further investigate the dispersal potential of exotic conifers, the realized dispersal of Tsuga heterophylla, Abies grandis and Thuja plicata was quantified from isolated forest trials. The monitoring of recruitment curves of three of these conifer species confirmed the high invasive potential of Tsuga heterophylla and Abies grandis if planted in favorable sites, especially under coniferous cover. However, Thuja plicata encountered more dispersal and regeneration limitations. A strategy of fast resource acquisition through high relative growth rate (RGR) and specific leaf area (SLA) has been highlighted in numerous studies comparing native to invasive species, or non-invasive to invasive species. However, this hypothesis has almost always been tested on light-demanding species. The relationship between seedlings developmental traits and invasiveness was tested for the two groups of emerging invasive trees identified in the old arboreta, i.e. Acer and conifer species. To allow a finer analysis, invasiveness was quantified on continuous gradient instead of the usual non-invasive/invasive dichotomy. Global invasiveness was calculated based on proxies extracted from the GBIF Database (number of regions and countries invaded) and the Global Compendium of Weeds (number of citations and risk score). Local invasiveness combined values of regeneration densities and dispersal distances measured in the eight forest arboreta. For maples, invasiveness was positively correlated to growth rates in biomass and height, SLA and number of leaves. For conifers, more invasive species displayed faster height increment, presumably in a strategy of fast light acquisition. A strategy of fast resource acquisition is therefore a key component of the invasion process, even in shaded forest ecosystems. The results of this PhD indicate that temperate forests are not immune to invasion by non-native trees, and that several tree species might still be in a lag-phase preceding invasion. Combining the results from the monitoring of forest arboreta and the growth experiments, a “forest invasion syndrome” emerges, combining shade-tolerance, high growth rate enhancing competitiveness in canopy gaps and long-distance dispersal ability. High planting intensity and enhanced habitat invasibility through frequent disturbance or silvicultural practices (e.g. abundant coniferous cover) may amplify this syndrome. This is consistent with recent studies indicating invasive tree species in forest adopt a “sit-and-wait” strategy, combining shade tolerance and high response to light after disturbances allowing them to outcompete native trees. Non-native trees, especially those alien to continental Europe, should not be promoted in afforestation program without a thorough risk assessment. Old forest trials and arboreta can act as sentinel sites and provide useful information for a wiser species selection and smarter management practices of temperate forests.15. Life on lan

Impact de la pollution lumineuse sur la biodiversité

" Chauves-souris, chouettes, crapauds, lucioles… Pour ces animaux et bien d’autres, la nuit n’est pas un temps de repos, mais au contraire, une période d’activité privilégiée. Des milliers d’années d’évolution leur ont conféré une parfaite adaptation à l’obscurité dont ils tirent avantage. Mais voilà que depuis quelques décennies, la lune et les étoiles ne sont plus les seules sources de lumière nocturne. Les bords de routes, les bâtiments et même les falaises sont éclairés tandis que les halos des villes illuminent le ciel. Naturalistes et astronomes tirent la sonnette d’alarme. Quelles sont les conséquences de cette profusion d’éclairage sur la faune nocturne et sur le fonctionnement des écosystèmes en général ? Que faire pour permettre aux animaux de se déplacer malgré ces innombrables barrières de lumière ? Alors que l’emprise de l’Homme sur son environnement n’a jamais été aussi forte, ne serait-il pas temps de rendre sa place à la nuit ? "3. Good health and well-being11. Sustainable cities and communities13. Climate action14. Life below water15. Life on lan

Impact de la pollution lumineuse sur la biodiversité

" Chauves-souris, chouettes, crapauds, lucioles… Pour ces animaux et bien d’autres, la nuit n’est pas un temps de repos, mais au contraire, une période d’activité privilégiée. Des milliers d’années d’évolution leur ont conféré une parfaite adaptation à l’obscurité dont ils tirent avantage. Mais voilà que depuis quelques décennies, la lune et les étoiles ne sont plus les seules sources de lumière nocturne. Les bords de routes, les bâtiments et même les falaises sont éclairés tandis que les halos des villes illuminent le ciel. Naturalistes et astronomes tirent la sonnette d’alarme. Quelles sont les conséquences de cette profusion d’éclairage sur la faune nocturne et sur le fonctionnement des écosystèmes en général ? Que faire pour permettre aux animaux de se déplacer malgré ces innombrables barrières de lumière ? Alors que l’emprise de l’Homme sur son environnement n’a jamais été aussi forte, ne serait-il pas temps de rendre sa place à la nuit ? "3. Good health and well-being11. Sustainable cities and communities13. Climate action14. Life below water15. Life on lan

Les traits fonctionnels de performance sont positivement corrélés au potentiel invasif global d'arbres exotiques : deux études sur les érables et conifères.

editorial reviewedDetermining the traits correlated with invasiveness in order to identify potentially invasive species remains a priority in invasion ecology. In trees, an important juvenile relative growth rate (RGR) and specific leaf area (SLA) under optimal conditions have been identified as performance traits promoting invasiveness. We tested the correlation between performance traits and a continuous quantification of invasiveness for two forest species groups: maples (Aceraceae) and conifers (Pinaceae and Cupressaceae). Eight maple species were grown indoor under non-limiting conditions for 8 weeks. Fifteen exotic coniferous species were grown for 10 weeks outdoor, under a shade sail, in southern Belgium. At several harvest times, SLA was measured and seedlings were dried to weigh the biomass and determine RGR. The global invasiveness was quantified using a combination of the number of regions and countries invaded in GBIF, and the number of citations and risk score in the Global Compendium of Weeds. For conifers, the local invasiveness in Belgian forests was also quantified as the density of regeneration observed in old arboreta. Phylogenetic effects were tested. For maples, as expected, we found a positive correlation between invasiveness and RGR, SLA, relative leaf production rate and height. For conifers however, RGR was not correlated to global and local invasiveness. Height of the seedlings was the only trait positively associated with global invasiveness, and species with higher SLA, height and number of leaves also had a higher local invasiveness in Belgium. We conclude that invasive maple species, mostly post-pioneer species, could benefit from an acquisitive strategy as already shown for other tree species. For conifers, the combination of traits promoting invasiveness might differ. Invasive conifers in forests might adopt a “sit-and-wait” strategy at the seedling stage

Fast height growth is key to non-native conifers invasiveness in temperate forests

peer reviewedMany exotic conifer species have been introduced for wood production in temperate regions. Some of these species can display an invasive behavior and negatively impact native ecosystems. It is therefore crucial to identify potentially invasive species before they are widely planted. Seedling high relative growth rate (RGR) and specific leaf area (SLA) have been associated with enhanced invasiveness of trees in previous studies. However, it has been mainly demonstrated for light-demanding species in disturbed habitats, less for shade-tolerant species in closed forests. Here, we investigated the link between seedlings growth traits of 15 exotic conifer species and invasiveness considered at both global and local scale. Seedlings were grown outdoor, under a shade net, and harvested after 4 and 10 weeks. RGR, SLA, Shoot–Root ratio, shoot relative growth rate, relative height growth rate (RHGR) and relative needles production rate were measured. We developed a continuous approach to position each species along a gradient of invasiveness. Local invasiveness consists of a value based on regeneration densities and dispersal distances observed in forest arboreta in Southern Belgium, and is therefore related to the ability of species to invade closed forest ecosystems. Global invasiveness was calculated based on the GBIF Database and the Global Compendium of Weeds, and encompasses all potentially invaded habitats. It appeared that RHGR was positively related to both local and global invasiveness, while SLA was positively related to local invasiveness only. However, RGR was not significantly related with local nor global invasiveness. This study indicates that preferential investment in rapid vertical growth associated with fast resource acquisition is a strategy enhancing invasiveness of non-native conifers, also in closed, shaded temperate forests

Arboreta reveal the invasive potential of several conifer species in the temperate forests of western Europe

peer reviewedIdentifying 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. ---- Identifier les espèces invasives émergentes est prioritaire afin de pouvoir mettre des actions de contrôle en place. En termes de nombre d'espèces d'arbres invasifs, la sylviculture est la deuxième voie d'introduction la plus importante, et une dette d'invasion existe probablement pour les conifères en Europe. Au début des années 1900, un réseau d'arboreta forestier fut établi en Belgique afin d'étudier le potentiel de production de diverses espèces résineuses et feuillues. Dans cette étude, nous avons utilisé huit arboreta pour identifier les espèces de conifères exotiques présentant un caractère invasif. A travers un échantillonnage systématique, nous avons quantifié la régénération naturelle de conifères exotiques et mesuré des données environnementales locales. Pour chaque espèce, la densité de régénération, les distances de dispersion et la structure d'âge furent analysées. Des modèles généralisés à effets mixtes furent utilisés pour tester l'effet de la surface de plantation et du type de peuplement sur la densité de régénération. L'espace environnemental de chaque espèce fut également délimité à l'aide d'une PCA. Sur les 31 espèces de conifères fréquement plantées, 15 (48 %) ont été identifiées dans la régénération naturelle, et huit (26 %) se régénèrent et se dispersent abondamment. Les espèces les plus invasives sont Tsuga heterophylla et Abies grandis, confirmant des observations de terrain, suivis de Thuja plicata et Pseudotsuga menziesii. De larges plantations et des peuplements résineux augmentent tout deux la densité de régénération de conifères exotiques. Les espèces se régénérant le plus tolèrent des conditions environnementales assez larges, dont un sous-étage ombragé, ce qui pourrait permettre l'invasion de forêts matures. Cette étude montre que 17 % des conifères étudiés sont potentiellement invasifs car ils se régénèrent abondamment, se dispersent à longue distance et produisent des descendants matures, capables de créer des populations satellites. En conclusion, notre étude fournit des recommandations pour de futures plantations, et nous appelons à la prudence concernant la plantation de ces espèces

La pollution lumineuse: quels impacts sur la faune sauvage ?

15. Life on lan

Evaluer le caractère invasif d’espèces exotiques arborescentes

editorial reviewe

The invasive potential of introduced exotic trees: what do arboreta tell us ?

An increasing number of woody species are being listed as invasive in Europe. Forestry is the second largest pathway of invasive tree introductions and it is likely that climate change will encourage forest managers to plant exotic tree species to maintain wood production. In the early 1900’s, several arboreta were established in Southern Belgium to assess the wood production potential of prospective exotic trees. However, they also offer the unique opportunity to assess the potential invasiveness of exotic tree species. A systematic sampling method was used to conduct surveys in eight arboreta and a buffer zone surrounding them. Regeneration of all exotic trees was recorded as well as biotic (herbaceous competition, composition of the tree stand) and environmental variables (soil type, pH, thickness of litter, canopy closure and climate). A descriptive approach allowed as to identify species showing an abundant regeneration. Linear regressions were implemented to assess whether the patterns in the regeneration of these exotic trees could be explained by their functional traits, dispersal modes, and environmental tolerances. Results revealed that several coniferous species from the North-American West coast exhibit rapid regeneration and/or dispersal, including Tsuga heterophylla, Abies grandis, Chamaecyparis lawsoniana, Pseudotsuga menziesii and Thuya plicata. We therefore recommend to exercise caution when planting these species in future forestry trials given their potentially invasive characteristics

Pourquoi un érable devient-il invasif ? Lien entre invasivité globale et traits de croissance.

editorial reviewedAfin d’identifier les espèces de plantes exotiques à haut risque invasif, déterminer les traits physiologiques corrélés à un caractère envahissant élevé reste une priorité dans le domaine de l’écologie des invasions. Un taux de croissance (RGR) important et une surface foliaire spécifique (SLA) élevée ont déjà été identifiés comme des traits permettant aux espèces invasives d’être compétitives dans des milieux riches en ressources. Nous avons mesuré plusieurs traits relatifs aux performances de croissance de huit espèces d’érables (Acer spp.) originaires de trois continents différents, sur huit semaines de croissance, en conditions optimales. Ces traits ont ensuite été comparés au caractère envahissant global des espèces, avec un contrôle de l’effet phylogénétique. Nous confirmons que le RGR, la SLA, la hauteur et le taux de production de feuilles sont corrélés avec le potentiel invasif global. Nous mettons également en évidence l’importance du timing pour la mesure de ces traits. Notre étude soutient que les érables invasifs pourraient bénéficier d’une stratégie compétitive leur permettant de concurrencer leurs homologues indigènes dans des environnements peu limitants.15. Life on lan