Biotic resistance represents an important natural barrier to potential invaders throughout the world, yet the underlying mechanisms that drive such resistance are still debated. In theory, native communities should repel both functionally similar invaders which compete for the same resources, and invaders which possess less competitive traits. However, environmental stress, trade‐offs across vital rates and competition‐induced plastic trait shifts may modify expected competitive outcomes, thereby influencing invasion dynamics. In order to test these theoretical links between trait distributions and biotic resistance, we performed a mesocosm experiment with 25 non‐native ornamental species invading native plant communities. Each non‐native species was grown with and without the native community under two watering treatments (regular and reduced). We measured biotic resistance as the difference in performance of non‐native individuals grown with and without the community in terms of their survival, growth and reproduction. We quantified overall functional dissimilarity between non‐native ornamental individuals and native communities based on the combination of plant height, specific leaf area and seed mass. Then, assuming each of these traits is also potentially linked to competitive ability, we measured the position of non‐natives on trait hierarchies. While height is positively correlated with competitive ability for light interception, conservative leaf and seed characteristics provide greater tolerance to competition for other resources. Finally, we quantified plastic trait shifts of non‐native individuals induced by competition. Indeed, the native community repelled functionally similar individuals by lowering the invader's survival rate. Simultaneously, shorter ornamental individuals with larger specific leaf areas were less tolerant to biotic resistance from the community across vital rates, although the effect of trait hierarchies often depended on watering conditions. Finally, non‐natives responded to competition by shifting their traits. Most importantly, individuals with more competitive traits were able to overcome biotic resistance also through competition‐induced plastic trait shifts. Synthesis. Our results highlight that both functional dissimilarity and trait hierarchies mediate biotic resistance to ornamental plant invaders. Nevertheless, environmental stress as well as opposing trends across vital rates are also influential. Furthermore, plastic trait shifts can reinforce potential invaders’ competitive superiority, determining a positive feedback.

Most naturalised and invasive alien plant species were originally introduced to regions for horticultural purposes. However, many regions now face an invasion debt from ornamental alien species, which have not yet naturalised. In this regard, climate change represents a threat as it may lower the barriers to naturalisation for some ornamental alien species. Identifying those species is extremely important for anticipating impending invasions. To identify predictors of naturalisation, we modelled the effects of climate, nursery availability and species characteristics on the current European naturalisation success of 2,073 ornamental aliens commonly planted in European gardens. We then used the resulting model together with climate projections for 2050 to forecast future naturalisation risks for the 1,583 species not yet naturalised in Europe. We found that non‐European naturalised range size, climatic suitability, propagule pressure, having a dioecious sexual system and plant height jointly explained current naturalisation success in Europe. By 2050, naturalisation probability projections increased by more than 0.1 for 41 species, and only decreased by more than 0.1 for one species. Policy implications. Using predictions based on our integrated model of alien ornamental naturalisation success, we identified species with high future naturalisation risk and species with high projected increases in naturalisation potential in Europe under climate change. This species list allows for prioritisation of monitoring and regulation of ornamental plants to mitigate the invasion debt.

Across the globe, invasive alien species cause severe environmental changes, altering species composition and ecosystem functions. So far, mountain areas have mostly been spared from large-scale invasions. However, climate change, land-use abandonment, the development of tourism and the increasing ornamental trade will weaken the barriers to invasions in these systems. Understanding how alien species will react and how native communities will influence their success is thus of prime importance in a management perspective. Here, we used a spatially and temporally explicit simulation model to forecast invasion risks in a protected mountain area in the French Alps under future conditions. We combined scenarios of climate change, land-use abandonment and tourism-linked increases in propagule pressure to test if the spread of alien species in the region will increase in the future. We modelled already naturalized alien species and new ornamental plants, accounting for interactions among global change components, and also competition with the native vegetation. Our results show that propagule pressure and climate change will interact to increase overall species richness of both naturalized aliens and new ornamentals, as well as their upper elevational limits and regional range-sizes. Under climate change, woody aliens are predicted to more than double in range-size and herbaceous species to occupy up to 20% of the park area. In contrast, land-use abandonment will open new invasion opportunities for woody aliens, but decrease invasion probability | MATERIALS AND METHODS | Study areaWe focused on a protected mountain area in the French Alps (Ecrins National Park-ENP), which covers 270,000 ha and is characterized by large environmental and altitudinal gradients (650-4,100 m a.s.l.), with lower altitudes mostly at the peripheries of the park (Fig. S1).The ENP is located at the crossroads of temperate and Mediterranean climates and harbours ca. 2,000 vascular plant species, with so far only very few occurrences of alien species. Currently, twothirds of the park consist of open habitats, managed mostly through traditional agro-pastoral practices such as extensive grazing (80%) and/or mowing (25%), while forests cover ca. 25% of the area. The department Hautes-Alpes (where the ENP is located) is currently the third least populated in France, but since 2006 its population has increased by ca. 1.2% each year, more than twice the national average (Insee, 2014), supporting more than 360,000 tourist beds.Within the department, the ENP is in itself a tourist destination, supported by a network of 740 km of mountain trails and more than 30 mountain huts. | Hybrid simulation modelWe used the spatially explicit hybrid model FATE-HD to simulate spatio-temporal dynamics of resident vegetation and plant invasions under different global change scenarios Boulangeat, Georges, Dentant, et al., 2014). FATE-HD combines species distribution models (SDMs) with process-based modelling to simulate population dynamics (dispersal, germinat...

Many organisms respond to anthropogenic environmental change through shifts in their phenology. In plants, flowering is largely driven by temperature, and therefore affected by climate change. However, on smaller scales climatic conditions are also influenced by other factors, including habitat structure. A group of plants with a particularly distinct phenology are the understorey herbs in temperate European forests. In these forests, management alters tree species composition (often replacing deciduous with coniferous species) and homogenizes stand structure, and as a consequence changes light conditions and microclimate. Forest management should thus also affect the phenology of understorey herbs. To test this, we recorded the flowering phenology of 16 early-flowering herbs on 100 forest plots varying in management intensity, from near-natural to intensely managed forests, in Central and Southern Germany. We found that in forest stands with a high management intensity, such as Norway spruce plantations, the plants flowered on average about two weeks later than in unmanaged forests. This was largely because management also affected microclimate (e.g. spring temperatures of 5.9 °C in managed coniferous, 6.7 in managed deciduous and 7.0 °C in unmanaged deciduous plots), which in turn affected phenology, with plants flowering later on colder and moister forest stands (+4.5 days per -1°C and 2.7 days per 10 % humidity increase). Among forest characteristics, the percentage of conifers had the greatest influence on microclimate, but also the age, overall crown projection area, structural complexity and spatial distribution of the forest stands. Our study demonstrates that forest management alters plant phenology, with potential far-reaching consequences for the ecology and evolution of understorey communities. More generally, our study suggests that besides climate change other drivers of environmental change, too, can influence the phenology of organisms.

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