1. Biotic resistance, the ability of species in a community to limit invasion, is central to our understanding of how communities at risk of invasion assemble after disturbances, but it has yet to ...translate into guiding principles for the restoration of invasion-resistant plant communities. We combined experimental, functional, and modelling approaches to investigate processes of community assembly contributing to biotic resistance to an introduced lineage of Phragmites australis, a model invasive species in North America. We hypothesized that (i) functional group identity would be a good predictor of biotic resistance to P. australis, while species identity effect would be redundant within functional group (ii) mixtures of species would be more invasion resistant than monocultures. 2. We classified 36 resident wetland plants into four functional groups based on eight functional traits. We conducted two competition experiments based on the additive competition design with P. australis and monocultures or mixtures of wetland plants. As an indicator of biotic resistance, we calculated a relative competition index (RCI avg ) based on the average performance of P. australis in competition treatment compared with control. To explain diversity effect further, we partitioned it into selection effect and complementarity effect and tested several diversity—interaction models. 3. In monoculture treatments, RCI avg of wetland plants was significantly different among functional groups, but not within each functional group. We found the highest RCI avg for fast-growing annuals, suggesting priority effect. 4. RCI avg of wetland plants was significantly greater in mixture than in monoculture mainly due to complementarity-diversity effect among functional groups. In diversity—interaction models, species interaction patterns in mixtures were described best by interactions between functional groups when fitted to RCI avg or biomass, implying niche partitioning. 5. Synthesis. Functional group identity and diversity of resident plant communities are good indicators of biotic resistance to invasion by introduced Phragmites australis, suggesting niche preemption (priority effect) and niche partitioning (diversity effect) as underlying mechanisms. Guiding principles to understand and/or manage biological invasion could emerge from advances in community theory and the use of a functional framework. Targeting widely distributed invasive plants in different contexts and scaling up to field situations will facilitate generalization.
Aim
Climate‐induced range expansion ultimately implies recruitment at sites that were previously unoccupied by a species (i.e. colonization events). Using evidence on abiotic conditions and biotic ...interactions at these migration sites, we aimed to identify migration pathways from northern temperate to boreal forests for species showing northward range expansion.
Location
Quebec, Canada.
Taxon
Trees of northern temperate/boreal forests.
Methods
Using past (1970–1977) and recently updated (2003–2015) forest inventories across 761,100 km2, we first quantified latitudinal shifts for saplings of eight tree species and investigated colonization events at migration sites. We used field evidence and a consensus modelling approach to determine environmental suitability and identify edaphic, climatic and disturbance conditions, as well as species co‐occurrence patterns, characterizing recent colonization events. The results were interpreted in relation to novel species associations facilitating species migration in unsuitable landscapes.
Results
All species showed northward latitudinal shifts driven by increased recruitment and colonization northward. Colonization events occurred largely at historically unsuitable sites. Migration sites showed a shift towards humus types characteristic of the boreal forest and not typically found in the core range of most temperate species. Climatic conditions at migration sites were initially colder than at occupied sites, but warming suggests recent climatic suitability. A decrease in conifer basal area at migration sites following disturbances reduced priority effects that possibly constrained deciduous species establishment. Co‐occurrence patterns pointed to deciduous species tolerant of boreal edaphic conditions, leading the way for other temperate species.
Main conclusions
Temperate tree species can recruit into sites typical of boreal forests, even under environmentally challenging conditions. Warming and disturbances open up the way for some novel species associations that in turn have the potential to facilitate the recruitment of temperate species into the boreal forest, revealing migration pathways.
Despite debates on the real impact of plant invasion on native biodiversity, there remain many situations where exotic invasive plants must be managed and habitats restored. Restoration practices ...that build on plant community assembly principles could be useful to delay or prevent re-invasion after control, but there are still few syntheses of the biodiversity theory, ecological mechanisms and experimental evidence relevant to invasive plant management, possibly delaying applications. To provide such a synthesis, we review current knowledge on three key determinants of invasion success: biotic resistance, abiotic constraints, and propagule pressure. We elaborate on the ecological mechanisms at play for each determinant and emphasize, using case studies, their relevance for invasive plant management and ecological restoration. We find evidence that restoring a plant cover can enhance invasion resistance, but the challenge for both research and field applications is to understand how multiple determinants interact in relation to species traits in the fields. Failure to recognize these interactions and their effect on community assembly processes may explain some of the mixed species responses observed. While we need control and restoration case studies with local species at different sites, the development of a coherent, dynamic and adaptive framework around biotic/ecological resistance will have to go beyond the idiosyncrasy of the many species and systems being tested. Emphasizing the functional diversity of the restored community seems a promising approach when facing potentially multiple invaders and/or fluctuating abiotic conditions.
Both climatic and edaphic conditions determine plant distribution, however many species distribution models do not include edaphic variables especially over large geographical extent. Using an ...exceptional database of vegetation plots (n = 4839) covering an extent of ∼55,000 km2, we tested whether the inclusion of fine scale edaphic variables would improve model predictions of plant distribution compared to models using only climate predictors. We also tested how well these edaphic variables could predict distribution on their own, to evaluate the assumption that at large extents, distribution is governed largely by climate. We also hypothesized that the relative contribution of edaphic and climatic data would vary among species depending on their growth forms and biogeographical attributes within the study area. We modelled 128 native plant species from diverse taxa using four statistical model types and three sets of abiotic predictors: climate, edaphic, and edaphic-climate. Model predictive accuracy and variable importance were compared among these models and for species' characteristics describing growth form, range boundaries within the study area, and prevalence. For many species both the climate-only and edaphic-only models performed well, however the edaphic-climate models generally performed best. The three sets of predictors differed in the spatial information provided about habitat suitability, with climate models able to distinguish range edges, but edaphic models able to better distinguish within-range variation. Model predictive accuracy was generally lower for species without a range boundary within the study area and for common species, but these effects were buffered by including both edaphic and climatic predictors. The relative importance of edaphic and climatic variables varied with growth forms, with trees being more related to climate whereas lower growth forms were more related to edaphic conditions. Our study identifies the potential for non-climate aspects of the environment to pose a constraint to range expansion under climate change.
Aim
Species distribution models typically project climatically suitable habitat for trees in eastern North America to shift hundreds of kilometres this century. We simulated potential migration, ...accounting for various traits that affect species' ability to track climatically suitable habitat.
Location
Eastern Canada, covering ~3.7 million km2.
Methods
We simulated migration‐constrained range shifts through 2100 using a hybrid approach combining projections of climatically suitable habitat based on two Representative Concentration Pathways (RCP4.5, RCP8.5) for three time periods and two species distribution modelling approaches with process‐based models parameterized using data related to dispersal ability and generation time. We developed a unique “migration kernel” that uses seed dispersal traits and observed migration velocities to obtain kernel shape and dispersal probabilities for each tree species. We then calculated lags between the migration‐constrained range limits obtained through simulations and limits of climatically suitable habitat.
Results
All species demonstrated northward range shifts at the leading edge of their simulated distribution through 2100, but the magnitude and rate of that shift varied by species and time period. Climatically suitable habitat limits were found to be north of simulated distribution limits across both RCPs, with lags increasing through time. On average, simulated distribution that remained within climatically suitable habitat declined more under RCP8.5 than RCP4.5, with large areas of the rear edge of the simulated distribution becoming partially or completely climatically unsuitable for many species.
Main conclusions
Climatically suitable habitat limits projected for 2100 far exceeded migration‐constrained range limits for all 10 tree species, particularly for temperate species. This study underlines the limited extent to which tree species will track climate change via natural migration. Integrating observed migration velocities, seed dispersal and generation time with SDM outputs allows for more realistic evaluations of tree migration ability under climate change and may help orient forest conservation and restoration efforts.
Climate change is expected to result in a reorganization of the continental distribution of tree species. Recent shifts in distribution patterns have been reported, but it is not always clear how ...climate change influences these patterns locally, especially in relation to other disturbances.
We investigated latitudinal shifts of four ecologically important tree species between 1970 and 2014 within a study area that encompasses their northernmost range limit in northeastern North America (Quebec, Canada; ~761,000 km2). Changes in latitudinal limits were defined in relation to changes in tree saplings’ occurrence patterns within forest plots resampled over two time periods (1970–1977 and 2003–2014). By examining changes in the frequency of occurrences in different portions of the study area along a latitudinal gradient, we were able to identify spatially explicit patterns of loss or gain (sapling recruitment) resulting in the shifts observed. We then estimated the probability of observing a recruitment event in response to changes in climate, disturbance and their interaction, using a multimodel selection approach.
Latitudinal limits of all four species shifted northward, but these shifts resulted from different patterns of plot occurrence changes, depending on the species and the location examined. Greater recruitment at northern locations than at southern ones drove shifts for Acer saccharum Marsh., Fagus grandifolia Ehrh. and Acer rubrum L., but less so for Betula alleghaniensis Britt. Climate variables indicating changes in early or late growing season conditions were most often selected in models. Warming tended to reduce recruitment probability in the south but increase it in the north, leading to divergent responses for a given species across the study area. Disturbance effects were generally less important than climate change effects, as was their interaction.
Synthesis. Spatially explicit and divergent responses to climate change and disturbance drive recruitment patterns underlying latitudinal shifts of tree species. The importance of early‐ or late‐season climate variables points towards biological processes being affected at critical stages of the life cycle. Understanding the factors that influence species’ migration capacity in a changing climate is crucial to inform adaptive management and conservation practices.
Spatially explicit and divergent responses to climate change and disturbance drive recruitment patterns underlying northward latitudinal shifts of four tree species at their range limit in northeastern North America. The importance of early‐ or late‐season climate variables points towards biological processes being affected at critical stages of the life cycle.
The interaction between landscape structure and spatial patterns of plant invasion has been little addressed by ecologists despite the new insights it can provide. Because of their spatial ...configuration as highly connected networks, linear wetlands such as roadside or agricultural ditches, can serve as corridors facilitating invasion at the landscape scale, but species dynamics in these important habitats are not well known. We conducted a landscape scale analysis of Phragmites australis invasion patterns (1985-2002 and 1987-2002) in two periurban areas of southern Québec (Canada) focusing on the interaction between the network of linear wetlands and the adjacent land-uses. Results show that, at the beginning of the reference period, the two landscapes were relatively non-invaded and populations occurred mostly in roadside habitats which then served as invasion foci into other parts of the landscape. The intrinsic rates of increase of P. australis populations in linear anthropogenic habitats were generally higher than those reported for natural wetlands. Riparian habitats along streams and rivers were little invaded compared to anthropogenic linear wetlands, except when they intersected transportation rights-of-way. Bivariate spatial point pattern analysis of colonization events using both Euclidian and network distances generally showed spatial dependence (association) to source populations. An autologistic regression model that included landscape and edaphic variables selected transportation rights-of-way as the best predictor of P. australis occurrence patterns in one of the landscapes. Given the high invasion rates observed, managers of linear wetlands should carefully monitor expansion patterns especially when roads intersect landscapes of conservation or economic value.
Habitat-corridors are assumed to counteract the negative impacts of habitat loss and fragmentation, but their efficiency in doing so depends on the maintenance of ecological processes in corridor ...conditions. For plants dispersing in linear habitats, one of these critical processes is the maintenance of adequate pollen transfer to insure seed production within the corridor. This study focuses on a common, self-incompatible forest herb, Trillium grandiflorum, to assess plant-pollinator interactions and the influence of spatial processes on plant reproduction in hedgerow corridors compared to forests. First, using pollen supplementation experiments over 2 years, we quantified the extent of pollen limitation in both habitats, testing the prediction of greater limitation in small hedgerow populations than in forests. While pollen limitation of fruit and seed set was common, its magnitude did not differ between habitats. Variations among sites, however, suggested an influence of landscape context on pollination services. Second, we examined the effect of isolation on plant reproduction by monitoring fruit and seed production, as well as pollinator activity and assemblage, in small flower arrays transplanted in hedgerows at increasing distances from forest and from each other. We detected no difference in the proportion of flowers setting fruit or in pollinator activity with isolation, but we observed some differences in pollinator assemblages. Seed set, on the other hand, declined significantly with increasing isolation in the second year of the study, but not in the first year, suggesting altered pollen transfer with distance. Overall, plants in hedgerow corridors and forests benefited from similar pollination services. In this system, plant-pollinator interactions and reproduction seem to be influenced more by variations in resource distribution over years and landscapes than by local habitat conditions.
Projecting suitable conditions for a species as a function of future climate provides a reasonable, although admittedly imperfect, spatially explicit estimate of species vulnerability associated with ...climate change. Projections emphasizing range shifts at continental scale, however, can mask contrasting patterns at local or regional scale where management and policy decisions are made. Moreover, models usually show potential for areas to become climatically unsuitable, remain suitable, or become suitable for a particular species with climate change, but each of these outcomes raises markedly different ecological and management issues. Managing forest decline at sites where climatic stress is projected to increase is likely to be the most immediate challenge resulting from climate change. Here we assess habitat suitability with climate change for five dominant tree species of eastern North American forests, focusing on areas of greatest vulnerability (loss of suitability in the baseline range) in Quebec (Canada) rather than opportunities (increase in suitability). Results show that these species are at risk of maladaptation over a remarkably large proportion of their baseline range. Depending on species, 5-21% of currently climatically suitable habitats are projected to be at risk of becoming unsuitable. This suggests that species that have traditionally defined whole regional vegetation assemblages could become less adapted to these regions, with significant impact on ecosystems and forest economy. In spite of their well-recognised limitations and the uncertainty that remains, regionally-explicit risk assessment approaches remain one of the best options to convey that message and the need for climate policies and forest management adaptation strategies.
In agricultural landscapes, linear habitats, such as hedgerows at field margins increase structural connectivity among forest patches, potentially providing dispersal corridors for forest herbs. The ...spatial structure of linear habitats, however, also results in edge effects and perturbations that can influence the individual and population performance of forest plants. This study compares the stage structure and components of growth and reproduction of 14 Trillium grandiflorum populations in hedgerows and forests. Hedgerow Trillium tended to grow faster and, when mature, produced more flowers and more ovules per flowers than forest Trillium, a pattern possibly associated to differences in nutrients and light availability between the two habitats. Seed production and germination rate, however, did not differ between hedgerows and forests. At the population level, seedlings and juveniles were proportionally less abundant in hedgerows than in forests. Although well-established plants can thrive in hedgerows, reduced recruitment may eventually limit the capacity to establish new populations and therefore hamper migration along hedgerow-corridors. Considering the strategies by which plants persist in linear habitats becomes particularly relevant at a time when species are expected to be much in need of dispersal corridors because of climatic stress.
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