Isabelle Aubin

Land-use intensification reduces functional redundancy and response diversity in plant communities.

Ecosystem resilience depends on functional redundancy (the number of species contributing similarly to an ecosystem function) and response diversity (how functionally similar species respond differently to disturbance). Here, we explore how land-use change impacts these attributes in plant communities, using data from 18 land-use intensity gradients that represent five biomes and > 2800 species. We identify functional groups using multivariate analysis of plant traits which influence ecosystem processes. Functional redundancy is calculated as the species richness within each group, and response diversity as the multivariate within-group dispersion in response trait space, using traits that influence responses to disturbances. Meta-analysis across all datasets showed that land-use intensification significantly reduced both functional redundancy and response diversity, although specific relationships varied considerably among the different land-use gradients. These results indicate that intensified management of ecosystems for resource extraction can increase their vulnerability to future disturbances.

Placing Forestry in the Assisted Migration Debate

Assisted migration (AM) id often presented as a strategy to save species that are imminently threatened by rapid climate change. This conception of AM, which has generated considerable controversy, typically proposes the movement of narrowly distributed, threatened species to suitable sites beyond their current range limits. However, existing North American forestry operations present an opportunity to practice AM on a larger scale, across millions of hectares, with a focus on moving populations of widely distributed, nonthreatened tree species within their current range limits. Despite these differences (and many others detailed herein), these two conceptions of AM have not been clearly distinguished in the literature, which has added confusion to recent dialogue and debate. Here, we aim to facilitate clearer communication on this topic by detailing this distinction and encouraging a more nuanced view of AM.

How resilient are northern hardwood forests to human disturbance? An evaluation using a plant functional group approach

ABSTRACT Evaluating forest ecological integrity remains a major challenge for ecologists. We analyzed understory vegetation using an approach that combined plant functional types and vertical stratification to evaluate the effects of human disturbances on the ecological integrity of sugar maple-dominated stands in southern Québec. Ecological integrity was evaluated by analyzing the divergence of understory species assemblages from those observed in comparable unmanaged forest. Multivariate analyses of biological traits revealed 13 emergent groups that share common traits associated with a similar life history strategy. Responses of these groups, of specific traits, and of understory structure to different human disturbances were tested. Nine of the 13 emergent groups varied in occurrence or diversity among disturbance types. Analyses also revealed a set of traits specifically associated with unmanaged old growth forest, indicating that species possessing these traits may be sensitive to human disturbance. Overall, the understory vegetation assemblage was found to be relatively stable among all human disturbances investigated. However, our results suggest some issues of possible long-term conservation concern given a continuation of human disturbances: (i) an increase of species associated with open environment, including exotic species; (ii) a decrease of spring geophytes; (iii) a decrease of certain shade-tolerant forbs; and (iv) a modification of understory structure by the development of a dense sapling stratum.

Can a trait-based multi-taxa approach improve our assessment of forest management impact on biodiversity?

Harvest impact on forest biodiversity has been widely studied, but for managers confronted with a need for integrated cross-taxa assessment, application remains a significant challenge. Using post-harvest boreal forest succession as a model system, we investigate the usefulness of a trait-based multi-taxa approach to improve our understanding of the community dynamics after disturbance. We assess the strength of response to forest harvesting and recovery patterns of four taxa with contrasting attributes (vegetation, carabids, spiders and birds) along a post-harvest chronosequence of jack pine stands in the boreal forests of Canada. We used a complementary set of functional and taxonomic diversity metrics to identify commonalities and dissimilarities in the community assembly processes and sensitivities to harvesting among taxa. Despite the overall similarity of community response for most pairs of taxa and metrics, the strength of cross-taxa congruency varied greatly among metrics, illustrating the complexity of taxa response to harvest as well as the necessity of including a variety of biodiversity metrics in impact assessments. Of the four selected taxa, spiders were found the most sensitive to harvesting, with a strong response to environmental changes after harvest and a slow community recovery process. Birds and carabids showed highly congruent response patterns, with a strong response to harvest followed by a marked recovery process. Ground vegetation was the most resilient to harvesting. We discuss the management implications of these contrasting recovery processes, outline the current limitations of this method and suggest steps toward the implementation of effective integrated multi-taxa monitoring programs.

Population structure and growth acclimation of mountain maple along a successional gradient in the southern boreal forest

ABSTRACT A range of stands was sampled in the southern boreal forest of eastern Canada to determine the impact of forest development stages and light availability on the dynamics of an abundant understory shrub: mountain maple (Acer spicatum). Mountain maple was studied at both the population and individual stem levels. At the population level, a total of 190 1-m2 quadrats were sampled in five forest types representing a successional gradient (young aspen, mature aspen, mixedwood, shade-tolerant conifer, and old spruce budworm-affected conifer forests). At the individual stem level, a total of 100 stems of different sizes were harvested in a sub-sample of the quadrats. Mountain maple stem biomass, density, and population structure were found to be affected by forest composition, the coniferous forest being the least favourable. At the individual stem level, light availability and stem size were found to have an important impact on stem morphology and growth, suggesting a high level of plasticity in relation to canopy opening. From these results, four developmental phases of mountain maple population dynamics were identified: 1) following a drastic disturbance, a first phase of rapid growth, and potentially intense competition with aspen suckers, when present, takes place; 2) following the development of a closed aspen canopy, the overall density, biomass, and presence of mountain maple in the understory increase; 3) following recruitment of conifer trees into the overstory canopy, mountain maple enters a suppression phase associated with a decrease in light; and 4) following the occurrence of small scale disturbances, mountain maple quickly responds to rapidly dominate the openings, which further ensures its maintenance in the understory.

Functional Responses and Resilience of Boreal Forest Ecosystem after Reduction of Deer Density

The functional trait-based approach is increasingly used to predict responses of ecological communities to disturbances, but most studies target a single taxonomic group. Here, we assessed the resilience of a forest ecosystem to an overabundant herbivore population by assessing changes in 19 functional traits for plant, 13 traits for ground beetle and 16 traits for songbird communities after six years of controlled browsing on Anticosti Island (Quebec, Canada). Our results indicated that plants were more responsive to 6 years of reduced browsing pressure than ground beetles and songbirds. However, co-inertia analysis revealed that ground beetle communities responded in a similar way than plant communities with stronger relationships between plant and ground beetle traits at reduced deer density, a pattern not detected between plant and songbird. High deer density favored plants species that reproduce vegetatively and with abiotic pollination and seed dispersal, traits implying little interaction with animal. On the other hand, traits found at reduced deer density mostly involved trophic interaction. For example, plants in this treatment had fleshy fruits and large seeds dispersed by birds or other animals whereas ground beetle species were carnivorous. Overall, our results suggest that plant communities recovered some functional components to overabundant herbivore populations, since most traits associated with undisturbed forests were reestablished after six years of deer reduction. The re-establishment of functional plant communities with traits involving trophic interaction induces changes in the ground-beetle trait community, but forest structure remains likely insufficiently heterogeneous to shift the songbird trait community within six years.

Functional Relationships Between Old-Growth Forest Canopies, Understorey Light and Vegetation Dynamics

The distinct structural and compositional features, understorey light conditions and understorey vegetation of old-growth forests in boreal, temperate and tropical biomes are discussed. Old-growth forests are characterised by gap dynamics that create a structurally complex and dynamic vertical and horizontal crown and understorey light environment. In both temperate and tropical forests, gaps tend to close quickly from both regrowth of lateral crown and understorey vegetation. In boreal forests, gaps tend to last longer and fill mainly from understorey vegetation. Understorey light is generally below 5% near the forest floor in all biomes, with very few microsites having high light levels. Above the main understorey vegetation, however, light is more variable and can reach up to 65% in some old-growth boreal forests. This heterogeneous, complex and unpredictable understorey light environment acts as a selective filter that promotes unique understorey plant adaptations.

Spatial processes structuring riparian plant communities in agroecosystems: implications for restoration

The disruption of hydrological connectivity by human activities such as flood regulation or land-use changes strongly impacts riparian plant communities. However, landscape-scale processes have generally been neglected in riparian restoration projects as opposed to local conditions, even though such processes might largely influence community recovery. We surveyed plant composition of field edges and riverbanks in 51 riparian zones restored by tree planting (565 1-m2 plots) within two agricultural watersheds in southeastern Québec, Canada. Once the effects of environmental variables (hydrology, soil, agriculture, landscape, restoration) were partialled out, three models of spatial autocorrelation based on Morans eigenvector maps and asymmetric eigenvector maps were compared to quantify the pathways and direction of the spatial processes structuring riparian communities. The ecological mechanisms underlying predominant spatial processes were then assessed by regression trees linking species response to spatial gradients to seed and morphological traits. The structure of riparian communities was predominantly related to unidirectional spatial gradients from upstream to downstream along watercourses, which contributed more to species composition than bidirectional gradients along watercourses or overland. Plant traits selected by regression trees explained 22% of species response to unidirectional upstream-downstream gradients in field edges and 24% in riverbanks, and predominantly corresponded to seed traits rather than morphological traits of the adult plants. Our study showed that even in agriculturally open landscapes, water flow remains a major force structuring spatially riparian plant communities by filtering species according to their seed traits, thereby suggesting long-distance dispersal as a predominant process. Preserving hydrological connectivity at the watershed-scale and restoring riparian plant communities from upstream to downstream should be encouraged to improve the ecological integrity of rivers running through agricultural landscapes.

Extreme Climate Variability Should Be Considered in Forestry Assisted Migration: A Reply

Extreme Climate Variability Should Be Considered in Forestry Assisted Migration Recently, Pedlar and colleagues (2012) stated that assisted migration in forestry (forestry AM) differs from species rescue assisted migration (species rescue AM) because the risks of invasiveness, hybridization with local species, and the spread of diseases are minimized in managed forests. The rationale behind this assertion for forestry AM is that it involves the translocation of populations within the existing geographic range of the species, whereas species rescue AM involves the introduction of exotic species. However, although we agree that forestry AM is less risky than species rescue AM for the recipient ecosystem, not only can forestry AM fail, but it can also incur enormous financial costs. The failure of efforts that involved planting maritime pine (Pinus pinaster Aiton) trees in southwestern France (Aquitaine) with seeds from more southerly populations from Portugal for production purposes is a textbook case. The climate variability in Aquitaine includes periods of intense frost that are sufficiently rare (every 10–20 years) to be overlooked when establishing tree populations. The frost of the winter of 1985 was the most intense frost event since records began, with temperatures dropping as low as –22 degrees Celsius (°C; Boisseaux 1986), affecting about 350 square kilometers of tree plantations in the region (Doré and Varoquaux 2006). The highest mortality related to frost was observed in populations harvested from Leiria, in Portugal, for which nearby records show that the absolute minimum temperature was only –7.8°C in the last 60 years. Climate averages over the last 30 years differ only slightly between Leiria and Aquitaine, which would erroneously suggest that samples from Portugal would have survived in the Aquitaine region. Newly emerging climates (Williams et al. 2007) and the uncertainty related to extreme climate events (Easterling et al. 2000) will make the search for southern locations with climatic conditions similar to those of northern populations of trees extremely difficult. Policies of forest adaptation to climate change should account for extreme cold events in the target populations, even if climate change will likely decrease the number of extreme cold events (Easterling et al. 2000), which remain, in our opinion, the hidden element behind the maladaptation of southern populations to northern locations. MARTA BENITO-GARZÓN MINH HA-DUONG NATHALIE FRASCARIA-LACOSTE JUAN F. FERNÁNDEZ-MANJARRÉS Marta Benito-Garzón (marta. benito@gmail.com) and Juan F. Fernández-Manjarrés are affiliated with the French National Center for Scientific Research’s (CNRS) Laboratoire d’Écologie, Systématique et Evolution (ESE), at the Université Paris-Sud, in Orsay, and MB-G and Minh Ha-Duong are affiliated with the CNRS Centre International de Recherche sur l’Environnement et le Développement, in Paris. Nathalie Frascaria-Lacoste is affiliated with both the ESE and AgroParisTech, in Orsay.