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We addressed two poorly understood aspects of plant response to climate change: the impact of extreme climatic events and the mediating role of biotic interactions, through a study of heatwave effects on tree seedling survival rates and ability of the tree canopy to alter seedling responses. Mountain belt of the northern French Alps (Maurienne Valley). The survival rates of two seedling cohorts from four tree species (Abies alba, Acer pseudoplatanus, Fraxinus excelsior and Picea abies) were measured during both the 2003 European heatwave and an average summer (2004) in deciduous broadleaf mountain forests. Seedlings were transplanted into two soil moisture conditions, and in experimental gaps or under the tree canopy. The heatwave strongly decreased tree seedling survival rates, while there was an important species-specific mediating role of biotic interactions. In the wettest conditions, the tree canopy strongly increased survival of Abies, buffering the negative impact of the heatwave. In contrast, in the driest conditions, the tree canopy decreased survival of Picea and Acer, amplifying the negative impact of the heatwave. We found evidence of increasing soil water stress in the understorey of the driest community, but further studies including vapour pressure deficit measurements are needed to elucidate the driving mechanism of facilitation. The high species specificity of the mediating role of biotic interactions and its variation along stress gradients leads to questions on our ability to predict large-scale responses of species to climate changes.

Question: How does Alnus alnobetula colonization affect plant communities in sub-alpine grasslands undergoing land abandonment on a regional scale? Location: Savoie, French Alps. Methods: Variations within and between communities were monitored in 243 plots within three types of stands representing an A. alnobetula colonization chronosequence from grassland to dense vegetation. They were tested for species richness, species strategies and species distribution through multivariate analysis. Regional variations were assessed at three sites along a rain shadow gradient. Results: We detected a convergence of the understorey communities along an A. alnobetula colonization gradient at all sites, associated with a reduction in species richness and the replacement of stress related strategists by competitor strategists. This convergence was unaffected by the rain shadow gradient although it was a key factor of plant distribution. Conclusions: The dense cover and the soil properties provided by A. alnobetula stands buffered the heterogeneity of sub-alpine communities induced by a rain shadow gradient throughout the study region. It facilitated the development a homogeneous understorey community dominated by competitor related strategists while excluding grasslands stress adapted species through competition processes. The resulting community, although species-poor, has an interesting conservation value. Accordingly, as a ‘native invader’, A. alnobetula does not display typical traits of biological invaders. Future experimental research on the interactions between A. alnobetula and components of the understorey would be particularly interesting in a severe environment that promotes facilitation among plants. Nomenclature: Kerguelen (1993) and Anon. (2004).

The European Alps harbours a unique and species-rich biodiversity, which is increasingly impacted by habitat fragmentation through land use changes, urbanization and expanding transport infrastructure. In this study, we indentified the 50 most important questions relating to the maintenance and restoration of an ecological continuum - the connectedness of ecological processes across many scales including trophic relationship and disturbances processes and hydro-ecological flows in the European Alps. We suggest new approches in takling the issue of an ecological continuum in the Alps by analysing and classifying the characteristics of the resulting questions. The priority setting exercise will support research and funding institutions in channelling their capabilities and resources towards questions that need to be urgently addressed in order to facilitate significant progress in biodiversity conservation in the European Alps.

Question: The relative importance of environmental vs. biotic resistance of recipient ecological communities remains poorly understood in invasion ecology. Acer negundo, a North American tree, has widely invaded riparian forests throughout Europe at the ecotone between early- (Salix spp. and Populus spp.) and late-successional (Fraxinus spp.) species. However, it is not present in the upper part of the Rhône River, where native Alnus incana occurs at an intermediate position along the successional riparian gradient. Is this absence of the invasive tree due to environmental or biotic resistance of the recipient communities, and in particular due to the presence of Alnus? Location: Upper Rhône River, France. Methods: We undertook a transplant experiment in an Alnus-dominated community along the Upper Rhône River, where we compared Acer negundo survival and growth, with and without biotic interactions (tree and herb layer effects), to those of four native tree species from differing successional positions in the Upper Rhône communities (P. alba, S. alba, F. excelsior and Alnus incana). Results: Without biotic interactions Acer negundo performed similarly to native species, suggesting that the Upper Rhône floodplain is not protected from Acer invasion by a simple abiotic barrier. In contrast, this species performed less well than F. excelsior and Alnus incana in environments with intact tree and/or herb layers. Alnus showed the best growth rate in these conditions, indicating biotic resistance of the native plant community. Conclusions: We did not find evidence for an abiotic barrier to Acer negundo invasion of the Upper Rhône River floodplain communities, but our results suggest a biotic resistance. In particular, we demonstrated that (i) additive competitive effects of the tree and herb layer led to Acer negundo suppression and (ii) Alnus incana grew more rapidly than Acer negundo in this intermediate successional niche.

Maintenance and monitoring of soil fertility is a key issue for sustainable forest management. Vital ecosystem processes may be affected by management practices which change the physical, chemical and biological properties of the soil. This study is the first in Europe to use electrical resistivity (ER) as a non-invasive method to rapidly determine forest soil properties in the field in a monitoring purpose. We explored the correlations between ER and forest soil properties on two permanent plots of the French long-term forest ecosystem-monitoring network (International Cooperative Program Forests, Level II). We used ER measurements to determine soil-sampling locations and define sampling design. Soil cores were taken in the A horizon and analysed for pH, bulk density, residual humidity, texture, organic matter content and nutrients. Our results showed high variability within the studied plots, both in ER and analysed soil properties. We found significant correlations between ER and soil properties, notably cation exchange capacity, soil humidity and texture, even though the magnitude of the correlations was modest. Despite these levels of correlations, we were able to assess variations in soil properties without having to chemically analyse numerous samples. The sampling design based on an ER survey allowed us to map basic soil properties with a small number of samples.

Biotic interactions are major drivers of biological invasion. However, the roles of direct and indirect positive and negative interactions at the different phases of an invasion process have rarely been addressed because most studies have focused on a particular stage of invasion. We analyzed the respective role of direct and indirect interactions during the colonization and establishment phases of the invasion of Acer negundo, a North-American tree species which has invaded European floodplains. In the Middle Rhone floodplain (France), where Acer forms monodominant stands at the ecotone between native early- and late-successional communities, we set up an experiment along a natural successional gradient. In the three communities we transplanted seedlings of the invasive and of three natives within the forest and in experimental gaps and with and without herbaceous species. We also quantified the effects of the tree canopies and of understory communities on light, available nitrogen and soil moisture. We made two main predictions: (i) native pioneer tree species directly facilitate Acer, and (ii) adult Acer have a higher competitive effect on all other species than native overstory species. Consistent with our first hypothesis, we observed strong direct facilitation for the survival of Acer seedlings in the early-successional Salix community, which was likely due to the mitigation of flooding disturbance. In contrast to our second hypothesis adult Acer did not have higher direct competitive effects on the other species than late-successional native Fraxinus. However, we found significant indirect facilitation of Acer canopies on the growth of Acer seedlings. This facilitative effect occurred for the invasive within its own community because adult Acer significantly reduced the abundance of highly competitive herbaceous competitors which decreased belowground competition. We conclude that the relative importance of direct and indirect interactions were highly variable during the invasion process, but that direct facilitation by native species was predominent during the colonization phase of Acer and indirect facilitation by adult Acer canopies was predominant during the establishment phase.

In forest ecosystems, the influence of landscape history on contemporary biodiversity patterns has been shown to provide a convenient framework to explain shifts in plant assemblages. However, very few studies have controlled for present human‐induced activities when analysing the effect of forest continuity on community structures. By cutting and removing trees, foresters substantially change stand ecological conditions, with consequences on biodiversity patterns. Disentangling the effect of past and present human activities on biodiversity is thus crucial for ecosystem management and conservation. We explored the response of plant and springtail species richness and composition to forest continuity (ancient vs. recent) in montane forests, while controlling for stand maturity (mature vs. overmature). We established 70 sites in landscapes dominated by unfragmented ancient forests where we surveyed plants and assessed springtails by analysing environmental DNA. Neither plant nor springtail species richness was influenced by forest continuity or by stand maturity. Instead, site‐specific characteristics, especially soil properties and canopy openness, were of major importance in shaping above‐ and below‐ground richness. For plant and springtail species composition, the effect of forest continuity was mediated by stand maturity. Thus, both plants and springtails showed a convergence in assemblage patterns with the increasing availability of overmature stand attributes. Moreover, soil and stand‐scale factors were evidently more important than landscape‐scale factors in shaping above‐ and below‐ground species composition. Synthesis. We clearly demonstrated that biodiversity patterns are more strongly influenced by present human‐induced activities than by past human‐induced activities. In the Northern Alps where our study sites were located, the colonization credit of most species has been paid off and the transient biodiversity deficit usually related to forest continuity has moved towards equilibrium. These findings emphasize the necessity to better control for local‐scale factors when analysing the response of biodiversity to forest continuity; we call for more research into the effects of forest continuity in unfragmented mountain forests.

Mountain regions and UNESCO Mountain Biosphere Reserves (MBRs) encapsulate broad elevational ranges, cover large gradients of geological, topographical and climatic diversity, and thus host greater biodiversity than the surrounding lowlands. Much of the biological richness in MBRs results from the interaction of climatic contrasts and gravitational forces along elevational gradients. External forces such as atmospheric change and human land use interact with these gradients, and result in distinct landscape patchiness, ie mosaics of land cover types within and across elevational belts. The management of MBRs influences land use and land cover, which affects biodiversity and ecosystem processes, both of which provide goods and services to society. Due to their broad environmental and biological diversity, MBRs are ideally suited for global change research and will be increasingly important in illustrating biodiversity conservation. This article summarizes the ecologically relevant results of an international workshop on elevational gradients that aimed to achieve a synthesis of the major ecosystem and biodiversity conditions and drivers in an altitude context. The workshop developed a core research agenda for MBRs that prioritizes long-term research and changes in land use across a broad elevational range.

Current land-use abandonment and the current rise in temperature in the Alps both suggest that tree limits may change. When it is assumed that the climate of the early mid-Holocene between 8000 and 5000 yr before present is analo-gous to that of the predicted climate of the late 21st century, palaeo-ecological studies of the early Holocene may providedata for the prediction of the vegetation pattern in a century from now. It appears that mid-Holocene charcoal assemblagesCorrespondence analysis (CA) of charcoal assemblages shows that an important ecological gradient is determined byelevation. However CA also shows that charcoal assemblages in profiles between 1700 and 2100 m a.s.l. are roughly strati-fied: the more recent assemblages from the topmost centime-tres of soil are intermediate between the lowermost assem-blages and assemblages from higher elevations. This suggests that the woody communities at the highest elevation werelocated at lower elevations at a later date. The taxonomic diversity of the soil charcoal assemblages has been comparedto that of present-day phytosociological relevés after transfor-mation to charcoal-equivalent data. This comparison revealedthat the vegetation pattern along the altitudinal gradient in the mid-Holocene was different from that at present. The assem-blages indicate that some communities disappeared, that Picea is a late-Holocene invading species, and that there isno strict modern analogue for the vegetation structure prior to that of 3000 yr ago. The past structure of the woodyvegetation was also different from that of today. Although past vegetation is not a good analogue for predicting futurevegetation patterns, it still has potential as an indicator for the potential presence of tree species where there is nonetoday. If we assume a temperature rise, and take into account current trends of landscape use abandonment, then we canexpect strong vegetation dynamics at the upper tree line in the future: Abies alba may expand to occupy elevations of ca 1800-2000 m in mixed communities with Picea abies, Pinus sylvestris and hardwood species, and Pinus cembra may ex-pand up to 2500-2700 m a.s.l.

Subalpine grasslands in the western Alps are currently facing major environmental changes induced by pastoral disuse. In such a context, the rapid spread of a shrub (Alnus viridis [Chaix] DC.) is expected to threaten biodiversity. The aim of this study was to assess the impact of its cover on vascular plant diversity. Data collected at several local levels (108 plots) in a representative area detected a linear decrease in species richness across an increasing A. viridis (green alder) cover gradient, whereas alder-stand β-diversity at the community level was particularly singular. We used constrained ordination methods based on Principal Components Analysis to partial out substantial variance explained by spatial structure of samples. The resulting partial Redundancy Analysis accounted for 62.3% of the initial inertia, and the environmental effect by A. viridis cover was identified as a major cause of variance in the whole species ordination (16.1%). The expansion of A. viridis on subalpine grasslands causes considerable environmental changes which have mostly a negative effect on the conservation of vascular plant diversity in the inner part of the western Alps. However, A. viridis appears to contribute to the diversity of the subalpine belt by inducing a peculiar floristic composition.