How do plant community ecologists consider the complementarity of observational, experimental and theoretical approaches ?

Background and aims -- A panoply of methods are used by ecologists for studies at the plant community level. While early studies were mainly descriptive, more manipulative experiments are now being undertaken because they provide a more functional approach and greater insight into underlying mechanisms. Mathematical models are also being increasingly used for prediction of biodiversity change under global change. The aim of this study is to highlight the forces, limitations, and advantages of these three approaches, namely observational, experimental and theoretical modelling. Methods -- We assessed 156 papers published during the last four years in three specialized disciplinary journals (DJ) and 186 papers in three generalist high impact journals (HIJ) dealing with plant ecology, and checked the methods that were used. We asked participants of the ECOVEG7 meeting held in Lausanne (April 2011) whether observational, experimental and theoretical modelling approaches can, or should, be used alone or in combination when studying plant communities and ecosystem functioning in the context of global change. Key results -- About 50 % of articles published in both journal types used only a single approach. Papers in HIJ used the approaches in similar proportions, while articles in DJ had 8 times more observational than modelling studies. Combined approaches represented only 8 % in DJ, while this percentage was more than double in HIJ. Conclusion -- Plant community ecologists favour a combination of several approaches to their studies, but for practical reasons single-approach studies are generally preferred. Obstacles to a more comprehensive approach include difficulties in communicating among people using different approaches and publication strategies, as it is believed to be difficult to publish observational studies at least in HIJ. Papers that combine different approaches are more suitable for publication in HIJ.

Effects of Subordinate Plant Species in Plant and Soil Community Structure and Ecosystem Functioning

Pierre Rémi Mariotte

The relevance of biodiversity to human health is an increasing international political issue as it causes concern for ethical and aesthetic reasons, but also has a strong impact on ecosystem properties and ecological goods and services utilised by humanity. However, humans have applied increasing pressure on worldwide biodiversity through pollution, land-use and climate change, over exploitation and the introduction of invasive species which can lead to major alterations of biological communities and a consequent decrease in biodiversity. When communities are assembled at random from a pool of species, more diverse mixtures have a higher probability to contain species or species-groups with high capacity to drive ecosystem processes. Therefore, it is relevant to ask "how may we classify species in a community in terms of functionality?", and "which species-group is important to maintain the productivity and stability of ecosystems?" In this thesis, I chose to classify species according their frequency and their cumulative relative cover in a plant community (i.e. plant hierarchy) and to differentiate three species-groups: dominant, subordinate and transient, which contribute towards biodiversity. Dominant species are clearly very important for ecosystem functioning, due to the large amount of biomass they produce, but there is growing evidence that subordinate species, which represent a low amount of plant biomass in grassland ecosystems, are also of considerable importance for ecosystem functioning. Given this recognition, a further objective was to explore the ecosystem level effects of these subordinate plant species, which is currently not well known. This research focused on two aspects: the persistence of subordinate species in the community, and their role in grassland ecosystems. The approach to satisfy these objectives encompassed four years of experimental field studies and glasshouse microcosm experiments. In order to explain the persistence of subordinate species in semi-natural grasslands, we performed two greenhouse experiments to test the effects of root competition exclusion and the inoculation of arbuscular mycorrhizal fungi (AMF) on the competitiveness of dominant and subordinate species, directly measured by their biomass production. The effects of subordinate species on ecosystem functioning were assessed through long-term field experiment in two sites in the Swiss Jura Mountains, where dominant, subordinate and transient species were preliminarily determined. At both site (Les Amburnex, La Frétaz), a removal experiment was carried out in randomly replicated plots with three different treatments: control without perturbations, removal of all subordinate species and partial removal of dominant biomass. In one site, a summer drought treatment was added using rainout shelters to simulate an extreme climatic event. Biomass production, litter decomposition and soil respiration were monitored during four years in each plot and complemented in some cases by measurements of soil dissolved inorganic nitrogen concentrations, soil microbial communities (bacteria and AMF) and carbon and nitrogen isotopes in plant leaves. The results of the different experiments conducted in this thesis highlight the importance of cattle activity and AMF on the persistence of subordinate plant species in semi-natural grasslands. Indeed, cattle activities (i.e. trampling) provide spatial heterogeneity, through gap creation (areas of root competition reduction), and thus favours the growth of less competitive subordinate species. While AMF acted as a parasite in the greenhouse experiment, the current findings suggest that the competitiveness of dominant species was reduced through the action of the fungus, which indirectly enhanced subordinate species. The interactions between subordinate species and soil organisms were confirmed in the field as bacterial and AMF communities shifted in plots where subordinate species were removed compared to the control treatments. Moreover, the absence of subordinate species had many negative effects on ecosystem functioning by reducing litter decomposition, soil respiration, nitrogen mineralization and community above-ground production. These findings suggest that plant-soil feedbacks explain the importance of subordinate species on ecosystem functioning despite only representing a very low quantity of biomass in plant communities. During the summer drought, subordinate species increased the resistance of the plant community and maintained productivity. Therefore, these outcomes confirm the role of drought-resistant subordinates in the functioning of grassland ecosystems threatened by climate change. The synthesis of this thesis has important implications for the way grasslands should be managed for biodiversity and confirms that extensive grazing must be maintained as it promotes species coexistence and the persistence of key subordinate species. Moreover, this work demonstrates the important role of subordinate species in ecosystem functioning, showing that not only dominant species determine ecosystem properties. This thesis serves as a contribution to the advances in our understanding of ecosystem functioning and above and below-ground linkages, and provides basis for future research in this domain.

EPFL2012

Combining short-term manipulative experiments with long-term palaeoecological investigations at high resolution to assess the response of Sphagnum peatlands to drought, fire and warming

Alexandre Buttler, Vincent Eric Jules Jassey, Mariusz Lamentowicz

Northern hemisphere peatlands are substantial carbon stores. However, recent climate change and human impacts (e.g., drainage and atmospheric nutrient deposition) may trigger the emission of their stored carbon to the atmosphere. Biodiversity losses are also an important consequence of those changes. Therefore, there is a need to recognise these processes in space and time. Global change experiments are often conducted to improve our understanding of the potential responses of various ecosystems to global warming and drought. Most of the experiments carried out in peatlands are focused on carbon balance and nitrogen deposition. Nevertheless, it is still unclear how fast peatlands respond to temperature changes and water-table lowering in the continental climate setting. This is important because continental regions account for a significant proportion of all northern hemisphere peatlands. A combination of short-term and long-term approaches in a single research project is especially helpful because it facilitates the correct interpretation of experimental data. Here we describe the CLIMPEAT project - a manipulative field experiment in a Sphagnum-dominated peatland supported by a high-resolution multi-proxy palaeoecological study. The design of the field experiment (e.g., treatments), methodology and biogeographical setting are presented. We suggest it is beneficial to support field experiments with an investigation of past environmental changes in the studied ecosystem, as human impacts during the past 300 years have already caused substantial changes in ecosystem functioning which may condition the response in experimental studies.

Int Peat Soc2016

Resistance of plant communities to invasive species

Aurélie Thébault

Invasive species are nowadays considered as one of the most important threat to biodiversity. By displacing native species, modifying ecosystem functioning and causing substantial losses to agricultural production, they represent a menace to natural and managed ecosystems. Although ecology of invasions has become an important research topic since the last decades, the mechanisms that determine why a given species may invade a given ecosystem and why some biomes are less resistant to invasion are still not clarified. Ecology of invasions is divided into 2 main topics: invasiveness and invasibility. While invasiveness refers to species ability to invade a community, invasibility focuses on the resistance of a community to invasion. Invasiveness may be the result of ecological processes, such as release from biotic constraints or human alteration of the environment (disturbance, stress…) or the consequence of evolutionary processes, such as hybridization or polyploidization that may increase genetic variation and therefore, enhance niche breadth. Invasibility has been said to be influenced by disturbance and biotic factors such as community diversity, dominant species identity, biotic interactions or community compositional stability. The invasion success is the consequence of the interaction between species invasiveness and community invasibility. Most studies in ecological invasions have focused on either invasiveness or invasibility, but hardly both together. By working at the same time and in the same conditions with native and introduced genotypes and by comparing their ecological performances, this thesis aims at a better understanding of both invasiveness and invasibility mechanisms. Two worldwide invasive species, Centaurea maculosa and Senecio inaequidens, were used in several experiments (pot, microcosm, field) to disentangle the importance of invasiveness and community invasibility in their invasion success. Both species encountered polyploidisation in their native range, leading to the presence of diploid and tetraploid populations, whereas only tetraploid populations have been found in the introduced range. Using native diploid, native tetraploid and introduced tetraploid genotypes of the two model-species, allows assessing the effects of genetic variation (diploid vs. tetraploid genotypes) and environmental variation (genotype from native vs. introduced range) on species phenotypic traits variations and consequently on fitness variation and invasiveness. In the community context, studying response of different genotypes to experimental factors and community change gives information on the interaction between invasiveness and invasibility. Plants were grown in pot, in field or in artificially built communities where (i) the management treatment, (ii) the community diversity and (iii) the spatial organisation of resident species were manipulated. In addition, (iv) community species composition, (v) community competitive ability and (vi) compositional and functional stability of the community were monitored. According to the experiments, data were gathered on survival, morphological traits (vegetative height, lateral spread, shoot and root biomass), leaf traits (specific leaf area, leaf dry matter content) and reproductive traits (probability of flowering, capitulum production) of the genotypes (native diploid, native tetraploid and introduced tetraploid) of both model species. Through the use of statistical models and multivariate approaches, the effects of management and biotic factors on survival, growth and reproduction of native and introduced genotypes of these two worldwide invasive species were assessed. Invasion strategy of the two model species was investigated through a growth experiment in optimal conditions in a pot experiment. High investment in seed production could explain invasive success of S. inaequidens through high propagule pressure, whereas C. maculosa's strategy seemed to be oriented towards interactions with belowground communities, as shown by the shift in rhizosphere bacterial communities between genotypes. For both species, polyploidisation in the native range could be linked to a specialisation towards higher competitive ability, which could have allowed the first step of invasion. Introduction in the new range could be related to a loss of specialisation through selection of traits allowing coping with various or changing environments, improving successful spread. Survival of both model species was highly affected by community spatial pattern, management and neighbouring competition. Responses of growth and reproductive output of native and introduced genotypes to management, community spatial pattern and community diversity were species-specific. Growth and reproductive output of both genotypes of S. inaequidens were affected by experimental factors whereas introduced genotypes of C. maculosa were less affected than native ones. Comparison of response of native and introduced genotypes to experimental factors allowed defining two strategies of invasion based on phenotypic plasticity. Centaurea maculosa was able to maintain fitness in all kinds of environments, either favourable or stressful ("Jack-of-all-trades" invader). Senecio inaequidens was able to deal with all kinds of environments and was also able to increase its fitness in favourable conditions ("Jack-and-Master" invader). The combination of the effects of management, community spatial pattern and community diversity on species genotypes allowed defining, for both invasive species, two invasion phases which were impacted by different factors. The introduction phase corresponds to the survival of seedlings and their ability to deal with neighbouring competition. If seedlings manage to survive despite neighbouring competition, they grow and reproduced in order to spread, which corresponds to the establishment phase. In terms of management perspectives, regular mowing or use of highly covering species could limit invasive success of S. inaequidens and C. maculosa respectively. The synthesis of all the experiments conducted in this thesis with C. maculosa and S. inaequidens highlights (1) the importance of polyploidisation in the invasion process as well as (2) the species-specific invasion strategies and consequently (3) the species-specific response of invasive species to abiotic and biotic factors. It also emphasizes on (4) the temporal evolution of the interaction between invasiveness and invasibility since the community factors that affected invasive species fitness changed according to the invasion stage (introduction vs. establishment phase) of the invader. The provided insights into the importance of the interaction between species invasiveness and community invasibility will contribute to improve knowledge in ecology of invasions, in addition to supply some clues for management efficiency.