How Does Plant Diversity Affect Soil Aggregate Stability and Erosion Processes in Disturbed Alpine Ecosystems?

Plant species diversity is hypothesized to be among the most relevant factors for enhancing soil stability in disturbed sites at high elevation. Because a more diverse plant community may comprise a high number of plant species of different growth forms, plant functional groups, and root characteristics, the chance of including a species that fulfills a specific key function may increase, thereby ensuring ecosystem integrity. Although plausible, the contribution of plant diversity to soil stabilisation in general, and to erosion control on alpine ski slopes in particular, has scarcely been demonstrated. This thesis investigated the relationship between plant diversity and soil stability in disturbed alpine ecosystems. The main objectives were (i) to determine root traits of 13 alpine pioneer species growing on a machine-graded ski slope without the in uence of neighbors, in order to address mechanisms of belowground diversity effects (Chapter 2), (ii) to quantify the contribution of plant diversity to soil stability by testing the aggregate stability of the soil from machine-graded ski slopes and from adjacent undisturbed vegetation (Chapter 3), (iii) to compare aggregate stability data from both graded and un-graded ski slopes and the associated control sites (Chapter 4), and (iv) to investigate the relationship between interrill erosion, vegetation cover, and plant functional diversity by applying rainfall simulations (Chapter 5). Both field and laboratory experiments were conducted. Chapter 2. Belowground diversity in root characteristics of alpine plants: key traits for soil restoration. Alpine plant species were highly diverse in functional root traits. Three of the 13 studied species were significantly different in root length and spreading, plant age and tensile strength. The results show that belowground diversity can be enhanced substantially by employing a few plant species with specific root traits. This study suggests that restoration attempts should aim at selecting species with a high belowground diversity to achieve a high diversity in functional root types which can positively affect slope stability. Chapter 3. Higher plant diversity enhances soil stability in disturbed alpine ecosystems. Plant diversity, vegetation cover and root density positively influenced aggregate stability, which was significantly lower on ski slopes compared to control sites. Out of all the tested variables, plant diversity explained considerably more variance than abiotic soil parameters. Plant species associated with more diverse communities were from various functional groups, i.e. grasses, forbs and shrubs. A higher density of fine roots and, to a lesser extent, coarse roots positively influenced aggregate stability. This study showed that, in addition to re-establishment of a functional vegetation cover with a dense root system, a high number of plant species with different growth forms is important. Chapter 4. Soil aggregate stability on alpine ski slopes. Aggregate stability was lower on graded ski slopes, but not on ungraded ski slopes compared with control plots . Root length density, number of plant species and vegetation cover were all positively correlated with aggregate stability. However, more than 13 plant species, 80% vegetation cover and 0.011 g cm-3 root density did not cause a further increase in aggregate stability. In multiple regression analysis, we determined that the effect of the number of plant species on aggregate stability was more pronounced on graded and gravelly ski slopes than on un-graded and sandy slopes. This study provided evidence that high plant diversity plays an important role in soil aggregate stability at disturbed alpine sites. Chapter 5. Interrill erosion at disturbed alpine sites: effects of plant functional diversity & vegetation cover. Interrill erosion was reduced the most when vegetation cover was high: 60% vegetation cover reduced the sediment yield by 83% compared to unvegetated ground. At 60% vegetation cover, sediment yield was significantly reduced in the presence of three functional groups compared to one plant functional group. Furthermore, combinations of plant functional groups including grasses reduced the sediment yield more than combinations without grasses. This study supports the view that, besides the re-establishment of a closed vegetation cover, high plant functional diversity is relevant for reducing interrill erosion at disturbed alpine sites. In conclusion, the resistance of topsoil to water-induced erosion at disturbed alpine sites depends not only on the degree of vegetation cover, but also on the presence of a high number of plant species that are highly diverse in functional traits above and below ground.

Functional traits and root morphology of alpine plants

Alexandre Buttler

Background and Aims Vegetation has long been recognized to protect the soil from erosion. Understanding species differences in root morphology and functional traits is an important step to assess which species and species mixtures may provide erosion control. Furthermore, extending classification of plant functional types towards root traits may be a useful procedure in understanding important root functions.

Oxford University Press2011

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

Theoretical and Experimental Studies of Dendritic Metacommunities

Francesco Carrara

The present thesis deals with the understanding of the origins and the mechanisms of maintenance of biodiversity in natural landscapes, in particular by identifying key processes that define large-scale patterns of abundance and diversity. Biological communities often occur in spatially structured habitats where connectivity directly affects dispersal and metacommunity processes. Recent theoretical work suggests that dispersal constrained by the connectivity of specific habitat structures affects diversity patterns and species interactions. This is particularly relevant in dendritic networks epitomized by fluvial ecological corridors. This thesis addresses whether connectivity alone can explain observed features of biodiversity and selectively promote different components of community composition in river-like landscapes, such as local species richness or the among-community similarity. The relevance of this thesis lies in the major ecological challenges posed by the topic, and its fundamental importance to conservation biology. The studies pursued herein are also deemed relevant because of the influence of the spatial connectivity and dispersal on population dynamics and of the relevance of biodiversity to ecosystem functioning. Mechanisms of species coexistence were investigated with a blend of theoretical tools (broadly related to statistical mechanics and the theory of stochastic processes) and experimental work using laboratory microbial communities. The research tools ranged from aspects of modern coexistence theory in a local perspective to the recent concept of the metacommunity in spatial ecology, within a unified framework. The study of biodiversity in riverine ecosystems guided by observational data has been addressed by combining theoretical metacommunity models with laboratory experiments. The results are diverse. First, they show experimentally that connectivity per se shapes key components of biodiversity in metacommunities. Local dispersal in isotropic lattice landscapes homogenizes local species richness and leads to pronounced spatial persistence. By contrast, dispersal along dendritic landscapes leads to higher variability in local diversity and among-community composition. Although headwaters exhibit relatively lower species richness, they are crucial for the maintenance of regional biodiversity. By suitably arranging patch sizes within river-like networks the effect of local habitat capacity (i.e., the patch size) and dendritic connectivity on biodiversity can be experimentally disentangled in aquatic microcosm metacommunities. It is shown in this thesis that species coexistence and community assembly depend on intricate, non-trivial interactions of local community capacity and network positioning. Furthermore, an interaction of spatial arrangement of habitat capacity and dispersal along river-like networks also affects a key ecosystem descriptor, namely regional evenness. High regional evenness in community composition is found only in landscapes preserving geomorphological scaling properties of patch sizes. In riverine environments some of the rarer species sustained regionally more abundant populations and were better able to track their own niche requirements compared to landscapes with homogeneous patch size or landscapes with spatially uncorrelated patch size. All the experimental results were supported and extended by a theoretical analysis where the above mechanisms have been generalized. This thesis provides the first direct experimental evidence that spatially constrained dendritic connectivity is a key factor for community composition and population persistence in riverine landscapes. As such, this thesis assesses a longstanding issue in spatial community ecology. It offers unique insights into the ecological forces structuring natural communities in a key ecosytem, and demonstrates principles that can be further tested in theoretical metacommunity models possibly to be extended to real riverine ecosystems. Taken together, the analyses show how the structure of ecological networks interacts with the spatial environmental matrix in determining biodiversity patterns and the functioning of biological communities. The analyses also suggest that altering the natural linkage between dendritic connectivity and patch size strongly affects community properties at multiple scales. The first part of this thesis (chapters 2 and 3) addresses key aspects of biodiversity-ecosystem functioning research where the combination of theory-guided experiments and theoretical investigations shows how a stochastic implementation of population dynamics proves fundamental for key community properties such as species persistence and community stability. The diversity-productivity and diversity-stability relationships are explored. Both experimental findings and the results of a stochastic model fitted to the experimental interaction matrix, suggest the emergence of strong stabilizing forces when species from different functional groups interact in the same environment, increasing species coexistence and community biomass production. The last part (chapter 6) provides a synthesis of this thesis work, in that it aims at unifying aspects from niche-theory, usually adopted in spatially implicit models, with those characteristic of a spatially explicit context from a typical real-life mountainous regions. It is dedicated to the possible explanation for a macroecological pattern routinely observed from organisms in different domains of life, that is, the mid-elevational peak in local species richness. Guided by empirical observations on diversity of macroinvertebrates in Swiss river basins, a theoretical ansatz is provided which is deemed to capture the essential geomorphological drivers and controls relating species-fitness to altitude. A set of overarching conclusions and perspectives for future research are discussed in the concluding chapter.