Causes and consequences of the rarity of plant species

The reasons why some plant species are rare, while some others are not, is a long-standing concern for biologists. A better understanding of drivers of plant rarity is central to broaden our knowledge of the distribution of species and is crucial for conservation practitioners to protect rare and threatened species. However, despite a large literature body dedicated to this question, we are lacking understanding and evidences on the effects of factors suggested to contribute to plant species rarity and factors suggested to increase success of conservation projects of threatened plants. In this thesis, I focused on investigating whether generalist herbivory and genetic processes could contribute to further plant species rarity and how genetic diversity, propagule pressure and plant origin alter reintroduction success. To achieve this, I have conducted multi-species experiments taking into account the local and regional rarity, habitat characteristics and several traits linked to life-history and resource-allocation strategies of the plant species. In the first chapter of this thesis, I investigated how the performance and preference of one below-ground and three aboveground invertebrate generalist herbivore species vary with the rarity, habitat characteristics and traits linked to resource-allocation strategies of 62 different plant species. I found that regionally and locally rare and common plant species did not generally differ in their defense against generalist herbivores. The results indicate that the hypothesis that rare plant species are less defended against herbivores does not hold for herbaceous plant species. Instead, they suggest that the ability of plants to allocate resources away from defense against herbivores to achieve a stronger competitive ability might have allowed plants to become locally and regionally common. In the second chapter, I tested whether selfing and between-population outcrossing affected five fitness-related traits for 16 species of different local and regional rarity, habitat characteristics and resource-allocation strategies. The results demonstrate that regional rarity, habitat characteristics and traits linked to resource-allocation strategies shape the outcome of selfing and between-population outcrossing in plants. This study highlights that inbreeding depression may not represent a major threat to species that are already rare. Moreover, as between-population outcrossing appears beneficial for plant fitness of many species, including rare ones, we suggest considering it for plant-conservation activities. In the last chapter, I investigated how genetic diversity, propagule pressure and population of origin alter fitness of reintroduced rare and threatened plant species. The results show that genetic diversity and population of origin interacted and were important determinants of early plant fitness. The effect of genetic diversity was positive or negative depending on how ecologically similar the populations of origin to the translocation site are. In addition, propagule pressure may affect negatively fitness of reintroduced plants, possibly due to intra-specific competition or plant antagonists. The study underlines the importance to match reintroduction and origin sites very carefully by conducting vegetation record to assess ecological similarity. Altogether, the results of this thesis challenge important hypotheses about rare plant species and suggest that plant species rarity may be driven by complex interactions between extrinsic and intrinsic factors. Finally, it highlights the importance of considering context dependency in multi-species experiments in order to disentangle patterns related to plant rarity from those related to plant habitat and resource allocation strategies

Environmental variation in sex ratios and sexual dimorphism in three wind‐pollinated dioecious plant species

Variation in plant sex ratios is often attributable to sex-specific mortality in heterogeneous environments that differentially limit male and female plant reproduction. Yet sexual dimorphism and plastic responses to environmental heterogeneity are common and may co-vary with variation in sex ratios. Here, we show that the sex ratio and the degree of sexual dimorphism for a number of plant traits varied along climatic and elevation gradients in three wind-pollinated dioecious species, Rumex lunaria, Urtica dioica and Salix helvetica. Some of the observed sex-specific responses to climatic variation are consistent with greater sensitivity of females to water scarcity, but most responses rather point to the greater sensitivity of males to ecological stress, consistent with larger male reproductive effort, as has been commonly reported for wind-pollinated plants. In contrast, we found no evidence for variation in either sex ratios or sexual dimorphism expected under sexual selection. Interestingly, sex ratios and sexual dimorphism varied both along distinct and the same ecological axes of variation, suggesting that the evolution of sexual dimorphism in the measured traits was not sufficient to prevent sex-specific mortality

A common soil temperature threshold for the upper limit of alpine grasslands in European mountains

Open Access funding provided by Université de Lausanne. Field inventories and temperature loggers were financially supported by 5th RTD Framework Programme of the European Union, UK Centre for Ecology and Hydrology, Département de la culture et des sports du Valais, Departamento de Medio Ambiente del Gobierno de Aragón, Foundation Dr. Joachim de Giacomi, Fondation Mariétan, Italian project of strategic interest NextData, MAVA Foundation, Frignano Regional Park (Modena), Norwegian Environment Agency, Ordesa & Monte Perdido National Park, Research Commission of the Swiss National Park, Scientific Grant Agency VEGA (project Nr. 2/0132/18), Scottish Natural Heritage, Sierra Nevada National Park, Société académique de Genève, Swiss Federal Office of Education and Science, Swiss Federal Office for the Environment, Tiroler Wissenschaftsfonds, Tuscan-Emilian Apennines National Park, and Wissenschaftsförderung der Südtiroler Landesregierung.We thank the numerous collaborators in each region who participated in field inventories, S. Jordan for his preliminary analyses, and A.-L. Aeby and F. Schütz for their assistance on statistical analyses. We are particularly grateful to C. Körner and two anonymous reviewers for their helpful comments on earlier versions of the manuscript.While climatic research about treeline has a long history, the climatic conditions corresponding to the upper limit of closed alpine grasslands remain poorly understood. Here, we propose a climatic definition for this limit, the 'grassline', in analogy to the treeline, which is based on the growing season length and the soil temperature. Eighty-seven mountain summits across ten European mountain ranges, covering three biomes (boreal, temperate, Mediterranean), were inventoried as part of the GLORIA project. Vascular plant cover was estimated visually in 326 plots of 1 x 1 m. Soil temperatures were measured in situ for 2-7 years, from which the length of the growing season and mean temperature were derived. The climatic conditions corresponding to 40% plant cover were defined as the thresholds for alpine grassland. Closed vegetation was present in locations with a mean growing season soil temperature warmer than 4.9 degrees C, or a minimal growing season length of 85 days, with the growing season defined as encompassing days with daily mean >= 1 degrees C. Hence, the upper limit of closed grasslands was associated with a mean soil temperature close to that previously observed at the treeline, and in accordance with physiological thresholds to growth in vascular plants. In contrast to trees, whose canopy temperature is coupled with air temperature, small-stature alpine plants benefit from the soil warmed by solar radiation and consequently, they can grow at higher elevations. Since substrate stability is necessary for grasslands to occur at their climatic limit, the grassline rarely appears as a distinct linear feature.Universite de Lausanne5th RTD Framework Programme of the European UnionFoundation Dr. Joachim de GiacomiFondation MarietanMAVA FoundationNorwegian Environment AgencyScottish Natural HeritageSociete academique de GeneveSwiss Federal Office of Education and ScienceSwiss Federal Office for the EnvironmentTiroler WissenschaftsfondsTuscan-Emilian Apennines National ParkWissenschaftsforderung der Sudtiroler LandesregierungOrdesa & Monte Perdido National ParkResearch Commission of the Swiss National ParkVedecka grantova agentura MSVVaS SR a SAV (VEGA) 2/0132/18Italian project of strategic interest NextDataFrignano Regional Park (Modena)UK Centre for Ecology and HydrologyDepartement de la culture et des sports du ValaisGobierno de Arago

Are rare plant species less resistant than common ones to herbivores? A multi-plant species study using above- and below-ground generalist herbivores.

Rare plant species are suggested to be less resistant to herbivores than common species. Their lower apparency and the fact that they often live in isolated populations, resulting in fewer herbivore encounters, might have led to the evolution of reduced defences. Moreover, their frequent lower levels of genetic diversity compared with common species could negatively affect their resistance against enemies. However, the hypothesis that plant resistance depends on plant regional and local rarity, independently of habitat and competitive and growth strategy, lacks evidence. To test this hypothesis, we assessed the performance and preference of one belowground and three aboveground generalist invertebrate herbivores from different taxonomic groups as indicators of plant resistance. Herbivores were fed a total of 62 regionally and locally rare and common plant species from Switzerland. We accounted for differences in a plant's growth and competitive strategy and habitat resource availability. We found that regionally and locally rare and common plant species did not generally differ in their resistance to most generalist herbivores. However, one herbivore species even performed better and preferred locally and regionally common plant species over rarer ones, indicating that common species are not more resistant, but tend to be less resistant. We also found that all herbivore species consistently performed better on competitive and large plant species, although different herbivore species generally preferred and performed better on different plant species. The latter indicates that the use of generalist herbivores as indicators of plant-resistance levels can be misleading. Synthesis: Our results show that rare plant species are not inherently less resistant than common ones to herbivores. Instead, our results suggest that the ability of plants to allocate resources away from defence towards enhancing their competitive ability might have allowed plants to tolerate herbivory, and to become locally and regionally common

A common soil temperature threshold for the upper limit of alpine grasslands in European mountains

While climatic research about treeline has a long history, the climatic conditions corresponding to the upper limit of closed alpine grasslands remain poorly understood. Here, we propose a climatic definition for this limit, the ‘grassline’, in analogy to the treeline, which is based on the growing season length and the soil temperature. Eighty-seven mountain summits across ten European mountain ranges, covering three biomes (boreal, temperate, Mediterranean), were inventoried as part of the GLORIA project. Vascular plant cover was estimated visually in 326 plots of 1 x 1 m. Soil temperatures were measured in-situ for 2–7 years, from which the length of the growing season and mean temperature were derived. The climatic conditions corresponding to 40 % plant cover were defined as the thresholds for alpine grassland. Closed vegetation was present in locations with a mean growing season soil temperature warmer than 4.9 °C, or a minimal growing season length of 85 days, with the growing season defined as encompassing days with daily mean ≥ 1 °C. Hence, the upper limit of closed grasslands was associated with a mean soil temperature close to that previously observed at the treeline, and in accordance with physiological thresholds to growth in vascular plants. In contrast to trees, whose canopy temperature is coupled with air temperature, small-stature alpine plants benefit from the soil warmed by solar radiation and consequently, they can grow at higher elevations. Since substrate stability is necessary for grasslands to occur at their climatic limit, the grassline rarely appears as a distinct linear feature