Responses and vulnerability of the genus viola to elevational gradients and alien invasive species in mountain systems

Abstract

Introduction Mountain ecosystems are biodiversity hotspots due to their geographical isolation, their role as a species refuge during past climate changes, and the wide range of environmental conditions present along their altitudinal gradients. The environmental gradients common in alpine systems allow the ecological responses of plants to different environmental conditions to be tested over short distances. They thus function as natural laboratories, especially useful for understanding plant responses to global change. For example, the presence of invasive species in these systems can lead to synergistic effects with those of climate change, having dramatic consequences for threatened native species or fragile ecosystems, such as those found in the high mountain zones of oceanic islands. Content of research The main objective of this thesis is to explore physiological, reproductive and phenotypic responses of entomophilous species (genus Viola) in mountain systems to different biotic and abiotic conditions found along their altitudinal gradients. In Chapters 1 and 2 the performance, reproductive success and floral traits of Viola cheiranthifolia (Tenerife) are assessed along its altitudinal gradient, as well as the interactive effects of an invasive herbivore (rabbits) on these characters. Chapter 3 explores how phenotypic features of the flowers and leaves of a high Andean violet (Viola maculata) vary in response to an altitudinal gradient. In this gradient, these two phenotypes may conflict, as large flowers may be more attractive to pollinators. However, if they are positively associated with larger leaves, a dilemma could arise in a drought scenario, as small leaves would be selected. In Chapter 4, we analyse how the production of chasmogamous (CH) and cleistogamous (CL) flowers in Viola jaubertiana varies according to size, water stress and herbivory per individual, and across the altitudinal gradient. In addition, we also evaluated the mechanisms that may be favouring the maintenance of outcrossing in this predominantly cleistogamous species. Conclusions Chapter 1 and 2. Despite increasingly adverse environmental conditions with altitude, V. cheiranthifolia did not show greater pollen limitation with altitude, which we attribute to compensatory mechanisms such as increased autogamy. The rabbits in the National Park are found to greatly reduce the performance and reproductive success of V. cheiranthifolia. This in turn alters the species-environment interactions along its altitudinal gradient and also the selection pressures that pollinators and abiotic conditions exert on their floral traits. Chapter 3. In this study, the inverse water stress gradient (i.e. higher to lower altitude) in the Central Andes was confirmed, but no decoupled correlation was detected between floral and vegetative traits in the population with highest water stress, plants with small leaves also having small flowers. This absence of decoupling may be due to the high self-fertilization capacity of this species, and to the strong selection pressure exerted by water stress, constraining both phenotypes. Chapter 4. Viola jaubertiana showed a great pollen limitation, besides a very low reproductive success of the CH flowers. In contrast, the CL flowers were much more successful; they were more economical to produce and provided 100 times more seeds than CH flowers. Despite this great imbalance, the greater mass, dispersion potential and germination of seeds from outcrossing, together with a probably positive effect of heterosis, could explain the maintenance of chasmogamy in this Mallorcan endemism.