PhD position (non-EU citizen / non-OECD citizen) At: The Resource Ecology Group, Wageningen University, The Netherlands
The Resource Ecology Group performs research and education in community ecology of large herbivores, with emphasis on their interactions with the vegetation. We consider aggregation levels ranging from plant part to ecosystem, with a geographical focus on Africa. The group has an outstanding international scientific status and all our graduate students face prosperous carrier outlooks. Within the general research field 'Sustainable management of natural resources' of Prof. de Bie, we now offer a PhD position for the period 1 June 2007 - 1 June 2011. PhD project The study focuses on the role large savanna trees have on nutrients and moisture for tree seedlings and grasses. How do these large trees alter the balance between competition and facilitation of trees and grasses in savannas? We will experimentally test whether large trees can enhance plant available nutrients (the nutrient-pump hypothesis) and moisture (the two-layer hypothesis) using isotope tracers in South Africa. Preparation, data analysis and the thesis defence will be carried out at Wageningen University. Requirements (Plant)Ecologist, agronomist, geo-botanist or soil scientist (MSc degree) with experimental experience in tropical fieldwork and with an interest in savanna ecology. Experience in isotope tracers in plants and statistical data analysis is preferred. In possession of a driving licence. Candidates with a publication record are preferred. Renumeration A full-time position for 4 years. The stipend will be 1100 Euro per month when in the Netherlands and 700 Euro when in the country of fieldwork. The position is open for non-EU citizens only. Information For information contact Dr. Frank van Langevelde, tel.: +31 317 484750; email [EMAIL PROTECTED], or Prof Steven de Bie, email: [EMAIL PROTECTED] Application Send your application, including CV, before April 15 to [EMAIL PROTECTED] Project PhD: Role of large savanna trees on plant available nutrients and moisture: testing the nutrient-pump hypothesis and two-layer hypothesis Summary of project Savannas are characterized by the co-dominance of trees and grasses. Savannas are economically important for livestock grazing and, especially in Africa, famous for their diversity of large mammals. These biomes are mainly characterised by water-limited plant growth, as annual evaporative demand exceeds precipitation. The co-occurrence of woody plants and grasses in savannas is determined by a complex set of interacting factors, mainly water, nutrients, fire and herbivory. The effect savanna trees exert on understorey grass productivity has been a focus of research for decades, and both negative and positive effects have been described. Trees reduce grass growth by decreasing light availability and by competing with grass for water and nutrients, but trees can also improve grass production by increasing soil moisture via hydraulic lift, by reducing evapotranspiration and through augmenting soil nutrient availability (trees could act as nutrient pump). The coexistence of savanna trees and grasses is largely debated, and is known as the "savanna problem". The dominant competition-based model for the stable tree-grass coexistence throughout the world is rooting niche partitioning, i.e. the so-called two-layer hypothesis. However, there is poor empirical evidence for the root niche separation, and resource competition alone appears to be insufficient to generate long-term tree-grass coexistence. Earlier studies showed that savanna trees affect grass growth by reducing water availability due to below ground competition and increasing soil nutrient concentrations. The combination of these two can sharply increase nutrient concentration of grasses and thus the forage quality. We hypothesise that large savanna trees are essential for the survival of large herbivores because large herbivores need the forage of high quality from under trees. The objective of this research is to test the nutrient-pump hypothesis and the two-layer hypothesis. Testing these hypotheses will help us to unravel the balance between competition and facilitation of trees and grasses in savannas. We will set up experiments by releasing isotope tracers at different depths to determine the origin of both water and nutrients that are taken up by large trees. Can large trees indeed ameliorate soil nutrient concentrations underneath their canopy? Do large trees use water from deeper soil layers, additional to the water from the top soil for which they compete with grasses? Moreover, we will compare the performance of tree seedlings and grasses underneath these large trees. We will use shading, addition of water and nutrients, and clipping to test what limits the survival and growth of these tree seedlings, whether there is competition for resources with the grasses, and whether grazing negatively affects tree recruitment.
