A) Evolution of heterocarpic plants
Many composites produce two or more types of seeds which differ in dispersal and germination characteristics. This heterocarpy is considered a type of bet-hedging strategy whereby different seed morphs are produced in environments which are spatially and/or temporally variable.
A1) Along a gradient of environmental unpredictability (i.e. an aridity gradient in Israel), the ratio of far vs. short dispersing seeds should change: more seeds with high dormancy should be produced towards the arid. There should be a trade-off with dispersal, i.e. dispersal distance and seed dormancy should be inversely related. This hypothesis will be tested using a collection seeds from of annual heterocarpic species. They will be grown in Winter 2021/22. Start: Sept-Nov 2021 (preparation), peak work March-May 2022 (i.e. between Nov and March there is time for taking courses).
A2) Plasticity should be more pronounced for plants coming from more variable (i.e. arid environments). Thus we assume that plants will plastically change the ratio between the two seed morphs more when coming from arid sites. This hypothesis will be tested with growing several species of annual plants from an aridity gradient under drought and well-watered conditions. Start: Sept-Nov 2021 (preparation), peak work March-May 2022 (i.e. between Nov and March there is time for taking courses)
B) Competition intensity and relatedness
In an equilibrium world, coexistence requires that plants differ from each other. Therefore, competition should be stronger between plants that are very similar. Thus, competition intensity should increase along a gradient of relatedness, ranging from between regions, between family within a region, between species within a family, between individuals within a species from different climates, between individuals from same climate and species, between sibling plants. Relatedness should also affect symmetry of interactions and should be more symmetric on an intraspecific level.
This topic is more relevant for an MSc level and will be tested with the collection of Israeli annuals, i.e. this is a winter topic (Nov-May).
C) Plant-soil feedbacks
Plants can modify the microbial community in the soil they are growing in and very often, there are negative plant-soil feedbacks. These result in intensified intraspecific negative interactions and have thus been proposed as a mechanism promoting coexistence among species. Plant-soil feedbacks have also been studied in the context of invasion biology, i.e. studies have tested whether conditioned soil may inhibit invasion.
In this topic, we can make use of a huge common garden experiment with 2800 pots that were conditioned with altogether 30 common grassland species. We would like to start a pilot study in Spring 2020, probably with Arabidopsis thaliana as this species grows fast and has no mycorrhiza, i.e. we can rule out positive plant-soil feedback effects. We could then also make a larger experiment where we test the use of native species for suppressing common invasives such as Solidago canadensis or Impatiens glandulifera.
D) Plant-microbe interactions under climate change
Climate change in our region is predicted to generate a higher frequency of extreme drought events. This may affect organisms directly, by decreasing resource availability, or indirectly, by modifying organismic interactions. This project, which is embedded in a long-term experiment entitled ‘Drier’, aims at studying the change in the microbial community associated with common grassland plants, when these are exposed to extreme drought events in the field.
Methods: The microbial community of several common grassland plant species has been sampled in Summer 2020, after a 1- to 3-year drought in a long-term field experiment. The first step in this project is to characterize this community in terms of differences among plant species and drought treatments. This will be done by high-end molecular methods, and will be mostly conducted in the lab of Eric Kemen (ZMBP).
E) Drought and overgrazing as drivers of a desertification tipping point in semi-arid rangelands (Bachelor thesis)
Background: Changing environmental conditions, such as frequent and intense drought events, coupled with unsustainable management practices like overgrazing, continue to degrade drylands. One main concern is the irreversible transition of productive rangelands into degraded states, a phenomenon known as tipping points. Because tipping points remain difficult to anticipate in real-world systems, they often come as an unpleasant surprise. Research under the NamTip project (https://rb.gy/owbk6d) aims to gain a better understanding of these shifts, by focusing specifically on desertification, which is the most common tipping point in dryland ecosystems.
Methods: Drone multispectral data were collected between 2021 and 2024 at an experimental site called ‘TipEx’. The experiment attempts to push a semi-arid rangeland in Namibia to cross a tipping point by simulating drought (using rainout shelters) and grazing (through clipping) at various intensities. The assumption is that once the system crosses the tipping point, the effects will cascade through various ecosystem components, usually with a fast response observed for primary production. The first step in this project is to analyze drone imagery to produce vegetation index maps from which primary production can be estimated. The results will contribute to understanding the effects of drought and overgrazing on forage provision and the potential to identify a desertification tipping point.