Research

Rising atmospheric CO2 is greatly altering the productivity and composition of plant communities across the world. Increasing CO2 stimulates the photosynthesis and growth of plants but the ability to respond to CO2 differs significantly across species and functional groups. For grass-dominated ecosystems, such as grasslands and savannas, which together cover 25% of the world's vegetated terrestrial area and have vast socio-economic value, rising CO2 is a particularly significant threat. Elevated CO2 is contributing to significant grassland community changes, such as the substantial increases in C3 tree and shrub densities seen in C4 grass-dominated ecosystems across the world in recent decades. However, changes in community dynamics are not solely down to the CO2-responsiveness of plant species, but also their competitive ability. How these two aspects of plant performance under a changing climate interact to determine community changes is unclear, making the future of grassland communities, and their associated array of ecosystem services and biodiversity, difficult to predict. Predicting grassland community interactions in future high CO2 climates requires an understanding of the interactions between a plant’s capacity to respond to eCO2 and its competitive ability. Along with Prof Peter Reich and other colleagues, I address this question by utilizing experimental data from grassland ecosystems around the world to assess how plant performance is influenced by its ability to respond positively to eCO2 stimulation and its competitive ability relative to its neighbors. We confirm that the high plant CO2 stimulation expected from theoretical models and experiments which exclude competition are unlikely to be seen in field scenarios with strong competitive dynamics and that CO2 stimulation will not provide significant greater tolerance of competition for most species studied. Both spatial and temporal variability will play a great role in determining competitive outcomes in unstable and dynamic systems which have tree-grass mixtures.

I am currently working as the Program Manager for the Institute for Global Change Biology at the University of Michigan. The community composition of grassland regions are particularly sensitive to climatic changes due to the alternate photosynthetic types present in many of these systems. Much of the grasslands around the world are home to both C3 and C4 grasses, each of which evolved at different times in the Earth's history and thus have different requirements for resources such as CO2 and water and different temperature tolerances. The Free-Air CO2 Enrichment experiment, BioCON, which is housed at the Cedar Creek Ecosystem Science Reserve in Minnesota provides a long-term (26 years to date) study of how an array of C3 and C4 grasses respond to changes in atmospheric CO2 concentrations, nitrogen availability, water availability and temperature. In this study I am exploring how changes in these factors influences the photosynthesis and growth of 4 species of C4 grass and 4 species of C3 grass, and how these responses influence competitive interactions. The outcomes of this study will provide important insight when predicting future community level changes in North American grassland systems.  

Fire plays an important role in maintaining the savanna tree-grass balance by limiting the recruitment of heat-sensitive tree seedlings. However, fire behaviour may change under increasing CO2 concentrations, due to altered flammability of the grassy layer. We determined the effect of predicted future CO2 concentrations, and how it interacts with water-availability, on grass flammability and traits influential to flammability, and uncovered the physiological mechanisms underpinning these responses. Using the widespread C4 savanna grass, Themeda triandra, as a model, we found that improved water-use efficiency under elevated CO2 (800 ppm) resulted in a larger (greater aboveground biomass), but wetter (higher moisture content) grass fuel load, that cured at a slower rate under drought conditions. These changes were associated with increased time to ignition, reduced flaming times and reduced predicted rate of spread. We modelled the effect of altered grass flammability on fire behaviour at a national level (South Africa), finding large-scale reductions in fire spread under elevated CO2, mitigating the converse effects of predicted increases in aridity, and marginal increases in fireline intensity. CO2-induced reductions in fire frequency, spread or intensity could have serious implications for savanna vegetation dynamics, possibly exacerbating the woody encroachment already seen in these ecosystems across the world. Publications linked to this project in the African Journal of Range and Forage Science.

The recent discussion around woody thickening in savannas has placed much emphasis on rising atmospheric CO2 as a driver of woody abundance increases. However, some of the complexity of these systems and their interactive responses to climate, disturbance and elevated [CO2] (eCO2) have been only partially considered. My thesis examined these interactions and provided insight into the effects of climate change on seedlings of a typical southern African C3 woody encroacher (Vachellia karroo). Seedling responses to eCO2 were highly dependent on interactions with levels of disturbance, competitive interactions, light availability, and water availability. The inconsistencies seen in C3 seedling eCO2 responses and the fact that C4 grasses can see an eCO2 benefit under water-limited conditions complicates the expectations that C3 plants will always have the advantage in high CO2 climates. I found that magnitude of C3 woody seedlings’ eCO2 responses will likely be site-specific (depending on resources and disturbance) and will occur with the greatest magnitude when conditions are suitable for CO2 to have a maximum effect. Publications linked to this project in the Journal of Ecology and Functional Ecology.

C4 grasses dominate much of the world’s warm-climate grassland and savanna regions, particularly those that are characterised by extreme dry seasons. C4 photosynthesis has evolved multiple times in several grass lineages and includes three variants of the photosynthetic mechanism, referred to as photosynthetic subtypes. Differences in species distributions related to both lineage and photosynthetic subtype have been observed and related to rainfall, but the physiological mechanism underlying these differences are poorly understood. To investigate these mechanisms, we studied an array of southern African C4 grass species representing different C4 photosynthetic subtypes (NADP-Me and NAD-Me) and lineages (Panicoideae and Aristidoideae). We used a combination of species distribution modelling (to determine the influence of rainfall on distribution patterns of each study species) and experimental drought manipulation to access differences in the response to, and recovery from drought in terms of leaf water relations, gas exchange and chlorophyll fluorescence. Panicoideae NADP-Me species were the most susceptible to drought due to apparent greater metabolic impairment. This study showed that drought susceptibility differs both phylogenetically and according to photosynthetic subtype, but that the role of phylogeny may outweigh the influence of photosynthetic subtype. Predicted increases in aridity in areas such as southern Africa make this understanding useful in determining how changes in drought might influence the distributions of C4 lineages and photosynthetic subtypes. Publications linked to this project

While at the Rhodes University Elevated CO2 Facility I assisted with a study looking at how rising CO2 concentrations could affect the growth of on of the most notorius invasive water weeds in the world, Water Hyacinth (Pontedaria crassipes). Elevated carbon dioxide (eCO2) and rising temperatures will have far-reaching effects on global plant-insect interactions, yet their implications for future biological control programs are not fully understood. Studies have shown that elevated CO2 will affect insect feeding guilds differently and these responses can be predicted with some confidence. This study used P. crassipes and interactions with two of its main biological control agents, providing two unique feeding guilds , to estimate if the agents will maintain their ability to control the weed in future climates and if a particular feeding type would be more effective that the other. The results of this study indicate that successful biological control of P. crassipes under conditions of elevated CO2 might rely on phloem-feeding insects, with chewers playing a lesser role. Publications linked to this project. 

In my third year of undergrad, I did my mini-thesis in the Smaldeel region of the Eastern Cape Province, South Africa. I looked at how light availability affected one of the dominant C4 grass species, Themeda triandra. In order to look at this, I looked at leaf area index (using a hemispherical camera) and understory composition along transects from open to full encroached tree canopies. This involved hours and hours of setting timers on the hemispherical camera and then running as far as I could and diving to the ground so that the camera didn't catch me and affect the readings. Photo cred: Colin Bennet. 

In addition to this, I also ran an experiment for my second major, Zoology. This project involved the digestion of freshwater crabs and frogs - and if you have studied digestion before, you will know that there is a lot of measuring poop and its not worth adding a photo!

South African BSc honours courses involve both coursework and a thesis. My course-work was joint between two departments, Botany and Environmental Science, to give me an honours degree in Biodiversity and Conservation. For my thesis, I was fortunate enough to be able to work in the first large-scale elevated CO2 research facility in Africa. I conducted research on the responses of Vachellia karroo, a species considered to be one southern Africa's most prominent woody encroaching species, to rising CO2 concentrations, shading and herbivory. I conducted experiments to study the photosynthetic, growth and allometric responses of V. karroo to rising CO2 concentrations, focussing on interactions with simulated herbivory and shading.