Freshwater wetlands are globally significant reservoirs of soil carbon. Diverse vegetation, inundation regimes, geomorphology, and climate affect the amount and persistence of soil carbon in these ecosystems. Attempts to quantify the relative impact of these drivers have relied upon datasets with only a few sites and a limited number of site-specific samples. Such data limitations led to an inability to model carbon density profile with depth, or include spatially variable predictors. In this study, we collected soil carbon data from 100 wetlands in Victoria, southeastern Australia, constituting one of the largest datasets of wetland soil carbon measurements from a single empirical study. These data allowed us to estimate the soil carbon density profile through depth using a curvilinear density depth response, and to integrate across that profile to generate a model of landscape-level drivers of carbon stock. We used this model to evaluate the importance of hydrology, geomorphology, climate, and vegetation structure using continuous spatial data. Our model demonstrates that, in general, organic carbon density decreases with depth, but that the shape of that decline varied considerably among wetlands. We found that carbon was highest among wetlands with intermediate levels of precipitation but with infrequent permanent water cover. The hierarchical model and out-of-sample prediction methods described here provide an analytical framework for future analyses of soil carbon stock in wetlands, which guard against overfitting and enable appropriate extrapolation. These advances could be important for extrapolation in global carbon cycle modelling, spatial conservation prioritisation, and carbon offset and mitigation efforts.

Australia is a global hotspot for orchid diversity but has one of the highest rates of orchid extinctions in the world. Epitomising this trend are the Leek Orchids (Prasophyllum), a large genus of endemic terrestrial orchids containing 140 species with 39 species currently listed as nationally threatened. Complicating conservation efforts, attempts to grow Leek Orchids in cultivation usually fail for reasons not yet understood. Our study investigates three factors that may be behind poor seed germination: 1) seed viability through a study comparing pollination treatments; 2) mycorrhizal fungi associations, encompassing differences between adult plants and seedlings, seasonal turnover of fungal communities, and the effect of habitat; and, 3) the effect of nutrient composition in germination media.

Tetrazolium staining showed that viability across three Leek Orchid species ranged from 12-43%, indicating that seed viability is not responsible for poor germination. Preliminary findings suggest that Leek Orchids are usually mycorrhizal generalists that associate with Ceratobasidiaceae fungi. Sequencing of fungal isolates is showing some segregation between fungi used by seedlings in the wild and fungi living in the roots of adult plants. This may explain why fungi collected from the roots of adult plants often do not germinate seed. In germination trials, the composition of nutrient media significantly affected germination. Asymbiotic trials showed that Leek Orchids will only germinate on very low nutrient asymbiotic media. Contrastingly, the symbiotic trial found that higher nutrient media resulted in the highest symbiotic germination rates. These results can improve ex-situ conservation efforts for threatened Leek Orchids.

“Least-cost theory” concerns the manner in which plants co-optimize investments in resources needed for photosynthesis. Two types of investments are crucial: enzyme costs of Rubisco carboxylation capacity (Vcmax), and the water costs of transpiration. The theory posits that plants adjust the balance of these investments to achieve a given photosynthetic income at least total cost and that, as a first approximation, the unit-costs of carboxylation and transpiration are set by site properties (climate and soils). Predictions from least-cost theory have strong support at regional and global scale. However, there is still unexplained variation in investment strategies among co-occurring species. In principle, differences in hydraulic traits of plants could affect transpiration costs and be responsible for this variation. Important hydraulic traits include sapwood capacitance, leaf area:sapwood area ratio, effective rooting depth, and sapwood tissue density. Through my project, I investigate the associated costs with these traits and how they affect photosynthetic traits. Predictions based on least-cost theory are tested using a combination of field studies in eastern Australia, and previously collected data from Macquarie University researchers. Preliminary results show sapwood capacitance and effective rooting depth significantly affect the ratio of unit costs and these relationships change across site rainfall gradient. These findings have implications for improving next-gen global vegetation models and predicting primary production in future climates.

Pastures are economically important and provide a range of ecosystem services. Grasses associate with several symbiotic fungi, including endophytes (Clavicipitaceae: Epichloë) and arbuscular mycorrhizal (AM) (Glomeromycotina), in roots and shoots, respectively. Endophytes and AM fungi often affect insect herbivores by influencing the overall chemistry of their host plant (e.g. alkaloid-production by endophytes). Furthermore, many grasses have the ability to accumulate large amounts of silicon from the soil. Silicification of plant tissues alleviates a wide range of stresses, including herbivory, and recent evidence suggests that both endophytes and AM fungi may facilitate silicon uptake. The consequences of this for herbivores, and whether endophytes and AM fungi interact in this regard, are currently unknown. We therefore conducted a factorial greenhouse experiment to evaluate whether these components, acting alone or in combination, altered population dynamics and fecundity of the bird-cherry oat aphid (Rhopalosiphum padi) feeding on Festuca arundinacea. Results confirm that endophyte-associations had the greatest effect and reduced all aphid performance parameters. However, the presence of AM fungi countered these reductions to the extent that aphids performed better on plants associated with both types of fungi. This potentially reflects reductions in alkaloids in these plants relative to plants with just endophytes. Despite evidence that both types of fungi increased silicon uptake, silicon had no discernible impacts on aphids. In conclusion, symbiotic fungal associations may be beneficial to plants, but they might have deleterious effects on herbivore defences when acting in combination. These interactions may, however, prove more effective against other herbivores (especially chewers).

Arbuscular mycorrhizal fungi (AMF) can reduce nutrient leaching by removing extra nutrients from soil and delivering these to their host plant. However, the extent to which AMF is able to mediate leaching under warming and water stress remains unknown. To test this, we grew Lucerne (Medicago sativa) and Fescue (Festuca arundinacea) with (M) and without (NM) AMF inoculation (Rhizophagus irregularis) under different temperature [ambient - 26 °C (aT); elevated - 30 °C (eT)] and water conditions [well-watered - 100% water holding capacity (WHC); water-stressed - 40% (WHC)]. After 4 months of growth, leached nutrients (PO4-, NH4+, and NO3-) were collected and analysed followed by a complete destructive harvest. Total plant biomass, nitrogen (N), phosphorus (P), and mycorrhizal parameters (colonization rate and extraradical hyphae biomass) were measured. We found warming did not affect mycorrhizal colonization nor mycorrhizal fungal growth within the soil for Lucerne. In contrast, mycorrhizal colonization of Fescue was significantly reduced. AMF significantly reduced P leaching by 46% for Lucerne, regardless of climate conditions. However, for Fescue, AMF only reduced P loss under aT by 48% and its effect disappeared under eT. Mycorrhizal colonization rate of Fescue was negatively correlated with P loss due to leaching across the different climatic conditions. We did not find any significant mycorrhizal effect on N leaching for either Lucerne or Fescue. Our results suggest although symbiotic fungi significantly reduce P leaching under current climatic conditions, this nutrient saving might be lost under warming depending upon the plant species with which they are associated.