Herbivore and mycorrhizal mediation of carbon cycling in heather moorlands

The multiple drivers of environmental change, such as climate change and pollution, have led to widespread negative impacts on ecosystem functioning and services that are critical for human well-being. Heather moorlands are highly sensitive to these drivers and it is critical that we understand the impacts, interactions and feedbacks of current drivers of change in order to sustainably manage and conserve these unique and globally important environments.
Moorlands are found in uplands of the temperate zone, with 75% of the world’s heather (Calluna vulgaris) moorland located in the UK (Holden et al. 2007). Moorlands support a unique diversity of flora and fauna upon which globally rare species are dependent. Given the decline in this habitat over the last 100 years, heather moorlands are also a habitat of high conservation priority in the UK.
Moorland habitats are usually associated with acidic, base deficient soils, such as peat, and therefore represent a significant terrestrial carbon store. However, UK moorlands may act as both a significant sink and source of carbon (Billett et al. 2011) depending on management, climate and atmospheric pollution, including nitrogen deposition. A fundamental, yet poorly-understood, component in this ecosystem that influences carbon and nutrient dynamics involves the intimate symbioses between the roots of heather and ericoid mycorrhiza-forming fungi (EMF). These associations are assumed to be mutualistic, with the fungus supplying nitrogen (N) from the soil - a critical limiting factor - to the host plant in exchange for photosynthetically-fixed carbon.
Periodic pest outbreaks are a clear biological signal that ecosystem processes are being disrupted. Outbreaks of insect herbivores on heather can lead to severe defoliation, but the effect on belowground processes and the carbon cycle is unknown. Defoliation may reduce symbiotic fungal diversity and slow nutrient cycling in birch forests in arctic ecosystems (Parker et al. 2017), but its effect on EMF and nutrient cycling remains untested in moorlands. The extent to which carbon cycling and nutrient availability may themselves drive the pest outbreaks, mediated by EMF and nitrogen deposition, are unknown. EMF may increase plant nutritional quality and enhance plant defences against herbivores. Whether the enhanced nutrient status of EMF-associated plants (Kowal et al. 2018) makes them more attractive to pests, or whether the greater access to resources makes them more resilient to pests is unknown (Thirkell et al. 2017). How these factors are themselves influenced by changes in the environment remains unexplored, despite their potentially critical implications for the conservation of these threatened habitats.
The focus of this project is on understanding of the impacts of herbivorous pests and nitrogen pollution on EMF, and thus carbon cycling, in heather moorlands across a pollution gradient in the UK. The research will identify the feedback mechanisms that link below-ground soil nutrients and above-ground productivity with herbivore abundance, integrating field and laboratory-based research techniques and combining the expertise of Steven Sait (community ecology of insects), Katie Field (mycorrhizal physiology) and Pippa Chapman (soil biogeochemistry). Measurements will be carried out on UK heather moorlands, including Yorkshire, Scotland and the Orkney Islands. Over 15 years of fieldwork in Orkney has revealed varying patterns in abundance of insect pests, including outbreaks, and their natural enemies (e.g. Graham et al. 2004; Hicks et al. 2015), but the below-ground soil-plant-fungal interactions in this system are entirely uncharacterised, representing a significant knowledge gap.
The fieldwork will be supported by lab experiments that make use of the world class facilities within the Schools of Biology and Geography, including quantification of CO2 fluxes from soil mesocoms using a portable gas analyser and a ground-breaking isotope tracing approach pioneered in earlier studies by co-supervisor Katie Field (see Field et al. 2015). This exciting and novel multi-disciplinary project will lead to new insights into the link and feedback loops between above- and below-ground biotic and abiotic processes and how this may influence the carbon cycle in these important ecosystems.