Loss of functional diversity and network modularity in introduced plant–fungal symbioses

Native and alien trees associate with a wide range of beneficial fungi, but the few studies of these interactions tend to focus only on a few plant species or locations at a time. This approach limits broad scale understanding of functional shifts and changes in interaction network structure that may occur following introduction. An ectomycorrhizal fungal species can associate with multiple, potentially distantly related, plant species. This means that the presence of one plant species may support a population of fungi that serves as inoculum, facilitating the establishment of a second plant species. The interactions of alien trees through shared mutualist species can therefore be analysed through network theory.

In a study published in AoBP (in the special issue on Evolutionary Dynamics of Tree Invasions), Dickie et al.  investigate plant–fungal interaction networks using two extensive datasets derived from fungal sporocarp observations and recorded plant hosts in two island archipelago nations: New Zealand (NZ) and the United Kingdom (UK). In New Zealand, fungi on alien trees are less functionally diverse than those associated with natives, whilst in the UK there is no functional difference in fungal associates of alien and native plant genera. In both New Zealand and the United Kingdom, however, the structure of the plant-fungi interaction network is simplified and “nested”, which suggests that beneficial fungi hosted by alien trees may help facilitate further tree invasion. Whether this is driven by fungal traits (e.g. lack of host-specific fungi; expanded host-range) or habitat drivers (e.g. planting into atypical soils) requires more detailed investigation to unravel. Regardless of the cause, the sharing of symbiotic fungi among alien trees may have important implications for the invasion process, if tree species which share fungal associates facilitate each other.