The mycorrhizal fungi involved in the tree invasion of lowland heathlands

Abstract

In England, the loss of lowland heathland, a habitat of high conservation importance, is primarily due to the invasion of birch and pine. This secondary succession has been researched in depth from a plant perspective but little is known about the role of mycorrhizal fungi, even though both trees and heather are mycorrhizal. In fact, tree encroachment onto lowland heathland can be regarded as the replacement of a resident ericoid mycorrhizal community by an invading ectomycorrhizal community. I determined the identity and distribution of the ectomycorrhizal fungi associated with birch and pine encroachment onto lowland heathlands. I established whether there are mycorrhizal fungi that mediate the invasion by a) comparing the mycorrhizal inoculum potential of soil and ectomycorrhizal fungal diversity at three levels of invasion (uninvaded heathland, invaded heathland and woodland), b) comparing the fungi forming mycorrhizas on tree seedlings and trees across diverse sites, c) determining the effect of proximity to trees on mycorrhization and seedling biomass, and d) identifying fungal dispersal methods. I established that in lowland heathlands i) seedlings have limited access to ectomycorrhizal fungi even within sapling rooting zones, ii) ectomycorrhizal inoculum potential increases as the level of tree invasion increases, iii) mycorrhizal seedlings accumulate more biomass than non-mycorrhizal seedlings, iv) there are five keystone ectomycorrhizal fungi that participate in tree invasion - Rhizopogon luteolus, Suillus bovinus, S. variegatus (pine symbionts), Laccaria proxima and Thelephora terrestris (primarily birch symbionts), v) some ectomycorrhizal fungi cannot colonise seedlings via spores, and vi) ectomycorrhizal communities differ between lowland heathland sites. This study is the first to identify the mycorrhizal fungi that associate with tree seedlings on lowland heathlands and it is one of the first biome-level mycorrhizal studies of secondary plant succession. The data presented provide the stepping-stones required for future ecologically-relevant modelling and experimentation aimed at understanding mycorrhizal invasions.