Contrasting responses of soil nematode communities to native and non-native woody plant expansion

Woody plant expansion into grasslands is widespread, driven by both successions to dominance by native woody species or invasion by non-native woody species. These shifts from grass- to woody-dominated systems also have profound effects on both above- and belowground communities and ecosystem processes. Woody-plant expansion should also alter the functional composition of the soil biota, including that of nematodes, which are major drivers of soil food-web structure and belowground processes, but such belowground impacts are poorly understood. We determined whether succession by a widespread native (Kunzea ericoides) and invasion by a non-native woody species (Pinus nigra) into tussock grasslands affect the composition of nematode functional guilds and the structure of nematode-based food webs. Although increasing dominance by woody species in both systems altered the functional guild composition of the nematode community, we found contrasting responses of nematode functional guilds to the different dominant plant species. Specifically, nematode communities reflected conditions of resource enrichment with increasing K. ericoides tree cover, whereas communities became structurally simplified and dominated by stress-tolerant nematode families with increasing P. nigra tree cover. Because nematodes regulate both bacterial- and fungal-dominated food webs in soils, these shifts could in turn alter multiple ecosystem processes belowground such as nutrient cycling. Incorporating species' functional traits into the assessment of habitat-change impacts on communities can greatly improve our understanding of species responses to environmental changes and their consequences in ecosystems.