Nematode communities change following woody plant invasion of grassland

Background/Question/Methods Woody plant encroachment is an important land cover change in dryland ecosystems throughout the world, and frequently alters above- and belowground primary productivity, hydrology, and soil microbial biomass and activity.  However, little is known regarding the impact of this geographically widespread vegetation change on the soil fauna.  Nematodes represent a major component of the soil microfauna whose community composition and trophic structure could be strongly influenced by the kinds of changes in ecosystem structure and function often observed following woody encroachment.  Our purpose was to characterize nematode community composition and trophic structure along a grassland to woodland chronosequence in the Rio Grande Plains of southern Texas.  Research was conducted at the LaCopita Research Area where woody encroachment has been documented previously.  Soil cores (0-10 cm) were collected in fall 2006 and spring 2007 from remnant grasslands and woody plant stands ranging in age from 15 to 86 yrs, and nematodes were extracted by sugar centrifugation.

Results/Conclusions
Neither nematode densities (320-1380 individuals 100 g^-1 soil) nor family richness (15-19 families 100 g^-1 soil) were altered by woody encroachment.  However, family evenness decreased dramatically in woody stands >20 yrs old.  The trophic structure of nematode communities was strongly modified following grassland to woodland conversion.  Although root biomass was 2-5X greater in wooded areas, plant parasites decreased from 40% of all nematodes in grasslands to <10% in the older wooded areas, suggesting the quality of woody plant root tissue could be limiting plant feeders.  In contrast, bacterivores increased from 30% of nematodes in grasslands to 70-80% in older woody patches.  This large increase in bacterivores may be a response to the 1.5-2.5X increase in soil microbial biomass following woody encroachment.  Therefore, while energy flow through grassland nematode communities appears to be based nearly equally on herbivory and bacterivory, energy flow through nematode communities in wooded areas appears to be based primarily on bacterivory.  We speculate that these shifts in nematode community composition and trophic structure could have important implications for ecosystem patterns and processes.  First, the low abundance of root parasites (and presumably root herbivory) under woody plants may be one mechanism by which woody plants are able to establish and compete effectively with grasses during succession from grassland to woodland.  Second, the large increase in bacterivores following woody encroachment likely accelerates microbial turnover and the mineralization of N, thereby providing a feedback that enables the persistence of N-rich woody plant communities.