Mycorrhizal legacies of plant invasion.
Mycorrhizal legacies of plant invasion - not just a story of doom and gloom.
The great majority of terrestrial plant roots are colonized by arbuscular mycorrhizal (AM) fungi in the phylum Glomeromycota, in which the obligate biotrophic fungi receive up to 30% of the total plant assimilated carbon (C) (Drigo et al., 2010) in exchange for phosphorus (P) and other putative services (Smith & Read, 2008). AM fungi have been referred to as keystone mutualists (O’Neill et al., 1991) due to their ubiquity and location in the root-soil interface where they mediate plant interactions (van der Heijden et al., 1997) and are important drivers of C and nutrient cycles (Rillig, 2004).  Previous work has shown that AMF abundance and richness can decrease with some (Hawkes et al., 2006; Mummey et al., 2006; Stinson et al., 2006; Wolfe et al., 2008), but not all plant invasions (Hawkes et al., 2006), although the majority of studies have been conducted on single invasive plants at one location, complicating comparisons among invasive plants.  To address this, we used 454 SequencingTM (Margulies et al., 2005) combined with phylogenetic distance analyses to determine if and how AM fungal communities differed among cheatgrass, knapweed and spurge invasions in six geographically distinct locations that also harbored native remnant grasslands.  Shifts in overall fungal abundances were indirectly assessed based on AM colonization on field collected roots.    Carbon allocation to AMF differs among plant species, so we hypothesize that invasion-mediated effects on fungal abundance will depend on the host quality of the invasive plant relative to the native plant community that it replaces.  Regardless of the invasive plant, however, we expect lower AMF diversity in invaded sites, because while plant richness may initially increase with invasion, plant diversity in severe invasions is most certainly lower and plant and fungal diversities are positively correlated (van der Heijden et al., 1998; Landis et al., 2004, van der Boorde et al., 2010).   An increased knowledge in this area is not only important to generate hypotheses about the potential role of AM fungi in plant invasions, but could also highlight invasion legacies that need to be considered during restoration.
Plant invasions alter ecosystem functions and incur costs in the billions of dollars.  While severe invasions reduce native plant diversity, relatively little is known about shifts in belowground microbial communities that could affect further invasions and create soil legacies that impair restoration success.  Some recent work supports the reduced mutualism hypothesis and show that some invasive plants either do not form a root symbiosis with arbuscular mycorrhizal fungi (AMF) or have a low mycorrhizal dependency.  However, knapweed (Centaurea stoebe) and leafy spurge (Euphorbia esula) are highly colonized by AMF and may gain increased growth and competiveness from the symbiosis, suggesting that broad generalizations among invasive plants should be avoided.  Sampling from six locations that each harbored sites dominated by knapweed, leafy spurge, cheatgrass (Bromus tectorum), and native plants, we used 454 sequencing to show that knapweed and leafy spurge invasions have a greater AMF abundance and taxa richness relative to native remnant plant communities, whereas cheatgrass invasions harbor fungal communities of lower abundance and richness.  Because AMF communities grouped according to plant community, invasion mediated-shifts appear predictable, and the lower evenness and greater phylogenetic dispersal of AMF taxa within cheatgrass invasions suggests a stronger fungal competition within this plant relative to the others. Whether or not the greater AMF richness in knapweed and leafy spurge invasions is due to a co-invasion of AMF taxa, or a loss of fungal taxa in native communities, is uncertain but could have functional consequences and suggests that native plant communities may not be islands of diversity to colonize restored areas.  Overall, we show great differences in belowground responses among invasive plants, which supports the recent plea to move away from the native/non-native dichotomy and instead focus on individual plants and their traits regardless of their origin.