Grassland invaders and their mycorrhizal symbionts: a study across climate and invasion gradients.

Rebecca A Bunn,Søren Rosendahl,Ylva Lekberg,Christopher Gallagher,Philip W Ramsey

doi:10.1002/ece3.917

Abstract

Controlled experiments show that arbuscular mycorrhizal fungi (AMF) can increase competitiveness of exotic plants, potentially increasing invasion success. We surveyed AMF abundance and community composition in Centaurea stoebe and Potentilla recta invasions in the western USA to assess whether patterns were consistent with mycorrhizal-mediated invasions. We asked whether (1) AMF abundance and community composition differ between native and exotic forbs, (2) associations between native plants and AMF shift with invading exotic plants, and (3) AMF abundance and/or community composition differ in areas where exotic plants are highly invasive and in areas where they are not. We collected soil and roots from invaded and native forb communities along invasion gradients and in regions with different invasion densities. We used AMF root colonization as a measure of AMF abundance and characterized AMF communities in roots using 454-sequencing of the LSU-rDNA region. All plants were highly colonized (>60%), but exotic forbs tended to be more colonized than natives (P < 0.001). We identified 30 AMF operational taxonomic units (OTUs) across sites, and community composition was best predicted by abiotic factors (soil texture, pH). Two OTUs in the genera Glomus and Rhizophagus dominated in most communities, and their dominance increased with invasion density (r = 0.57, P = 0.010), while overall OTU richness decreased with invasion density (r = −0.61, P = 0.006). Samples along P. recta invasion gradients revealed small and reciprocal shifts in AMF communities with >45% fungal OTUs shared between neighboring native and P. recta plants. Overall, we observed significant, but modest, differences in AMF colonization and communities between co-occurring exotic and native forbs and among exotic forbs across regions that differ in invasion pressure. While experimental manipulations are required to assess functional consequences, the observed patterns are not consistent with those expected from strong mycorrhizal-mediated invasions.

Highlights

Non-native plant invasions can reduce plant diversity and alter ecosystem processes (Levine et al 2003)
We asked whether (1) arbuscular mycorrhizal fungi (AMF) abundance and community composition differ between native and exotic forbs, (2) associations between native plants and AMF shift with invading exotic plants, and (3) AMF abundance and/or community composition differ in areas where exotic plants are highly invasive and in areas where they are not
There was a significant interaction between site and host plant (F4,36 = 7.50, P < 0.001), in which colonization was higher in C. stoebe and P. recta than native forbs at the MPG2 and MPG3 sites, but there was no difference in colonization between host species at MPG1

Summary

Introduction

Non-native plant invasions can reduce plant diversity and alter ecosystem processes (Levine et al 2003). Despite substantial research efforts (Richardson and Pysek 2008), we still lack a predictive understanding of how some exotic plants establish and outcompete native plants. The two most popular hypotheses state that exotic plants become invasive due to an escape from natural enemies in their native range (enemy release hypothesis; Keane and Crawley 2002) or fail to become invasive because they encounter new enemies in their exotic range (biotic resistance hypothesis; Elton 1958). Invasions may fail due to an absence of specific mutualists in the exotic range (Richardson et al 2000) or succeed because the invader either brings its mutualists or is able to associate with native or cosmopolitan mutualists (Dickie et al 2010).

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