Interactive Effects of Fungal Community Structure and Soil Moisture on Wyoming Big Sagebrush Performance
Authors: Engel Arden Marlene 
Published: September 2023

Large swathes of the North American sagebrush steppe have been converted to grazing and agricultural use. Remaining fragmented areas are threatened by invasive annual grasses and associated perpetuation of increased fire frequency, with efforts to restore sagebrush after fire largely unsuccessful. Lack of success may be due, in part, to poor interactions between sagebrush seed sourced from distant locations and unfamiliar soil fungi.

A greenhouse experiment was designed to evaluate effects of soil fungal community composition on sagebrush reestablishment by answering: (1) How does inoculation with ‘coevolved’ versus ‘foreign’ fungal inoculum impact sagebrush fitness? (2) How does moisture availability modify the advantage different fungal communities provide?

Wyoming big sagebrush seed was collected from a single population at the wet end of its range and grown in sterilized soil inoculated with either its coevolved wet-site fungi or foreign fungi from a drier sagebrush-dominated site, with sterilized-inoculum controls. Seedlings received either a wet-precipitation treatment simulating that of the seeds’ and wet-site inoculum’s source site or a dry-precipitation treatment simulating that of the drier foreign inoculum’s source site, producing eight treatment combinations. Quantification of δ13C in sagebrush tissue (a drought stress proxy) established that plants given dry-site precipitation experienced more drought stress and photosynthesized less than those given wet-site precipitation. Sagebrush biomass, root morphology, and visible fungal root colonization were assessed after 6-7 months of growth. Generalized linear models were fitted to evaluate plant response variables as functions of inoculum source site, viability, and precipitation regime.

ITS fungal DNA sequence data demonstrated that the in-situ fungal rhizosphere communities of mature sagebrush at the sites from which inoculum
was derived differed markedly in fungal taxonomic composition. Seedling rhizosphere community composition at harvest also differed by treatment
group, confirming that different inoculation treatments were effectively applied. Arbuscular mycorrhizal fungal taxa, which this experiment
initially aimed to assess specifically, however, were entirely absent from all but one seedling’s rhizosphere by the end of the growth period,
instead requiring assessment of the fungal community broadly. Rhizospheres of seedlings inoculated with live inoculum from either site were
enriched in fungi of Phlyctochytrium sp. (and family Chytridiaceae) as well as fungi belonging to unknown families and genera,
compared to sterilized inoculum, and dry-site inoculum treatments also had significantly greater abundances of Paraphoma sp. (and
family Phaeosphaeriaceae). The majority of root-colonizing fungi visible under light microscopy were dark septate endophytes, a functional group
with variable documented effects on plant growth and morphology – though, root colonization by DSE was not correlated with growth responses,
consistent with some prior studies.

Live fungal inoculum from both sites enhanced sagebrush seedling biomass when the precipitation regime simulated that of the inoculum’s source site, when compared to sterilized counterparts. Live dry-site inoculum also produced longer, finer roots under dry-site conditions. Both findings suggest adaptation of the soil fungal community to local environmental conditions and reflect that the positive impacts of inoculation on sagebrush growth and morphology are contingent on a match between the inoculum and soil moisture. Applying local-environmentally-adapted fungal inoculum, derived from the intended restoration site, could potentially assist sagebrush in optimizing water uptake and maintaining productivity under drought conditions, thereby encouraging sagebrush establishment. Further research into the role of the differentially abundant taxa and DSE on sagebrush responses, and classification of as-yet unidentified fungal taxa may elucidate mechanistic explanations of the observed effects of inoculation.