Evidence for mutualist limitation: the impacts of conspecific density on the mycorrhizal inoculum potential of woodland soils |
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Authors: | Email author" target="_blank">Kristin?E?HaskinsEmail author Catherine?A?Gehring |
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Institution: | (1) Department of Biological Sciences and the Merriam-Powell Center for Environmental Research, Northern Arizona University, Flagstaff, AZ 86011-5640, USA |
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Abstract: | The ability of seedlings to establish can depend on the availability of appropriate mycorrhizal fungal inoculum. The possibility
that mycorrhizal mutualists limit the distribution of seedlings may depend on the prevalence of the plant hosts that form
the same type of mycorrhizal association as the target seedling species and thus provide inoculum. We tested this hypothesis
by measuring ectomycorrhizal (EM) fine root distribution and conducting an EM inoculum potential bioassay along a gradient
of EM host density in a pinyon–juniper woodland where pinyon is the only EM fungal host while juniper and other plant species
are hosts for arbuscular mycorrhizal (AM) fungi. We found that pinyon fine roots were significantly less abundant than juniper
roots both in areas dominated aboveground by juniper and in areas where pinyon and juniper were co-dominant. Pinyon seedlings
establishing in pinyon–juniper zones are thus more likely to encounter AM than EM fungi. Our bioassay confirmed this result.
Pinyon seedlings were six times less likely to be colonized by EM fungi when grown in soil from juniper-dominated zones than
in soil from either pinyon–juniper or pinyon zones. Levels of EM colonization were also reduced in seedlings grown in juniper-zone
soil. Preliminary analyses indicate that EM community composition varied among sites. These results are important because
recent droughts have caused massive mortality of mature pinyons resulting in a shift towards juniper-dominated stands. Lack
of EM inoculum in these stands could reduce the ability of pinyon seedlings to re-colonize sites of high pinyon mortality,
leading to long-term vegetation shifts. |
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Keywords: | Pinus edulis Juniperus monosperma Pinyon-juniper woodland Bioassay Drought |
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