Evidence for mutualist limitation: the impacts of conspecific density on the mycorrhizal inoculum potential of woodland soils

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.

Root-Associated Ectomycorrhizal Fungi Shared by Various Boreal Forest Seedlings Naturally Regenerating after a Fire in Interior Alaska and Correlation of Different Fungi with Host Growth Responses

“…It is known that the presence of particular mycorrhizal species in a given soil can facilitate the establishment of plant species that benefit from that fungus in that soil (4,8,18,25,35,36,51,55,59). There are also many examples of relationships between mycorrhizal fungal infection by specific fungal species and the growth rate and/or competitive dynamics of particular plant species (6,12,36,38,43,54,55) and ECM or plant ability to acquire N and other nutrients (16,21,35).…”

Consequences for ectomycorrhizal fungi of the selective loss or gain of pine across landscapes

“…The situation is more complicated if EM hosts are present in pine-invaded regions as the specificity between pines and EM fungi becomes important. Mature conspecifics are important sources of EM inoculum for establishing pine seedlings (Haskins and Gehring 2005) and understanding pine regeneration in native and non-native regions requires an understanding of the availability of EM fungal associates and their host specificity.…”

Evidence for competition and facilitation trade-offs: effects of Sitka alder density on pine regeneration and soil productivity

“…Interspecific competition can negatively affect conifer resource acquisition and growth (Wagner et al 2004). However, plant communities can also have positive effects on conifer regeneration (facilitation) through long-term nutrient accretion, coinoculation with beneficial soil microbes, or protection from environmental or biotic elements, such as drought, frost, herbivores, or pathogens (Berkowitz et al 1995;Callaway and Walker 1997;Powell and Bork 2004;Haskins and Gehring 2005;Simard et al 2005). Such balancing between competitive and facilitative effects is important to understanding the effects of vegetation removal on forest productivity, composition, and diversity over the long term (Kimmins 2003).…”