Testing the ecological stability of ectomycorrhizal symbiosis: effects of heat, ash and mycorrhizal colonization on Pinus muricata seedling performance

Abstract

Understanding how abiotic conditions mediate the outcome of biotic interactions is a key question in community ecology. This is particularly interesting in the case of mutualisms because changing environmental conditions may be a source of evolutionary or ecological instability in the relationship between symbiotic partners. For the mycorrhizal symbiosis, elevated nutrient levels may make the carbon cost to plants of supporting mycorrhizal fungi outweigh the benefits of mycorrhizal nutrient acquisition. In this study, we simulated the effects of post-fire nutrient enrichment on the ectomycorrhizal symbiosis between Rhizopogon occidentalis and Pinus muricata by using factorial combinations of soil heating, ash addition and mycorrhizal colonization. In general, the presence of R. occidentalis and the heat and ash treatments increased growth of P. muricata seedlings. However, significant interactions between the treatments indicated that there were contextdependent effects on seedling growth. Soil heating increased soil mineral nitrogen content, but did not decrease the benefit of mycorrhizal colonization relative to unheated controls. This appeared to be driven by a decrease in the proportion of the root system occupied by R. occidentalis, indicating that host-plants can optimize colonization in a way that maximizes growth. Because seedlings in heat-treated soils were larger than unheated controls, reduced percent colonization did not cause any decrease in fungal biomass in heat-treated soils. Since there was no decrease in the performance of either partner, our results show that, in this system, ectomycorrhizal mutualisms are likely robust within the natural degree of environmental variability caused by moderate intensity wildfires.

Competition-function tradeoffs in ectomycorrhizal fungi

“…Although Kipfer et al ( 2012 ) found that the most enzymatically active fungus in their study, Suillus granulatus , had a positive effect on seedling growth, we did not observe statistically significant differences in growth by treatment in our study. In part, this was likely due to the homogeneous, high-quality soil environment created by autoclaving the experimental soils, which can release substantial amounts of nutrients into plant-accessible pools, reducing the impact of EMF on seedling growth ( Peay, Bruns & Garbelotto , 2010 ) and plant carbon allocation to EMF ( Hobbie , 2006 ). Other studies have found no relationship between EMF competitiveness and seedling growth ( Kennedy, Peay & Bruns , 2009 ).…”

Convergence in mycorrhizal fungal communities due to drought, plant competition, parasitism, and susceptibility to herbivory: consequences for fungi and host plants

“…The cost to benefit ratio of the ectomycorrhizal symbiosis has been shown to change under different environmental conditions ( Kennedy and Peay, 2007 ), and host plants have been shown to regulate their EMF partners under changing environmental conditions. For example, Peay et al (2010) found that seedlings were able to maintain high growth rates under experimental nutrient enrichment by reducing colonization by EMF of the genus Rhizopogon . Across a natural environmental gradient, Moeller et al (2013) found that the traits of EMF reflected the nutritional needs of their host plants, with communities composed of efficient foragers with high carbon requirements dominating in nutrient deficient soils.…”

“…Across a natural environmental gradient, Moeller et al (2013) found that the traits of EMF reflected the nutritional needs of their host plants, with communities composed of efficient foragers with high carbon requirements dominating in nutrient deficient soils. Because Geopora is a common member of the EMF communities found on pinyons, trees on which less efficient mutualists were eliminated were able to maintain higher growth rates as in Peay et al (2010) . The strong positive relationship observed between dominance by Geopora and pinyon growth suggests that although community disassembly was often considered detrimental ( Zavaleta et al, 2009 ), negative effects may not always be observed, at least in the short term.…”

Peer Review #2 of "Competition-function tradeoffs in ectomycorrhizal fungi (v0.1)"

“…Although Kipfer et al (2012) found that the most enzymatically active fungus in their study, Suillus granulatus, had a positive effect on seedling growth, we did not observe statistically significant differences in growth by treatment in our study. In part, this was likely due to the homogeneous, high-quality soil environment created by autoclaving the experimental soils, which can release substantial amounts of nutrients into plant-accessible pools, reducing the impact of EMF on seedling growth (Peay, Bruns & Garbelotto, 2010) and plant carbon allocation to EMF (Hobbie, 2006). Other studies have found no relationship between EMF competitiveness and seedling growth (Kennedy, Peay & Bruns, 2009).…”