Effect of brush mastication on mycorrhizas and hypogeous fungi in mixed hardwood chaparral

Belowground mycorrhizal communities perform critical water, mineral, and nutrient transport functions in terrestrial ecosystems. Hypogeous fungi serve as inoculum for mycorrhizas and as food for small mammals. While the importance of these fungi to ecosystem health is clear, their response to fuels management practices, particularly in oak dominated chaparral, is largely unknown. This study examines the effects of brush mastication in combination with prescribed burning on mycorrhizas and hypogeous fungi in the upper layers of the mineral soil. Brush mastication is widely used as an alternative fuel reduction technique in urban interface areas of the Western U.S. with narrow burn windows and high wildfire risk. Mastication can achieve fuel management goals rapidly, but its ecological effects are unclear. The dense layer of chipped debris that results from mastication alters soil moisture conditions and contributes to increased fire severity. To assess changes in the mycorrhizal community and in hypogeous fungi, plots were established in experimental treatment units containing unburned masticated debris, masticated debris subjected to a prescribed burn, and natural standing brush. Roots were sampled in soil cores taken at two depths (0-10 cm and 10-20 cm), and mycorrhizas were described by morphology and DNA sequencing. Mycorrhizal density was greater in the upper 0-10 cm than in the lower 10-20 cm of soil. Abundance of mycorrhizas per soil volume decreased following brush mastication and burning.  Species richness of mycorrhizas decreased following mastication, and increased slightly following burning. Hypogeous fungal fruiting bodies were sampled by scuffling the soil surface. Species richness of hypogeous fungi increased following mastication, but decreased with burning of masticated debris. Hypogeous fungal fruiting bodies differed on masticated, and masticated-burned, and untreated plots. This study provides valuable information on the relationships between belowground mycorrhizal fungi and fire surrogate treatments.