Long‐term effects of burning slash on plant communities and arbuscular mycorrhizae in a semi‐arid woodland

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Abstract
Summary: Burning of slash (woody debris) piles resulting from the harvest of fuel wood is a common management technique designed to reduce fire risk and increase establishment of understorey vegetation in many semi‐arid woodlands. However, the consequences of slash burning on plant communities and their below‐ground fungal mutualists are poorly understood. We examined how the burning of slash piles affected understorey plant communities and arbuscular mycorrhizal (AM) fungi in a pinyon–juniper woodland in northern Arizona, USA, 5 years after harvesting. We analysed plant communities and AM inoculum potential along 16 transects in: (i) burned slash pile sites; (ii) interspaces between burned slash piles; (iii) non‐burned canopy sites; and (iv) non‐burned interspaces. We quantified AM colonization of dominant native and exotic plants in burned and non‐burned areas. Burned areas had significantly fewer understorey plant species than non‐burned areas, and exotic species were four times more abundant at burned slash sites than at other sites. Exotic plants from burned and non‐burned areas exhibited levels of AM colonization that were 50% greater than native plants. Bioassay and field‐collected plants exhibited similar levels of AM colonization and there were no significant treatment differences. The total biomass of bioassay plants grown in soil from burned slash sites averaged 19% greater than plant biomass from other sites, and had 27% more phosphorus in their shoot tissue; however, tissue nitrogen contents were similar. Synthesis and applications. These results suggest that either fire did not reduce AM inoculum potential, or that AM fungal populations recovered in the 5 years after the slash was burned. Changes in understorey plant communities and increases in exotic species with burning could result from other soil changes (e.g. species of AM fungi present), reduction of native species in the seed bank or greater dispersal ability of exotic plants compared with native plant species. We suggest that burning slash piles as a management tool in pinyon–juniper woodlands can result in plant communities that are persistently dominated by exotic species. Management approaches that utilize fuel wood harvest alone or that incorporate seeding of native plants may achieve the desired results.