Long‐term interval burning alters fine root and mycorrhizal dynamics in a ponderosa pine forest

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Abstract
Summary: Plant roots and their mycorrhizal symbionts are critical components of forest ecosystems, being largely responsible for soil resource acquisition by plants and the maintenance of soil structure, as well as influencing soil nutrient cycling. Silvicultural treatments should be guided by knowledge of how these below‐ground components respond to different forest management practices. We examined the cumulative effects of 20 years of prescribed burning at 2‐year intervals. We measured fine root length density and fine root and mycorrhizal root biomass in the upper 15 cm of mineral soil in a south‐western ponderosa pine forest over a complete burn cycle. Repeated burning reduced fine root length, fine root biomass and mycorrhizal root biomass, as well as the amount of nitrogen and phosphorus stored in these below‐ground pools. Estimates of fine root production, fine root decomposition and nutrient dynamics were similar in burned and control plots. Synthesis and applications. Although repeated‐prescribed fire may be an effective, low‐cost approach for reducing fuel loads and lessening the chance of a catastrophic wildfire in ponderosa pine forests, our results suggest that this strategy may negatively affect below‐ground biomass pools and nutrient cycling processes in the long term. We recommend that mechanical reductions in fuel loads be conducted in these and similar forests that have not experienced fire for decades, before fire is reintroduced as a management tool.