Above- and belowground responses to tree thinning depend on the treatment of tree debris

Mechanical mastication is increasingly prescribed for wildfire mitigation, yet little is known about the ecological impacts of this fuels treatment. Mastication shreds trees into woodchips as an alternative to tree thinning and burning the resulting slash, which can create soil disturbances that favor exotic plants. Previous research on mastication has not simultaneously considered both the responses of soil organisms and understory plant communities. We compared mastication to slash pile burning (both 6-months and 2.5-years post-treatment) and untreated controls in pinyon–juniper (Pinus edulis–Juniperus osteosperma) woodland and measured soil properties, arbuscular mycorrhizal fungi (AMF) and understory plant composition. Our results showed that slash pile burns had severely degraded soil properties and low AMF abundance and richness compared to untreated or mastication plots. Pile burns were dominated by exotic plant species and had approximately 6× less understory plant abundance and richness than untreated plots. Only two variables differed between mastication and untreated plots 6-months post-treatment: mastication had lower soil temperature and higher soil moisture. Mastication plots 2.5-years post-treatment had more plant cover and richness than untreated plots or pile burns, although non-native Bromus tectorum cover was also greater and AMF spore richness was lower than untreated plots. The structural equation model (SEM) we developed showed that plant cover strongly influenced AMF abundance (0.50) and both plant cover (0.36) and AMF (0.31) positively influenced soil stability. In the short-term, mastication is a preferable method as it creates fewer disturbances than pile burning; however long-term impacts of mastication need further study as this practice could affect native plant communities. Our results suggest that the manner in which woody debris is treated following tree thinning has an important influence on soil stability and native plant biodiversity.