Long-Term Impacts of Biochar and Mulch in a Compound Disturbed Lodgepole Pine Forest

Abstract

Fires are a natural and important element of lodgepole pine forests, however severe fires can adversely impact soil productivity, or a soils’ ability to sufficiently support nutrient availability and plant growth, for decades after a wildfire. Compound disturbances, for example, when fire occurrences follow severe mountain bark beetle outbreaks, may reduce ecosystem resiliency by influencing soil biogeochemical processes and destroying seed sources. Organic soil amendments are often applied to manage post-fire erosion, but they also have short-term impacts on soil moisture and nutrient retention that may enhance recovery of native vegetation. Little is known about how woody residue treatments such as mulch and biochar impact long-term soil productivity in compound disturbed coniferous forests. This research compared decade scale post-fire change in soil nutrients and understory plant cover and composition on six replicate burned hillslopes treated with 1) biochar, 2) wood mulch, 2) biochar + mulch and 4) an untreated control in a severely burned, beetle-killed lodgepole pine ecosystem in Colorado, USA. The site was in the gray-phase at the time of the 2010 Church’s Park fire and experienced significant overstory mortality. Both mulch and biochar were still evident a decade after treatment when they were resampled in 2023. Six replicate 5 x 5 m experimental plots that were moderately to severely burned and had similar pre-fire forest compositions were established in 2012, two years following the Church’s Park fire. Treatments of 37 t ha−1 of wood mulch, 20 t ha−1 of biochar, their combination, and an untreated burned control were randomly assigned within each treatment block. Soil moisture, nutrient pools, and plant cover were sampled in each plot in 2016 and 2023. As observed in the first years after treatment, mulch increased soil moisture compared to unamended controls ten years later, though it had few residual impacts on soil N or cations. Conversely, biochar amendments increased total soil carbon, dissolved organic C in soil leachate and C:N ratios in both soil and leachate. Though biochar also elevated various dissolved and extractable soil N forms, it reduced net nitrification rates. Total understory plant cover has remained at ~40% since 2016 and the amendments had no significant effects on overall graminoid and forb cover. However, biochar doubled the cover of the dominant shrub Vaccinium scoparium in areas where it occurred. Conversely, mulch reduced the cover of most common forb, Oreochrysum parryi, by more than 50%. These post-fire rehabilitation treatments had lasting impacts on water, N and C soils, and significant impacts on the abundance and cover of key understory species, but the long-term recovery of this ecosystem remains uncertain. As seen elsewhere in the Southern Rocky Mountain region where fire has burned bark beetle-killed forests, tree regeneration is extremely sparse within the burn scar and completely absent from the plots. Managers addressing changing climatic conditions and more frequent, severe disturbances would benefit from further research that evaluates whether these rehabilitation treatments enhance tree, shrub, and herbaceous plant establishment during active revegetation projects.