Long-term soil nutrient and understory plant responses to post-fire rehabilitation in a lodgepole pine forest

Authors: KAISER, SOPHIE - Harvard University; FEGEL, TIMOTHY - Us Forest Service (FS); Barnard, David; Mahood, Adam; SPARKS, KYA - Colorado State University; WILKINS, MICHAEL - Colorado State University; RHOADES, CHARLES - Us Forest Service (FS)

Submitted to: Forest Ecology and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/24/2024
Publication Date: 11/9/2024
Citation: Kaiser, S., Fegel, T.S., Barnard, D.M., Mahood, A.L., Sparks, K., Wilkins, M., Rhoades, C.C. 2024. Long-term soil nutrient and understory plant responses to post-fire rehabilitation in a lodgepole pine forest. Forest Ecology and Management. 575(1). https://doi.org/10.1016/j.foreco.2024.122359.
DOI: https://doi.org/10.1016/j.foreco.2024.122359

Interpretive Summary: Wildfires are natural for lodgepole pine forests, but they can affect the soil quality and how plants grow back afterwards. We studied different treatments applied to the soil after a wildfire in 2010 and found that wood mulch helped the soil retain moisture and nitrogen for a few years, but its effects faded over time. Biochar, a charcoal-like substance, had a longer-lasting effect on the soil, increasing carbon storage and some forms of nitrogen. Biochar also helped a specific shrub grow more, while mulch seemed to suppress the growth of a common wildflower. Overall, these soil treatments had some benefits for the soil and some plants, but they didn't seem to help new lodgepole pine trees grow back in the most severely burned areas. The study suggests that planting new trees might be a good idea along with these soil treatments, especially since wildfires are becoming more frequent and intense.

Technical Abstract: Wildfires and other disturbances play a fundamental role in regenerating lodgepole pine forests. Though severe, stand-replacing fires are typical of this ecosystem, they can have dramatic impacts on soil properties and biogeochemical processes that influence the rate and composition of vegetation recovery. Organic soil amendments are often applied to manage post-fire erosion, but they can also benefit soil moisture and nutrient retention and potentially the trajectory of post-fire revegetation. We compared decade-scale change after the 2010 Church’s Park fire in soil nutrients and understory plant cover and composition on six burned hillslopes treated with 1) biochar, 2) wood mulch, 2) biochar + mulch, and 4) an untreated control. Wood mulch increased soil moisture and N retention the first three years following treatment. Mulch and biochar were still visible when we resampled in 2023. Mulch continued to increase soil moisture compared to unamended controls, though it had few lasting effects on soil N or cations. Conversely, biochar increased total soil C, dissolved organic C in soil leachate and C:N in soil and leachate. Biochar also elevated various dissolved and extractable soil N forms but reduced net nitrification. Understory plant cover changed little between 2016 and 2023, and the amendments had no general effect on total graminoid, forb, or shrub cover. However, biochar doubled cover of the dominant shrub Vaccinium scoparium, where it occurred. Conversely, mulch reduced cover of the most common forb (Oreochrysum parryi) by more than 50%. The combined biochar and mulch treatment had additive effects on both soil and plant responses. Though these post-fire rehabilitation treatments had decade-long impacts on soils and the understory plant community, tree regeneration is scarce in high-severity burn patches where the likelihood of conifer forest recovery remains uncertain. These findings suggest that coupling rehabilitation treatments with active reforestation may be a worthwhile option for managers confronting more frequent, severe disturbance.