225-5 Effects of floodplain logjams on soil geochemistry and plant communities

Session: Critical Zone Science: Intersection of Processes Linked to Geomorphology, Ecology, Fire and Climate (Posters)

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Abstract:

The effect of floodplain logjams on soil geochemistry is largely unknown, particularly in post-fire landscapes. Previous research indicates that decaying large wood alters upland soil chemistry by transferring organic carbon and nutrients to the underlying soil. Additionally, logjams enhance soil water retention by increasing surface roughness and reducing upland erosion during high flows. These events create moist, nutrient-rich soil that supports diverse understory vegetation. However, the effects of logjams are poorly understood in floodplain environments, particularly those altered by wildfire. Our objective is to understand the geomorphic and ecological effects of floodplain logjams on soil and plant communities at burned and unburned sites by assessing spatial patterns of soil moisture, nutrient content, and plant community structure in proximity to floodplain logjams. We hypothesize proximity to floodplain logjams will be positively correlated with higher soil moisture, greater nutrient content, and more diverse plant communities. Measurements were conducted along Little Beaver Creek (LBC), a forested montane stream in northern Colorado. A 2020 wildfire burned a large percentage of LBC at high severity, followed by a major flood in July 2022 that formed the logjams observed in this study. Fieldwork focused on two transects within LBC that differed by burn status. Soil was sampled for both water content, based on bulk density, and nutrient content (C, NO₃, P, K, SO₄-S). Plant communities were surveyed via quadrat method. For each square within a quadrat surrounding a logjam, the total number of plants, the number of different species, and the percentage of area covered by vegetation was recorded. Preliminary results suggest that (i) soil moisture is greater in unburned areas, (ii) soil nutrient content varies spatially across the floodplain, and (iii) plant species richness tends to increase with greater proximity to a floodplain logjam.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-11027

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