Effects of Floodplain Logjams on Soil Biogeochemistry and Riparian Vegetation

Abstract

The effect of floodplain logjams on soil biogeochemistry and riparian vegetation is largely unknown, particularly in post-fire landscapes. Previous research indicates that decaying, downed wood on upland surfaces can alter the underlying soil through the transfer of nutrients, increase retention of soil moisture, and influence the survival, diversity, and succession of surrounding vegetation. However, these effects are poorly understood in floodplain environments, and particularly floodplains altered by wildfire. The objective of this study is to understand the geomorphic and ecological effects of floodplain logjams on soil chemical properties and riparian vegetation in a post-fire landscape. Field measurements were conducted along Little Beaver Creek (LBC), a forested montane stream in northern Colorado. A large percentage of LBC burned in 2020, followed by a major flood in 2022 that formed approximately 700 floodplain logjams along a 9-kilometer length of the river corridor. Six of the logjams formed by the 2022 flood were analyzed in this study, three of which were located on a burned floodplain, and the other three on an unburned floodplain. Four soil samples were collected at sequential distances away from a logjam and tested for key soil properties that influence nutrient availability and impact biogeochemistry (e.g., SOC, NO3, P, K, SO4-S). The soil moisture was quantified utilizing soil cores that were collected across both floodplains. Thirty cores per floodplain were collected, dried, and weighed to determine the volumetric soil water content. Floodplain vegetation was surveyed via quadrat method to a maximum of three meters around each logjam. The total number of plants, number of different plant species, and percentage of vegetated area were recorded for each plant survey. I found that floodplain logjams supported the retention of soil C, N, and P, particularly in the unburned floodplain. I anticipate that the trends between nutrient concentrations and logjam proximity will become more pronounced with time as logjam wood decay increases and as microbial communities recover post fire. Additionally, there was a significant relationship between soil texture and logjam proximity. Finer soils with higher clay and silt content were found underneath the logjams compared to more coarse soil farther away from the logjam, which further supports the retention of soil nutrients. There was no significant difference in soil nutrient content between the upper and lower horizons of the soil profile, except for potassium and nitrate concentrations. I found that local topography is the main control on floodplain soil moisture along LBC. There was a significant difference in soil moisture content between the burned and unburned floodplains, where the unburned floodplain had greater soil volumetric water content. Additionally, there is an inverse relationship between soil moisture and logjam proximity between the burned and unburned floodplains – there is a positive relationship between logjam proximity and soil moisture on the burned floodplain, whereas there is a negative relationship on the unburned floodplain. There was no significant effect from floodplain logjams on vegetation density or richness. Solar exposure was the primary predictor of plant abundance and species richness, and due to the lack of canopy cover in the burned area, there was greater plant density and species richness on this floodplain. These results will help inform restoration practitioners of the utility of floodplain logjams to achieve various river restoration objectives and to improve overall soil health. This research expands our understanding of how these floodplain features support spatial heterogeneity and increased resilience to future disturbances amidst climate warming.