Seeing the forest for the soil: topographic controls on soil carbon dynamics in the boreal mixedwood forest

• Author / Creator

  Sewell, Paul

• Boreal forest soils store an estimated 272 Pg of carbon. Due to a high degree of spatial heterogeneity, there is a wide range in carbon stores in this ecosystem. Changes in topography and forest structure are important to carbon distribution, influencing the soil microclimate and the chemical quality and quantity of litter inputs. With increasing pressure from a changing climate, developing our understanding of carbon dynamics and adapting our management strategies in the boreal forest is paramount. The goal of this research was to investigate how soil moisture gradients influenced soil organic carbon storage and stability, in both natural and harvested stands in the boreal mixedwood forest. I investigated the long-term (17 years) evolution of soil properties following variable retention harvest at the EMEND project, where aspen had primarily regenerated in conifer-dominated, and deciduous-dominated stands. The topographic Depth-to-Water (DTW) index was used as a proxy for soil moisture to model relationships in the forest floor and mineral soil (0-7 cm) at the stand level, and to see if harvest and aspen regeneration had altered these relationships compared to uncut control stands. In undisturbed stands, relationships between soil properties and the DTW index were more strongly expressed in the mineral soil compared to the forest floor, with increased carbon stocks, and increased carbon and nitrogen concentrations at the wet end of the gradient. Relationships between the DTW index and forest floor properties were altered to a greater extent by harvest, but these effects varied between cover types. In order to gain a deeper understanding of the topographically-driven distribution of carbon quality and stocks at the stand-scale, I performed an in-situ soil respiration study on a hillslope, which featured well-drained Orthic Gray Luvisols with an aspen dominated canopy upslope, and transitioned to poorly-drained Gleysols with a white spruce dominated canopy, at downslope positions. Measurements were taken from the surface of the forest floor, as well as from the exposed mineral soil in order to partition forest floor respiration from that of the underlying mineral profile. In addition, a controlled laboratory incubation (210 days) was conducted on forest floor materials to further investigate the linkage between soil organic carbon quality and respiration fluxes. Over the course of the in-situ experiment, mineral soil respiration was consistently lower at downslope positions due to lower temperatures and higher water content. In-situ forest floor respiration was approximately equal along the hillslope, with different microclimatic controls on fluxes. At downslope positions, respiration was controlled by temperature, while respiration upslope was related to water content. Respiration rates measured during the laboratory incubation were greater than in-situ forest floor respiration by a factor of ten. The labile carbon pool at downslope positions was nearly double upslope, suggesting that under warming, the forest floor at these lower positions may be a much larger carbon source. Both the stand-scale and hillslope scale studies indicated that topographic variation had greater influence over mineral soil properties, while the forest floor was also affected by canopy composition.

• Subjects / Keywords

  • Boreal
  • Soil
  • retention harvest
  • Carbon
  • Mixedwood
  • forest floor
  • soil respiration
  • Topography
  • Depth to water

• Graduation date

  Fall 2018

• Type of Item

  Thesis

• Degree

  Master of Science

• DOI

  https://doi.org/10.7939/R3VT1H592

• License

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