Hydrological events transport large proportions of annual or seasonal dissolved organic carbon (DOC) loads from catchments to streams. The timing, magnitude and intensity of these events are very sensitive to changes in temperature and precipitation patterns, particularly across the boreal region where snowpacks are declining and summer droughts are increasing. It is important to understand how landscape characteristics modulate event-scale DOC dynamics in order to scale up predictions from sites across regions, and to understand how climatic changes will influence DOC dynamics across the boreal forest. The goal of this study was to assess variability in DOC concentrations in boreal headwater streams across catchments with varying physiographic characteristics (e.g., size, proportion of wetland) during a range of

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<p>Canada’s boreal zone is a complex mosaic of forests, wetlands, streams and lakes.  The pool of carbon (C) stored in each of these ecosystem components is vast, and significant to the global C balance.  However, C pools and fluxes are heterogeneous in time and space, which contributes to uncertainty in predicting how a changing climate will affect the fate of C in these sensitive ecosystems. The objective of this study was to investigate factors controlling spatial variability in soil C stocks and stream C export and assess the sensitivity of these stocks and fluxes to climatic factors. We conducted a detailed examination of soil C stocks and stream dissolved organic C (DOC) export from a 320 ha boreal forested catchment located in northwestern Ontario, Canada. High-frequency stream chemistry and discharge samples were collected from three inflow streams during snowmelt and rain events from 2016-2017. An intensive soil C sampling campaign resulting in 47 surface (0 – 30 cm) samples were collected during the summer of 2019. Stream hysteresis analysis revealed marked differences in flowpaths among sub-catchments during snowmelt and rain events. In the wetland-dominated catchment, near-stream sources contributed most of the DOC export during both rainstorms and snowmelt events, but in upland-dominated catchments, the sources of DOC depended on antecedent moisture conditions. Rainstorms in these catchments following prolonged droughts resulted in DOC flushing from distal regions of the catchment. Soil C stocks were also highly spatially variable, with much of the variability being explained by local-scale factors (e.g. gravel content, soil depth, distance to the nearest ridge). Taken together, these two findings emphasize the need to consider sub-catchment scale variability when calculating C pools and fluxes in boreal catchments. This is also important when predicting how C dynamics will shift in the future as a result of shorter winters, longer droughts and more intense rainstorms.</p>