Vegetation and elevation influence the timing and magnitude of soil CO₂ efflux in a humid, topographically complex watershed

Abstract. In topographically complex watersheds, landscape position and vegetation heterogeneity can alter the soil water regime through both lateral and vertical redistribution, respectively. These alterations of soil moisture may have significant impacts on the spatial heterogeneity of biogeochemical cycles throughout the watershed. To evaluate how landscape position and vegetation heterogeneity affect soil CO₂ efflux (F_SOIL), we conducted observations across the Weimer Run watershed (373 ha), located near Davis, West Virginia, for three growing seasons with varying precipitation. An apparent soil temperature threshold of 11 °C for F_SOIL at 12 cm depth was observed in our data, where F_SOIL rates greatly increase in variance above this threshold. We therefore focus our analyses of F_SOIL on instances in which soil temperature values were above this threshold. Vegetation had the greatest effect on F_SOIL rates, with plots beneath shrubs at all elevations, for all years, showing the greatest mean rates of F_SOIL (6.07 μmol CO₂ m⁻² s⁻¹) compared to plots beneath closed-forest canopy (4.69 μmol CO₂ m⁻² s⁻¹) and plots located in open, forest gap (4.09 μmol CO₂ m⁻² s⁻¹) plots. During periods of high soil moisture, we find that CO₂ efflux rates are constrained, and that maximum efflux rates occur during periods of average to below-average soil water availability. While vegetation was the variable most related to F_SOIL, there is also strong interannual variability in fluxes determined by the interaction of annual precipitation and topography. These findings add to the current theoretical constructs related to the interactions of moisture and vegetation in biogeochemical cycles within topographically complex watersheds.