Spatial controls on boreal soil carbon fluxes across a vegetation and permafrost transition
The association of permafrost with vegetation species and soil carbon fluxes is dynamic and complex, and critical to understand given rapid climate changes occurring in the boreal region. We measured tree net primary production (NPP), a suite of soil physical and biological characteristics, soil surface CO2 flux (RS), and active layer depth (ALD) across replicated transects in Poker Creek Watershed, Alaska. Each 75 m transect encompassed a permafrost transition as upland vegetation (Betula papyrifera with no permafrost within 1.5 m depth) changed to lowland (Picea mariana with permafrost at 0.4 m). We measured each variable at 16-25 points on each of the six transects using an efficient cyclic sampling design, from mid-summer to late autumn of 2014, and then used a combined regression-kriging approach to examine the spatial dependence and relationships of these ecosystem properties and processes.
Soil CO2 fluxes (0.2-5.0 µmol m-2 s-1) and NPP were strongly spatially correlated with ALD and each other, and exhibited nonlinear breakpoints with respect to soil temperature. Soil moisture was not a strong control on RS. The RS residuals' autocorrelation structure changed markedly between summer and autumn measurements, with the latter exhibiting stronger fine-scale (down to 1 m) spatial correlation. Tree density, basal area, and NPP peaked in the transition zone between B. papyrifera and P. mariana, an area in which P. glauca also occurred, and in which soil fertility measurements were highest as poorly-drained organic soils gave way to well-drained mineral ones. These results emphasize the complex ecological and biogeochemical relationships between the abiotic and biotic factors controlling carbon fluxes at a variety of scales, particularly in spatially-complex permafrost ecosystems.