Context Varying altitudes and aspects within small distances are typically found in mountainous areas. Such a complex topography complicates the accurate quantification of forest C dynamics at larger scales. Objectives We determined the effects of altitude and aspect on forest C cycling in a typical, mountainous catchment in the Northern Limestone Alps. Methods Forest C pools and fluxes were measured along two altitudinal gradients (650–900 m a.s.l.) at south-west (SW) and north-east (NE) facing slopes. Net ecosystem production (NEP) was estimated using a biometric approach combining field measurements of aboveground biomass and soil CO2 efflux (SR) with
allometric functions, root:shoot ratios and empirical SR modeling. Results NEP was higher at the SW facing slope
(6.60 ± 3.01 t C ha-1 year-1), when compared to the NE facing slope (4.36 ± 2.61 t C ha-1 year-1). SR was higher at the SW facing slope too, balancing out any difference in NEP between aspects (NE: 1.30 ± 3.23 t C ha-1 year-1, SW: 1.65 ± 3.34 t C ha-1 year-1). Soil organic C stocks significantly decreased with altitude. Forest NPP and NEP did not
show clear altitudinal trends within the catchment. Conclusions Under current climate conditions, altitude
and aspect adversely affect C sequestering and releasing processes, resulting in a relatively uniform forest NEP in the catchment. Hence, including detailed climatic and soil conditions, which are driven by altitude and aspect, will unlikely improve forest NEP estimates at the scale of the studied catchment. In a future climate, however, shifts in temperature and precipitation may disproportionally affect forest C cycling at the southward slopes through increased water limitation. Keywords: Mountain forest C cycle, Soil CO2 efflux, Net primary production, Net ecosystem production, Aspect Altitude LTER Zo¨belboden
