Is Landscape Affecting Carbon Cycling Processes in Mountain Forest Catchments?

Johannes Kobler, Environmental Agency of Austria – Austria

A Review of the ECOPOTENTIAL paper: 

Kobler, J., Zehetgruber, B., Dirnböck, T., Jandl, R., Mirtl, M., Schindlbacher, A. 2019. Effects of aspect and altitude on carbon cycling processes in a temperate mountain forest catchment. Landscape Ecology 34: 325-340. https://doi.org/10.1007/s10980-019-00769-z 

Mountainous catchments are characterized by complex topographies, which cause steep gradients of altitude and aspect within small distances. By determining microclimatic patterns and gravity driven element fluxes, they render the variation in forest carbon (C) fluxes and pools. While the forest C dynamics are well studied at the plot scale, detailed knowledge about their spatio-temporal structure at the catchment scale is still missing. In this study, we evaluated the effects of altitude and aspect on C cycling in a mountainous forested catchment within the National Park Kalkalpen, Austria. We combined recent field measurements with existing long-term soil- and forest inventory data of eight plots located along two altitudinal gradients at south-west (SW) and north east (NE) facing valley slopes and quantified plant- and soil-related C fluxes and pools.

We expected lower C fluxes with increasing altitude due to lower temperature and shorter vegetation periods. We hypothesized differences in soil water supply as the major aspect-related driver. Higher radiation input at the SW facing slopes combined with the typically shallow soils may cause frequent water shortage and negative effects on forest C fluxes. However, we found that forest net primary (NPP) and net ecosystem production (NEP) did not show clear altitudinal trends within the relatively narrow altitudinal gradient (~500 m – 900 m) within the catchment. Annual NPP was higher at the SW facing slope (6.6±3.01 tC /ha), when compared to the NE facing slope (4.4±2.61 tC /ha). However, as SR was higher at the SW facing slope too, difference in annual NEP between aspects were balanced out (NE: 1.3±3.23 tC /ha, SW: 1.6±3.34 tC /ha). 

Water limitation was most likely prevented by regular and extended precipitation events during the years, whereas the relatively narrow altitudinal gradient within the catchment combined with higher radiation input at the higher than lower parts of the slopes may have prevented altitudinal-related effects on C fluxes. Under current climate conditions, altitude and aspect adversely affect C sequestering and releasing processes, resulting in a relatively uniform forest NEP within the catchment. Under a future climate, however, stronger shifts in temperature and precipitation may disproportionally affect forest C cycling at the southward slopes through increased water limitation.

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