Some forests like it cold

Forests cover roughly one third of the global land area and currently remove around one quarter of anthropogenic CO₂ emissions, thus slowing down the increase in atmospheric CO₂ concentrations. Forests account for approximately 90% of all terrestrial biomass, which corresponds to about 400 Gt C.

However, the ratio of carbon uptake to release is a very fragile one and is determined by many factors such as water and nutrient availability, the amount and quality of light or stand age. In addition to the factors mentioned above, temperature is one of the most important factors that determine this ratio. Both fluxes determining the net ecosystem exchange (NEE), the gross uptake of CO₂ by photosynthesis (GPP) and ecosystem respiration (Reco) are sensitive to temperature. Thus, we investigated the temperature sensitivity of NEE at a newly established pine forest field site in Austria at 960 m a.s.l. Applying an understory and an ecosystem scale eddy covariance system we were able to disentangle temperature effects on understory and tree crown CO₂ exchange.

We found a clear temperature optimum for CO₂ uptake on ecosystem scale at around 20°C and a decrease in uptake on higher temperatures. This decrease was caused by (i) the understory turning from a slight sink for CO₂ into a source of CO₂ at higher temperatures, and (ii) a reduction of CO₂ uptake in the tree canopy layer. Furthermore, we compared carbon flux data with continuous tree growth data from dendrometer measurements.