Browsing by Author "Unger, Stephan"
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- Disentangling drought-induced variation in ecosystem and soil respiration using stable carbon isotopesPublication . Unger, Stephan; Máguas, Cristina; Pereira, João S.; Aires, Luis M.; David, Teresa S.; Werner, ChristianeCombining C flux measurements with information on their isotopic composition can yield a process-based understanding of ecosystem C dynamics. We studied the variations in both respiratory fluxes and their stable C isotopic compositions (δ13C) for all major components (trees, understory, roots and soil microorganisms) in a Mediterranean oak savannah during a period with increasing drought. We found large drought-induced and diurnal dynamics in isotopic compositions of soil, root and foliage respiration (δ13Cres). Soil respiration was the largest contributor to ecosystem respiration (Reco), exhibiting a depleted isotopic signature and no marked variations with increasing drought, similar to ecosystem respired δ13CO2, providing evidence for a stable C-source and minor influence of recent photosynthate from plants. Short-term and diurnal variations in δ13Cres of foliage and roots (up to 8 and 4‰, respectively) were in agreement with: (1) recent hypotheses on post-photosynthetic fractionation processes, (2) substrate changes with decreasing assimilation rates in combination with increased respiratory demand, and (3) decreased phosphoenolpyruvate carboxylase activity in drying roots, while altered photosynthetic discrimination was not responsible for the observed changes in δ13Cres. We applied a flux-based and an isotopic flux-based mass balance, yielding good agreement at the soil scale, while the isotopic mass balance at the ecosystem scale was not conserved. This was mainly caused by uncertainties in Keeling plot intercepts at the ecosystem scale due to small CO2 gradients and large differences in δ13Cres of the different component fluxes. Overall, stable isotopes provided valuable new insights into the drought-related variations of ecosystem C dynamics, encouraging future studies but also highlighting the need of improved methodology to disentangle short-term dynamics of isotopic composition of Reco.
- Partitioning carbon fluxes in a Mediterranean oak forest to disentangle changes in ecosystem sink strength during droughtPublication . Unger, Stephan; Máguas, Cristina; Pereira, João S.; M. Aires, Luis; S. David, Teresa; Werner, Christiane; Igreja Aires, Luis MiguelNet carbon flux partitioning was used to disentangle abiotic and biotic drivers of all important component fluxes influencing the overall sink strength of a Mediterranean ecosystem during a rapid spring to summer transition. Between May and June 2006 we analyzed how seasonal drought affected ecosystem assimilation and respiration fluxes in an evergreen oak woodland and attributed variations in the component fluxes (trees, understory, soil microorganisms and roots) to observations at the ecosystem scale. We observed a two thirds decrease in both ecosystem carbon assimilation and respiration (Reco) within only 15 days time. The impact of decreasing Reco on the ecosystem carbon balance was smaller than the impact of decreasing primary productivity. Flux partitioning of GPP and Reco into their component fluxes from trees, understory, soil microorganisms and roots showed that declining ecosystem sink strength was due to a large drought and temperature-induced decrease in understory carbon uptake (from 56% to 21%). Hence, the shallow-rooted annuals mainly composing the understory have a surprisingly large impact on the source/sink behavior of this open evergreen oak woodland during spring to summer transition and the timing of the onset of drought might have a large effect on the annual carbon budget. In response to seasonal drought Reco was increasingly dominated by respiration of heterotrophic soil microorganisms, while the root flux was found to be of minor importance. Soil respiration flux decreased with drought but its contribution to total daily CO2-exchange increased by 11.5%. This partitioning approach disentangled changes in respiratory and photosynthetic ecosystem fluxes that were not apparent from the eddy-covariance or the soil respiration data alone. By the novel combination of understory vs. overstory carbon flux partitioning with soil respiration data from trenched and control plots, we gained a detailed understanding of factors controlling net carbon exchange of Mediterranean ecosystems.