Water balance and productivity of a Mediterranean oak woodland : Quantifying understory vegetation impacts by development of a stable oxygen isotope partitioning approach
Semi-arid ecosystems cover large areas world-wide and contribute about 40% to global gross primary production (GPP). The major driving factor of GPP in these ecosystems is water availability; since annual precipitation pattern show periodical summer droughts and evapotranspiration losses are high. Hence, understanding seasonal vegetation-soil-water feedbacks is vitally important. This thesis aims at disentangling vegetation impacts on water and carbon cycling of a Mediterranean savanna-type oak woodland. Special focus was laid on the seasonal understory impact on net ecosystem fluxes, soil water distribution and tree-understory interactions. Moreover, the impact of altered precipitation pattern and drought intensity on ecosystem functioning was assessed, as these phenomena are predicted to increase by climate change scenarios in these ecosystems. To achieve these aims, classical ecophysiological measurements, plant community and structural observations as well as eddy-covariance technique were combined with a novel stable oxygen isotope partitioning approach. Stable oxygen isotopes are valuable tracers for water movements within the ecosystem, but due to methodological restrictions in the past, it was necessary to validate modeling approaches and derive field sampling protocols for model input parameters. Therefore, a measurement set-up was developed coupling branch and soil chambers with an H2O laser spectrometer, enabling direct, high frequent measurements of δ18O signatures of evaporative fluxes and, to my best knowledge, the first validation of the Craig and Gordon model under heterogeneous field conditions. Thereby it was possible to assess multiple effects of understory vegetation on ecosystem water cycle and productivity. Although understory transpiration strongly contributed to ecosystem evapotranspiration, beneficial effects of the understory vegetation were dominant as herbaceous biomass strongly increased rain infiltration, diminished soil evaporation and significantly added to the ecosystem carbon sink strength. However, the understory was also vulnerable towards drought: development, species composition, transpiration and carbon gain of the understory plants were strongly influenced by competition for water with cork-oak trees which shortened the understory longevity and reduced the overall understory productivity in spring. The years 2011 and 2012, contrasting in annual precipitation amount and pattern, offered an ideal opportunity to study effects of drought on ecosystem functioning. The pronounced winter/spring drought and reduced precipitation in 2012 led to strongly reduced development of the understory layer. Ecosystem carbon sink strength and net ecosystem evapotranspiration were severely reduced and more water was used by the ecosystem through evapotranspiration than was introduced by precipitation in 2012. Decreased ecosystem productivity was caused by stomatal regulations and decreased maximum carboxylation rate (Vcmax) of cork-oaks. Moreover, the drought in 2012 led to strong alterations in tree phenology: annual tree diameter increment and fruit production were severely reduced. These findings suggest that the herbaceous understory, although vulnerable to drought, plays a vitally important role for ecosystem fluxes, rain infiltration and hence also cork-oak productivity and ecosystem resilience towards drought. At the same time, the increased drought and altered precipitation pattern predicted in future climate change scenarios for the Mediterranean basin does not only threaten understory development and annual cork growth. It also very likely decreases rain infiltration and ground water recharge, which in turn can severely affect cork-oak productivity and the resilience of the ecosystem towards drought.
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