Disentangling seasonal vegetation effects on ecosystem evapotranspiration and water use efficiency of Mediterranean savannah-type oak forest
Final Report Abstract
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 project aimed at disentangling vegetation impacts on water and carbon cycling of a Mediterranean savanna-type oak woodland with special focus on seasonal understory impact on net ecosystem fluxes, soil water distribution and treeunderstory 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, 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 still require validation of modeling approaches and input parameters under field conditions. Therefore, a measurement set-up was developed coupling branch and soil chambers with an H2O laser spectrometer, enabling direct, high frequent measurements of δ18O of evaporative fluxes and, to our 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 estimation of canopy structure and leaf area index is of great importance for scaling of observations to ecosystem scale and modeling purposes. Great effort has been made on method development of leaf area index measurements under particular consideration of sparse canopies and affordable effort. Height and angularly distributed digital cover photography (DCP) could successfully be applied here for the first time. Precision of results could be enhanced by 30% compared to standard techniques. In 2011 and 2012 contrasting annual precipitation amount and pattern offered an ideal opportunity to study the effects of severe 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 ecosystem evapotranspirational water losses exceeded the amount received by precipitation in 2012. Decreased ecosystem productivity was caused by stomatal regulations and decreased maximum carboxylation rate (Vcmax) of cork-oaks. Moreover, the 2012 drought led to strong alterations in tree phenology: annual tree diameter increment and fruit production were severely reduced. These findings suggest that the projected increased drought and altered precipitation pattern under future climate change for the Mediterranean basin does not only threaten annual cork growth and understory development, which plays a vitally important role for ecosystem fluxes. 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.
Publications
- (2010) Soil-Litter-Iso: A one-dimensional model for coupled transport of heat, water and stable isotopes in soil with a litter layer and root extraction. Journal of Hydrology, 388 438-455
Harverd, V, Cuntz, M
(See online at https://doi.org/10.1016/j.jhydrol.2010.05.029) - (2012) Linking carbon and water cycles using stable isotopes across scales: progress and challenges., Biogeosciences, 8, 2659-2719
Werner, C, Schnyder, H, Badeck, F, Brugnoli, B, Cuntz, M, Dawson, T, Ghashghaie, J, Grams, T, Kayler, Z, Keitel, C, Lakatos, M, Lee, X, Máguas, C, Ogee, J, Rascher, K, Siegwolf, R, Unger, S, Welker, J, Wingate, L, Zeeman, M, Gessler, A
(See online at https://doi.org/10.5194/bgd-8-2659-2011) - (2013) Partitioning evapotranspiration – Testing the Craig and Gordon model with field measurements of oxygen isotope ratios of evaporative fluxes. Journal of Hydrology 496, 142-153
Dubbert, M, Cuntz, M, Piayda, A, Maguas, C, Werner, C
(See online at https://doi.org/10.1016/j.jhydrol.2013.05.033) - (2014) Influence of tree cover on herbaceous layer development and carbon and water fluxes in a Portuguese cork oak woodland. Acta Oecologica
Dubbert, M, Mosena, A, Piayda, A, Cuntz, M, Correia, A, Pereira, J S, Werner, C
(See online at https://doi.org/10.1016/j.actao.2014.05.007) - (2014) Oxygen isotope signatures of transpired water vapor – the role of non-steady-state transpiration under natural conditions. New Phytologist
Dubbert, M, Piayda, A, Cuntz, M, Werner, C
(See online at https://doi.org/10.1111/nph.12878)
