Impact of recurrent extreme drought events and shrub invasion on Mediterranean cork-oak ecosystem functioning and resilience

• Applicants: Professor Dr. Joaquim G. Pinto; Professorin Dr. Christiane Werner
• Subject Area: Ecology and Biodiversity of Plants and Ecosystems
• Term: from 2017 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 322068324

• Final Report Year: 2023

Final Report Abstract

Mediterranean ecosystems are simultaneously threatened by extreme drought and plant invasion. Both stressors can have substantial effects on ecosystem water and carbon fluxes. However, the interaction of both stressors is rarely investigated and effects on ecosystem processes unknown. Thus, this project aimed at investigating the interaction effects of extreme drought and plant invasion on Mediterranean cork oak ecosystem functioning and resilience. Moreover, we wanted to quantify the recurrence of drought events under present and future climate conditions for the Iberian Peninsula. To achieve these aims, we established a rain exclusion and shrub invasion (Cistus ladanifer) experiment combined with continuous soil water content and sap flow measurements, which were complemented by a large range of ecophysiological and ecohydrological measurements. Interaction effects of two environmental stressors are rarely investigated under field conditions. The long-term measurements within a cork oak ecosystem revealed that the interaction of extreme drought and shrub invasion is highly dynamic and non-linear. While we observed neutral interactions in the wet year 2018, this shifted to an amplifying interaction in the dry year 2019. Thus, shrub invasion and additional drought were having a stronger impact on the ecosystem than one stressor alone. However, drought legacy effects of the extreme drought 2019 led to a buffering interaction in the recovery period of 2020, meaning, that the interaction of shrub invasion and additional drought was less pronounced than invasion alone. These results shed light onto the dynamics of such interaction effects. Nevertheless, shrub invasion was dominating as stress factor over the additional drought. This was especially evident in the hydraulic behavior of the cork oak trees. Compared to cork oak trees under ambient or rain exclusion conditions, trees under shrub invasion, independent of the additional drought, deployed a different hydraulic strategy under dry conditions. This more conservative behavior of invaded trees led to lower levels of leaf area, trunk increments, water fluxes and gas leaf exchange. In the future, this strategy could prove detrimental for cork oak trees under invasion, especially under recurrent drought events. Recurrent drought events in consecutive years have rarely been evaluated for the Iberian Peninsula. Thus, we focused on the quantification of recurrent drought events under present and future climate conditions. By developing two novel drought indices (Recurrent Dry Year Index and Consecutive Drought Year Index), we demonstrate that the Iberian Peninsula is regularly affected by extreme droughts under present climate conditions. Especially the southern and central parts are affected by recurrent drought of two to three years (up to six years). The main driver for these (recurrent) drought events is precipitation deficits in the winter. Under future conditions (RCP8.5), drought events will become more severe with a higher frequency and duration of recurrent droughts. If the global warming should exceed +3° global warming level, the majority of models predicts and almost permanent state of drought for the Iberian Peninsula with severe consequences for the population and ecosystems, such as cork oak woodlands. These findings suggest that extreme drought combined with plant invasion can have substantial impacts on cork oak functioning and resilience. Although the interaction of both stressors is highly dynamic, invasion has a clear negative effect on cork oak hydraulic behavior and carbon fluxes. Under future conditions with more recurrent drought events, cork oak ecosystems are severely threatened, especially under the influence of shrub invasion. Thus, this project suggests that both, extreme drought and shrub invasion can contribute to the already observed cork oak decline.

Publications

• Terpenoid Emissions of Two Mediterranean Woody Species in Response to Drought Stress. Frontiers in Plant Science, 9 (2018, 7, 23).
  Haberstroh, Simon; Kreuzwieser, Jürgen; Lobo-do-Vale, Raquel; Caldeira, Maria C.; Dubbert, Maren & Werner, Christiane
  
• Volatile diterpene emission by two Mediterranean Cistaceae shrubs. Scientific Reports, 8(1).
  Yáñez-Serrano, A. M.; Fasbender, L.; Kreuzwieser, J.; Dubbert, D.; Haberstroh, S.; Lobo-do-Vale, R.; Caldeira, M. C. & Werner, C.
  
• A pool‐weighted perspective on the two‐water‐worlds hypothesis. New Phytologist, 222(3), 1271-1283.
  Dubbert, Maren; Caldeira, Maria C.; Dubbert, David & Werner, Christiane
  
• Natural Carbon Isotope Composition Distinguishes Compound Groups of Biogenic Volatile Organic Compounds (BVOC) in Two Mediterranean Woody Species. Frontiers in Forests and Global Change, 2 (2019, 9, 18).
  Haberstroh, Simon; Kreuzwieser, Jürgen; Boeddeker, Helena; Eiblmeier, Monika; Gutte, Helene; Lobo-do-Vale, Raquel; Caldeira, Maria C. & Werner, Christiane
  
• Nonlinear plant–plant interactions modulate impact of extreme drought and recovery on a Mediterranean ecosystem. New Phytologist, 231(5), 1784-1797.
  Haberstroh, Simon; Caldeira, Maria C.; Lobo‐do‐Vale, Raquel; Martins, Joana I.; Moemken, Julia; Pinto, Joaquim G. & Werner, Christiane
  
• Plant invasion modifies isohydricity in Mediterranean tree species. Functional Ecology, 36(9), 2384-2398.
  Haberstroh, Simon; Lobo‐do‐Vale, Raquel; Caldeira, Maria C.; Dubbert, Maren; Cuntz, Matthias & Werner, Christiane
  
• Recurrence of Drought Events Over Iberia. Part I: Methodology and Application for Present Climate Conditions. Tellus A: Dynamic Meteorology and Oceanography, 74(2022), 222.
  Moemken, Julia & Pinto, Joaquim G.
  
• Recurrence of Drought Events Over Iberia. Part II: Future Changes Using Regional Climate Projections. Tellus A: Dynamic Meteorology and Oceanography, 74(2022), 262.
  Moemken, Julia; Koerner, Benjamin; Ehmele, Florian; Feldmann, Hendrik & Pinto, Joaquim G.
  
• Soil VOC emissions of a Mediterranean woodland are sensitive to shrub invasion. Plant Biology, 24(6), 967-978.
  Meischner, M.; Haberstroh, S.; Daber, L. E.; Kreuzwieser, J.; Caldeira, M. C.; Schnitzler, J.‐P. & Werner, C.
  
• The role of species interactions for forest resilience to drought. Plant Biology, 24(7), 1098-1107.
  Haberstroh, S. & Werner, C.