1. Das Jena Trockenstress Experiment - warum sterben Bäume wenn das Wasser knapp wird? Henrik Hartmann Waldemar Ziegler Susan Trumbore

2. Outline Henrik Hartmann? Introduction –Forests and the carbon cycle –Effects of drought on trees –Potential mechanisms of drought-induced tree mortality Research hypotheses Experimental design First results Scientific conclusion Non-scientific conclusions Questions and discusssion

3. Henrik Hartmann * Haiger, Hessen (1968) Immigration to Canada (1992) B.ForSci, Edmundston, N.B. (2003) Ph.D. Forest ecology, Montreal (2008) Postdoc, Jena (2009 - )

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5. Sugar maple mortality following partial harvest Do disturbances from partial harvest cause physiological stress strong enough in residual trees to kill them? Nope! What caused mortality? – Malacosoma disstria (forest tent caterpillar)

6. Forests in the modern world Forests cover ~ 30% of land surface (4 billion ha) and are “essential to economic development and maintenance of all life forms” (UNO 1992) They foster the bulk of terrestrial biodiversity and are a major driving force of the cycling of life-supporting elements (e.g., carbon) (Gaston 2000, FAO 2007) Forests contain ~90% of living biomass and ~2/3 of atmospheric carbon (Körner 2003)

7. Forests and the carbon cycle Forests circulate about 8% of atmospheric CO 2 back and forth into the biosphere every year (Malhi et al. 2002) Forest net carbon uptake is the small difference between two large fluxes – photosynthesis and respiration (Litton et al. 2007) However, forest mortality from disturbances and environmental change liberates large amounts of stored carbon into the atmosphere (Kurz et al. 2008) … and thus can fuel feedback loops between atmospheric CO 2 concentrations and climate change

8. Forests and the carbon cycle

9. Forest and drought Anthropogenic GHGs, mainly CO 2, have been linked to climate change …... which is predicted to cause alterations in precipitation patterns (IPCC 2007) Frequencies of severe and extreme drought events have increased during the last decades of the 20th century and… … the area affected by extreme drought events will further increase (from 3% to ~ 30%) and drought events will be more frequent and longer-lasting (Burke et al. 2006) Elevated temperatures and drought have been linked to tree and forest mortality in many parts of the world (Allen et al. 2010)

10. Weltweites Baum- und Waldsterben Source: Allen et al. 2010

11. A model tree CO 2 O2O2 Photosynthesis O2O2 CO 2 Respiration Carbohydrate translocation Water transport Water uptake Main processes of tree carbon and water cycle Transpiration

12. Drought impacts on main processes Carbon gain vs. water loss Xylem and phloem transport Respiration vs. temperature

13. Linking water and carbon cycle

14. Hydraulic framework

15. Experimental evidence

16. The Jena Drought Stress Experiment Field experiment: 2 seasons, 2 species (P.abies, P.sylvestris), rainroof, WTC Treatment: water availability Greenhouse experiment: 3 seasons, 3 species (P.abies, P.sylvestris, T.occidentalis), above- and belowground chambers Treatments: water and CO 2 availability

17. Hypotheses 1. Trees ‘dry out’ Water potential decreases below specific threshold and causes hydraulic/symplastic failure 2. Trees starve to death after depleting all stored carbon Trees respire all stored carbon 3. Trees starve to death because they cannot access stored carbon Trees respire local carbon stores but decreasing water potential empedes long-distance carbon translocation

18. Measurements

19. Custom-made equipment

21. First results (pre-experiment) Strong decrease in sapflow, maybe even cavitation, after < 2 weeks of drought

22. First results (pre-experiment) Pronounced stem shrinking from drought stress –Reduced phloem hydration  lower C-translocation capacity

23. First results (pre-experiment) Higher C-investment in root turnover

24. First results (pre-experiment) C-balance negative after < 2 weeks of drought stress

25. First scientific conclusions Drought –Creates very low water potential in xylem and maybe even cavitation –Increases C-expenditures due to higher fine root turnover –Cause a rapid decline of NPP All 3 hypotheses are supported by our data  We‘re on the right way but mortality has not occured yet.

26. Non-scientific conclusions Technical equipment has been costom-made to serve our needs and to reduce costs Purchase of equipment has to follow low-bidding rules More collaboration between suppliers and customers would facilitate research and may be economically intersting for both sides

27. Thank you!!