1. Ecology and Soil Moisture: Using an Oak-Grass Savanna as a Model System for Studying the Effects of Soil Moisture Dynamics on Water and Carbon Exchange Dennis Baldocchi Siyan Ma, Naama Raz-Yaseef, Laurie Koteen, Joe Verfaillie, Trenton Franz ESPM UC Berkeley COSMOS Workshop Tucson, AZ, Dec 2012

2. Oak-Savanna Model System for Studying the Effect of Soil Moisture on Ecosystem Ecology Structure/Function – Oak and grasses provide contrasting life forms, woody/herbaceous, perennial/annual – The Canopy is open and heterogeneous, giving us a opportunity to test the applicability of ecosystem and biogeophysical models, mostly developed for ideal and closed canopies Environmental Biology – The Mediterranean climate provides distinct wet/ cool and dry/hot seasons to examine the ecosystem response (photosynthesis, transpiration, respiration, stomatal conductance) to a spectrum of soil moisture and temperature conditions Global Change – The Mediterranean climate experiences great extremes in inter-annual variability in rainfall; we experience a wider range in ppt over a few years than long-term predicted changes.

3. Tonzi Ranch Flux Tower

4. Oak-Grass Savanna: A Two Layer System Summer: Trees green; grass dead Spring: Trees green;grass green Winter: Trees deciduous; grass green

5. Oak Savanna consists of Heterogeneous and Open Canopy with Low LAI (< 2.0)

6. Objectives Examine fluxes of water and carbon with changes in soil moisture – Role of Seasonal Moisture Deficits – Role of Rain Pulses Explore spatial/temporal variation of soil moisture – Temporal variation with TDR – Depth of soil with GPR – Root distribution with GPR – Soil moisture spatial patterns with EMI

7. A Decade of Evaporation Measurements

8. InterAnnual Variation in Water Balance Ppt: 350 to 900 mm ET savannna: 380 to 520 mm ET grass: 280 to 520 mm

9. In Semi-Arid Regions, ET is Conservative: The Most ET lost, scales with Precipitation during the Drier Years ‘Mediterranean Ecosystems Don’t evaporate more than 500 mm per year’, Serge Rambal

10. Evaporation from an Oak Savanna > Annual Grassland How and Why?

11. Soil Moisture Dynamics at Oak Savanna Differ from Near by Annual Grassland Smaller Water Reservoir Contributes to Lack of Trees

12. Eco-hydrology: ET, Functional Type, Physiological Capacity and Drought

13. Effects of Functional Types and R sfc on Normalized Evaporation R c is a f(LAI, N, soil moisture, Ps Pathway)

14. ET and Soil Water Deficits: Root-Weighted Soil Moisture Baldocchi et al., 2004 AgForMet

15. Measuring Spatial/Temporal Variation in Soil Moisture

16. Hourly Sampling, Few points and Depths, Theta Probe Poor Vertical and Horizontal Sampling

17. How Deep is the Soil? Trenton Franz, EMI

18. ESI, Moisture Point Many Locations, Discrete Depths, Bi-Weekly, Manual Sampling

19. Marry the Two Sensor Types Calibrate Theta-Probe with Moisture Point Better Spatio-Temporal Resolution

20. Where are the Roots and How Many? Remote Sensing Coarse Roots with GPR Ground Truth with Soil Pits

21. Vertical Distribution of Roots with Ground Penetrating Radar Raz-Yaseef et al. JGR Biogeosciences, in press

22. Radial Distribution of Coarse Roots, with Ground Penetrating Radar Raz-Yaseef et al. JGR Biogeosciences, in press

23. Baldocchi et al., 2004 AgForMet ET and Soil Water Deficits: How do Trees stay Alive with such Low Water Potentials? Root-Weighted Soil Moisture Matches Pre-Dawn Water Potential

24. G. Miller, X Chen, Y. Rubin, D. Baldocchi WRR 2010 But, Oak Trees Seem to Tap Ground Water

25. Pre-Dawn Water Potential Represents Mix of Dry Soil and Water Table Miller et al WRR, 2010 During Summer MidDay Water Potential is Less Negative than Shallow Soil Water Potential, Evidence the Trees are tapping Groundwater

26. ‘Soil Moisture’ Maps with EMI Trenton Franz, U Arizona

27. Water Fluxes are Coupled with Carbon Fluxes

28. Interannual Variation in Net Carbon Exchange: Grassland is Carbon Neutral; Savanna is a C Sink

29. Carbon Fluxes Scale with Spring Rainfall Ma et al, 2007 AgForMet

30. Soil Moisture Controls on Respiration Xu + Baldocchi, AgForMet 2004

31. Sustained and Elevated Respiration after Fall Rain

32. Impact of Rain Pulse, Timing of Rain and Photodegradation on Ecosystem Respiration Baldocchi et al, JGR, Biogeosciences, 2006

33. Synthesis and Conclusion

34. Conclusions Savanna woodlands need about 80 mm more water to function than nearby grasslands – Trees tap ground-water to sustain themselves during the summer Year to year variability in Carbon Uptake is due to length of wet season. – Oaks are risk adverse and experience less inter-annual variability in NEE than grasslands Photosynthesis and Respiration are tightly linked – Oaks need high N levels to attain sufficient rates of carbon assimilation for the short growing season Oaks are darker, warmer and use more water than grasslands

36. Biometeorology Team Funding: US DOE/TCP; NASA; WESTGEC; Kearney; Ca Ag Expt Station