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

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.

Tonzi Ranch Flux Tower

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

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

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

A Decade of Evaporation Measurements

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

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

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

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

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

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

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

Measuring Spatial/Temporal Variation in Soil Moisture

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

How Deep is the Soil? Trenton Franz, EMI

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

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

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

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

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

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

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

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

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

Water Fluxes are Coupled with Carbon Fluxes

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

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

Soil Moisture Controls on Respiration Xu + Baldocchi, AgForMet 2004

Sustained and Elevated Respiration after Fall Rain

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

Synthesis and Conclusion

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

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