Assessing Douglas-fir water-use history using stable isotope ( 13 C and 18 O) in tree rings: principles and potential J. Renée Brooks Western Ecology Division, Corvallis OR Environmental Protection Agency

Stable Isotopes in Tree rings Isotopes indicate the magnitude of key ecological processes Isotopes indicate the magnitude of key ecological processes  13 C – intrinsic water-use efficiency  13 C – intrinsic water-use efficiency  18 O – RH, stomatal conductance  18 O – RH, stomatal conductance Isotopes record these responses to changing environmental condition. Isotopes record these responses to changing environmental condition. Tree rings are formed incrementally creating a record over time. Tree rings are formed incrementally creating a record over time. Isotopes integrate ecological processes over time Isotopes integrate ecological processes over time An annual ring integrates over the year. An annual ring integrates over the year.

Carbon isotope discrimination and its relationship to leaf physiology Where a = 4.4 (diffusion of CO 2 ); b = ~27 (enzymatic fractionation), c i internal [CO 2 ], c a = ambient [CO 2 ]

transpiration rate water stress humidity photon flux canopy leaf area CO 2 leaf conductance cicacica  productivity Growth, reproductive output photosynthetic rate Nitrogen

Carbon Isotope Discrimination a measure of Intrinsic Water-Use Efficiency Where a = 4.4 (diffusion of CO 2 ); b = ~27 (enzymatic fractionation), c i internal [CO 2 ], c a = ambient [CO 2 ]

Interpreting  13 C Same  13 C Value Increased  13 C Value Decreased  13 C Value

Interpreting  13 C and  18 O power of dual isotopes Grams et al PCE, Scheidegger et al Oecol.

Oxygen isotopes in plant tissues

What happens to leaf water? Craig Gordon (1965), Farquhar and Lloyd (1993)  + Equilibrium fractionation  k Kinetic fractionation  18 O v Water vapor e a /e i Atmosphere - leaf vapor gradient  18 O e enrichment of leaf water (above the source)

Transpiration Leaf surface Mass flow of leaf water Back diffusion of 18 O enriched water 18 O enrichment according to Craig-Gordon Model (1965) (preferential loss of H 2 16 O) Péclet Effect Stronger Transpiration  less 18 O in leaf water Stomatal conductance, g H2O [mmol m -2 s -1 ]  18 O [per mil]

Bulk Water vs. site of evaporation the Péclet effect Where C = molar density of water, D = diffusivity of H 2 18 O in water, E = transpiration rate L = effective path length Barbour et al. (2007)

Model for Cellulose  18 O ƒ o = fraction exchanged with xylem water ƒ o = fraction exchanged with xylem water wl = leaf water wl = leaf water wx = xylem water wx = xylem water cx = xylem cellulose cx = xylem cellulose ε o = fractionation factor (+27 ‰) ε o = fractionation factor (+27 ‰) Roden et al. 2000

Isotopic applications to field studies

Effects of soil Water Dupouey et al PCE Relative Extractable Water (%)  13 C (‰)

Transpiration Livingston and Spittlehouse 1993  13 C (‰)

Effects of Thinning McDowell et al PCE Thinned Control 200 year-old Ponderosa Pine Thinning

McDowell et al PCE Effects of Thinning

Effects of Fertilization Brooks & Coulombe in review

Fertilization effects on  Brooks & Coulombe in review

Leaf Gas-Exchange Brooks & Coulombe in review

 18 O response to Fertilizer  13 C effect Leaf Area effect Brooks & Coulombe in review

N Fertilization Created Hydraulic Imbalance Leaf area increased Leaf area increased Roots and sapwood insufficient to support increased leaf area Roots and sapwood insufficient to support increased leaf area Fertilized trees experience drought at the end of summer. Fertilized trees experience drought at the end of summer. Increase in leaf area offset decrease in leaf gas-exchange – Growth increased. Increase in leaf area offset decrease in leaf gas-exchange – Growth increased. Hydraulic imbalance lasted 10 years Hydraulic imbalance lasted 10 years

Multiple Fertilizer Applications

Unresponsive Site

Tree rings records Extent and duration of growth response is recorded in ring width data. Extent and duration of growth response is recorded in ring width data.  13 C and  18 O allow for understanding the leaf physiology and whole tree hydraulics.  13 C and  18 O allow for understanding the leaf physiology and whole tree hydraulics. Control trees necessary for separating management treatments from climate signals. Control trees necessary for separating management treatments from climate signals.

Tree rings provide added insights into long-term experiments.