Kathryn A. Berger Master of Science, Candidate Dept. of Natural Resources

1. Evaluation and Improvement of Model Algorithms for Predicting Belowground Carbon Allocation in Forests Kathryn A. Berger Master of Science, Candidate Dept. of Natural Resources University of New Hampshire Research & Discover Student Presentations August 2008

2. Total Belowground Carbon Allocation • Forests allocate C belowground to: • Roots, root respiration, exudates & mycorrhizae • Termed Total Belowground Carbon Allocation (TBCA) • Why study TBCA? • A large component of global C cycle • Considerable potential for future C sequestration • Connects belowground soil processes to forest canopy • The mechanism is still poorly understood • Difficulty in directly measuring TBCA restricts our understanding of belowground C cycling • Ecosystem models have proven useful to study effects of forest response to global climate change, but TBCA measurement challenges limit model development and simulation

3. The PnET-CN Model PnET-CN: Daily-to-monthly time step model of C, N, and H2O forest ecosystem fluxes Relies on relatively few input parameters Minimal reliance on calibration Carbon allocation: Central Plant C pool allocates to: foliage, wood, and fine root pools Fine roots predicted using Raich and Nadelhoffer (1989) mass-balance approach for estimation of TBCA (Aber et al. 1997)

4. Raich and Nadelhoffer (1989) Approach Fine Root Carbon (g C) = 130 + 1.92 * Leaf Carbon (g C) PnET-CN model: RootAllocA: Intercept of relationship between foliar and root allocation RootAllocB: Slope of relationship between foliar and root allocation RootCAdd = RootAllocA * (DaySpan / 365) + RootAllocB * FolProdCMo RootAllocB RootAllocA (Raich and Nadelhoffer 1989)

5. Objectives • To evaluate the performance of the current PnET-CN model in predicting TBCA under e[CO2] • If necessary, develop a more realistic simulation to capture TBCA dynamics

6. Methods • Validation performed using data from 3 U.S. Free-Air CO2 Enrichment (FACE) experimental forests • FACE site objectives: • Quantify CO2 effect on NPP in a manner that will inform ecosystem and global models • Three U.S. forest FACE experiments • Stand treatment plots exposed to ~550 ppm CO2 for 3 to 8 years • NPP from all plots and years post closed canopy

7. FACE Site Results Duke FACE ORNL FACE wood CO2 on fine root (Norby 2005)

8. PnET-CN Model Simulation • Model simulation of FACE experiments • Single-step increase in CO2 • Disturbance history as identified by each site • Site-specific vegetation parameters • Site-specific climate records

9. PnET-CN Simulations:Elevated CO2 Initial evaluation of the PnET-CN model found: NPP (g C m-2 y-1) is predicted by the PnET-CN model into 3 biomass pools: foliage, wood & fine roots Under elevated CO2 additional C is allocated predominantly to wood Experimental evidence suggests otherwise... Example: Aspen FACE, Rhinelander, WI

10. Development of Modified TBCA Mechanism • “The use of constant partitioning coefficients in static models is unlikely to provide a realistic picture of carbon cycling.” (Litton et al. 2007) • Resource availability always caused shifts in C partitioning between TBCA and Wood C pools • Fertilization increased partitioning to wood and decreased partitioning TBCA for all forests • Based on concept that variability around the Raich and Nadelhoffer (1989) relationship is due to availability of belowground nutrients (N) suggested by: • Functional Carbon Allocation Theory (Root:Shoot Ratio) • Haynes and Gower 1995; Gower et al. 1996; Nadelhoffer et al. 1998; Giardina and Ryan 2002; Palmroth et al. 2006; Franklin 2007

11. Functional Equilibrium Models Hi CO2 Carbon is allocated to the shoot when C supply is reduced Plant shifts C allocation to roots when water or nutrient supply (predominantly N) is reduced C allocation shifts in such a way that relative growth rate is maximized. (Thornley and Johnson 1990) High Nutrients High Nitrogen (Adapted from: http://ecology.botany.ufl.edu/ecologyf02/homeostasis.html)

12. Modified TBCA mechanism Dataset: Mature forests ≥ 45 years old (Raich and Nadelhoffer et al. 1989; Davidson et al. 2002) Y-intercept of zero 95% confidence interval Upper slope 3.2 Lower slope 2 R&N (1989)derived slope= 2.59 r2 = 0.3449

13. Development of Modified TBCA Mechanism • Introduction of internal PnET-CN variable: NStatus • NStatus a function of internal model variable (NRatio) and input parameter (FolNConRange) • Removal of static TBCA mechanism • No longer dependent on RootAllocA and RootAllocB input parameters • Result: A fluctuating TBCA mechanism based upon N availability

14. Results FACE Site validation Application of the Modified model to future climate scenarios

15. PnET-CN NStatus : UnderElevated CO2 Predicted PnET- CN NPP (g C m2 y-1) versus published FACE site values under elevated CO2 conditions for each of the three simulated biomass pools: foliage, wood and fine roots

16. PnET-CN NStatus : UnderElevated CO2 Largest increase in % fine root stimulation at ORNL FACE Enhanced model nearly doubles fine root estimates

17. Application of the Modified TBCA Mechanism • Model simulations at 5 northeastern forests under a variety of projected climate scenarios (originally performed by Ollinger et al. 2008) • Climate projections for 1990-2099 using the PCM (Parallel Climate Model) and HadCM3 (Hadley Centre Coupled Model version 3) models • Generated using CO2 (and other greenhouse gas emissions) scenarios: higher [A1] and lower [B1] projections • Greatest warming: HADA1 projections 6.3°C warming • Least warming: PCMB1 projections 1.5°C warming

18. PnET Predictions of Total NPP PCMA1 NPP (g C m-2 y-1) at Howland Forest , ME under Parallel Climate Model (PCM) future climate scenarios Total NPP estimates between models are very similar PCMB1

19. PnET Predictions of ANPP NPP (g C m-2 y-1) at Howland Forest , ME under Parallel Climate Model (PCM) future climate scenarios Estimates of ANPP decrease with PnET-CN: NStatus model

20. PnET Predictions of BNPP NPP (g C m-2 y-1) at Howland Forest , ME under Parallel Climate Model (PCM) future climate scenarios Estimates of BNPP increase with PnET-CN: NStatus model

21. Effect on C Storage • How does the modified TBCA mechanism influence estimates of net ecosystem production (NEP)? • NEP = NPP – Rh • Difference between NPP and heterotrophic respiration (Rh) • Determines the amount of carbon lost or gained by an ecosystem • PnET-CN output can also be used to produce predictions of future NEP • Does increased allocation to fine roots (with faster turnover times) result in decreased carbon storage in the future?

22. NEP Predictions

23. Results • Modified TBCA mechanism suggests increased NEP over long-term (1990-2099) • Additional C allocated to fine roots resulted in more long-term C storage than wood C pool

24. Discussion • Increased carbon flow belowground has increased intensity of roots foraging for soil nutrients • C allocation shift of fine root and mycorrhizal fungi into deeper soils may allow forests to acquire additional N to offset increased nutrient limitations (Pritchard et al. 2008) • Root foraging concept provides evidence to indicate e[CO2] will stimulate productivity (and carbon storage) in N-limited ecosystems more than previously expected • Unlikely that that ecosystem productivity will be stimulated by e[CO2] fertilization indefinitely

25. Acknowledgments • Special thanks to: • My committee: Scott Ollinger, Mary Martin, Andrew Richardson, & Christy Goodale • UNH-NASA Research and Discover Fellowship • Rita Freuder and Julian Jenkins • My colleagues in Rm 251 • And most importantly my friends and family for their support • Thanks very much for listening, any questions?