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Grant, R.F. 1 , Baldocchi, D.D. 2 and Ma, S. 2

Ecological Controls on Net Ecosystem Productivity of a Seasonally Dry Annual Grassland under Current and Future Climates: Modelling with Ecosys. Grant, R.F. 1 , Baldocchi, D.D. 2 and Ma, S. 2 1 Department of Renewable Resources, University of Alberta, Edmonton, AB, Canada T6G 2E3

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Grant, R.F. 1 , Baldocchi, D.D. 2 and Ma, S. 2

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  1. Ecological Controls on Net Ecosystem Productivity of a Seasonally Dry Annual Grassland under Current and Future Climates: Modelling with Ecosys Grant, R.F.1, Baldocchi, D.D.2and Ma, S.2 1 Department of Renewable Resources, University of Alberta, Edmonton, AB, Canada T6G 2E3 2 Department of Environmental Science, Policy and Management & Berkeley Atmospheric Science Center, University of California, Berkeley, CA

  2. Ecological controls on NEP of annual grasslands in Mediterranean climate zones • Net C uptake • determined by the duration and intensity of precipitation in the rainy season during which soils are wet enough to sustain net C uptake by plants. • Net C emission • slow during the dry season when grasslands have senesced • rapid, rainfall-induced pulses at the start of the next rainy season before grasslands regrow, • rapid at the end of the rainy season if grassland growth terminates before the start of the following dry season.

  3. How can we model these controls? • (1) Climatic and phenological signals that induce germination during soil wetting at the start of the rainy season, and senescence during soil drying at the end of the rainy season • (2) Soil-root-canopy-atmosphere hydraulic scheme by which soil and atmospheric water status determine plant water status, and hence GPP during soil wetting and drying between germination and senescence • (3) Stimulation or suppression of Rh during wetting or drying of surface residues and soil evident in precipitation-driven pulses that characterize C emissions in seasonally dry ecosystems

  4. (1) Climatic and phenological signals • requirements for time accumulated at yc above or below set thresholds to be met during earlier and later plant growth stages respectively • 480 h above -0.2 MPa for germination, and 240 h below -2.0 MPa for senescence. • avoid premature germination or senescence and hence wastage of resources during false starts or ends to the rainy season

  5. (2) Soil-root-canopy-atmosphere hydraulic scheme: derive yc at which root water uptake + capacitance = transpiration Rn LE H atm. vapor density atmosphere stomatalsunlit,1 boundary stomatalshaded,1 canopy layer 1 canopy vapor density stomatalsunlit,n canopy layer n c stomatalshaded,n axial1 axial2 capacitance s,1 axial3 r,1 soil1 radial1 s,2 radial2 r,2 soil2 s,n r,n radialn soiln CO2 fixation, turgor, Ca - Ci residue layer soil hydraulic resistance, root length density root surface area root axis length soil layer 1 soil layer 2 soil layer n

  6. (3) Stimulation or suppression of Rh during wetting or drying • Scheme water and vapor transfer through soil surface and surface residue to solve for qr and yr. • Specific microbial respiration in soil and residue constrained by aqueous concentration [M/q] and y.

  7. Vaira Ranch (38.418N, 120.958W) MAT 16.3 oC Precipitation 559 mm

  8. Intercepts (a), slopes (b), correlations (R2), root mean square of differences between modelled and measured fluxes (RMSD), root mean square of error in measured fluxes (RMSE)

  9. Rainy seasons shorter, less intense vs. longer more intense Soil drying earlier vs. later Net C emission at start of rainy season End of net C uptake sooner vs. later Large interannual variation in NEP caused by precipitation

  10. Later start to rainy season lowers net C uptake in late winter of 2004 vs. 2005

  11. Earlier end to rainy season lowers net C uptake in spring 2004 vs. 2005

  12. Wetting of dry surface soil and residue at the start of the rainy season drives rapid pulses of CO2 emission

  13. Annual NEP is closely associated with the duration of net C uptake

  14. increased productivity with rising Ta, Ca, but … earlier maturity in longer growing seasons leaves wetter, respiring soil during dry season

  15. pattern of IAV changes after 35 to 70 years, avg. NEP declines NEP mean and IAV are stable under current climate SOC stops rising, declines gradually after IAV changes

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