Quantification of the 2018 drought for European forests and impacts - - PowerPoint PPT Presentation

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Quantification of the 2018 drought for European forests and impacts - - PowerPoint PPT Presentation

Mar 04, 2023 291 likes β€’433 views

Quantification of the 2018 drought for European forests and impacts of stomatal and non stomatal limitation of photosynthesis European 2018 drought European Drought Observatory, combined drought indicator (CDI) Drought taskforce ->

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Quantification of the 2018 drought for European forests and impacts of stomatal and non stomatal limitation of photosynthesis

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European 2018 drought

European Drought Observatory, combined drought indicator (CDI) Drought taskforce -> Philosophical transaction of the royal society B

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A bit of theory

Figure from Zhou et al., 2019 Ca gs gm Ci

Cc

Non-stomatal Stomatal 𝐻𝑄𝑄

𝑑 = π‘Š 𝑑𝑛𝑏𝑦(𝐷𝑗 βˆ’ Ξ“ βˆ—)

(𝐷𝑗 + 𝐿𝑛) 𝐷𝑗 = 𝐷𝑏 βˆ’ 𝐻𝑄𝑄 𝑕𝑑,𝑑𝑝2 𝑕𝑑,𝐼2𝑃 = 𝑕0 + 1.6(1 + 𝑕1 βˆšπ‘Šπ‘„πΈ) 𝐻𝑄𝑄 𝐷𝑏 At light saturation : gs,𝐼2𝑃 = LEGaΞ³ s Rn βˆ’ G βˆ’ S + ρCpCaVPDa βˆ’ LE(s + Ξ³) gs can be obtained from PM equation

Medlyn et al.,2011

𝑕𝑑,𝐼2𝑃 1.6 = 𝑕𝑑,𝐷𝑃2

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Stomatal and non stomatal limitation

  • f photosynthesis : models

𝐻𝑄𝑄 = π‘Š

𝑑𝑛𝑏𝑦(𝐷𝑗 βˆ’ Ξ“ βˆ—)

(𝐷𝑗 + 𝐿𝑛) 𝐷𝑗 = 𝐷𝑏 βˆ’ 𝑯𝑸𝑸 𝒉𝒅 𝑕𝑑 = 𝑕0 + 1.6(1 + 𝑕1 βˆšπ‘Šπ‘„πΈ) 𝐻𝑄𝑄 𝐷𝑏 Non stomatal limitation Stomatal limitation Changes in apparent Vcmax with measured Ci values Changes in Ci which are associated with changes in g1 (changes in the GPP-gc slope) g1 is inversely proportional to iWUE 𝐻𝑄𝑄 π‘Šπ‘„πΈπ·π‘ 𝑕𝑑 𝑕1

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Quantification of drought

  • In lack of soil and pre-dawn leaf water potential at flux tower

sites , Relative Extractable Water (REW):

REWt = SWCt βˆ’ SWCWP SWCFC βˆ’ SWCWP Soil humidity sensors REW varies from 1 (Field capacity) and 0 (wiliting point) Cumulated over the root zone

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Ecosystem stations

Site ID Country Latitute Longitude Dominating species Soil texture Precipitation Mean 2018 Difference BE-BRA Belgium 51.308 4.52Pinus Syvlestris sand 819 724 95 BE-VIE Belgium 50.305 5.998 Fagus sylvatica/Pseudotsuga Menziesii silty clay loam 898 592 305 CZ-LNZ Czech 48.682 16.946 Quercus robur/Fraxinus angustifolia/Carpinus betulus/Tilia cordata sandy loam 445 438 7 CZ-RAJ Czech 49.444 16.697Picea abies sandy loam 622 474 148 CZ-STN Czech 49.036 17.97Fagus Sylvatica sandy loam 644 457 187 DE-HAI Germany 51.079 10.453Fagus Sylvatica clay loam 734 511 223 DK-SOR Denmark 55.486 11.645Fagus Sylvatica sandy clay loam 837 399 438 FR-BIL France 44.494

  • 0.956Pinus pinaster

sand 892 884 8 FR-HES France 48.674 7.065Fagus Sylvatica silty clay loam 827 737 90 IT-SR2 Italy 43.732 10.291Pinus Pinea sandy 1078 908 170

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Results : stomatal limitation

οƒ  No consistant stomatal behavior across ecosystems Recall : g1 is inversely proportional to iWUE

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Results : non stomatal limitation

οƒ  Non stomatal limitations are observed at almost all sites where REW felt < 0.4

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Degree of limitation

We quantify the degree of limitation by :

  • Fixing Vcmax at unstressed value and computing GPP with
  • bserved Ci
  • Fixing G1 at unstressed value and compute GPP with observed

Vcmax values Compute the ratio of GPPmodelled/GPPobserved

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Degree of stomatal and non stomatal reduction

οƒ  In most ecosystems, non-stomatal limitation is the dominant mechanism οƒ  Decrease of apparent Vcmax could be the result of both diffusional effects (mesophyll conductance) or biochemical effects

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Focus on 3 beech forests

  • FR-HES, DK-SOR and DE-HAI are 3 beech forests
  • We observe non-stomatal limitation at all 3 sites

In term of water use efficiency (iWUE) we observe :

  • Constant g1 at DK-SOR (constant iWUE)
  • Decreasing g1 at FR-HES (increased iWUE) which has a visible impact on GPP
  • Increasing g1 at DE-HAI (decreased iWUE) but with no visible impact on GPP

(GPP is already too low)

  • > unsolved question !
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Implications for drought modeling

𝑕𝑑,𝐼2𝑃 = 𝑕0 + 1.6(1 + 𝑕1 βˆšπ‘Šπ‘„πΈ) 𝐻𝑄𝑄 𝐷𝑏 How should plante regulate stomata ? (Cowan & Farquhar, 1977) Stomata regulate both photosynthesis and transpiration Stomata should maximise : 𝐡 βˆ’ Ξ»E where Ξ» is the carbon cost of water. g1~

1

Ξ» If Ξ» = πœ€π΅

πœ€πΉ = π‘‘π‘π‘œπ‘‘π‘’π‘π‘œπ‘’ (water spent now can’t be spent later) but

does not apply when water availbility decrease !

  • > when soil water depletes, the cost should increase (Ξ» β†— and

g1 β†˜) MakΓ«la et al., 1996 Results from this study do no support this ! οƒ  the costs of stomatal opening are probably not well identified Ideas :

  • Loss of hydraulic conductivity
  • Limit non-stomatal limitation

Dewar et al., 2018

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Conclusions

  • Non stomatal limitation was the dominant short

term mechanism limiting GPP in forest at flux tower sites

  • Apparent Vcmax has proven a useful way of modeling

these NSL

  • Future optimal conductance models should take NSL

into accounts

  • REW has proven a very useful index of edaphic

drought at flux tower sites

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Thank you !