ICOS Science Conference, Session 8

1. Drought 2018: impacts of stomatal and non stomatal
limitation for European forests photosynthesis
Gourlez de la Motte et al. (Longdoz Bernard)
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2. Lessons learn from 2003 drought:GPP(gCm-2month-1)
Ciais et al., 2005 (Nature)
 Significant GPP reduction for almost all the EC sites
 Which photosynthesis/photorespiration processes are
impacted by edaphic drought ?
2003
2002
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3. European 2018 drought
August 2003 SPEI06
August 2018 SPEI06
 Same intensity but
with different spatial
pattern
 What is the relationship
between drought intensity and
GPP processes ?
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4. GPP processes
Non-stomatal
processes
Stomatal processesTransfer through stomata
Transfer through mesophyll
Carboxylation
PGA Phosphorylation
RuBP regeneration
Oxygenation
…
 How edaphic drought modulates stomatal & non-
stomatal processes?
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5. Potential influence of soil water status
PPFD
Ca
T°
ea
Ysoil
O2
Ylf
ABA
Ci
Cm
Tr
CO2+H2O → CH2O +O2
Photorespi
gs
gm
5 How Ysoil impact gs and biochemical reactions?

6. Determination of Vcmax ,Gs Ci and g1 from
EC station data
𝑪𝒊 = 𝑪 𝒂 −
1.6 𝑮𝑷𝑷
𝑮 𝒔,𝑯𝟐𝑶
𝑮 𝒔,𝑯𝟐𝑶 = 𝐺0 + 1.6(1 +
𝒈 𝟏
√𝑽𝑷𝑫
)
𝑮𝑷𝑷
𝑪 𝒂
4. Gross assimilation model by Farquhar et al. 1980 (at Light saturation, no gm → Vcmax will
be an apparent one = 〔Vcmax + gm〕 ):
𝑮 𝒔,𝑯𝟐𝑶 =
𝑳𝑬 𝑮 𝒃𝒍−𝒂𝒕𝒎 𝛾
Δ 𝑹 𝒏 − 𝑮 + 𝜌 𝐶 𝑝 𝑮 𝒃𝒍−𝒂𝒕𝒎 𝑽𝑷𝑫 − 𝑳𝑬(Δ + 𝛾)
1. Inversion of Penman-Monteith equation (big leaf & LE = Transpiration):
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2. Stomatal conductance parameterization (Medlyn et al.,2011):
3. Fick law through stomata:
4 equations based on assumptions to solve 4 unknowns using EC station data (half-hour)
𝑮𝑷𝑷 =
𝑽 𝒄𝒎𝒂𝒙(𝑪𝒊 − Γ ∗)
(𝑪𝒊 + 𝐾 𝑚)
 What is the dependence of 𝒈 𝟏 and 𝑽 𝒄𝒎𝒂𝒙 on soil water ?

7. Stomatal and non stomatal limitation of GPP
Non stomatal limitationStomatal limitation
Is 𝒈 𝟏 varying during the 2018 drought ?
𝐺𝑃𝑃
𝑉𝑃𝐷𝐶 𝑎
𝑔𝑠
𝒈 𝟏
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𝒈 𝟏 is inversely
proportional to
iWUE
Medlyn et al.,2011
Is 𝑽 𝒄𝒎𝒂𝒙 varying during the 2018 drought ?
Drought quantification : Ysoil → Relative extractable water 𝑅𝐸𝑊 =
𝑆𝑊𝐶 − 𝑆𝑊𝐶 𝑊𝑃
𝑆𝑊𝐶 𝐹𝐶 − 𝑆𝑊𝐶 𝑊𝑃
𝑅𝐸𝑊 varies from 1 (FC) and 0 (WP). Allow comparison of drought level between sites
Soil water content
(m³w m-3
s)
Field Capacity
Wilting Point
Summation over the
rooting depth
 What is the dependence of 𝒈 𝟏 and 𝑽 𝒄𝒎𝒂𝒙 on 𝑅𝐸𝑊
When 𝑅𝐸𝑊 < 0.4 forest GPP seems to be lowered (Granier et al., 2007)
Need rooting depth of
the sites
Need granulometry of the sites
Stomatal limitation
Is 𝒈 𝟏 varying during the 2018 drought ?

8. Forest stations
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Brasschaat
Vielsalm
Hesse
Soro
Hainich
Bilos
San Rossore 2
Rajec
Lanzhot
Stitina

9. Results : stomatal limitation
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No clear variation of 𝒈 𝟏 when 𝑅𝐸𝑊
decrease at all sites except at DE-HAI
→ No clear influence of REW on
iWUE
→ Impact of drought on 𝑮 𝒔 pass
through reduction of biochemical
reaction activity ?
or/and
→ Ylf not (yet) impacted by the
decrease of 𝑆𝑊𝐶 ?
Dewar et al., 2018

10. Results : non stomatal limitation
SWC sensors failure
(overestimation)
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→ Reduction of 𝑽 𝒄𝒎𝒂𝒙 (apparent one)
observed when 𝑅𝐸𝑊 dropped <
0.4 (non stomatal limitations)
Exception : IT-SR2 & BE-VIE (no
reduction); DK-SOR (higher threshold)
Ylf not (yet) impacted by the
decrease of 𝑆𝑊𝐶 ?
SWC sensors
profile not deep
enoughGPP measured
coming from plot
≠ SWC sensors

11. Comparing the degree of GPP limitation
exercised by stomatal and non-stomatal
processes
We quantify the Degree of Limitation (DoL) by :
• For each data, 𝑽 𝒄𝒎𝒂𝒙 was replaced by the unstressed
value 𝑽∗
𝒄𝒎𝒂𝒙 to compute a new 𝑮𝑷𝑷∗ (Farquhar
equation)
• DoL for non-stomatal processes = 𝑮𝑷𝑷∗ 𝑮𝑷𝑷
• For each data, 𝒈 𝟏 was replaced by the unstressed
value 𝒈∗
𝟏
to compute a new 𝑮𝑷𝑷∗ (Medlyn
equation)
• DoL for non-stomatal processes = 𝑮𝑷𝑷∗ 𝑮𝑷𝑷
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𝑽∗
𝒄𝒎𝒂𝒙
𝒈∗
𝟏

12. Stomatal and non-stomatal Degree of Limitation
non-stomatal
mechanism =
dominant reason
for
𝑮𝑷𝑷 limitation
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Decrease in apparent 𝑽 𝒄𝒎𝒂𝒙
Diffusional effects
(mesophyll conductance)
Biochemical effects

13. Conclusions
• Non-stomatal limitation (NSL) was the dominant short
term mechanism decreasing 𝑮𝑷𝑷 at forest flux tower sites
during 2018 drought
• No clear short term influence of soil water status on
𝒊𝑾𝑼𝑬 for European forest
• Apparent 𝑽 𝒄𝒎𝒂𝒙 dependence on 𝑹𝑬𝑾 should taken into
account to model correctly drought event (apply on 𝒈 𝒎
and/or actual 𝑽 𝒄𝒎𝒂𝒙….WIP)
• 𝑹𝑬𝑾 is a very useful index of edaphic drought at flux
tower sites but need of 𝑺𝑾𝑪 & granulo profiles + rooting
depth
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14. Thank you !
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