ICOS Science Conference 2022: Day 3, Session 21

1. Increased forest resistance to extreme
atmospheric dryness: a case study of
two characteristics Swiss forests
Ankit Shekhar1, Lukas Hortnagl1, Nina Buchmann1, Mana Gharun1,2
1 Department of Environmental Systems Science, ETH Zürich, 8092 Zürich, Switzerland
2 Faculty of Geosciences, University of Münster, 48149 Münster, Germany
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2. Day to day weather – Air temperature
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∆T ≈ 8°C
∆T ≈ -7°C

3. Day to day weather – VPD
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4. Day to day: Daytime NEP
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NEP = Net ecosystem productivity
CO2 release
CO2 uptake

5. Forest productivity & functioning
 Forest productivity ≈ f(Biotic factors, Weather, non-weather abiotic…)
 Tree physiology (leaf-level gas exchange processes) adjusts with
environmental fluctuations -> A process of physiological acclimatization.
 Sudden or abrupt changes might affect the process of physiological
acclimatization -> change in forest NEP
Objective & Hypothesis
 Assess the impact of abrupt dryness incident on forest NEP.
 Hypothesis - Forest’s NEP is affected more by abrupt changes in
dryness than by progressive changes in dryness.
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6.  CH-Lae:
- Montane Mixed Forest.
- Elevation of 689 m a.s.l.
- Site dominated by European Beech.
- EC based flux measurement from 2004
- 2020
 CH-Dav:
- Sub-alpine Coniferous Forest (Norway
Spruce)
- Elevation of 1639 m a.s.l.
- EC based flux measurement from
1997-2020
Two characteristic forest sites
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Zweifel et al., 2021

7. Normal weather condition
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Retrospective LOWESS - Local weighted regression based on only
past days.
(VPDnw)

8. Abrupt and progressive dryness incidents
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Abrupt dryness:
∆VPD > 95th percentile
∆VPD = VPD - VPDnw
Progressive dryness:
VPDi – VPDi-1 < 1𝛔
Progressive dryness incident –> days with progressive increase
in VPD within its normal day to day variability.

9. Occurrences of Abrupt dryness
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Site/days Dav Lae
Abrupt Dryness 308 171
Progressive dryness 296 186
Forest experience abrupt
dryness up to 9% of its CUP

10. Hypothesis testing
 Forest’s NEP is affected more by abrupt changes in dryness than by
progressive changes in dryness
∆NEPabrupt > ∆NEPprogressive
∆NEPabrupt = NEPobs,abrupt – NEPnon-abrupt
∆NEPabrupt = NEPobs,progressive – NEPnon-abrupt
NEPobs,abrupt & NEPobs,progressive is observed NEP on days with abrupt
and progressive dryness.
NEPnon-abrupt is expected NEP (modelled) under normal conditions.
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11. Expected NEPnon-abrupt
 Use of Random Forest regression to estimate NEPnon-abrupt
 Train: Days excluding abrupt or progressive incidents (normal
days).
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NEP ~ RFR{DOY, Tair, VPD,
Rg, Precipitation}normal days
NEPnon-abrupt = RFR{DOY, Tair,
VPD, Rg, Precipitation}
R2 = 0.86
Lae
R2 = 0.91
Dav

12. Impact of Abrupt Dryness
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∆𝑁𝐸𝑃 = NEPobs – NEPnon−abrupt
Hypothesis is true
Abrupt dryness results in about 9-10% decrease in NEP
Dav Lae

13. Resistance to Abrupt dryness
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 Resistance – Impact of abrupt dryness in comparison to normal weather
dryness
𝑅𝑒𝑠𝑖𝑠𝑡𝑎𝑛𝑐𝑒 =
𝑁𝐸𝑃𝑜𝑏𝑠,𝑎𝑏𝑟𝑢𝑝𝑡 𝑉𝑃𝐷 − 𝑁𝐸𝑃𝑚𝑜𝑑𝑒𝑙,𝑤𝑒𝑎𝑡ℎ𝑒𝑟 𝑉𝑃𝐷
𝑉𝑃𝐷𝑎𝑏𝑟𝑢𝑝𝑡 − 𝑉𝑃𝐷𝑚𝑒𝑎𝑛 𝑤𝑒𝑎𝑡ℎ𝑒𝑟
𝑁𝐸𝑃𝑚𝑜𝑑𝑒𝑙,𝑤𝑒𝑎𝑡ℎ𝑒𝑟 𝑉𝑃𝐷
NEP at:
VPD = mean weather VPD
Tair = temperature at normal
VPD with RH at abrupt VPD

14. Resistance to Abrupt dryness
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𝑁𝐸𝑃𝑚𝑜𝑑𝑒𝑙,𝑤𝑒𝑎𝑡ℎ𝑒𝑟 𝑉𝑃𝐷 - Takes into account the impact of abrupt VPD also
on temperature change.
 Resistance: Dav > Lae
 Increasing trend in
Resistance at Lae

15. Europe-wide analysis 1970-2020
Discussion & Remarks
 Forest NEP decreased by ~10% due to abrupt dryness incidents.
 Occurrence of abrupt weather ≈ Variance in day-to-day weather.
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 The occurrences of sudden
weather days has increased
over last 50 years in Europe.
 Relevance of abrupt changes in
weather conditions for identifying
physiological extremes.