ICOS Science Conference 2020, Session 14

1. Impact of the 2018 drought on
carbon, water and energy
exchange of a mature Sitka spruce
and a restock site on organo-
mineral soil
Climate Change Research Group
Forest Research
Northern Research Station
Roslin EH25 9SY
Twitter: @DrGeorgeXenakis
Visit our web site: www.forestresearch.gov.uk/harwood
Follow us on twitter @FRHarwoodTower
Funded by the Forestry Commission
Partly funded by the Natural Environment Research Council (NERC) - GREENHOUSE project
Georgios Xenakis, Adam Ash, Lukas Siebicke, Mike
Perks and James I. L. Morison

2. 2
Harwood Forest GHG monitoring site
From Bastos et al. (2020) – Sci.Adv., pp. 13

3. 3
Harwood Forest GHG monitoring site
From Kendon et al. (2019) – IJC., p. 1-55

4. 4
Question
How did the 2018 drought affect the sink/source
dynamics for carbon, water and energy at the two
sites?
The question
From Forestry Statistics 2019 – Forestry Commission
• In the UK, 50% of forest area is conifers
• 50% that area growing Sitka spruce on highly
organic carbon content (peaty gley).
• Commonly managed as even-aged, single-
species plantations
• rotation length ~50 years
• “patch clear-felling” harvesting system

5. 5 02/09/2020
Harwood Forest GHG monitoring site
Two long-term monitoring sites
• Mature stand (MS)
• 1st rotation single species commercial
plantation Sitka spruce (Picea sitchensis (Bong.)
Carr.) – 42 ha
• P73, YC18
• Peaty gley soil (cambic stagnohumic gley, WRB,
FAO)
• Mean temperature 7.6 oC, mean total annual
precipitation 904 mm (1981-2011)
• Elevation 290 m, slope 2o
• Tower 32 m height (since 2014)
• LAI 5.7 – Stem density 1348 trees ha-1
• Clear-felled/restock site (CFS)
• 2nd rotation – 42 ha
• P58, YC16
• Felled 2015 – Re-planted winter 2017 with Sitka
spruce
• Peaty gley soil (cambic stagnohumic gley, WRB,
FAO)
• Mean temperature 7.3 oC, mean total annual
precipitation 940 mm (1981-2011)
• Tower 6 m height (2015)
• Measurements
• CO2 & energy fluxes with eddy covariance (10Hz
- CSAT3 & modified LI7500)
• Soil fluxes (manual & automated – LI8100)

6. 6
• Turbulent flux [EddyPro – LiCOR]
• CO2 canopy storage flux
• Correction for canopy decoupling at the MS
(Jocher et al. 2017, 2018; Thomas et al. 2013)
• QC, u* filtering and gap-filling for CO2, H2O and
energy fluxes
• Partition ecosystem respiration to above- and
below-ground
• Calculated albedo (α) and Bowen ratio (β)
• Calculated inherent WUE for dry days (Beer et
al. 2009 – GBC): iWUE= GPP x VPD /ET
From Thomas et al. (2013) – AFM, p 14-27
Data analysis and calculations

7. 7
Wind direction, footprint & energy balance closure
Wind direction West & West-South-West
Footprint [500 m fetch] 75% within 500 m
Energy balance closure 79 – 91%
Canopy decoupling
average correction
NEE: reduced by 23%
Reco: increased by 33%
GPP: increased by 2%

8. 8
Meteorology
Ta – Air temperature [oC]
Ts – Surface (2 cm) soil temperature [oC]
P – Precipitation [mm]
FP – Photon flux density [μmol m-2 s-1]
D – Vapour pressure deficit [kPa]
Change in April-July 2018 to the April-July
mean of the previous 3 years
CFS MS
Ta 3% 6%
Ts 3% -2%
P -16% -16%
FP 6% 8%
D 6% 24%

9. 9
Energy fluxes
Change [%] in April-July 2018 to the April-July
mean of the previous 3 years
CFS MS
QRn -7% 12%
α -3% 10%
QH 18% 20%
QLE 41% 120%
β -15% -49%
QG 23% 46%
QRn– Net radiation [MJ m-2]
α – Albedo
QH – Sensible heat [MJ m-2]
QLE – Latent heat [MJ m-2]
β– Bowen ratio
QG – Soil heat [MJ m-2]

10. 10
Carbon and water fluxes
• Drop in soil water because of high
evapotranspiration and low
precipitation
• Drop in GPP and Reco in May 2018 at
the CFS possibly due to water stress
• CFS was a CO2 sink only in April 2018
then neutral for May & June turning
source thereafter
• Low iWUE at the MS between March –
May
• Water efficiency haven’t recover to
previous years levels suggesting water
stress

11. 11
Year
NEE
[tC ha-1 yr-1]
GPP
[tC ha-1 yr-1]
Reco
[tC ha-1 yr-1]
Rabg
[tC ha-1 yr-1]
Rblg
[tC ha-1 yr-1]
MS CFS MS CFS MS CFS MS CFS MS CFS
2015 -7.09 7.05 22.8 1.51 15.7 5.38 8.08 2.95 7.61 5.61
2016 -8.09 3.05 24.7 5.14 16.6 8.2 10.3 4.5 6.34 3.7
2017 -8.62 0.97 25.05 8.48 16.4 9.46 10.3 5.19 6.11 4.26
2018 -5.58 2.30 23.03 8.85 17.4 11.2 11.8 6.13 5.62 5.03
Change* -30% - -5% - 7% - 24% - -16% -
MS = Mature Stand, CFS = Clear-fell Site
Carbon budget
*Percentage change from the mean of the previous three years

12. 12
Conclusions
• Large impact on the tree CO2 sink
• The restock site returned to a strong CO2 source
• Increased net radiation at the mature due to increase in incoming SW radiation because of
less cloudiness
• High heat loses mainly as latent heat. We believe mainly lost due to increase in
transpiration.
• Reduced net radiation at the clear-fell/restock likely due to increased in LW radiation from
increased surface temperature, despite decreased albedo.
• Low precipitation and high evapotranspiration led to soil water depletion, which never
recovered to pre-drought levels at the end of 2018. Stronger for mature trees.
• High water losses during spring likely due to high stomatal conductance.
• Due to high losses of water, spring photosynthesis was less efficient.
• Drier soils in the summer caused stomatal closure to prevent further water loss, limiting
photosynthesis.

13. 13
Thank you
Thank you for your attention