This document discusses the effects of extreme weather events on stem CO2 effluxes in a Mediterranean beech forest. It finds that only spring frost negatively affects stem carbon fluxes by reducing the amount of carbon fixed in stem biomass and related growth respiration. The study also finds that ecosystem respiration and stem CO2 effluxes show strong seasonal variability, with stem efflux contribution to ecosystem respiration being lowest during leaf development and highest during xylem formation when growth respiration occurs. The key is to understand how different respiration components are affected by environmental variables to better predict ecosystem carbon exchange responses to climate change.

1. Effects of extreme weather events on stem CO2 efflux
Negar Rezaie, Gabriele Guidolotti,Ettore D’Andrea, Giorgio Matteucci, Carlo Calfapietra
Institute of Research on Terrestrial Ecosystems (IRET) National Research Council (CNR) of Italy

2. I. How the weather extreme events can influence the stem
CO2 Effluxes?
II. How big is the contribution of the stem Effluxes to the
Ecosystem Respiration in a Mediterranean beech forest?
Questions
E.D’Andrea et al.2020 Plant,Cell & Environment

3. Above-ground biomass
Below-ground biomass
Soil Organic
Carbon(SOC)
CO2 Sinks
CO2 Sources
Autotrophic Respiration RA
- LEAF
- BRANCHES
- STEM
- ROOT
Heterotrophic Respiration RH
Photosynthesis(GPP)
Litterfall
Root loss
Ecosystem respiration(RECO)
o In order to predict how NEE and RECO will
respond to climate change it is necessary
to understand how different
respiration components will be affected
by environmental variables.
RECO= RA+ RH

4. o Meanwhile, we need to be careful about invoking the process
‘respiration’ when really we are measuring Stem CO2 Efflux!
1. Growth Respiration
2. Maintenance respiration
3. CO2 transported in transpiration
stream
4. CO2 transported in phloem sap
5. Dissolution/ Exsolution (T,PH)
6. Gas Exchange
7. CO2 fixation through photosynthesis
8. CO2 fixation by PEP-C or Rubisco in
xylem
9. E CO2 measured with chamber
S.Trumbore et al. 2013
Negar Rezaie_CO2 efflux & xylem formation
Stem Respiration(RS) ≠ Stem CO2 Efflux
What we measured
• The most commonly
measured proxy, stem CO2
efflux (ES), is likely to
underestimate local
respiration.

5. o The necessity to unify
the two carbon cycle
phase in Forest…
Negar Rezaie_CO2 efflux & xylem formation
Carbon fluxes
Negar Rezaie_CO2 efflux & xylem formation
Effect of Short-term Extreme Events on physiological processes and C cycle
Plant level
Ecosystem level
M.Zohner et al.2020 PNAS
Rita et al.2019 GCB
Photosynthesis (radial growth,
Reserve compounds)
Respiration
(maintenance respiration,
Growth respiration)
BVOCs
?

6. Negar Rezaie_CO2 efflux & xylem formation
Collelongo , 1570 ml
D’Andrea et al. 2019 New Phyt
2015, Late Frost of 2016 & Summer drought of 2017
Three-month Standardized
Precipitation
Evapotranspiration Index
(SPEI) at the experimental
site of Collelongo in 2015,
2016 and 2017
- Collelongo, 1570m

7. From single tree to stand level
* Phenological xylem formation phases :
i. cambium reactivation,
ii. beginning of cell enlargement (bE);
iii. beginning of cell wall thickening (bW);
iv. beginning of cell maturation (bM);
v. cessation of cell enlargement (cE)
vi. cessation of cell wall thickening and lignification (cW)
*The total duration of Xylem formation; dX = cW–bE
Eco2 measured with Chamber
CO2
Esm=ES15m*Q10m
(Tair -15)/10
Esmg=ES15mg*Q10mg
(Tair -15)/10
Esg=Esmg-ESm
Transverse section of a sample (July 15, 2015)
cambial cell division
cell Enlargement
wall thickening
(lignification)
Maturation(cell death)
**CO2 Efflux
***Upscaling
Allometric equation for
calculating stem Lateral Surface

8. Radial growth and xylem phenology
o Cambial activity and radial growth are not entirely linked to leaf phenology.
Leaf & Cambium phenology
Cambium
LAI
Leaf unfolding

9. o Stem CO2 efflux cannot uniquely be estimated from temperature data!
Q10= ES overall temperature
sensitivity for a 10°C increase
X Q10 and ES15, were
sensitive to wood formation
processes, showing an increase
during the growing period.
ES 15= The stem CO2 efflux at an
air temperature of 15C

10. o Effluxes from stem are not entirely synchronised to radial growth
Relationships between ring widths and the
mean annual ES measured in 2015 (green),
2016 (blue) and 2017 (red).
Daily stem increment and stem carbon effluxes
contribution of maintenance respiration to daily stem CO2 efflux calculated as
ESm/ESmg

11. o Overview of RECO and Stem CO2 Efflux in Collelongo
The contribution of Growth(ESg), maintenance (ESm) and total
(ESmg) stem CO2 efflux to RECO at seasonal scale
Seasonal dynamic of RECO(19years), Growth(ESg), maintenance
(ESm) and total stem CO2 effluxes (ESmg) at seasonal scale
(ESg)
(ESm)
(ESmg)
(ESg)
(ESm)
(ESmg)
RECO

12. KEY RESULTS
- How the weather extreme events can influence the stem CO2 Effluxes?
ONLY SPRING FROST AFFECTS NEGATIVELY STEM C FLUXES.
During frost year, Beech trees fixed less C in the stem biomass, showing also a reduction of
the stem carbon efflux related to growth respiration.
- How big is the contribution of the stem Effluxes to the Ecosystem Respiration in a
Mediterranean beech forest?
RECO AND STEM CO2 EFFLUXES SHOWED STRONG SEASONAL VARIABILITY IN A BEECH
FOREST IN CENTRAL ITALY.
At seasonal scale, the contribution of ES to RECO is highly variable. Showing the lowest
value during the leaf development and the highest one during the xylem formation period,
when growth respiration occurs.

13. o Interested in unravelling fine scale C dynamic (e.g. BVOCs, xylogenesis, stem emission, C allocation) to
evaluate medium- long-term response to actual changing climate?
Please contact me, we can try together!
Thank you!
Be Resilient as Beech!
Negar.rezaeisangsaraki@iret.cnr.it
@negar_rezaie
@CalfapietraLab
Gas exchange Lab_CNR IRET
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