Net ecosystem CO2 exchanges between a dwarf A.marina mangrove

and the atmosphere-

Application of the eddy-covariance flux tower to « Le C♥ur de Voh »

mangrove (New Caledonia)

Leopold, A., Marchand, C., Renchon, A., Deborde, J., Quiniou, T., Allenbach, M.2016. Agricultural Forest Meteorology 223, 217‐232

Mangroves: Net sink or source for CO2?Daytime Nighttime

Vertical flux (F)As a result of covariance between gaz

concentration (CO2, ρC) and vertical wind component (w)

F(CO2) = w.ρC

Eddy-covariance,A powerful tool,

to measure atmosphere-ecosystem exchanges

From Burba,  2005

USA,Florida

ChinaIndia

Mangroves and Eddy-Covariance…A rare love story…

Northern hemisphereHumid tropical and sub‐tropical climates

Modified from Giri et al. 2011

The story of the New Caledonian flux tower

Sourthern hemisphereSub‐tropical but semi‐arid climate

Measurements:  Net Ecosystem CO2 Exchange (NEE)

Determination:    Net Ecosystem Productivity (NEP), ‐NEEEcosystem Respiration (Reco)Gross Ecosystem Productivity (GEP)

© A.LEOPOLDModified from Giri et al. 2011

GEP = −NEE + Reco

BACKGROUNDBACKGROUND

35 000 ha

88%

New Caledonian Mangroves

Total Area

East Coast

West Coast

Modified from Virly, 2006

A famous mangrove grows on the New Caledonia’s West Coast« Le Cœur de Voh »

©1990, Y. Arthus‐Bertrand ©2012, CNES/ASTRIUM

Avicennia marina var. australasica- Dwarf- Hmax < 2m- Hmean= 57.2±30.3cm- 3.3±0.2 trees m-2

‐IRGA‐ closed path, LI‐7200 ‐Sonic anemometer‐Windmaster

‐Data logger CR1000 + meteorological sensors‐ air temperature and humidity‐ PAR / solar radiation‐ soil heat flux plates‐ sediment temperature‐ rain gauge

‐Tide gauges custom built

H= 3.10m

© A.LEOPOLD

‐500

0

500

1000

1500

2000

2500

‐12

‐10

‐8

‐6

‐4

‐2

0

2

4

6

8NEE (µ

mol m

‐2s‐1)

PAR (µmol photons m

‐2s ‐1)Photosynthesis (day) vs. Respiration (day & night)

Data from Leopold et al.  2016

Mangroves,

As a unique forested ecosystem… 

Seasonal variations of NEE

Seasonal variations of NEE

‐6.0

‐5.0

‐4.0

‐3.0

‐2.0

‐1.0

0.0Mar‐14

Apr‐14

May‐14

Jun‐14

Jul‐1

4

Aug‐14

Sep‐14

Oct‐14

Nov

‐14

Dec‐14

Jan‐15

Feb‐15

Mar‐15

0.00.51.01.52.02.53.03.54.04.5

1 2 3 4 5 6 7 8 9 10 11 12 13

µmolCO

2m

‐2s‐1

Daytime NEE 

µmolCO

2m

‐2s‐1

Nighttime NEE

90% values :‐5 to +2 µmol m‐2 s‐1

Min: ‐10.7 µmol m‐2 s‐1

90% values :0.38 to +3 µmol m‐2 s‐1

Max: 8.80 µmol m‐2 s‐1

Florida mangrove*Min: ‐10 to ‐25 µmol m‐2 s‐1

Florida mangrove*Max: up to 10 µmol m‐2 s‐1

*Data from Barr et al. 2010

Seasonal variations of NEE

‐10

‐8

‐6

‐4

‐2

0

2

4

6

NEE

(µmol m

‐2  s 

‐1)

End of dry and cold season (October)

End of wet and warm season (April)

Wet and warm season (February)

Seasonal variation of both daytime and nighttime NEE minima and maxima

Modified from Leopold et al.  2016

Seasonal variations of NEE,subsequent to seasonal variation of driving factors

VPDopt = 2.25 kPa Topt = 26.8 °C

Minimal NEE, i.e. maximal CO2 fixation rate at the ecosystem scale

Mangroves,

As a unique forested ecosystem… 

The impact of tidal cycle on NEE

Mangroves, As a unique forested ecosystem… 

The impact of tidal cycleDecrease of NEE during flooding waters

(a) As a consequence of lower CO2 released by respiration

‐12

‐10

‐8

‐6

‐4

‐2

0

2

4

6

8NEE (µ

mol m

‐2s‐1)

Decrease of NEE during flooding waters

0

1

2

3

4

5

6

7

8

9

10

0 5 10 15 20 25 30 35 40

Nighttim

e NEE

 (µmol m

‐2s‐1)

Low Tide

High Tide

Tair (°C)

(a) As a consequence of lower CO2 released by respiration

Mangroves, As a unique forested ecosystem… 

The impact of tidal cycleDecrease of NEE during flooding waters

(a) As a consequence of lower CO2 released by respiration

(b) As a consequence of higher CO2 absorbed byphotosynthesis because of water availability

Decrease of NEE during flooding waters(b) As a consequence of higher CO2 absorbed byphotosynthesis because of water availability

Any flooding waters

Decrease of NEE during flooding waters(b) As a consequence of higher CO2 absorbed byphotosynthesis because of water availability

At saturing PAR, NEE minimum decreases with flooding waters.

Trees seem stressed by missing water

Mangroves,

As a unique forested ecosystem… 

The impact of a semi‐arid climateon NEP

The impact of semi arid climate on mangrove productivity

NC West Coast:Annual rainfall < 1,000 mm, with a high seasonality

NEP of a dwarf A.marina mangrove growing under semi arid climate

‐20

‐15

‐10

‐5

0

5

10

15

20

Mar.14‐Mar.15

Relative low annual NEP= 72.9 gC m‐2Mon

thly NEP

 (gC m

‐2) 

Minima during the dry season:‐ Low water inputs & high porewater salinity‐ Reduced stomatal conductance & leaf photosynthesis?

Sink

Source

Low water inputs = High porewater salinity

Decrease of gas exchanges by stomatal closure?See Carine Bourgeois ‘s poster (N° 78)(PhD candidate, AUT Auckland, IRD New CaledoniaSupervisors: Dr. A. Alfaro and Dr. C. Marchand)

NEP of a dwarf A.marina mangrove growing under semi arid climate

‐20

‐15

‐10

‐5

0

5

10

15

20

Mar.14‐Mar.15

Relative low annual NEP= 72.9 gC m‐2Mon

thly NEP

 (gC m

‐2) 

Minima during the rainy season‐ high freshwater inputs‐ high photosynthesis ‐ high respiration: resumption growth?

Sink

Source

NEP decreases with rainfall,When seawater is not a limiting factor 

‐20

‐15

‐10

‐5

0

5

10

0 50 100 150 200 250 300

Mon

trhly NEP

 (Tno

rm.) 

(gCO

2m

‐2)

Monthly cumulative rainfall (mm)

Do frehswater inputs promote the growth resumption and subsequent respiratory metabolism?? 

(see Robert et al., 2014 and Santini et al., 2015)

< 80% of time> 80% of time

Air‐exposed sediments

R²a = 0.995p<0.001

Perspectives

(a) Inter‐annual variation of NEP?

(b) May the NEP be lower?Carbon exports: DOC, DIC, POC?

(e.g., 25% to 70% of NEP (Barr et al. 2010))

(c) NEE Partitioning: what is the contribution of carboncycling components on the NEE measured?

‐ Soil‐ Vegetation (root, leaves, pneumatophores, trunks)‐ Water column

Mar.14‐Mar.15

Relative low annual NEP= 72.9 gC m‐2

Vietnam

NCNZ

Latitudinal, climatic and biodiversity gradients: ‐ New Zealand (36°S), temperate climate, 1 mangrove species‐ New Caledonia (21°S), semi‐aride climate, 25 mangrove species‐ Vietnam (10°N), tropicale climate, 60 mangrove species

Eddy‐covariance network in the Indo‐Pacific area

© KARI, ESA image

In situ installation of flux tower:

- Wind rose and available distances

- Daytime: Trade winds (« Alizees »): SE- Nighttime: Land breeze: NE

Data acquisition and processingSampling rate 10Hz

Quality control:‐ Qc =2 according to Goulden et al. 1996‐ U* threshold‐ Footprint

Relationships with environmental data (30‐min interval)(solar radiation, PAR,  Air temperature, VPD) 

« Gap‐filling »(Online tool from Max Planck Institut)

30‐min mean CO2 fluxes (Eddy‐pro software©) : NEE

Daytime / nighttime datasetsLow tide/ high tide datasets

Partitionning: GEP and RecoAnnual Budgeting: NEP

Low water inputs = high porewater salinity

05

101520253035404550

40 60 80 100

Dep

th (cm)

Porewater salinity

Range from 40 to 100i.e., 1.4 à 2.8 fold the seawater salinity

Wet and warm seasonDry and cold season

High salnity of porewater along the year=

Dwarf trees

0

20

40

60

80

100

120

140

160

180

200

0 20 40 60 80 100

N = 899Léopold, unpublished data

Heigh

t (cm

)

Basal perimeter (cm)‐

Rainfall seasonality controls mangrove immersion rate by seawater at high intertidal locations

The impact of semi arid climate on mangrove productivity

Rainy season:‐ high rainfall & immersion rates  

Dry season:‐ low rainfall & immersion rates 

Dry season:‐ Low rainfall and immersion rate‐ Very high porewater salinity (>100)

=Low photosynthesis

Mon

trhlyGEP

 (gCO

2m

‐2  m‐1)

0

20

40

60

80

100

120

March14

March15

Wet and warm seasonDry and cold season

Rainfall seems to have benefit effects on mangrove photosynthesis…

But also on ecosystem respiration… 

0

20

40

60

80

100

120

0

50

100

150

200

250

300

Mon

thly Rainfall (mm)

Mon

thly Reco (gC m

‐2)

Global climate changes…As a consequence of anthropogenic GHG increase.

Years before 2005

CO2(ppm

)375 ppm

250

300

10000 5000 0

280 ppm

modified from IPCC, 2005

0

200

400

600

800

1000

1200

0 500 1000 1500 2000

Rainfall rate,as a driver of  subtropical mangrove NEP

Annual cumulative rainfall (mm)

Annu

al NEP

 (gCO

2m

‐2.an‐

1 )

Voh, NC

Chine

Everglades, USA