Ankasa flux tower: a new research facility for the study of the carbon cycle of primary tropical forests

in Africa

Ankasa flux tower: a new research facility for the study of the carbon cycle of primary tropical forests

in AfricaL. Belelli Marchesini1 , A. Forgione1, F. Mazzenga1, E. Grieco1, E.Vittorini1, A. Bombelli1, R. Zompanti1, P. Stefani1, D. Papale1 and R.Valentini1

(1) Department of Forest Science and Resources (DISAFRI) – University of Tuscia, Viterbo (Italy)

66ththFramework Framework Programme of European Programme of European CommissionCommission

University of Cape Coast

University of Tuscia

THE ANKASA EDDY COVARIANCE SITETHE ANKASA EDDY COVARIANCE SITE

A new station for the monitoring of CO2 and energy fluxes over a primary tropical forest in Ghana is operative as part of the CarboAfrica eddy covariance network. The facility, located in the Ankasa Conservation Area (05° 16’ 11.20’’N; 02° 41’ 41.55’’ W), includes a 65 m tall steel tower equipped with a system enabling the measurements of fluxes at the top of the structure, of CO2, air temperature and humidity along a vertical profile and of relevant physical parameters of the forest ecosystem.

The Ankasa flux tower is the first in the African continent collecting data on CO2 exchanges over a tropical primary forest, and from its activity a breakthrough in the understanding of the carbon cycling in this kind of environment is expected.

ANKASA CONSERVATION AREAANKASA CONSERVATION AREA

The Ankasa Conservation Area lies in Southwest Ghana on the border with the Ivory Coast. It covers 509 km2 (fig.1) and it hosts an ancient rainforest and the most biodiverse in Ghana. Ankasa represents the only wet evergreen protected area in almost pristine state, being home to over 800 vascular plant species. The topography is characterized by rugged, deeply divided terrain in the north and west with flatter swampy ground associated with the Suhien watershed in the East. Its maximum elevation is 150m, though most lies below 90m. The climate of the area is characterized by a distinctive bi-modal rainfall pattern occurring from April to July and September to November. The average annual rainfall is 1700 to 2000mm. Mean monthly temperatures are typical of tropical lowland forest and range from 24°C to 28°C. Relative humidity is generally high throughout the year, being about 90% during the night falling to 75% in early afternoon.

Other eddy covariance sites of the Carboafrica network

Ankasa flux tower

THE INSTRUMENTAL SET UPTHE INSTRUMENTAL SET UP

The eddy covariance (EC) system consists of a Gill Wind Master sonic anemometer mounted on the top of the tower and a LiCor 7000 closed path CO2/H2O analyzer placed at 56 m sucking air samples along a 9 m rilsan tube. EC data are acquired at 20Hz by ECCOCatch software developed by DISAFRI. An additional EGM-CIRAS gas analyzer serves a CO2 concentration vertical profile system made up of measurements at 6 levels ( 0.2, 2, 14, 18, 30, 40 m). A Campbell CR1000 logger collects data from the CO2 profile, and from a variety of sensors measuring soil temperature and moisture, solar radiation (global, direct and diffuse PAR), radiative properties of the forest (net radiation, reflected PAR, soil heat flux), and more typical meteorological variables such as air temperature and moisture along a vertical profile, precipitation and atmospheric pressure. A set of 9 solar panels, with a power 190 W each, supplies the energy for the system functioning.

Figure 1. Map of Ankasa Consevation Area (left) with the location of the flux tower marked (yellow dot) and distribution of eddy covariance stations in Africa (right).

Contact person: Dr. Paolo Stefani Contact person: Dr. Paolo Stefani (pstefani@unitus.it)(pstefani@unitus.it)

Figure 2. a: diurnal evolution of the CO2 vertical profile characterized every 30 minutes in the canopy space from 0.2 to 40 m in the period 31 July – 7 August 2008; b: trend of carbon dioxide flux (Fc) and momentum flux (τ) measured at the tower top; c: diurnal evolution of the vertical profile of air temperature from 2 to 48 m height.

CO2 vertical profile within the canopy space

date

31/Jul 01/Aug 02/Aug 03/Aug 04/Aug 05/Aug 06/Aug 07/Aug

hour

4 8 12 16 20 4 8 12 16 20 4 8 12 16 20 4 8 12 16 20 4 8 12 16 20 4 8 12 16 20 4 8 12 16 20

Hei

ght

[m]

0

5

10

15

20

25

30

35

40

340 360 380 400 420 440 460 480 500 520 540 560

CO2 and momentum flux measured at 65 m

date

31 Jul 01 Aug 02 Aug 03 Aug 04 Aug 05 Aug 06 Aug 07 Aug

4 8 12 16 20 4 8 12 16 20 4 8 12 16 20 4 8 12 16 20 4 8 12 16 20 4 8 12 16 20 4 8 12 16 20

Fc

[m

ol C

O2

m-2

s-1

]

-50

-40

-30

-20

-10

0

10

20

[

kg m

-2 s

-1]

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Fc

Air temperature vertical profile

day

31/Jul 01/Aug 02/Aug 03/Aug 04/Aug 05/Aug 06/Aug 07/Aug

4 8 12 16 20 4 8 12 16 20 4 8 12 16 20 4 8 12 16 20 4 8 12 16 20 4 8 12 16 20 4 8 12 16 20

Hei

ght

[m]

0

5

10

15

20

25

30

35

40

45

50

23 24 25 26 27 28 29 30 31

a

b

c

2

hour

0 2 4 6 8 10 12 14 16 18 20 22

mol

m-2

s-1

-25

-20

-15

-10

-5

0

5

10

15

FcNEE

0 2 4 6 8 10 12 14 16 18 20 22

mol

m-2

s-1

-15

-10

-5

0

5

10

St. CO2

hour

0 2 4 6 8 10 12 14 16 18 20 22

mol

m-2

s-1

-25

-20

-15

-10

-5

0

5

10

15

FcNEE

0 2 4 6 8 10 12 14 16 18 20 22

mol

m-2

s-1

-15

-10

-5

0

5

10

St. CO2

a

b

3

ACKNOWLEDGEMENTSACKNOWLEDGEMENTS

The research activities in the Ankasa Conservation Area are carried out with the collaboration of the Wildlife Division and the of the Forestry Commission of Ghana. We gratefully acknowledge the help of Mr. Justice John Mensah in the field work and maintenance of the flux tower of Mr. Abdul Kareem, park manager of Ankasa Conservation Area, and the all the park rangers.

Pictures below:

Vegetation storeys of the Ankasa forest photographed at different levels of the flux tower indicated by the arrows

Pictures below:

Vegetation storeys of the Ankasa forest photographed at different levels of the flux tower indicated by the arrows

Figure 3.

Daily trends of storage carbon dioxide flux (St.CO2) (a), of carbon dioxide flux (Fc) measured at the tower top and of the net ecosystem exchange (NEE = Fc+St.CO2) (b) averaged at hourly time step for the period 31 July – 7 August. Dashed lines delimit the interval of the mean± standard error.

ANALYSIS OF PRELIMINARY DATAANALYSIS OF PRELIMINARY DATA

The analysis of preliminary data collected in the first week of August 2008 shows a daily uptake of 1.33±0.73 gC m-2 d-1 (mean±s.e.) with the CO2 flux measured above the canopies at 65 m (Fc) ranging from a night efflux of 2.3 to a day-time uptake of -14.8 mol m-2 s-1 (fig. 2b). The build up of the CO2 concentration in the closed canopy space at night (fig. 2a) determines an underestimation of the ecosystem respiration on average by a factor 2-2.5 and by a maximum of 4. The stored CO2 is released towards the atmosphere in the early morning once the turbulent motions in the surface layer are activated, as denoted by a positive peak in Fc occurring around 8 a.m (fig. 2a). However the net ecosystem exchange (NEE) denotes an assimilation activity of CO2 starting just after the dawn at 6 a.m. The discrepancy in the trend of Fc compared to NEE (fig. 3) highlights the importance of characterizing the magnitude and sign of the storage term to correctly evaluate the carbon exchanges with the atmosphere.

Department of Forest Environment and ResourcesDipartimento di Scienze dell’Ambiente Forestale e delle

sue Risorse

DISAFRIDISAFRI