Primary productivity of a Hevea forest in the Ivory Coast
B.A. Monteny
ORSTOM,
Bioclimatologie-INRA,
78850Thiverval-Grignon,
FranceIntroduction
The
objective
of thestudy
was to under-stand the
impact
of climaticparameters
and soil-water factors on Hevea brasilien- sisproductivity (rubber).
Theprimary
pro- duction rates of a Hevea forest located in the humidtropical
zone of theIvory
Coasthave been calculated from a mathematical model based on determined
experimental equations.
Carbon dioxide fluxdensity
above the forest is measured
by
the ener-gy balance method
coupled
with theBowen ratio. This
ratio, /3,
and the ratio ofphotochemical
heat fluxdensity
forC0 2
fixation to latent heat flux
density, f,
are determined from verticalprofiles
ofpoints.
The
light-CO 2
assimilation responsecurve of the Hevea
forest,
in relation to rain distribution and solar radiation avail-ability,
is used to calculate thedaily
es-timates of
organic
carbon accumulationand, hence,
theyearly dry
matterproduc-
tion.
Materials and Methods
The
equations describing
individual inflow andoutput fluxes of forest stand
productivity
require:
1) thephotosynthetic
response of aplant
stand and its evolution under the mostimportant
factors which controlphotosynthesis
(radiation and water); 2) the daily and seasonal distributions of these factors; 3) the stand bio-mass b (aerial and root) and the evolution of leaf area.
Net
primary productivity,
NP, results from the difference between the leaf carbon assimiliation rate A and biomassrespiration
Rb (Jones, 1983):NP=
E(A-Rb)=E(A-Rbd-Rb,!
=I(NA-Rb,,) ( 1 )
where
Rb d
= biomassrespiration
daytime rate;Rb"
= biomassrespiration nighttime
rate; A = grossphotosynthesis
rate; NA = net assimila- tion rate (daytime hours). Biomassrespiration during
the daytime (12 h) is assumed to beequal to that at night (Rbd
Rb").
The
respiration
rate R,extrapolated
from the light response curve at zero solar radiation, cor-responds
to:R = Rbn+
C0 2
soil =Rbaerial
+Rr!!i
a’R so i l
(2) Soil carbon flux,C0 2
soil,originates
from thedecomposition
of litter and subsurface root hairs + rootrespiration: C0 2
soil = E(R so il
+f!
root )
’
SoilCO, !
measurements(involving
anenclosure and
an
air flow system, Perrier et al., 1976) show th,at the fluxes do notchange during
a 24 hperiod
but thatthey
vary moreparticularly
withchanges
in subsurface soil water content.Measurements were carried out in a SAPH Hevea
plantation,
at Dabou-Ousrou, near Abid-jan
(5°19’ N, 04"30’ W),Ivory
Coast. Thetropi-
cal forest zone is under the influence of the
intertropical convergence motion
(ITC)
whichdetermines the climate. The climate of the forest zone has 2
dry
seasons: December-March and
August-September
and 2 wet sea-sons with the most
important
fromApril
to July.The
plantation
is a 7000 ha more or less flat land, situated in the southern part of the Ivoriantropical
forest zone, 50 km from the Guinea Gulf coast.From air and soil
C0 2
flux measurementsusing micrometeorological
methods(Allen
et al., 1974;Saugier
andRipley,
1974), experi- mentalequations
have been determined which express the net assimilation rate as a function of stand characteristics in relation to environ- mental factors(for
more details, seeMonteny,
1987).Results and Discussion
The
light-photosynthesis
curves for theHevea forest canopy
present
a maxi-mum net
photosynthesis
at 1.85 mgC0
2 - M - 2 - S -
1
for a young canopy(2-3
mo, curve1)
and decreases to 0.5 mgC0 2’
m- 2
’ s-
1 for an older one
(Dec.-Jan.),
bothwithout soil-water deficit
(Fig. 1), fitted
results from different
days
of measure-ment
during
2 mo.The
light
response curves show a ten-dency
towardslight
saturation. The carbonassimiliation is sensitive to soil-water sta-
tus, as seen for the
photosynthesis
curve2 due to the increase of the stomatal resistances. The observed decrease in net
C0
2
assimiliation(Dec.-Jan.)
ispartly
dueto the
deposition
of cuticular wax(leaf ageing)
and to the effect of a water stressperiod during
the shortdry
season inAugust.
The
global outgoing C0 2
flux atnight
isequal
to 0.44 mgCO 2 ’m- 2 ’ s- 1 .
Rootrespi-
ration is assumed to be
proportional
toroot
dry weight,
as it is for shootrespira-
tion. From the soil
C0 2
flux and the bio-mass measurements, the calculated
respi-
ration of
aboveground
biomassRb&dquo;
is0.15 mg
C0 2 .m- 2 ’ s- 1 . During
the wet sea-son, the ratio of
C0 2
soil flux to theC0 2
canopy fixation is
nearly unity, indicating that,
in humidtropics,
carbon turnover isvery fast until the
ground
floorhumidity
limits litter
decomposition
or when the leafphotosynthesis
decreases.The
daily
netproductivity, NP,
of the Hevea forest is the difference between the netC0 2 absorption
calculated at 15 min intervals from 6:00-18:00 h(based
onsolar radiation data available on 16
days
and the
light
response curves inFig. 1)
and the
night
biomassrespiration
from18:00-6:00 h. The
depletion
of soil waterin the root zone affects the
photosynthetic
leaf
capacities (Fig. 1)
and the water-vapor
exchanges (Fig. 2).
We considerthe ratio of actual
evapotranspiration
toequilibrium evaporation, ETRlEto,
as amodulation factor
representing
the effectof water stress on net stand
productivity (Monteny, 1987).
The coefficient of conversion from carbon dioxide todry
matter is 0.56 g
DM-g C02 !.
The annual evolution of the calculated
primary production
rate of a 19 yr old rub- ber forest ispresented
inFig.
2. It showsimportant
variationsduring
the year:1) May
-
June: high dry
matterproduction
rates after leaf
regrowth
without soil waterlimitation; 2) July-August-September:
de-creased
dry
matterproduction
rates, in relation to the attenuation of solar radia- tionby
clouds from the ITC and thedeple-
tion of soil water at the end of the short
dry
season;3)
October-November: thephysiological ageing
of leaves associated with stomatabecoming increasingly plugged
with cuticular wax reduced thedaily
net canopy assimilation rates forC0 2
; 4) January-February:
soil wateravailability
was the main factor respons-ible for reduced leaf
photosynthetic activity
andphysiological
modification before leaf fall. At the end of the leaf span,global respiration
rate of the stand washigher
than the assimilation rate of the canopy, which
explains
thenegative
rates ofdry
matter
production.
The estimated annual net
productivity
ofthe Hevea foirest is 13.8 T
DM ’ ha-1’yr 1 compared
with the current annual in- crement: 8.1 TOM ’ ha- 1 ’yr 1
measuredbetween 11 and 19 yr.
Taking
into accountleaf and shooi: litter fall and latex
sampling (2.4
+ 3.8 + 1.6 TOM ’ ha- 1 ’yr 1 ,
respec-tively),
the total annual increment would be 15.9 TOM.ha- 1 ’yr 1
or 15%higher
than the estimated net
productivity.
Theefficiency
of solar radiation conversion into net annualproduction
is 1.7%.Conclusion
In the humid
tropical regions
of West Afri-ca, two climatic factors affect the forest
dry
matterproduction:
rain distribution and radiationquantities.
Thesefactors,
depending
upon the shift of theITC,
areresponsible
for the lowatmospheric C0 2
assimilation rate
by
forests. The wet sea-sons are
commonly
cloud-covered and it is the litterdecomposition
whichsupplies
most of the
C0 2
to the canopy. Leaf lifes- pan activities are influencedby morpho- logical
modifications withageing, reducing,
on the otherhand,
Heveaphoto- synthetic efficiency.
References
Allen L.H., Hanks R.J. & Gardner H.R.
(1974)
Carbon dioxide
uptake by
wide rowgrain
sor-ghum computed
by the profile Bowen ratio.Agron. J.
66, 35-41Jones H.G. (1983) In: Plants and Microclimate.
Cambridge University Press, Cambridge, pp.
323
Monteny B.A.
(1987)
Contribution a t’etude des interactionsvegetation-atmosphere
en milieutropical
humide.Importance
du r6le dusyst6me
forestier dans le
recyclage
des eaux depluie.
Ph.D. Thesis, Universit6 de Paris Xi,
Orsay
Perrier A., ltier B. &
Jaussely
B.(1976)
Etudede la
photosynthbse
en plein champ. In: Lesprocessus de la
production végétale primaire.
(Moyse
A., ed.), Gauthier-Villars, Paris, pp. 113- 136Saugier
B. &Ripley
E.A.(1974)
A sensi-tive device for