Diurnal and seasonal course of monoterpene emissions from Quercus ilex (L.) under natural conditions application of light and temperature algorithms

Pergamon

Atmospheric Enuironmenf

PII: S1352-2310(97)00080-0

Vol. 31, No. SI, pp. 135-144, 1991 0 1997 Elsevier Science Ltd All rights reserved, Printed in Great Britain

1352-2310/97$17.00+ 0.00

DIURNAL AND SEASONAL COURSE OF MONOTERPENE EMISSIONS FROM QUERCUS ILEX (L.) UNDER NATURAL CONDITIONS-APPLICATION OF LIGHT AND TEMPERATURE ALGORITHMS N. BERTIN,*,

M. STAUDT,* U. HANSEN,* G. SEUFERT,*, 11P. CICCIOLI,t P. FOSTER,1 J. L. FUGIT and L. TORRESg

*Joint Research Centre of the European Commission, Environment Institute, I-21020 Ispra (VA), Italy; tconsiglio Nazionale delle Ricerche, Istituto sull’Inquinamento Atmosferico, Area della Ricerca di Roma, via Salaria km 29300, C.P.10,100016 Monterotondo Scala, Italy; SGroupe de Recherche sur TEnvironnement et la Chimie appliqued, IUT de Chimie - UJF, 39-41 Boulevard Gambetta, F38000 Grenoble, France; and §INP-Ecole Nationale Supirieure de Chimie de Toulouse, 118 Route de Narbonne, F31077 Toulouse Cedex. France (First received 16 February 1996 and injinal form 23 June 1996. Published November 1997)

Abstract-Quercus ibex is a common oak species in the Mediterranean vegetation and a strong emitter of monoterpenes. Since the short-term control of monoterpene emissions from this species involved both temperature and light, the usual exponential function of temperature may not be sufficient to model the diurnal and seasonal emission course. In the frame of the BEMA-project (Biogenic Emissions in the Mediterranean Area), we investigated the tree-to-tree, branch-to-branch, diurnal, and seasonal variability of monoterpene emissions from Q. ibex over one and a half years at Castelporziano (Rome, Italy). In addition, w,: checked the suitability of the model developed for isoprene by Guenther et al. (1991, 1993) to simulate the short- and long-term variations of monoterpene emissions from this particular species. We found that the tree-to-tree variability was rather small compared to the experimental error during air sampling and analysis by diverse laboratories. The branch-to-branch variability was noticeable between sun- and shade-adapted branches only. 80% of total emissions were represented by cc-pinene, B-pinene and sabinene, whose proportions were stable over the year and independent of light exposure. The emission ) estimated by the isoprene model or extrapofactor (emission rate at 30°C and 1000 ~molphotonm-Zs-l lated from measurements was similar: it was about 22 pggdw -r h-r for sun-exposed branches and 2.3 pggdw.-’ h-r for shade-adapted branches. It was rather stable over the seasons except during leaf development. The diurnal and seasonal emission patterns from Q. ibex were simulated in a satisfying way by Guenther’s algorithms especially if we excluded the laboratory variability. For shade-adapted branches, an emission factor 17 times lower had to be applied, but temperature and light responses were unchanged. 0 1997 Ehevier Science Ltd. Key word index: Quercus ilex, Holm oak, monoterpene, Mediterranean vegetation. INTRODUCTION

Quercns ilex is a very common species of the Mediterranean vegetation (Bacilieri et al., 1993) and in particular at Castelporziano (Manes et al., 1997), one of the experimental sites of the BEMA-project (Biogenic Emissions in the Mediterranean Area, Versino, 1997). Contrary to many deciduous oak species emitting large amounts of isoprene (e.g. Steinbrecher et al., 1997), Q. ilex was found to be a strong emitter of monoterpenes although it has no storage pool in leaves or bark (Staudt et al., 1993; Seufert et al.,

7 Present address: INRA Bioclimatogie, F-84914 Avignon Cedex 9, France. 11 Author to whom correspondence should be addressed.

modelling, light and temperature

algorithm,

1995a). Moreover, the influence of light in the shortterm control of monoterpene emissions from Q. ilex has been recently revealed by laboratory and field experiments (Staudt et al., 1993; Staudt and Seufert, 1995; Kesselmeier et al., 1996). Loreto et al. (1996a) complemented this information at leaf level and concluded that there is a strong similarity between isoprene emissions from isoprene emitters and monoterpene emissions from Q. ilex. These authors also provided evidence by ’3C02-labelling, that monoterpenes are formed from photosynthesis intermediates and that they are likely to share the same synthetic pathway as isoprene (Loreto et al., 1996b). The short-term control of monoterpene emissions by light was also observed on coniferous trees like Picea abies (Steinbrecher, 1989), suggesting the existence of a small photosynthetic pool besides the

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N. BERTIN et UI

large monoterpene reservoir (Schiirmann et al., 1993). Recently, similar indications were given by measurements made on other species present in the Mediterranean area, like Pinus pinea (Staudt et al., 1997) or more hypothetically Quercus coccifera (Hansen and Seufert, 1996; Steinbrecher and Hauff, 1996). These results suggest that a single function of temperature (Tingey et al., 1980) may be inadequate to model monoterpene emissions in areas like Mediterranean forests and macchia shrublands, where these species predominate. Up to now, emission models calculate the instantaneous emission rate by multiplying the emission factor with functions of temperature and light for isoprene and function of temperature for monoterpenes (Guenther et al., 1991, 1993). The emission factor which accounts for long-term adaptations, is species or ecotypes dependent and it is usually determined from experimental measurements under standard light and temperature conditions (Guenther et al., 1995). Light and temperature functions account for the short-term variations of the emission rate, and they are assumed to be stable over long-term climatic changes (Monson et al., 1995). In the frame of the BEMA project, emissions from Q. ilex have been measured by four laboratories over one and a half years at Castelporziano (near Rome, Italy). Measurements were performed on different branches at the top and the base of the canopy in a dynamic branch enclosure system. The numerous data collected at this natural site, allowed to discuss the tree-to-tree and branch-to-branch variability and the seasonal changes in the emission factor. In a second step, the light and temperature algorithms developed for isoprene emitters (Guenther et al., 1993) were applied during the course of the year to simulate the short- and long-term variations of monoterpene emissions from Q. ibex. The suitability of these algorithms were discussed, both with the original parameters given for isoprene, and with a set of parameters estimated by non-linear best fit procedure from our experimental data.

MATERIAL

AND METHODS

Site description, measurement periods and vegetal material

Experimental site, climate and vegetation have been described elsewhere (Manes et al., 1997; Enders et al., 1997).All

measurements were performed on three Q. ibex trees (Ql, Q2 and Q3) at the Santo Quercio site of Castelporziano (near Rome, Italy), a 30-yr old pine-oak forest. A 8 m high scaffold allowed access from bottom to top of the canopy. Monoterpene emissions were measured in dynamic mass balance chambers installed for each of the five measuring campaigns on sun-exposed branches at a 9 m height. During three campaigns simultaneous measurements were performed at 3 to 5 m height, on branches of the same tree exposed in half or in full shadow. After each measuring campaign, the branch enclosed in the chamber was cut to determine the leaf area and dry weight. For this reason, seasonal differences also included the natural branch-to-branch variability. Leaf area was measured with an image analysis system equipped with a conveyor belt (Delta-T, U.K.). Leaf and wood dry weights were determined after drying at 80°C in a ventilated oven until weight stabilised (Table 1). The average leaf size varied between 3.5 cm’ in the sun and 13.1 cm’ in full shade. Corresponding values of the specific leaf weight ranged between 173gm-2atthetopofthecanopyand71gm-Zatthebase of the canopy in full shade, reflecting a canopy gradient in leaf characteristics. At the end of May 94, leaves of the current year were almost completely developed and their specific leaf weight increased from 103 g m-’ in May to 173 g mm2 in October. Gas exchange chambers

The enclosure system was composed of a cylindrical Plexiglas frame sustaining a 0.05 mm thick Teflon bag. A controlled flow (pump and mass flow controllers MKS) of charcoal-filtered air (ozone free) entered in the upper part and came out through the stem insertion port. The air flow was set from 30 to 50 Ndmin-’ (0.7-2 min air residence time) according to chamber volume (20-50 e), foliage mass and season. A stainless steel impeller, driven by an external stepping motor, was placed at the top of each chamber. Photosynthetically-active radiation (PAR) was measured by external PAR ouantum sensors (SB 190. Licor. U.S.) nositioned at the obtside top of the chambers. Reddction’df the incident radiation by the Teflon film was less than 10% both in the PAR and UV-B range of the spectrum. Air temperature was measured by PTlOOOsensors placed in full shadow in the chamber outlet port, where the air flow rate was about l-2 m s-l. In June 93, the installation of micro-thermocouples inserted in the abaxial leaf epidermis, showed that air temperature measured in the stem insertion port was somewhat lower than leaf surface temperature although average differences were small (0.5-2°C). COZ and HZ0 concentrations of inlet and outlet air were determined by an absolute COZ analyser (BINOS 100, Leyboldt, Germany) and a dew point mirror (MTS-2, Walz). Climatic and gas data were scanned every 5 and 1 s and recorded every 5 and 1 min, respectively, on a Datalogger (type DL2, Delta-T, UK). Net CO2 assimilation and transpiration rates were calculated according to the Diagas-software (Walz, Germany). Monoterpene emission rate was calculated as the difference between outlet and inlet air concentrations

Table 1. Characteristics of the branches enclosed in the chambers during each measuring campaign June 93 Branch height (m) Sun exposure$

9 Sun

Proj. leaf area (m’) Leaf dry weight (g) Average leaf size (cm’) Specific leaf weight (pm-*) Wood dry weight (g)

0.62 106.4 17:‘4 79

5 Half shade 0.49 63 5.8 129 53

Oct. 93

May 94

9 Sun

9 Sun

0.294 50.8 3.3 173 41

0.099 10.2 4.9 103 9

3 Full shade 0.226 16.0 13.1 71 22

Aug. 94 9 Sun 0.075 11.7 4.2 157 8

3 Half shade 0.079 8.53 9.7 109 3

Oct. 94 9 Sun

9 Sun

0.178 30.7 3.5 172.6 33.9

0.179 27.8 3.5 155.3 23

Monoterpene emissions from Quercus flex multiplied by chamber flow and divided by leaf dry weight or projected area. Emission rates were expressed both in pgg dw-i h-i and ng m-’ projected leaf area s-i in order to compare branches with different sun-exposure and different specific leaf weight. Monoterpene sampling and analysis

Monoterpenes were alternately or simultaneously sampled in the same branch chambers by four laboratories (JRC, CNR, ENSCT, GRECA). The JRC group trapped monoterpenes in glass tubes (Chrompack, 15 cm long and 3 mm inner diameter) filled with 125 mg Tenax TA (Aldrich, 20-35 mesh) and placed in a cooled sampling device (Staudt et al., 1995). Defined volumes of air (3-6 f) were sampled at the inlet and outlet ports of the chamber at a mass-flow-controlled rate of 150-200 ml mini after a prepurging time of 5-10 min on a bypass line. Air samples were analysed by a gas chromatograph (GC CP9001, Chrompack) fitted out with a desorption unit (TCTIPTI CP4001. Chromnack). a fused silica capillary column (25 m x 0.32’mm, dE i.2 pm CP-Sil 8 CB, Chrompack) and a flame ionisation detector. The carrier gas was helium (85 kPa1 and the desorption and separation programs presented the following sequence: 3 min precooling at - lOO”C,10 min desorption at 200°C 1 min injection at-200°C. GC-oven: 4min at 65”C, 2.5”Cmin-’ to 8o”C, 2.o”C min-’ to 100°C and 20°C min-’ to 240°C. Gaseous and liquid calibration standards prepared from commercial authentic monoterpenes of high purity (Fluka Aldrich, 95-99% purity) allowed the peak identification and quantification. The whole sampling and analysing system control and calibration procedures have been described by Staudt et al. (1995). The ENSCT group used an on-line system, consisting of a Tenax TA-trapping and a fully automated adsorptiondesorption device, allowing an analysis every hour as described by Clement et al. (1993). The CNR group collected monoterpenes on traps filled with graphite carbons and analvsed by GC-MS (Ciccioli et al., 1992). The GRECA group sampled on stainless-steel tubes packed with Tenax TA and analvsed bv GC-FID. The variabilitv due to monoterpene sampling and analysis was evaluated by comparing measurements made at the same time by different laboratories in the same branch chamber. It was calculated as the difference between the maximum and minimum emission rate expressed in percentage of the average (all laboratories). We found that this variability ranged from 27 to 96% during the successive campaigns and the average was about 50% on all campaigns, indicating a relatively large scattering of data. Application of isoprene algorithms

Current light and temperature algorithms usually used to simulate isoprene emission rates have been described by Guenther et al. (1991, 1993). The instantaneous emission rate, E, is calculated by multiplying the emission factor, E,, with a function of temperature, CT, and a function of light, CL: E=EsxCTxCL

(1)

Temperature and light functions are defined by the following equations:

x (T - TJI(R x T x T,))

c =

w-G1

T

1 + exp(&

x (T - T,,,)/(R x T x T,))

(2)

where T is the leaf temperature, Ts the standard temperature (303 K or 3O”C),R the gas constant (8.134 JK-’ mol-‘) and CT1 (95,000 J mol-I), CT2 (230,000 J mol-‘) and T, (314 K or 41°C) are empiric,31 parameters: CL =

axC,,xPAR 1 + a**’ + PAR**2

(3)

where a (0.0027) and CL1 (1.066) are empirical parameters.

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The application of the isoprene algorithms requires an accurate estimation of the species-specific emission factor Es. It can be measured under standard climatic conditions (PAR of 1000~molm-2s-1 and temperature of 30°C) or it can be estimated by an emission model. For instance, assuming that the light and temperature algorithms correctly describe the short-term variations of monoterpene emissions, Es should be accurately estimated by the slope of the linear regression between Cr x CL calculated with the isoprene model and the emission rates measured at temperature T and light L (equation (1)). We estimated Es in this way with the original

parameters of Guenther’s model to compare the emission factor observed on different branches, different trees and different seasons. The five parameters (C,,, CTZ,T,,,, a and CL1)define the light and temperature functions which describe the shortterm influence of climatic changes on Es. As done for isoprene emitters (Guenther ef al., 1993), this set of parameters can be deduced by nonlinear best fit procedure from experimental normalised emission rates (E/E,). Two versions (“G93” and “Fit”) of the isoprene model will be discussed in this paper. In a first step, emission rates were simulated by Guenther’s model with its original parameters (“G93”) from the experimental PAR and temperature measured during diverse periods of the year. In a second step we looked for a new set of parameters by model adjustment (“Fit”) on about 280 experimental measurements. For this, the emission factor (Es) was extrapolated from experimental measurements made under light and temperature close to standard ones (PAR > 600 pmolm-*s-l and temperature = 30 ) 1°C) during each campaign and used to normalise the emission rates to a value of one under standard conditions (E/E,). Assuming a good accuracy in the determination of Es, the set of parameters found by best-fit procedure should optimise the performance of the model in simulating the short-term response of monoterpene emissions to light and temperature. The nonlinear regression procedure that we applied uses the Marquardt-Levenberg algorithm; it seeks the values of parameters that minimise the sum of the squared differences between observed and predicted values (Marquardt, 1963).

RESULTS

Plant gas exchanges

and water status

Net CO2 assimilation and transpiration rates observed in the chambers indicated a diurnal and seasonal physiological activity similar to that described for Q. ibex at other forest or shrubland sites in the Mediterranean area (e.g. Sala and Tenhunen, 1994). In spring and summer, daily maximum transpiration and net photosynthesis rates ranged from 0.5 to 1.1 mmol rn-‘s-’ and 5 to 10 pmol m-‘s-i, respectively. A clear midday depression of photosynthesis and transpiration was observed only in August 94 with a maximum around 9 a.m. in the morning and a second peak in the afternoon. In fall, temperature and PAR were lower and maximum transpiration and photosynthesis rates were about 0.3 mmol m-* s-r and 5 pmolm-* s-l, respectively. Xylem water potential was measured during each campaign with a Scholander-bomb on leaves sampled nearby the chambers. The predawn values never exceeded -0SMPa in spring and fall, but were lower than -1.5 MPa in August. At this time, leaf water

N. BERTIN

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potential peaked around (Seufert et al., 1995b). Variability of monoterpene season

midday

at

-3.5 MPa

emissions: branch, tree and

cc-pinene, sabinene and /I-pinene accounted for more than 80% of total monoterpene emissions. The remaining 20% were composed of limonene, trans+ ocimene, myrcene, camphene, a-thujene, tricyclene, a-phellandrene, p-cymene, a-terpinene, /I-phellandrene, cr-terpinene, a-terpinolene and linalool. For branches at the top or at the base of the canopy, cc-pinene, sabinene and /I-pinene, represented, respectively, 40-50%, 20-30% and 25-30% of the main emissions. These proportions were independent on branch, tree and season (Fig. 1). Even at the beginning of May, during leaf development, absolute emission rates were low but the composition was unchanged. Thus, in the following results, only the sum of these three compounds will be considered. The tree-to-tree variability was assessed by comparing branch emissions of two different trees (Ql and Q3 during October 94) measured at the same time by one laboratory at the top of the canopy. The variability was calculated as the difference between two measurements expressed in percentage of the average. Small temperature differences between the two chambers were compensated for by temperature normalisation, assuming a log-linear relationship between temperature and emissions with a slope of 0.12 (Staudt and Seufert, 1995). The tree-to-tree variability,

El20

A

::c

100

f

60

I

a g so

;

40

5B

20

E

0 June93

oct93

May34

Aug94

oc194

OctS4

Ql

Ql

Ql

03

01

93

June93

my94

Aug94

02

Ql

83

Fig. 1. Relative proportions of the main three compounds cc-pinene (black), sabinene (grey) and P-pinene (white) emissions from Quercus i[ex during June 93, October 93, May 94, August 94 and October 94, measured at the top (A) and at the base(B) of the canopy on different trees (Ql, Q2 and 43). Averages + standard deviations.

et a[

calculated in this way, was 7.7% in the morning, 4.4% at midday and 19% in the afternoon. The branchto-branch variability was evaluated in the same way, confronting different branches of a same tree. In this case both temperature and light normalisation were applied, using the functions CT and CL of Guenther’s algorithm. We observed that the branch-to-branch variability was of the same order of magnitude than the tree-to-tree variability when comparing sunexposed branches but was as high as 190% when comparing sun-exposed and shade-adapted branches. Variability of the emission factor from Quercus ilex

Emission factors are given both on the basis of leaf dry weight (pggleafdw-‘h-l) and leaf area (ngm-’ projected leaf areas-‘) together with the 95% confidence interval of the absolute mean (1a5%).Nevertheless, destructive measurements were made only at the end of each measuring campaign and this could induce some inaccuracy especially in May (3-week campaign) during leaf development. As mentioned in M&M, Es was estimated during each campaign from the model itself and from experimental measurements. Estimations from the model indicated a fast increase of the emission factor during leaf development, from 5.1 pggdw-‘h-’ (Z95Qh=0.8) or 150ngm-2s-‘(I 95%=23) at the beginning of May to 36.4 pggdw-‘h-l (Igs0/,=2.4) or 1040 ngm-*s-i 950h=70) at the end of May. Except the leaf develop(I ment period, Es was rather stable from May to October when expressed in ngm-’ s- ’ but presented a large peak (two times higher) at the end of May when expressed in pg gdw- ’h- ’ because of the low specific leaf weight and high emission rate at this time. From end of May to end of October Es was on average 21.4pggdw-‘h-’ (lg5% =2.9) or 872 ng m112s11 (1950h=120) at the top of the canopy. At the base of the canopy, the branch completely under shade (Ql) clearly presented a lower emission factor (2.3pggdw-‘h-l, I 950h=0.45 or 45 ngmm2sm1, lg5”/=9) than the two other branches partly exposed to sun light which emitted on average 20.4pggdw-‘hh’ (Ig5”,”= 3.1) or 661 ngm-*s-l (1950,0=100) under standard conditions. Taking into account the data scattering and the relative inaccuracy of these estimates (large 95%-interval of confidence), it did not seem appropriate to discriminate branches according to their position in the canopy, but rather the shaded leaf area from the rest. Merging top and base of the canopy, excluding shade-adapted leaf biomass, the average emission factor Es estimated by the original isoprene model was 21.7 pg g dw ’h ’ (Ig5%=3.6) or 840ngm-* s-l (19,,=140) on the measuring period. For each campaign, the selection of measurements made under light and temperature conditions close to the standard ones (PAR > 600 pmol photon m-* s- ’ and temperature between 29 and 31°C) was possible only at the top of the canopy. Mean Es deduced by

Monoterpene emissions from Quercusilex

this method was 21.7pggdw-‘h-’ 862 ngm-‘s-’ (Z,,,=81).

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(Z95Y 0 =2.0) or

Application of light and temperature algorithms to simulate the monocerpene emissions from Quercus ilex In order to discuss the suitability of the.model to predict the short-term variations of emissions, all data were normalised to a value of one under standard conditions, dividing by the experimental value of ES (21.7pggdw-‘h-’ or 862ngm-‘s-l). As explained above, the isoprene model (Guenther et al.,

1993) was first applied with its original set of parameters (“G93”). In a second step, model parameters were estimated try a best nonlinear fit procedure (“Fit”) on about 280 data, mixing laboratories, seasons (except beginning of May), trees and branches (except shade adapted). Fitted parameters are: CT1= 74,050 J mol-‘, &,=638,600 J mol-‘, T,=311.6 K (38.6”C), cr=O.O01,47and &=1.21 (CL1 was set to force CL to converge to 1 for standard PAR as mentioned by Guenther et al., 1993). The dependency of the estimated parameters is relatively high (0.6-0.8) probably due to the light and temperature co-variation under field conditions. The variation coefficient of CTZ was about 58% but lower than 20% for the other parameters. Light and temperature functions corresponding to the original parameters (“G93”) and to the estimated parameters (“Fit”) are plotted in Fig. 2. The optimum temperature is about 35°C instead of 40°C in the original model, and a sharp negative effect of ‘temperature is observed above this threshold. In the “Fit” model, the initial slope of the light function is somewhat lower but light effect slightly increases until higher intensities. Both versions of the model (“Fit” and “G93”) were applied to simulate the diurnal emission course of Q. ilex at Castelporziano on different periods of the year. For light-exposed branches, measured and simulated data are plotted in Fig. 3. Globally, emissions were underestimated by models on May and 3 August 94 and overestimated on June 93 and 4 August 94. On 9 June and 5 0a:ober 93, data scattering was high (inter-laboratories variability) and models simulated an intermediate emission rate. During October 93 and 94, simulations and measurements agreed fairly well. In all cases, the day/night transition was well simulated by both models. During daytime, the model dynamic generally agreed with the experimental measurements, al though absolute values were not always well simulated especially in summer. On 3 August, the emiss,ion rate presented a double-peak pattern, one before midday and the other around 5 p.m., despite stable sunny conditions. As the chamber temperature raised above 35”C, the “Fit” model simulated a slight temporary decrease around noon which is however too weak compared to the actual dynamic of the measured emission rate. On average, over the five measuring campaigns, the correlation coefficients (R’) of the linear regressions between simulated and experimental emission rates were 0.48

5

10

15

20

25

30

35

40

45

Temperature [“Cl 1.2 0.9 d

0.6 0.3

0

400

800 1200 PAR [pmol m” s’(]

1600

2000

Fig. 2. Temperature (A) and light (B) functions of the isoprene emission model developed by Guenther et al. (1993). Solid lines correspond to the original parameters of the model (“G93”) and dotted lines correspond to the best nonlinear fit of the model to monoterpene emission rates from Q. ilex measured over one year at Castelporziano (“Fit”).

(“Fit” model) and 0.45 (“G93” model) during daytime periods, and 0.72 and 0.69, respectively, on whole days. Nevertheless, considering only one laboratory (JRC data) to suppress the influence of data scattering, these coefficients were as high as 0.87 and 0.81 for “Fit” and “G93”, respectively. For other branches at the base of the canopy which still received sun light for some hours, model precision was similar to that obtained at the top of the canopy (data not shown). For the shade-adapted branch (Fig. 4) which was shaded for most of the time (May 94), models also succeeded to simulate the daily course when an emission factor of 2.3 pg g dw- ’h- ’ (45 ng mm2 s- ‘) was applied.

DISCUSSION

Q. ilex is a quite particular monoterpene emitter, rather similar to isoprene emitters than to other monoterpene emitters. Both light and temperature are involved in the short-term control of its emissions (Staudt and Seufert, 1995; Kesselmeier et al., 1996) and the pathway of monoterpene biosynthesis in leaves seems to be very close to that of isoprene (Loreto et al., 1996a,b). For these reasons, we expected that the mechanistic model developed for isoprene (Guenther et al., 1991, 1993) would be more appropriate than the model of Tingey (Tingey et al., 1980) to simulate the monoterpene emissions from Q. ilex under natural conditions. Over the year the

N. BERTIN et nl

140(X) 2000

40

1750

F 30 k .z ‘;ur % 20 $ t % IO ; d

_r 1500 3 1250 ‘3 ‘BlOoO E 0) 750 E. w 500 250 0

0

2YMay 00

25iMay 12

25May 00

29May 12

2900

Q\

a

t

09/Jun 09

C August 94

IQ

“E & 1200 E w

27lMay 00

n

09/Jun 12 m

WJun 00

WJun 12

IlNun 00

40

-7

\

a

i

20 =

I

d 909

9 IO p 5

03lAug

OYAug 12.

00

D October 93

OIuAug

04IAug

OYAug 40

E- October 94

c

,I500 3 1250 .i ~1000 ‘E ol 750 L w 509

0

0

woct 00

OYOct 12

05/act 05loct 071oct 00 Day and Time

12

00

2YOct 03

2YOct 07

2YOCl 2YOct 11 15 Day and Time

woct 19

Fig. 3. Daily courses of PAR (grey lines), cuvette air temperature (solid lines) and monoterpene emissions from Q. ibexmeasured at different periods of the year by many laboratories: JRC (circles), ENSCT (squares), CNR (diamonds) and GRECA (triangles). On each period emission rate was simulated from experimental PAR and temperature by Guenther’s model with its original parameters (bold dotted lines, “G93”) and with parameters deduced from our experimental measurements (normal dotted lines, “Fit” see Fig. 2).

light and temperature functions of Guenther’s model accounted for 45% of the emission variations during daytime and 69% during both day and night-time. Indeed, the absence of emissions in the night was obviously well described by the light function, whereas the accuracy of the model was lessened during daytime, especially around noon at high temperature. We proposed a new set of parameters for Q. ilex but the global improvement was low (+3%) compared to the original parameters deduced from isoprene emissions (Guenther et al., 1993).

Deviations from the model could indicate that the estimation of Es was inaccurate or that the light and temperature algorithms are not totally sufficient to predict the short-term variations of monoterpene emissions from Q. ibex under field conditions. Nevertheless, many other reasons could be evoked. One main point raised by this international exercise during the BEMA campaigns, is the variability of the experimental emission rate measured simultaneously by different laboratories in a same enclosure system (up to 50%). It integrates sampling and analysis error which

Monoterpene emissions from Quercus ibex

26May MI

*&May 04

*Way 06

26iMay 12

2BIMay 16

26iMay 20

2T/hlay 06

Day and Time

Fig. 4. Daily courses,of PAR (grey lines), cuvette air temperature (solid lines) and monoterpene emissions measured on a shade adapted branch of Quercus ilex by the JRC (circles) in May 94. Emission rate was simulated from experimental PAR and temperature by Guenther’s model with its original parameters (bold dotted lines, “G91”) and with parameters deduced from our experimental measurements (normal dotted lines, “Fit”) with an emission factor of 45 ngm-* s-l.

mainly relates to l:he different methods used (Larsen et al., 1997). The model accuracy calculated for only one laboratory railsed up to 87% (“Fit”), which clearly indicates that data scattering underrated the model performance. Light and temperature measurements may also contribute to the experimental error, in that they are not fully representative of the biomass enclosed in the chamber. In particular, PAR sensors were placed at the outside top of the chamber and did not account for leaf shadowing inside the chamber due to branch position and architecture. In the same way, leaf temperalure was substituted by air temperature measured in the shadow of the chamber outlet. We checked on short periods, that leaf temperature was underestimated by l-2°C on average, but the underestimation could be much higher on hot sunny days. Actually, the low-temperature optimum (35°C) of the “Fit” model (Fig. 2) compared to that of Guenther’s model (41°C) may result from a leaf temperature underestimation during hot days and may partly explain the large underestimation of the emissions on 3 August when temperature raised above 35°C (Fig. 3). A main difference between original and adjusted models is the emission response to high temperature, although it should be cautiously interpreted since very few data were available above 35°C. In the “Fit” model, the sharp drop of monoterpene emissions above 35°C was probably caused by the measurements made on 3 August. On this sunny day, emission rate peaked at 10 a.m. (2600 ngm-‘s-l) and 5 p.m. with a slight decrease around midday which was considered as a negatl:ve temperature effect in the model. Gas exchanges and leaf water vapour conductance presented a simil.ar pattern with a clear midday depression (data not shown). Valentini et al. (1997)

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simultaneously measured canopy fluxes of COz and water vapour by eddy correlation and found the same daily course. Considering these facts, one could assume that stomata1 conductance was involved in the short term control of the emissions on this day. However, according to the literature, the limitation of emissions by stomata1 closure has never been reported even if emissions occur through the stomata (Fall and Monson, 1992; Loreto et al., 1996a; Bertin and Staudt, 1996; Steinbrecher et al., 1997). At this period, measurements of low soil and xylem water potential indicated a water shortage but its negative effect on monoterpene emissions is unlikely. Indeed, Bertin and Staudt (1996) observed that an 18-day water stress period imposed to young potted Q. ilex trees reduced the emissions only under severe stress when gas exchanges completely stopped, which was not the case in August. Moreover, these authors did not observe any water stress effects on the short-term variations of monoterpene emissions. For these reasons, it is likely that the midday reduction of emissions observed in August, was rather the result of high temperature as assumed by the temperature algorithm of Guenther et al. (1993). Actually, the stability of the optimum temperature as assumed in the model is not proved. As reported for isoprene emissions from aspen leaves (Monson et al., 1992) an acclimation to growth conditions may shift this optimum by a few degrees. Unfortunately, we could not estimate the five best fit parameters on individual measuring campaigns because of the low number and high scattering of data. As underlined by Guenther et al. (1993), a sensitive point for model application is the accuracy of the emission factor. Our measurements indicated an average emission factor of 21.7 f 2 pg monoterpene gdw-’ h-’ (860 f 80ngm-2s-‘). For Mediterranean ecotypes, Guenther et al. (1995) proposed an emission factor of 16 pg C g-’ h-’ for isoprene (Es) and 1.2 PgCg-l h-’ for monoterpene emitters (MS). Considering an average specific leaf weight of 154 gmm2 at the top of the canopy and 103 gmm2 at the base of the canopy, Es and MS given by Guenther ef al. (1995) correspond to about 734 ng isoprene mm2 s-l and 59 ngmonoterpene rn-‘s-l at the top of the canopy and about 491 ng isoprene rnw2 s-l and 39 ngmonoterpene mm2 s-l at the base of the canopy. Loreto et al. (1996a) reported for Q. ilex a leaf emission factor of about 619 ng a-pinene me2 s-l which would correspond to about 1350 ngmm2 s-l for the sum of u-pinene, sabinene and B-pinene according to the relative proportions we measured among the main emitted compounds (Fig. 1). For the same species and the same experimental site, Kesselmeier et al. (1996) measured in June an emission factor ranging from 15 to 28 pgg-‘dw h-’ under saturating light and 30°C. According to our specific leaf weight values, this would correspond to 642 to 1198 ngmonoterpene m-2s-1 at the top of the canopy. As observed by Litvak et al. (1996) the unit in which the emission rate is expressed (pggdw-‘h-l or ngmM2s-‘) may

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involve high relative differences according to the specific leaf weight. For instance at the end of May, the emission rate of the current year leaves presented a small seasonal peak when expressed on the basis of leaf area but a high peak when calculated on the basis of leaf dry weight because of a low specific leaf weight. For this reason, the comparison of data from the literature expressec’ in different units is difficult. However, variations in E, may result from many factors like ge’qotype, phenology or growth conditions (Tingey et al., 1991). Sharkey et al. (1991) observed that shade-adapted leaves of aspen emitted two times less isoprene than sunny leaves of the same tree. Indeed, we measured a low variability of E, (5520%) between trees and sun-exposed branches, whereas the shaded branch inside the canopy had an emission factor 17 times lower than that found for sunny branches. Comparing branches at the base of the canopy, sun-exposed leaves emit 15 times more than shade leaves when emission rates are expressed on an area basis but only 9 times more when emissions are expressed on a dry weight basis. As explained by Litvak et al. (1996) for isoprene emission, this result means that about 60% of the sun/shade differences may be attributed to differences in specific leaf weight and 40% to changes in the biochemical and/or physiological properties that is photosynthate availability for monoterpene synthesis and/or enzymatic activity. These proportions are rather close to those observed for many isoprene emitters (Litvak et a/., 1996). The long-term adaptation to light exposure seemed to involve only a decrease of the emission factor without modifying the light and temperature responses (Fig. 4). One could expect that long-term acclimation to light and temperature regimes would also contribute to the variability of Es during the year. Nevertheless, after leaf development, our data did not exhibit any clear seasonal changes in the emission factor until end of October. As proposed for isoprene (Monson et a[., 1994), Es may be controlled for a large part by the amount of specific enzymes for monoterpene synthesis (e.g. cc-pinene cyclase) and its potential level could be mostly determined at an early stage during leaf development. The few measurements performed at the beginning of May, indicated an emission factor of about 3.2pggdw-’ h-‘(90ngmonoterpenem-Zs-‘),that is one order of magnitude lower than that measured at the end of May. Although we could not estimate the leaf biomass increase during these three weeks of measurement, it seems clear that young developing leaves have a much lower emission factor. Pio et al. (1993) also reported that emission rate from Q. ibex increases as leaves become more developed. Unfortunately, we did not perform any measurements in early season to characterize the possible onset of monoterpene emission during leaf ontogeny as reported for isoprene emission (Monson et al., 1994; Harley et al., 1994). The induction of isoprene emissions by high temperature was also reported by Sharkey and Loreto

et a/

(1993), which let think that an index of accumulated heat may trigger the onset of the isoprene synthase in spring during leaf development (Monson et al., 1994). Contrary to other species like Pinus pinea (Staudt et al., 1997) the relative proportions of z-pinene, sabinene and P-pinene, the main emitted compounds from Q. ibex, were rather stable over the year, independent of season, leaf ontogeny or branch position. Concerning canopy modelling, the application of one single emission factor to the whole leaf biomass and the whole year may be somewhat erroneous. For closed homogeneous canopies a light interception mode1 may help determining homogeneous canopy layers to which different emission factors may be applied. In this case it could appear that the upper layer accounts for the major part of total canopy emissions. For open or heterogeneous canopy as this was the case at Castelporziano, a good mapping of light interception ‘is necessary (Lenz et al., 1997) to estimate the proportion of leaves which significantly contributes to total emissions. Although the emission factor was stable from end of May to October, the possibility of a seasonal induction of monoterpene emissions in early spring linked to leaf development should be seriously considered in the future, at least for Q. ibex. On the whole, the yearly course of Q. ibex emissions was well simulated by the light and temperature algorithms of Guenther et al. (1991, 1993). The inaccuracy of the model was high during the hottest periods in summer. Nevertheless, it is still not clear whether the measurement errors or the intrinsic model properties are responsible for the discrepancies between modelled and actual emission rates measured in the field. For instance, the leaf age effect and the influence of other environmental factors than light and temperature should be examined. Acknowledgements-This work was realised at the Joint Research Centre of Ispra (Italy) in the frame of a post-dot grant of the EEC. The authors thank D. Droste and M. Diirr for their helpful technical assistance.

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