Modelling leaf gas exchanges to predict functional

Ecological Modelling 166 (2003) 123–134

Modelling leaf gas exchanges to predict functional trends in Mediterranean Quercus Ilex forest under climatic changes in temperature Marcello Vitale a,∗ , Mauro Scimone b , Enrico Feoli c , Fausto Manes a a

Dipartimento di Biologia Vegetale, Università “La Sapienza”, P.le A. Moro, 5, I-00185 Roma, Italy b SISSAD snc, Viale Campi Elisi, 62, I-34100 Trieste, Italy c Dipartimento di Biologia, Università di Trieste, Via Giorgieri 10, I-34100 Trieste, Italy Received 26 July 2002; received in revised form 31 January 2003; accepted 19 March 2003

Abstract A simple model based on the “big leaf” assumption and calibrated with field eco-physiological measures of gas exchanges is used to simulate the effects of temperature increase on net primary production, total canopy transpiration and the dimensionless decoupling coefficient Ω of Holm oak forests. Two different annual average air temperatures: 14.6 and 18.0 ◦ C are considered, they are respectively the average current temperature and the one expected in the next 50 years in the Mediterranean area if the trend of global warming will continue. The model simulates the behaviour of the three parameters by assuming no changes in the effects of water constraints at both the temperatures. The model has been implemented by STELLA® II software. According to the model, the increase of air temperature affects both the net primary productivity (6.3%) and the water losses by canopy transpiration (37.2%). The model predicts an average decoupling factor, Ω, of about 0.26 at both temperatures of 14.6 and 18.0 ◦ C. This value is in the range between Heathlands and Forest, suggesting that at the average annual temperature of 18.0 ◦ C the Holm oak forest will start to respond in a similar way to more xeric plant communities. © 2003 Elsevier B.V. All rights reserved. Keywords: Primary production; Transpiration; Photosynthesis; Decoupling coefficient; Holm oak; Modelling; Mediterranean area

1. Introduction The interest in modelling reflects the growing attraction in using models as vehicles to integrate knowledge, research activities, experimental results, and to test hypothesis (Goudriaan et al., 1999) and as the most feasible means to address how climate change will affect the process-based forest functionality (Bossel, 1996; Fosberg, 1990). Many mathemati∗ Corresponding author. Tel.: +39-06-499-12451; fax: +39-06-499-12448. E-mail address: [email protected] (M. Vitale).

cal models have been proposed to simulate vegetation functional responses such as net primary production and transpiration to climatic changes (Schwalm and Ek, 2001; Ågren et al., 1991). However, the majority of the models cannot easily be applied because they require many input parameters that are frequently not available. The data requirements of a model have implications for its applicability and scope. Recently it is being attempted to estimate the key factors of functional processes with the purpose of creating models relatively simple but capable to give useful predictions (Kickert et al., 1999; Manes et al., 1998a). An example of simplification is the use of the so-called

0304-3800/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0304-3800(03)00129-7

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M. Vitale et al. / Ecological Modelling 166 (2003) 123–134

“big leaf” model (Magnani et al., 1998; Lloyd et al., 1995), that describes the response of vegetation as a whole (big leaf) in terms of variations of the main environmental factors (Sellers et al., 1992; Amthor et al., 1994). The application of the “top-down” model to a given plant community implies that bulk canopy surface is aerodynamically well coupled to the atmosphere (McNaughton and Jarvis, 1991), allowing thus to approximate measurements concerning conditions at the notional canopy surface with measurements taken at few meters above. But, this assumption is not so valid for the tropical or temperate broad-leaf forests in relation to the dimension of their leaves and higher stomatal conductance, and, as a consequence, broad-leaf canopies are partially decoupled from the atmosphere (Meinzer et al., 1997; Herbst, 1995). In a modelling context, among the direct effects of climate change is the change in temperature. Average global temperature is expected to raise although the exact extent is still uncertain. ICPP Report (1995) foresees an increase of 0.8–3.5 ◦ C in the next century. It is important, therefore, to include this environmental parameter into the mechanistic functions of the models which are basically important such as photosynthesis, respiration, stomatal conductance and hydrology (Schwalm and Ek, 2001) to better simulate the functioning of plant communities under changing climate. The aim of this paper is to present an application of a relatively simple model based on the “big leaf” assumption to simulate for an evergreen Holm oak (Quercus ilex L.) forest, a forest-type widely diffused in the Mediterranean area, the behaviour of three eco-physiological parameters: 1. The net primary production based on the net photosynthesis. 2. The canopy transpiration. 3. The sensitivity of canopy transpiration to change in stomatal conductance using the dimensionless decoupling coefficient. The simulation is carried out considering two scenarios of average annual temperature the current one of 14.6 ◦ C and the one of 18 ◦ C foreseen within the next 50 years as a consequence of global warming in the Mediterranean area (Houghton et al., 1990). The decoupling coefficient Ω is very important to make predictions of vegetation changes, since it is related

to the effects of different vegetation structures in coupling with the atmosphere (Jarvis and McNaughton, 1986; Jarvis, 1995; Harley, 1995). The choice to adopt a mechanistic model characterised by a small number of eco-physiological variables derives from the knowledge that the more a model attempts to simulate, the more complexity and feedback loops included, the higher cost in terms of loss of resolution (Schwalm and Ek, 2001).

2. Materials and methods 2.1. The data collection site The measurement in the field were carried out in the Presidential Estate of Castelporziano located at 41◦ 44 N, 12◦ 25 E along the Mediterranean coast near Rome (Italy). The sampling area is located in Santo Quercio site, 6 km from the seashore. The climate is typically Mediterranean (Nahal, 1981) with the mean annual precipitation of approximately 753 mm and a typical Summer drought period lasting from May to August (Table 1). The rainfall pattern presents two maxima in Spring (March–April) and Autumn (November–December). This area is characterised by red sand soils of eolian origin and volcanic ashes mixed with clays (Manes et al., 1997a). The forest is dominated by 30- to 40-year-old trees of Holm oaks (Q. ilex L.), with a cover percentage of about 60–70% and 8–10 m in height, however other trees of Pinus pinea L., Q. frainetto Ten., Q. cerris L. and Q. robur L. are present. 2.2. Gas exchange measurements The Ciras Auto 1 (PP Systems, UK) was used to measure some physiological parameters on Holm oak leaves (Q. ilex L.). The instrument is equipped with a leaf chamber connected to an infrared red gas analyser (IRGA) and with a microcomputer that allows to obtain in digital form gas exchange data. Based on equations reported by von Caemmerer and Farquhar (1981) the microcomputer calculates the net photosynthesis (A), leaf transpiration (E), stomatal conductance to water vapour (gs ), sub-stomatal concentration of CO2 (Ci). Moreover, the instrument gives in digital form also some environmental parameters such as


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Table 1 Monthly average values of rainfalls (Pavg , mm) and the maximum (Tmax ) and minimum (Tmin ) of air temperature values (◦ C) Months

Pavg Tmax Tmin

January

February

March

April

May

June

July

August

September

October

November

December

76 12.7 3.6

81 13.5 3.9

70 15.6 5.9

51 17.9 7.9

31 22.3 11.4

18 25.7 14.1

9 30.1 17.8

35 30.5 18.4

76 27.0 15.8

106 22.8 12.8

107 16.9 8.2

93 13.6 5.1

Data by meteorological station in the Castelporziano Estate between 1955 and 1985.

light intensity (PAR), air (Tair ) and leaf temperature (Tleaf ), and indirectly the relative humidity (RH). 2.3. The model The model is structured in compartments (Fig. 1). The equations of the model to simulate the eco-physiological parameters are the following:

1. The net productivity rate: The main equation describing the light response curve of net photosynthesis considers the daily photosynthetic activity (A(i)) calculated in accord to De Wit et al. (1978): A(i) = (Amax − Rd )[1 − exp(−QY × Qi (i)A−1 max )] + Rd

[␮mol CO2 m−2 s−1 ]

Fig. 1. Simplified scheme of the model showing the main components and their connections.

(1)

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where Amax is the maximum rate of net photosynthesis at light saturation (␮mol CO2 m−2 s−1 ), Rd is the dark respiration rate (␮mol CO2 m−2 s−1 ), QY is the quantum yield efficiency of photosynthesis (␮mol CO2 ␮mol−1 photons) and Qi (i) is the average light radiation inside the canopy at the i-th day of year. The dark respiration rate is assumed to be a constant rate (0.3 ␮mol CO2 m−2 s−1 ), integrated for the darkness period and divided by the Leaf Area Index of the i-th day (LAI(i); m2foliage m−2 ground ): Rd = 0.3(24 − ph(i))LAI(i)−1

(2)

where ph(i) is the photoperiod (h). QY has been calculated by empirical measurements of quantum yield efficiency responses to leaf temperatures carried out in laboratory by using the Ciras Auto 1 gas analyser. QY fitted well in function of leaf temperature (Tleaf ) by following polynomial equation: 3 2 QY = −(2×10−6 × Tleaf ) + (4.4 × 10−5 × Tleaf )

+ (0.0019 × Tleaf )

(3)

The attenuation of light inside the canopy Qi (i) is assumed to be dependent on LAI(i) according to the Beer–Lambert law (Björn and Vogelmann, 1993): Qi (i) = Qo (i) e[−k(i)LAI(i)]

(4)

where Qo (i) is the light intensity above the canopy (␮mol photons m−2 s−1 ) for the i-th day of the year and k is the typical coefficient of light extinction for a given woody vegetation structure. In our case k was defined as empirical function of the previous i-th day leaf area index (LAI(i − 1)) and calculated by an iterative way:

Pn (i) = {A(i) × 3600 × ph(i) × (1−dro(i))−R(i)} × 10−6 × 30 × [g CH2 O m−2 day−1 ] (7) The average diurnal net photosynthesis in Eq. (7) is converted into biomass by multiplying the CO2 assimilated for the molar mass of carbohydrates (CH2 O = 30). The annual biomass production is obtained by cumulating the daily biomass for each of the year. Maintenance respiration rate R(i) is given by a Q10 temperature dependence: R(i) = 1.5Q10 [(Tleaf −Tref )/10]

(5) and fluctuating between 0.75 and 0.79 during the year. LAI is calculated for each day of year by the following equation: (6)

where SLA is the specific leaf area (in our case SLA = 0.0061 m2 g−1 DW, from experimental data), Pn (i − 1) is the net productivity rate of

[␮mol m−2 s−1 ] (8)

where Q10 is the increasing factor of a biological mechanism as response to a temperature increase of 10 ◦ C, Tleaf is the actual leaf temperature and Tref is the reference temperature below which the photorespiration stops. In our case, Tref = 10 ◦ C and Q10 = 1.5 (Hay and Walker, 1989). The empirical value of 1.5 derives by experimental measurements carried out on Holm oak plants at the Castelporziano forest (unpublished data). The aridity index of Mitrakos is given by: dro(i) = −0.02 × pre(i) + 1

k(i) = 0.91[LAI(i − 1)(0.54 + LAI(i − 1))−1 ]

LAI(i) = SLA × Pn (i − 1) × α

the (i-th − 1) day of year, and α is the partition coefficient of the accumulated biomass related to the leaf compartment, that is assumed to be 0.23 (Bruno et al., 1977). Pn (i), is then calculated as function of the average diurnal net photosynthesis rate A(i) from Eq. (1), photoperiod, the aridity index (dro(i)) of Mitrakos (1980) and the maintenance respiration rate (R(i), ␮mol CO2 m−2 s−1 ):

(9)

where pre(i) is the monthly rainfall (mm) referred to the i-th day of the year (Mitrakos, 1980). Daily values of monthly rainfalls (pre(i)) are estimated in relation to the characteristic of STELLA® II software package (High Performance Systems Inc., USA) to convert discrete input (as monthly rainfalls) into continuous curves input. Attributing each monthly value to the median day of the month, for each day a rainfall value is linearly interpolated in the interval between the previous and the following monthly values.


Finally, the annual biomass production is splitted in three compartments: leaves, trunk+branches and roots (t CH2 O ha−1 year−1 ) by means of proper partition coefficients, derived from previous field analysis carried out in the same experimental site (Bruno et al., 1977). 2. Canopy transpiration: E(i) (g H2 O m−2 day−1 ) is calculated as function of diurnal average leaf temperature, of vapour pressure difference (between the inside and the outside of the canopy), of photoperiod, of LAI and of the aridity index of Mitrakos. Equations are: cp × ρ ET = gc VPD (10) λ×γ E(i) = ET × 3600 × ph(i) × (1 − dro(i))

(11)

where ET is the average instantaneous canopy transpiration (g H2 O m−2 s−1 ), VPD is the diurnal average vapour pressure difference (mbar) derived from empirical functions of the air and leaf temperature, gc is the diurnal average stomatal conductance of the canopy (m s−1 ), cp is the specific heat of air at constant pressure (1.012 J g−1 ◦ C−1 ), ρ is the air density (g m−3 ), λ is the latent heat of vaporisation of water (J g−1 ), and γ is the psychrometric constant (mbar ◦ C−1 ). The ρ and λ are calculated as empirical functions of the air temperature (Friend and Woodward, 1990): ρ = 1288.4 − 4.103Tair

(12)

λ = 2500 − 2.367Tair

(13)

whereas γ is function of cp and λ (Friend and Woodward, 1990): γ(λ, cp) = P × cp × (0.622λ)−1

(14)

where P is the atmospheric pressure (mbar) and 0.622 is the ratio of densities of dry air and water vapour (dimensionless). The water vapour conductance of the canopy, gc , is calculated as function of the stomatal conductance, gs , and the LAI, for each i-th day of year: gc (i) = gs (i) × LAI(i) × fc−1 × 10−3

[m s−1 ] (15)

where fc transforms the stomatal conductance expressed in molar units (mmol m−2 s−1 ) to

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non-molar units (mm s−1 ) as function of the leaf temperature: fc = −0.0001Tleaf + 0.045

(16)

gs (mol m−2 s−1 ) is the leaf stomatal conductance to water vapour for each i-th day of the year, and it is calculated as function of the assimilation rate (␮mol m−2 s−1 ), the partial pressure of carbon dioxide (pCO2 ) outside the leaf boundary layer (␮mol/mol) and the air relative humidity, h, after Ball et al. (1987) and Harley et al. (1992): Pn (i) −1 gs (i) = gs0 + m× h(i)×pCO2 (i) 30×106 × (3600 × ph(i))−1

(17)

(=0.015 mol m−2 s−1 )

where gs0 is the minimum stomatal conductance to water vapour when Pn (i) = 0, and m is an empirical coefficient which represents the composite sensitivity of conductance to assimilation, pCO2 , humidity and temperature. 3. The dimensionless decoupling coefficient: Ω (McNaughton and Jarvis, 1983; Magnani et al., 1998) is defined as: ∗ ∗ −1 s s ga Ω= (18) +1 +1+ γ γ gc where s∗ is the slope of the saturation vapour pressure–temperature curve (mbar ◦ C−1 ), γ is the psychrometric constant, gc is the canopy conductance as defined before, and ga is the canopy aerodynamic conductance (mmol H2 O m−2 s−1 ). This latter parameter results from the sum in series (analogous of the electric circuits) of two components: the canopy boundary layer conductance (gb , mmol H2 O m−2 s−1 ) and the turbulent conductance (gt ) due to the movement of air eddies between the canopy and the atmosphere. Because gt is not related to the leaf dimensions but depends only on the canopy aerodynamic roughness and, as we are interested to evaluate the influence of air temperature on the gas exchanges occurring inside the boundary layer, in the calculation of ga , we have considered only gb , that is calculated as: gb = gc − gs

[mmol H2 O m−2 s−1 ]

2.3.1. Model assumptions The assumptions of the model are:

(19)

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1. no changes in the effects of water constraints at both the temperatures; 2. the dark respiration is expressed as a constant rate (Eq. (2)) which was derived from literature and our measurements (Manes et al., 1998b) and it is not expressed as function of temperature; 3. the stomatal conductance is expressed as function of relative humidity (Ball et al., 1987), assuming a stomatal sensitivity to levels of relative humidity values; 4. SLA is assumed as constant during the year in the LAI calculation (Eq. (6)). These constraints are rather restrictive, and we should consider SLA as function of time, taking also into account the phenological stages. Moreover, regard to the sensitivity of stomatal conductance to relative humidity, we should consider the contribute of the light radiation, the leaf water potential and the hormonal effect, all affecting the opening and closure of the stomata. But, one of the qualifications of this paper is to introduce a simple model keeping the functional comparisons at the community level, without to consider variables and relationships hard to obtain and to apply. 2.3.2. Programming software The model has been developed by STELLA® II software package. This software is based on a programming environment which provides a graphical interface that allows the user to model complex systems by a preliminary analysis of the most important system components and their interactions (object-oriented modelling). In the program, the user builds a model using a set of object icons. Each object represents a state variable, a parameter or process, and has specific attributes that define how it interacts with other objects in the system. Relations between objects are established by graphically drawing proper connections. Once these connections are established, the user specifies the functional relationships between components and initial values to complete the model.

3. Results The trend of net productivity simulated by the model (Fig. 2a) for the average year temperature of

14.6 ◦ C is characterised by two maximal peaks, one in Spring and the other in Autumn, this latter coincides with rainfalls occurring in October and November at our latitudes (Table 1). Annual productivity estimated by the model is 12.66 t CH2 O ha−1 year−1 (equal to 5.09 t C ha−1 year−1 ) whereas annual total water transpired by the canopy is 207 kg H2 O m−2 year−1 that is 2070 × 103 kg H2 O ha−1 year−1 . The net productivity obtained by the model fits very well with the results obtained on the same vegetation stands by Tirone et al. (2000) and Baldocchi et al. (2001) using eddy covariance technique (5.72 and 6.6 t C ha−1 year−1 , respectively) and by Tirone et al. (2001) using destructive sampling (4.95 t C ha−1 year−1 ). It is worthy to note that the annual productivity estimated by the model is closer to the destructive sampling value than to the eddy covariance technique. Interestingly, there are differences between the eddy covariance measurements for the same plant community, likely due to the variation of environmental parameters that could affect the CO2 fluxes. Moreover, the difference between model output and eddy covariance technique could be due to the effect of the canopy aerodynamic roughness on the gas exchanges, that we did not consider in the model (Van Gardingen and Grace, 1991). Canopy transpiration exhibits higher values in September– October months, when air temperatures are still elevated, and the underground water reserve is recharged by autumnal rainfalls (Fig. 2b). The average daily water use is 0.571 × 103 kg H2 O ha−1 day−1 . The water use efficiency values (WUE, mg C g−1 H2 O) show a trend characterised by the reduction of values (Fig. 2c) concurrently to the canopy transpiration peaks, in relation also to the rainfall periods (2.79 and 1.12 mg C g−1 H2 O in April and September, respectively). LAI values show an increasing trend with different slopes during the year. In particular, an increase of slope is observed in the Spring–Summer months in which LAI reaches 3.68 at the end of August (Fig. 3). During the Autumn and Winter months, taking into account the falling of the leaves, LAI decreases to the value of 2.50 following an empirically found trend for the Holm oak community of Castelporziano, with a reduction rate of 0.3% per day. Fig. 4a shows the daily values of three conductance components gs , gb , and gc . The average value of gs is 43 mmol H2 O m−2 s−1 (or 0.0011 m s−1 ), which is similar to average values measured at different


129

Fig. 2. Annual trends of (a) net photosynthesis (Pn ), (b) canopy transpiration (E), and (c) the WUE, calculated by the model at the two temperatures of 14.6 and 18 ◦ C.

heights of the canopy during previous measurement campaigns carried out in the Castelporziano Holm oak forest (Manes et al., 1997b). The annual mean value for the canopy boundary layer conductance is 87 mmol H2 O m−2 s−1 (or 0.0022 m s−1 ) whereas

for the canopy conductance we estimated an average value of 130 mmol H2 O m−2 s−1 (or 0.0032 m s−1 ). The seasonal aridity also affects the canopy conductance which shows the year’s lower values during the Summer period. In fact, the highest value of the

130


Fig. 3. Annual trend of LAI calculated on daily basis. It is evident the decrease of values after the Summer period due to the turn over of foliage, and quantified in a decreasing rate of 0.3% per day.

Fig. 4. Annual trend of water vapour conductance components such as stomatal conductance (gs ), boundary layer conductance (gb ) and canopy conductance (gc ) expressed as m s−1 , and calculated at 14.6 ◦ C (a) and 18 ◦ C (b).


131

Fig. 5. Annual trends of decoupling coefficient values, Ω calculated at the two temperatures 14.6 and 18 ◦ C.

year, 190 mmol H2 O m−2 s−1 (or 0.0047 m s−1 ), was estimated in April. The shape of the net productivity curve for 18 ◦ C average yearly temperature similar to that of Fig. 2a, but, as consequence of a higher temperature, the total net productivity is of about 13.46 t CH2 O ha−1 year−1 with an increase of 6.5% respect to Pn estimated at 14.6 ◦ C. This justifies the increase of LAI values that reached 3.77 (+2.25%). Moreover, net productivity values, at two different temperatures, show the greatest differences in Spring. At the same time, canopy transpiration values show a strong increase (Fig. 2b). In fact, the total canopy transpiration is 284 kg H2 O m−2 year−1 that is 2840 × 103 kg H2 O ha−1 year−1 , showing an increase of 37.2% with respect to the cumulative canopy transpiration estimated at lower air temperature. The average daily water use is 0.784×103 kg H2 O ha−1 day−1 . In this context, WUE values are lower during the year, with a reduction in April and September (Fig. 2c). The annual average value is 2.61 mg C g−1 H2 O respect to 3.67 mg C g−1 H2 O simulated at 14.6 ◦ C of annual average temperature. Stomatal conductance values are lower of 1.1% (42.6 mmol H2 O m−2 s−1 ) than those estimated at lower air temperature (Fig. 4b). Change in boundary layer conductance is 88 mmol H2 O m−2 s−1 and canopy conductance is estimated to be 131 mmol H2 O m−2 s−1 . The annual trends of gs , gb and gc are similar to those drawn at 14.6 ◦ C. Fig. 5 shows the annual trends of Ω values during the year at the two annual average temperatures. It is

noteworthy that these trends are very similar, keeping a decreasing trend in the first part of the year till to the end of August and then an increase of the values in the second part. At 14.6 ◦ C the average Ω is estimated to be 0.261, which is equal to the average value calculated at 18 ◦ C.

4. Discussion and conclusion The net productivity, predicted by our model for Castelporziano Holm oak forest, increases as consequence of the increasing of air temperature, thus confirming what is reported by Rochefort and Woodward (1992) for the Mediterranean forests. This increase in net productivity (6.3%) will only occur if there is a sufficient water and nutrient availability (Debano and Conrad, 1978), since as shown by Mooney et al. (1991), Oechel et al. (1994) and Diaz et al. (1993) photosynthesis and standing biomass increase at increasing CO2 concentrations and air temperature only if soils are fertilised. Moreover, the increasing temperature conditions will lead to a remarkable increase of water transpiration (37.2%), and hence to a greater consumption of water by the forest throughout the year. Studies carried out by Pinzari et al. (2001) on the soils of the Holm oak forest of Castelporziano highlighted a good quality of the soils. Moreover, the assumption made about the water availability also during the Summer drought stress is proved by the hydrogeological studies carried out by Anselmi et al. (1995) in the Castelporziano Estate, which confirm the ability to

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sustain the increase of canopy water losses in higher evaporative demand simulated at 18 ◦ C. Nevertheless, recent studies in this site (Busuoli et al., 2001) indicate that a negative trend of rainfalls in the last years could lead to a reduction of water availability in relation to increasing evapotranspiration. The evolution of this process could result into serious threats for the stability of the Holm oak forest system, as lowering of water table and the consequent intrusion of sea water. The progressive reduction of ground water availability could lead, in more arid Mediterranean areas, to alterations of the dynamic processes of vegetation with the establishment of plant species better adapted to xeric conditions (ICPP-Special Report, 1998). The canopy conductance (gc ) showed similar values at the two temperatures; probably this is due to a concurrent decrease of stomatal conductance (gs ) values (−1.1%). At this regard, neglecting the turbulent conductance (gt ) in ga calculation would have underestimated Ω, although the values found are similar to those reported for other broad-leaves plant communities. The tight and inverse relationship between vapour pressure difference (VPD) and Ω is consistent with high values of gb , preventing thus accumulation of transpired water vapour at the leaf surface and thereby coupling the evaporative demand at the leaf surface with that in the bulk air. Moreover, gb , and therefore evaporative demand sensed by the stomata, are influenced either by external factors such as wind speed and proximity of neighbouring trees, or by intrinsic factors such as the leaf dimensions, crown structure and height of the canopy. The annual trends of Ω at the two temperatures show the higher vegetation–atmosphere coupling during the Summer months, whereas a poor coupling is observed in Winter. Similar relationships are reported in studies carried out on different plant communities such as beech forest (Magnani et al., 1998) and Nothofagus forest (Kostner et al., 1992). Mature broad-leaves forests appear to be not as well coupled to the atmosphere as coniferous forests of similar structure (Jarvis and McNaughton, 1986). The average values of Ω are estimated to be of about 0.26 at both average temperatures of 14.6 and 18 ◦ C, which are between Heathland and Forest (Jarvis and McNaughton, 1986), and they should be known as bushland (garrigue) in the Mediterranean areas. This value (and the considerations previously made about water losses) suggests that at the average annual

temperature of 18.0 ◦ C the Holm oak forest likely will not be able to endure the strong water request from the bulk air, shifting thus to more xeric plant communities, such a pseudo-steppe or a garrigue. So, we hypothesise a progressive modification of the dominant Holm oak community toward to a more simple structure that is able to endure the strong water deficits between soil and air compartments. The flexible modular typology and the ability of the model to characterize as a whole the functional response of the plant community to the variation of an important parameter like the temperature, it is promising in view of further applications. Recently, allometric studies carried out on plants, irrespective of grade or habitat, highlighted very interesting relationships between biomass production rates (G) and body size (M) (Niklas and Enquist, 2001; Enquist and Niklas, 2001). More specifically, the annual growth rates, G, is proportional to photosynthetic biomass, 3/4 Mp , (G ∝ Mp ) and this latter, also considered as light-harvesting capability of plants, is proportional to the total non-photosynthetic biomass, Mn , (Mp ∝ 3/4 Mn ). Again, Mp shows a relationships proportional to the 2-power of stem diameter D, (Mp ∝ D2 ) and the stem biomass (Ms ) is proportional to the 8/3-power of stem diameter (Ms ∝ D8/3 ); as consequence G ∝ D3/2 . It is also demonstrated that the total community resource use (Rtot ) is predicted to be proportional to the product of the number of individuals N and the rate of resource use per individual Q (Rtot ∝ NQ), it follows that Rtot ∝ M (Niklas and Enquist, 2001). It is, therefore, easy to foresee that the linkage of the model outputs (i.e. net primary productivity) with the allometric theory could give interesting information about the structure characteristics of the Mediterranean plant communities and their modifications as response to the changing environmental variables. Moreover, “modelled” structural and functional plant community information and the use of G.I.S. with remote sensing techniques should be able to produce spatial information at regional scale (Running et al., 1999). Acknowledgements This research has been supported by grants from the Italian Ministry of Education, University and Research


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