Stomatal responses to drought of mature trees and seedlings of two co-occurring Mediterranean oaks

Stomatal responses to drought of mature trees and seedlingsof two co-occurring Mediterranean oaks

Sonia Mediavilla*, Alfonso EscuderoDepartamento de Ecologia, Facultad de Biologia, Universidad de Salamanca, 37071 Salamanca, Spain

Received 16 July 2002; received in revised form 11 June 2003; accepted 14 July 2003

Abstract

We studied stomatal responses to decreasing predawn water potential (Cpd) and increasing leaf-to-air water vapour pressure

deficit (VPD) of seedlings and mature trees of two co-occurring Mediterranean oaks with contrasting leaf habits: the evergreen

Quercus rotundifolia and the deciduous Quercus faginea. Our objective was to define and to compare the stomatal strategies of both

speciesfordrought resistanceandto identify thepossibledifferencesbetweengrowthstages inselecteddroughtadaptationattributes.

Among the mature trees, Q. rotundifolia exhibited a water-use behaviour that was more conservative than that of Q. faginea:

lower maximum stomatal conductances and greater sensitivity to VPD than the deciduous species. As a result, the leaf water

potential of the evergreen species never decreased along the day and along the growth season as much as in the deciduous

species; this may help to guarantee longer leaf longevity by avoiding irreversible damage during the summer drought. The

seedlings of the two species showed a less conservative water-use strategy in comparison with adult trees: a relatively high

stomatal conductance and lower stomatal sensitivity to soil and atmospheric drought. As a consequence, leaf water potential

decreased more in the seedlings along the day than in the adults. Q. rotundifolia was the species for which the most pronounced

differences between growth stages were obtained. Thus, interspecific differences in response to drought disappeared in the first

stages of the life of the trees, and the seedlings of the two species showed a common strategy, probably as a response to the

competition from the herbaceous layer. A low stomatal sensitivity in benefit of an increase in growth would probably be a more

successful strategy under the competitive conditions that seedlings experience during their establishment.

# 2003 Elsevier B.V. All rights reserved.

Keywords: Drought; Growth stages; Leaf water potential; Mediterranean Quercus species; Stomatal conductance; Vapour pressure deficit

1. Introduction

Quercus rotundifolia Lam. and Quercus faginea

Lam. are two species with clearly contrasting leaf

habits and characteristics (Mediavilla et al., 2001),

whose adaptations to drought were studied and com-

pared here. Both species are widely distributed in

areas of the interior of the Iberian Peninsula with a

cold Mediterranean climate. In these regions, low

winter temperatures and late frosts reduce the duration

of photosynthetic activity to a short period which,

moreover, to a large extent coincides with the most

important constraint imposed by the Mediterranean

climate: the summer drought. The high temperatures

and low rainfall in summer lead to a reduced avail-

ability of soil water and a high evaporative atmo-

spheric demand during a large part of the potentially

active period. Under these conditions, the different

Forest Ecology and Management 187 (2004) 281–294

* Corresponding author. Tel.: þ34-23-294464;

fax: þ34-23-294515.

E-mail address: [email protected] (S. Mediavilla).

0378-1127/$ – see front matter # 2003 Elsevier B.V. All rights reserved.

doi:10.1016/j.foreco.2003.07.006

strategies in response to drought may to a large extent

contribute to explaining the differences in producti-

vity, competitive ability, and distribution patterns of

different species (Tretiach, 1993; Damesin et al.,

1998).

The many studies that have been conducted on

Mediterranean oaks (Larcher, 1960; Salleo and Lo

Gullo, 1990; Acherar and Rambal, 1992; Nardini et al.,

1999) reveal that although stomatal closure is a com-

mon response to drought stress, different species may

exhibit different levels of sensitivity and response

rates. In principle, the differences in the areas of

distribution of the two Quercus species studied here

suggest differences in their strategies for drought

resistance. Thus, although both species often coexist,

the evergreen Q. rotundifolia is predominant in the

interior zones of the Iberian Peninsula with a more

severe Mediterranean climate, whereas the deciduous

Q. faginea is more abundant in regions with a milder

or oceanic climate (Tutin et al., 1964). This suggests

a greater tolerance to drought in the evergreen

species with respect to the deciduous one. Neverthe-

less, unlike Quercus ilex subsp. ilex, a widely studied

Mediterranean oak (Larcher, 1960; Tretiach, 1993;

Damesin et al., 1998; Tognetti et al., 1998), no studies

have been carried out on the patterns of response to

drought stress in Q. rotundifolia and very few works

have investigated Q. faginea (Acherar and Rambal,

1992). An initial aim of our study was thus to analyse

the sensitivity of stomatal response to soil and atmo-

spheric drying in co-occurring mature specimens of

these two species growing under natural conditions

in order to characterise their water-use strategies

and to test for possible differences in their tolerance.

We predicted that in order to compensate for shorter

leaf longevity, the deciduous species should show

greater stomatal conductance and less sensitivity to

soil and atmospheric water stress than the evergreen

species, whereas the latter, with a longer leaf long-

evity, should maintain lower stomatal conductance in

order to reduce the risk of desiccation and the loss of

leaves under drought conditions (Bond and Kavanagh,

1999).

Some authors (Davis, 1989), however, have sug-

gested that it is particularly important to study differ-

ences in adaptability to water stress at the seedling

stage because of the high mortality rate observed in

the seedling establishment phase. Indeed, seedling

establishment and juvenile growth are critical periods

in the life cycles of tree species (Kozlowski et al.,

1991), and morphological and physiological attributes

during these periods are key factors for the recruitment

and survival of the tree populations (Donovan and

Ehleringer, 1991, 1992; Ashton and Larson, 1996;

Ashton et al., 1999). We therefore also analysed

adaptability to drought during the first developmental

stages of the two study species to test whether the

stomatal strategies of both species differ between the

different stages of its life cycle. Although age-related

changes in water relations have been analysed for

several woody species (Donovan and Ehleringer,

1991, 1992; Bond, 2000; Kolb and Stone, 2000), there

are few works that have investigated the water-use

behaviour of seedlings and mature specimens of oaks

growing in the field under similar conditions (Bragg

et al., 1993; Donovan and Pappert, 1998; Cavender-

Bares and Bazzaz, 2000). Also, to our knowledge no

studies have been carried out in Mediterranean cli-

mates, where the summer drought exacerbates the

effect of the differences between both stages in aspects

such as the degree of development of the root system.

Seedlings have smaller and shallower root systems

than mature trees and occur in soil layers most sus-

ceptible to soil drying from transpiration of neighbour-

ing trees and of the herbaceous layer (Weltzin and

McPherson, 1997; Cavender-Bares and Bazzaz,

2000). By contrast, the roots of mature trees can

penetrate into deeper soil layers, where water may

be more abundant. This should allow mature trees to

have higher stomatal conductance and consequently

higher rates of transpiration and photosynthesis than

smaller trees, as observed by some authors (Donovan

and Ehleringer, 1991; Cavender-Bares and Bazzaz,

2000). However, under the competitive conditions that

the seedlings undergo during their establishment, a

greater stomatal conductance and photosynthetic rates

would be a more successful strategy, providing the

growth potential they need for establishment. In fact,

in a previous study of the same species carried out at

the same sites of the present study, seedlings exhibited

a much greater stomatal conductance than adult trees

during the well-watered part of the growth season

(Mediavilla and Escudero, in press). Many authors

have obtained the same results on studying changes in

leaf gas-exchange rates during tree maturation (Yoder

et al., 1994; Kolb and Stone, 2000). Accordingly, our

282 S. Mediavilla, A. Escudero / Forest Ecology and Management 187 (2004) 281–294

second hypothesis is that the leaves of seedlings

in Mediterranean climates should maintain a less

conservative water-use strategy than adults owing to

the need to compete for soil water reserves with the

herbaceous layer, and that this should contribute to

reducing interspecific differences in stomatal response

to drought at the seedling stage.

2. Materials and methods

2.1. Study species and area

The species selected—the evergreen Q. rotundifolia

Lam. (¼Q. ilex L. subsp. ballota (Desf.) Samp.) and

the deciduous Q. faginea Lam.—were studied over 2

years (1996–1997) in four plots located close to the

city of Salamanca, in central–western Spain, between

latitudes 418100N and 408500N and longitudes

between 68350W and 58400W. Both species were pre-

sent at each of the selected plots.

Two plots, consisting of sparse populations of iso-

lated mature trees over 100 years old, with open

pasture areas among them, were selected (see sample

trees characteristics in Table 1). These savannah-like

formations (‘‘dehesas’’) are very common in the

western part of the Iberian Peninsula. The seedlings

were studied in two other plots, which were planted

in November 1994, so that the seedlings had likely

recovered from plantation stress when the study was

carried out. Before planting, the seedlings had been

grown in a nursery during their first growth season,

so our study was conducted during their third and

fourth years of life. The acorns used to propagate the

seedlings had been obtained from areas nearby the

study stands. The two planted plots were frequently

subjected to weed control, which greatly reduced the

density of the herbaceous layer.

The climate characteristics (precipitation and tem-

perature) were fairly homogeneous for the whole of the

study area (Table 2). The climate is cold Mediterranean,

with cold wet winters and a period of summer drought

that occurs each year. The soils, which were dystric

cambisols in all cases, were poor in organic matter and

in nutrient content, having a low pH and medium/low

water retention capacity (Dorronsoro, 1992). The sites

could be classified in two types depending on the

and and clay contents, but both growth stages were

represented in the two soil types (Table 2).

2.2. Leaf gas-exchange measurements

Gas-exchange measurements were carried out

weekly in the field during the spring and summer

(April–August) of 2 years of study (1996–1997) and

throughout the diurnal period on each selected sam-

pling date. Measurements were taken on completely

expanded leaves receiving full sunlight during the

measurement period. In adult trees, on each sampling

date the leaves were selected at mid height in the

canopy from three specimens of each species at each

of the plots. In seedlings, however, the number of

specimens selected from each species in each plot was

increased to 6, owing to the greater variability among

individuals observed during this growth stage. In the

evergreen species, the two most abundant age classes

in the crown (current-year leaves, C, and the leaves

produced the year before, C þ 1) were studied.

Measurements were taken with a portable photosyn-

thesis system (LI-6200, Li-Cor Inc., Lincoln, NE, USA)

at ambient CO2 concentration (around 360 ml l�1)

and saturating photosynthetic photon flux density.

Table 1

Sample trees characteristicsa

Species Age class Trees (ha�1) H (m) D (cm) LAI* LAI**

Q. rotundifolia Mature trees �50 8.35 � 0.50 46 � 3.86 2.46 � 0.29 0.7380

Q. faginea Mature trees �50 8.55 � 0.41 40 � 3.83 1.95 � 0.06 0.5850

Q. rotundifolia Seedlings �1000 0.51 � 0.03 0.55 � 0.02 0.74 � 0.08 0.0061

Q. faginea Seedlings �1000 0.56 � 0.02 0.85 � 0.03 0.39 � 0.04 0.0189

a Sample tree characteristics are based on the mean (�S.E.) of 10 sample trees for each study area and for each species. D: stem diameter

at breast height (base of crown for the seedlings); H: average height; LAI*: leaf area index below the crown of the sample trees; LAI**: stand

level leaf area index.

S. Mediavilla, A. Escudero / Forest Ecology and Management 187 (2004) 281–294 283

Each measurement was completed within approxi-

mately 60 s to prevent excessive rise in leaf temperature.

Gas-exchange parameters were calculated according

to Caemmerer and Farquhar (1981). Besides the

necessary information for estimating transpiration

rates (E) and stomatal conductance to water vapour

(g), the measuring instrument provided air and leaf

temperature and leaf-to-air vapour pressure deficit

(VPD). Although gas-exchange rates measured inside

a cuvette obviously differ from true gas-exchange

rates due to the increased boundary layer conductance

in the cuvette, our estimates of transpiration rates still

may be used for comparative purposes. Immediately

after the gas-exchange measurements, each leaf was

harvested, taken to the laboratory, and the area of each

leaf was determined with a Delta-T Image Analysis

System (Delta-T Devices Ltd., Cambridge, UK).

Because of strong stomatal closure at midday (data

not shown), a large part of total assimilation takes

place during the morning. For this reason, we com-

pared the different species by calculating the max-

imum absolute values (gmax) and maximum daily

values of stomatal conductance (gday) reached by each

species, and the corresponding transpiration rates

(Emax and Eday). To reduce the effects of sampling

error, we took the average of the 10 highest values of

stomatal conductance and transpiration rates over the

entire study period as estimates of gmax and Emax,

respectively. Likewise, the maximum daily stomatal

conductance and transpiration rate were defined as the

average of the two or three highest measurements of g

and E, respectively, made under ambient field condi-

tions at light saturation on each measurement day.

2.3. Leaf water potential measurements

On all sampling dates, predawn water potentials

(Cpd) were measured as an estimate of plant water

availability (Schulze and Hall, 1982) using a pressure

chamber (PMS Instruments Co., Mod. 1002, Corvallis,

OR, USA) in twigs sampled from the same specimens

and from the same position in the crown as those used

for the gas-exchange measurements. However, in the

seedlings, owing to their small size, it was necessary

on some occasions to sample different individuals in

order to reduce the damage. The seasonal variation in

predawn water potential was estimated as the difference

between the mean maximum and mean minimum

predawn water potentials of the 2 years of study. In

addition, midday leaf water potentials (Cmidday),

assumed to be the minimum diurnal leaf water potential

value, were also measured on some days. Then,DCwas

calculated as the difference between Cpd and Cmidday.

2.4. Data analysis

Boundary line analysis was used to study the rela-

tionships between gday andCpd and between g and VPD.

Table 2

Sites characteristics

Characteristics Plot A Plot B Plot C Plot D

Age class Mature trees Seedlings Mature trees Seedlings

Elevation a.s.l. (m) 850 900 900 750

Climatea

Average annual temperature (8C) 11.52 12.50 12.78 13.25

Average July–August temperature (8C) 21.25 22.48 21.72 23.73

Average December–January temperature (8C) 4.12 5.28 4.45 5.52

Average annual precipitation (mm) 465 445 507 568

Average summer precipitation (mm) 92 88 96 98

Soilb

Sand content (%) 54.9 54.5 63.7 66.3

Clay content (%) 24.1 32.1 17.3 17.5

Silt content (%) 21.4 13.9 19.0 16.3

Available water capacity (%) 10.3 11.6 9.12 8.53

a Climatic data are provided by the National Institute of Meteorology (Valladolid Centre).b Details of soil characteristics are given by Dorronsoro (1992).


The data were pooled over the 2 years of the study

after checking that there were no significant differ-

ences among years for any of the variables (data not

shown). Following Chambers et al. (1985), all values

for stomatal conductance and the independent variable

in each case were plotted and then a line was drawn

above all the uppermost points. Thus, it was assumed

that all values below this line were the result of other

controlling variables.

Two-way analysis of variance (using species and

growth stages as sources of variation) and Fisher’s

protected LSD test were used to establish significant

differences at P � 0:05 between means after applying

the Levene test to check for homogeneity of variances.

The SPSS statistical package (SPSS Inc., Chicago, IL)

was used for data analysis.

3. Results

3.1. Leaf gas-exchange: interspecific comparisons

and age-related differences

Among the adult specimens, the evergreen species

showed significantly lower maximum stomatal con-

ductance and transpiration rates in the old leaf cohort

than in the current-year leaves and in both cases these

were lower than in Q. faginea (Fig. 1).

The seedlings showed a much higher mean maxi-

mum stomatal conductance and transpiration rate

than the adult trees of the same species (Fig. 1). The

differences between seedlings and adults were espe-

cially marked for gmax and Emax. Q. rotundifolia was

the species showing the greatest differences between

Fig. 1. Mean (�S.E., n � 50 for mature trees and n � 85 for seedlings) maximum daily stomatal conductance (gday) and transpiration rate

(Eday), and mean (�S.E., n ¼ 10) absolute maximum stomatal conductance (gmax) and transpiration rate (Emax) in mature trees (dotted bars)

and in seedlings (open bars). For each leaf type different numbers within the bars indicate significant differences (P < 0:05) between adults

and seedlings. Dotted bars with different uppercase letters above indicate significant differences (P < 0:05) among leaf types for mature trees.

Open bars with different lowercase letters above indicate significant differences (P < 0:05) among leaf types for seedlings. In Q. rotundifolia

current (C) and 1-year-old (C þ 1) leaves are included.


growth stages. Thus, for example, average gday and

gmax values in current-year leaves of mature Q. rotun-

difolia trees represented only 49 and 38%, respectively,

of the values reached by the leaves of the seedlings,

while in Q. faginea the percentages rose to 72 and 63%,

respectively (Fig. 1). Thus, the differences in stomatal

conductance and transpiration rate between Q. rotun-

difolia and Q. faginea and between the C and C þ 1

leaves of Q. rotundifolia almost disappeared in the

seedlings. The results of all the comparisons were the

same in both years (data not shown).

3.2. Leaf water potential: seasonal patterns

In all cases Cpd underwent a strong decrease during

the growth season (Table 3). However, some differences

among species and among growth stages occurred. Both

among mature trees and among seedlings, Q. faginea

showed slightly lower mean predawn water potentials

than those of Q. rotundifolia in both spring and summer

(Table 3), although the differences did not reach sig-

nificance (data not shown). Q. faginea also showed

lower absolute minimum predawn water potentials

than the evergreen species, both among seedlings

(�2.84 MPa for Q. faginea and �2.67 MPa for Q.

rotundifolia) and especially in the case of mature trees

(�3.88 and �3.05 MPa, respectively). Consequently,

the seasonal range of Cpd was wider in the deciduous

species than in Q. rotundifolia, although the inter-

specific differences were hardly appreciable in the

seedlings (Table 3).

In both species, Cpd was on average always lower in

the adult trees than in the seedlings (Table 3), although

the differences between both growth stages were only

significant for the summer (data not shown). By con-

trast, the seedlings reached much lower midday leaf

water potentials than the adults both in spring and in

summer (Table 3). The seasonal range of Cpd was

greater in adult trees than in seedlings, especially for

Q. faginea (Table 3).

3.3. Stomatal responses to Cpd: boundary line

analysis

The boundary line plot revealed a non-linear rela-

tionship between gday and Cpd for all species, with

steeper decreases in stomatal conductance at high Cpd,

and more gradual ones for lower Cpd values (Fig. 2).

However, there seemed to be differences in the inten-

sity of the response of gday to water stress between

species and between the two growth stages. Among

the mature trees, maximum stomatal conductances

were higher in Q. faginea than in Q. rotundifolia

for a high Cpd. Despite this, the decrease in g with

Cpd was more marked in Q. faginea. Thus, the differ-

ences between maximum stomatal conductances of

both species were more reduced for Cpd values lower

than approximately �1.6 MPa (Fig. 2).

The seedlings showed much higher maximum sto-

matal conductance values than the adult trees for all

values of Cpd (Fig. 2). Furthermore, although maxi-

mum conductances decreased with Cpd, the decrease

was never as marked as in adult trees. For example,

among the adult trees the decrease in the maximum

stomatal conductance values associated with a

decrease in Cpd from �0.5 to �2.5 MPa was approxi-

mately 70 and 77% for Q. rotundifolia and Q. faginea,

respectively. By contrast, in the seedlings conductances

were only reduced by 60% in both species (Fig. 2).

3.4. Relationship between Cpd and DC

In both species and in the two growth stages the

relationship between Cpd and DC was linear, positive

Table 3

Mean � S:E: (n in parentheses) predawn leaf water potential values (Cpd, MPa) throughout the growth season over two study years, and

seasonal range of Cpd and mean � S:E (n in parentheses) minimum diurnal leaf water potentials (Cmidday) on some selected days during spring

and summer over two study years

Species Age class Cpd spring Cpd summer Cpd seasonal

range

Cmidday spring Cmidday summer

Q. rotundifolia Mature trees �0.86 � 0.06 (22) �2.28 � 0.12 (22) 2.64 �2.13 � 0.05 (14) �2.41 � 0.06 (15)

Q. faginea Mature trees �0.89 � 0.07 (21) �2.47 � 0.16 (22) 3.58 �2.46 � 0.03 (14) �2.63 � 0.03 (15)

Q. rotundifolia Seedlings �0.69 � 0.07 (48) �1.33 � 0.08 (46) 2.46 �3.35 � 0.09 (38) �3.55 � 0.07 (38)

Q. faginea Seedlings �0.76 � 0.07 (47) �1.45 � 0.08 (48) 2.68 �3.40 � 0.08 (38) �3.42 � 0.07 (39)


and highly significant (Fig. 3). For both species, the

regression slopes were steeper and close to unity in the

seedlings (Fig. 3) and the differences between adult

trees and seedlings were significant according to an

analysis of covariance (data not shown).

The values of the X-intercepts of the regression line

between Cpd and DC differed between the mature

trees and the seedlings. Thus, among adults the inter-

cept was obtained at a lower Cpd value in Q. faginea

(�3.81 MPa) than in Q. rotundifolia (�3.20 MPa).

The intercept value was very similar for adults and

seedlings in Q. faginea. However, in Q. rotundifolia

seedlings the intercept of the regression line corre-

sponded to a lower Cpd than in adults. Thus, among

the seedlings the intercept value for Q. faginea

(�3.70 MPa) was very similar to that of the evergreen

species (�3.74 MPa).

3.5. Stomatal response to leaf-to-air VPD:

boundary line analysis

In all cases, boundary line analysis revealed that

the maximum conductances remained close to maxi-

mum values for each species at low and moderate

VPD values and strongly decreased after a VPD

threshold was reached (Fig. 4). However, some dif-

ferences were also observed between species and

growth stages. Among the mature trees, the two leaf

age classes of Q. rotundifolia showed a considerably

lower VPD threshold for inducing stomatal closure

Fig. 2. Relationships between predawn leaf water potential (Cpd) and maximum daily stomatal conductance (gday). The boundary line is taken

to indicate maximum gday at a given level of Cpd.


(�20 Pa kPa�1) than that of the deciduous species

(�35–40 Pa kPa�1) (Fig. 4). For seedlings, however,

the critical levels of VPD for stomatal closure tended

to be similar or slightly higher for Q. rotundifolia

(�40–45 Pa kPa�1) in comparison with Q. faginea

(�35–40 Pa kPa�1) (Fig. 4). Accordingly, in the case

of the evergreen species, stomatal closure also occurred

in the seedlings for a much higher critical VPD value

than that of mature trees. The results of all the com-

parisons, both at interspecific level and between growth

stages, were the same in all years, regardless of the

conditions of each particular year (data not shown).

4. Discussion

4.1. Mature trees: Q. rotundifolia versus Q. faginea

Q. rotundifolia and Q. faginea adopted clearly

different stomatal strategies for drought resistance,

despite the fact that they coexist in broad areas and

are hence subject to similar conditions. Q. rotundifolia

showed lower maximum stomatal conductances than

the deciduous species (Fig. 1) and greater stomatal

sensitivity to drought, especially to the increase in the

atmospheric evaporative demand (Fig. 4). Moreover,

Cpd was never reduced in Q. rotundifolia to minimum

values as low as those found for Q. faginea and,

therefore, Q. rotundifolia did not undergo such a high

seasonal variation in Cpd as that seen in the deciduous

species (Table 3). A greater seasonal range of Cpd in

Q. faginea is probably the result of a less intense

stomatal response to decreases in Cpd, although to a

certain extent the predawn water potential also

depends on the root capacity of each species. The

greater stomatal conductances of the deciduous spe-

cies would lead to a more rapid consumption of soil

water reserves and hence additional decreases in Cpd

and a greater seasonal variation in the predawn water

potential.

Fig. 3. Relationships between predawn leaf water potential (Cpd) and the difference between predawn and minimum daily leaf water

potential (DC).


Additionally, the variation along the day in the leaf

water potential supports the assumption of a lower

degree of stomatal control in Q. faginea. Thus, the

X-intercept of the regression line between Cpd and DCcorresponded to a lower Cpd in Q. faginea than in

Q. rotundifolia (Fig. 3). The X-intercept of the regres-

sion line between Cpd and DC can be interpreted as

an estimate of the theoretical value of Cpd for which

stomatal conductance would approach zero, as a

result of total stomatal closure (Rambal, 1992). This

minimum value of C possibly represents the water

potential below which a loss of turgor in the leaves

and irreversible damage to the photosynthetic appa-

ratus would occur (Damesin and Rambal, 1995).

Accordingly, a lower X-intercept would indicate that

total stomatal closure requires a lower potential in

Q. faginea and that this species is therefore able

to maintain its stomata open with more reduced

Fig. 4. Relationships between leaf-to-air VPD and g. The boundary line is taken to indicate maximum g at a given level of VPD.


Cpd values than Q. rotundifolia. Probably for this

reason, Q. faginea reached lower mean Cmidday than

Q. rotundifolia (Table 3).

All our results thus suggested a higher stomatal

sensitivity in Q. rotundifolia trees and hence a more

conservative water-use strategy than the deciduous

species. Since the strategy in Q. rotundifolia seems

to be designed to delay desiccation through an extre-

mely conservative water-use, the evergreen species

could be classified as a drought-avoiding water-saving

species (sensu Levitt, 1972). However, in comparative

terms, Q. faginea could be classified as a water-

spending plant (sensu Levitt, 1972) because this spe-

cies was more able to tolerate atmospheric and soil

water stress than Q. rotundifolia and showed the least

conservative water-use strategy. It is usually assumed

that water-spending plants have better access to soil

water thanks to a more developed root system (Levitt,

1972). In fact, although Q. faginea showed the greatest

daily maximum conductances and transpiration rates

(Fig. 1), in this species Cpd did not undergo as marked

a decrease in summer with respect to spring, as would

be expected from its transpiration rates. Thus, whereas

Eday in Q. faginea was on average 22% higher than in

Q. rotundifolia (Fig. 1), the predawn water potential

for Q. faginea during the summer was on average only

7% lower than in Q. rotundifolia (Table 3). This lower-

than-expected decrease in Cpd for Q. faginea could be

due to a greater soil water availability for the decid-

uous species. However, the smaller LAI of Q. faginea

(Table 1) would also contribute, evidently, to reducing

the total transpiration rate per unit ground surface.

Although the ecophysiological responses of woody

plants to water stress and their repercussions on the

competitive success of the different species in differ-

ent environments have been addressed in many stu-

dies, the results obtained by different authors are very

different. Unlike what was observed in the present

study, several authors have concluded that many spe-

cies typical of dry zones adopt a tolerant strategy

against stress in the sense of being able to maintain

an active gas exchange under conditions of low water

availability (Abrams et al., 1990; De Lucia and Schle-

singer, 1991; Prior et al., 1997). However, other

authors have reached different conclusions on study-

ing, like us, the strategies employed against drought in

different Quercus species from Mediterranean envir-

onments. Aussenac and Valette (1982) and Damesin

et al. (1998), for example, found similar responses

to limited water conditions in adult specimens of

the evergreen Q. ilex subsp. ilex and the deciduous

Q. pubescens and thus proposed similar drought tol-

erances in both species. Despite this, many other

studies have concluded, also in Mediterranean oaks,

that the species most typical of dry environments tend

to show a more conservative behaviour (Larcher,

1960; Tretiach, 1993; Nardini et al., 1999), and hence

their results are similar to our own.

Tretiach (1993) even interpreted the strong stomatal

sensitivity of Q. ilex subsp. ilex as a manifestation of

the low capacity of this species to inhabit more arid

areas of the Mediterranean climate and, according to

this author, this species is limited to areas with a

subhumid and cool climate. However, this cannot

be applied to the similar Q. rotundifolia, which covers

somewhat arid areas in the centre of the Iberian

Peninsula, replacing the deciduous oaks there, which

are restricted to more rainy areas and, normally, to

greater altitude. The fact that at the sites where both

species coexist Q. rotundifolia continues to show a

clearly conservative behaviour suggests that this strat-

egy is efficient for guaranteeing competitive success in

environments subject to strong water stress.

4.2. Q. rotundifolia and Q. faginea: mature trees

versus seedlings

As has been observed by other authors in different

woody species (Donovan and Ehleringer, 1991; Dono-

van and Pappert, 1998; Cavender-Bares and Bazzaz,

2000), here we observed important physiological dif-

ferences between growth stages that seem to suggest

developmental shifts in water-use strategies. The seed-

lings showed a clearly lower stomatal sensitivity to the

enhancement of the soil and atmospheric water deficit

than the adult trees (Figs. 2–4) and therefore much

higher maximum conductances than the adults (Fig. 1),

regardless of the intensity of the drought stress. The

lower stomatal sensitivity of the seedlings probably

leads to a less rigorous control of transpiration and

hence a less conservative water-use strategy in seed-

lings with respect to adults, such that seedlings could

also be encompassed within the spender strategy class

proposed by Levitt (1972). However, the seedlings

showed higher predawn water potentials than the

mature trees (Table 3), despite the similarity of the


soil and climate characteristics between the sites stu-

died (Table 2). The main reason for this was the

extremely low LAI of the seedling stands and the

virtual absence of herbaceous competitors in them,

which would have allowed soil water to be conserved

for longer. However, differences in height between

seedlings and adults could also lead to differences

in soil-to-leaf hydraulic conductance between both

stages. If hydraulic conductance changes with transport

distance, the difference in height between seedlings

and adults would be reflected in a lower hydraulic

conductance in mature trees as compared to seedlings

(Yoder et al., 1994; Mencuccini and Grace, 1996a,b;

Ryan and Yoder, 1997; Hubbard et al., 1999). The low

hydraulic conductance of adults may limit overnight

equilibration of leaves with the water potential of the

soil (Kolb and Stone, 2000).

However, although differences in the stomatal

responses to drought between growth stages were

observed in the two species studied, the extent of

the differences varied between Q. rotundifolia and

Q. faginea. Q. rotundifolia was the species for which

the most pronounced differences between growth

stages were observed. Thus, the seedlings of this

species showed much higher stomatal conductances

and transpiration rates than the adults of the same

species (Fig. 1). The maximum conductance observed

in Q. rotundifolia seedlings reached values that can

even be considered quite high for xeromorphic leaves

(Korner, 1995). Nevertheless, similar high conduc-

tances were also obtained by Acherar and Rambal

(1992) in 2-year-old seedlings of the evergreen Q. ilex

subsp. ilex and by other authors for other oak species

also at young stages (Crunkilton et al., 1992; Bragg

et al., 1993). However, also in Q. rotundifolia, the

greatest differences were observed in stomatal sensi-

tivity to drought between adults and seedlings, such

that in contrast to an extremely conservative water-use

in adults the seedlings do not seem to show any

‘‘prudence’’ in water expenditure. The consequence

of these more marked shifts between growth stages in

Q. rotundifolia than in Q. faginea is that in the

seedlings the interspecific differences disappear and

both species show a common strategy in water-use.

One problem associated with the approach used in

this study is the possible genetic differences between

mature specimens and the seedlings used in the affor-

estation, which could evidently contribute to the

physiological differences between growth stages, as

well as the differences between the sites. This limits

the validity of the comparisons between growth stages.

However, the differences observed in the present study

between the two growth stages were so large that it is

unlikely that they arose only from the above-men-

tioned possible artefacts. Within a growth stage, the

differences in stomatal behaviour between different

stands were not significant (data not shown) and the

interspecific comparisons yielded similar results in all

stands, although the site characteristics differed more

within a growth stage than between growth stages

(Table 2). Similarly, there is no reason to expect that

genetic distance between the seedlings and the adults

would be higher than between the adult specimens

studied in the different sites.

The reduction in stomatal conductance with tree age

in many species has been explained as the conse-

quence of a decrease in hydraulic conductance as

the xylem path length increases (Ryan and Yoder,

1997). Although this size-related reduction in hydrau-

lic conductance has been reported on several occa-

sions (Mencuccini and Grace, 1996b; McDowell et al.,

2002), the hydraulic limitation hypothesis has also

been criticised (Becker et al., 2000). The main reason

for such criticism is that if hydraulic conductance

limits stomatal conductance, it may be expected that

several compensatory mechanisms would have

evolved to minimise size-related constraints on leaf

gas-exchange (McDowell et al., 2002). However,

although the existence of compensatory mechanisms

that increase hydraulic conductivity has been demon-

strated (Mencuccini et al., 1997), these are not usually

efficient enough to fully compensate the decrease in

hydraulic conductance associated with the increase

in path length (Mencuccini and Grace, 1996b). One

of the most efficient compensatory responses would

be increasing the water potential difference (DC)

between soil and leaf (McDowell et al., 2002). If

the stomatal conductance of mature trees is indeed

limited by a low water supply, it should be expected

that a greater water potential difference would develop

to compensate for the low hydraulic conductance of

the xylem. In contrast, DC values at midday for a

given Cpd were actually lower in mature trees than

in seedlings (Fig. 3), suggesting that other factors

would limit stomatal conductance and transpiration

rates in mature trees.


We believe that the stomatal behaviour of seedlings

can be better interpreted in the light of the selective

pressures that operate at the seedling stage as a con-

sequence of the intense competition that seedlings

usually have to cope with during the early stages after

planting, especially from the herbaceous vegetation

(Bragg et al., 1993). Although in the stands planted

with seedlings the herbaceous layer had been almost

completely removed, stomatal behaviour is probably

genetically programmed (Bond, 2000), and the seed-

lings have a fixed behaviour, although under these

experimental conditions they do not really undergo

competition. It has been shown that herbaceous vege-

tation reduces the soil moisture available for woody

plants in a large variety of ecosystems (Collet et al.,

1996; Davis et al., 1999). Oaks have rapid root growth

and produce deep taproots as seedlings (Danner and

Knapp, 2001). However, it may take some years to

pass the maximum penetration depth of the roots of the

herbaceous plants and thus reduce their competitive

effects (Bragg et al., 1993; Weltzin and McPherson,

1997; Mediavilla, personal observation). Maintaining

high water-use efficiency through strong stomatal

control would, in principle, imply lower net CO2

assimilation rates and, therefore, a reduction in

growth. Furthermore, conservation of the soil water

reserves would increase the availability of soil water

for potential competitors (De Lucia and Schlesinger,

1991). A low stomatal sensitivity to drought in benefit

of an increase in growth would probably be a more

successful strategy under the competitive conditions

that the seedlings experience during their establish-

ment (De Lucia et al., 1988). In semiarid environments

tree populations usually become more open as the

trees grow up, resulting in open woodlands, such as the

typical Spanish ‘‘dehesas’’. The large distances

between individuals contribute to considerably redu-

cing the competition for water in these environments

at the mature stage. Under these conditions, saving

water reserves by means of low stomatal conductances

would contribute to increasing the length of the grow-

ing season (Bond and Kavanagh, 1999), and to main-

taining relatively high leaf water potentials, thereby

reducing the risk of xylem cavitation. If this inter-

pretation is correct, it would explain why mature trees

do not exhibit compensatory mechanisms as a

response to the increase in hydraulic resistance as

trees grow in height. The decrease in hydraulic con-

ductance would be compensated by a parallel decrease

in stomatal conductance, thus maintaining the water

status of the leaves.

One risk of the water spending strategy of the

seedlings is the probable increase in mortality during

drought periods, owing to the depletion of the soil

water reserves. Minimum leaf water potentials in the

seedlings were low and similar to the values found by

other authors for significant losses of hydraulic con-

ductivity (Tognetti et al., 1998; Bond and Kavanagh,

1999). Furthermore, midday water potentials were

almost constant on every day throughout the growth

season (Table 3), as could be also inferred from the

values of the regression slopes between Cpd and DC(Fig. 3). A regression slope close to unity for the

seedlings indicates that there is no relationship

between Cpd and Cmidday because the plants tend to

reach the same minimum daily value of C regardless

of the availability of soil water. These results suggest

that at the seedling stage the two species seem to

operate routinely near the point of catastrophic xylem

failure (Tyree and Sperry, 1988). However, the mor-

tality costs associated with the strategy adopted by the

seedlings may be considered relatively small for the

fitness of the species, given that a mature tree is

capable of producing an enormous number of seeds

throughout its life and thus of replacing dead seed-

lings. The pattern of water-use of the seedlings would

thus be another manifestation of the compromises in

the life cycle between the allocation of resources to

growth or to survival (Crawley, 1986).

Acknowledgements

This paper has received financial support from the

Spanish Ministry of Education (Project nos. FOR89-

0845 and AMB95-0800) and from the Junta de Castilla

y Leon (Project nos. SA47/95 and SA72/00B).

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Stomatal responses to drought of mature trees and seedlings of two co-occurring Mediterranean oaks