Impact of stand density on water status and leaf gas exchange in Quercus ilex

Forest Ecology and Management 254 (2008) 74–84 www.elsevier.com/locate/foreco

Impact of stand density on water status and leaf gas exchange in Quercus ilex Gerardo Moreno *, Elena Cubera Departamento de Biologı´a, Ingenierı´a Te´cnica Forestal, Universidad de Extremadura, Centro Universitario, Avenida Virgen del Puerto 2, 10600 Plasencia, Spain Received 5 December 2006; received in revised form 28 April 2007; accepted 29 July 2007

Abstract Tree thinning reduces tree-to-tree competition and likely contributes to the improvement of tree water status and productivity in water-limited systems. In this study, we examined the importance of competition for water among Quercus ilex trees in open woodlands by comparing the water consumption and physiological status of trees located along stand density gradients which ranged from 10% (low density; LD) to 100% (high density; HD) of canopy cover. The study was carried out at two sites which differed in mean annual rainfall (506 and 816 L m2; Dsite and Wsite, respectively). Predawn and midday leaf water potential (cd and cm, respectively) and CO2 assimilation rate (A) were measured every two weeks from mid May to mid September, in eight trees located along a stand density gradient at each site. Sap flow and soil moisture were measured only at Dsite. Sap flow was continuously recorded by sap flowmeters (constant heating method) installed in 12 trees along two stand density gradients. Soil moisture (U) was measured every 20 cm for the first meter and then every 50 cm up to 250 cm. Measurements were conducted in 18 soil profiles, 6 located in HD and 12 in LD (six beneath and six out the canopy). At Wsite, differences among stand densities for c and A were very small and emerged only at the end of the dry season. At Dsite, c (both predawn and midday), A, U, and sap flow density were significantly higher in LD trees than in HD ones. At Dsite, some water remained unused in the soil at the end of the dry season beyond the canopy in the LD areas, and trees did not experienced such an acute water deficit (cd > 1 MPa) as the HD trees did (cd < 3 MPa). Summer tree transpiration at the stand level (Estand) tended to saturate with the increase of canopy cover. Estand increases by 32% when canopy cover goes from 50% to 100%. Results confirmed that the increase of tree-to-tree competition with stand density was much more significant at dry sites. In these sites, tree thinning is recommended as a way to maintain tree functioning. # 2007 Elsevier B.V. All rights reserved. Keywords: Leaf water potential; Open woodland; Photosynthesis; Sap flow; Soil moisture

1. Introduction Evergreen forests dominated by the Holm oak (Quercus ilex L.) are one of the most important vegetation types in the Mediterranean basin (Terradas and Save´, 1992). In Spain, Q. ilex forests cover 25% of the total forested area (Terradas, 1999). Mediterranean evergreen oaks have been defined as ‘regulator’ species in terms of water use (Rambal, 1993), having three major mechanisms that contribute to drought resistance: stomatal control, deep rooting and reduced leaf area. This set of functional strategies enables these species to avoid or tolerate water stress. A strong stomatal control over water loss, preventing low water potentials, has been shown for

* Corresponding author. Tel.: +34 927 427000; fax: +34 927 425209. E-mail address: [email protected] (G. Moreno). 0378-1127/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.foreco.2007.07.029

Q. ilex (Sala and Tenhunen, 1996), but under extremely dry conditions, stomatal control can be insufficient to prevent extensive loss of hydraulic conductivity as a result of embolism (Martı´nez-Vilalta et al., 2003). Indeed, Q. ilex has been shown to be more sensitive to drought in comparison to other concurrent woody plants (Ogaya et al., 2003), and in dense forests, episodic Q. ilex diebacks have been detected during periods of severe drought (Pen˜uelas et al., 2001). These episodic diebacks could act as a self-thinning mechanism to adapt tree density to decreased soil water resources. In the Mediterranean basin, which is characterized by an acute summer drought, drought episodes may become more frequent in the present century due to climate change (IPCC, 2001). With this problem in mind, the forest manager must aim to improve tree condition. One of the main concerns is the effect of stand density on tree vitality, growth, and survival (Bre´da et al., 1995; Misson et al., 2003; Barton and Montagu, 2006;

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Ma¨kinen et al., 2006). Man-induced thinning can be used as a management tool to avoid strong intraspecific competition and to increase tree resistance to drought stress (Aussenac and Granier, 1988; Ducrey and Toth, 1992; Gracia et al., 1999; Misson et al., 2003). Gracia et al. (1999) showed that thinning treatments of Q. ilex help to overcome severe drought episodes. Evidence of the relationship between leaf area and evapotranspiration has been provided by several authors; for instance, Greenwood et al. (1985) for Eucaliptus sp. in Australia, and Gupta et al. (1998) for Prosopis cineraria in the arid zone of India. However, the subsequent establishment of grasses after thinning, which use soil water more rapidly than trees, could prevent remaining trees from benefiting from increased available soil water (Smit and Rethman, 2000). Dense forests have been traditionally thinned to increase the growth of native grasses throughout the world, and many of them have been progressively transformed into grazed open woodlands (silvopastoral systems; Etienne, 1996). Although forest thinning initially increases the availability of light for pasture, this clearance could have also important consequences for tree functioning, particularly in water-limited regions. This is presumably the case of Mediterranean open woodlands; for instance in Central Spain, Pulido and Dı´az (2005) reported that Q. ilex yields significantly more acorns in open woodlands than in dense ones (both at tree and stand levels). Man-made open woodlands occupy large areas in Mediterranean countries; for example, these woodlands cover more than 3 million ha in Spain and approximately 1.5 millions ha in Greece (Eichhorn et al., 2006). Mediterranean open woodlands, which in some cases are structurally similar to North-American and Central-African savannas (Joffre et al., 1999), are characterized by the coexistence of a continuous grass layer and a discontinuous tree layer, and display a high heterogeneity of vegetation structure. As in most savanna systems, they receive a low rainfall with high variability within and between years as well as a high evaporative demand during the summer. Indeed, water availability is one of the major ecological factors influencing either natural savannas (McPherson, 1997; Scholes and Archer, 1997) or man-made open woodlands (Infante et al., 2003; David et al., 2004). Joffre et al. (1999) showed that southern Iberian open woodlands (named dehesas in Spain) mimic the water dynamic of natural savannas. They found that mean tree density increased as rainfall increased (from 12 up to 40 trees ha1 with 450–500 and 750–800 L m2, respectively). However, dehesas and other European silvopastoral systems are facing several threats in the last several decades; for instance, the extensification process imposed by socioeconomic change has produced a gradual transformation of open woodlands into wooded shrublands and dense forests (Papanastasis, 2004). Indeed, tree encroachment in savannas is a worldwide phenomenon (Archer, 2003), creating a substantial uncontrolled fire hazard (Myers et al., 2004). These changes could be also having important consequences on soil water dynamics, tree water status, tree transpiration, and hence on tree functioning. In this context, predicting water balance in both natural and man-made savannas is necessary to assess the role of savanna in global production, to link vegetation cover and

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hydrology for water resource management, and to manage silvo-pastoral farming systems. Several authors have described the positive influence of trees on microclimate, soil physical parameters and soil water dynamics (see Moreno et al., 2007 for a revision), but little attention has been given to the implications of tree thinning on tree functioning and productivity of waterlimited open woodlands. The objectives of this work were to determine whether the decrease in stand density minimised the drought effect on the water status of trees, and to establish the importance of tree thinning on evergreen oak functioning in both semiarid and subhumid open woodlands. To achieve these objectives, leaf and whole plant responses to intraspecific water competition were compared in two contrasting Q. ilex stands of western Spain. Sites differed in mean annual rainfall (across-site variability). In addition to, at each site, a gradient of stand density (within-site variability) was studied, with canopy cover ranging from 10% to 100%. 2. Materials and methods 2.1. Study system The focus of this study is Spanish open oak woodlands which are known as dehesas. Dehesas occupy 3.1 million ha in the Iberian Peninsula (Diaz et al., 1997), and are considered to be the most extended agroforestry system in Europe (Eichhorn et al., 2006). Dehesas are in reality, a simplification of Mediterranean forests and shrublands in terms of their structure and woody species number. This is attained by reducing tree density, eliminating shrub cover, and favouring the grass layer by means of grazing and crop culture in long periods of rotation (Montero et al., 1998). As a result, most dehesas are characterised by a twolayered vegetation structure, with a savanna-like open tree layer (mostly oaks; 10–40 trees ha1) and an understory pasture or crop in the same land unit. Nevertheless, many dehesas have a mosaic-type structure, with a combination of grazed, shrubby, and cultivated open woodlands and dense forest plots. Typically, tree density ranges from 10 to 200 mature trees ha1 depending on its primary use (lower densities are found in intercropped areas and higher densities in areas reserved for hunting). 3. Study area The study was conducted in two contrasting Q. ilex dehesas located in the north of Extremadura, Central-Western Spain. In both sites the climate is typically Mediterranean, with hot, dry summers and cool, rainy winters. However, these sites differed in the amount of rainfall, with mean annual values of 506 and 816 L m2 and are therefore defined hereafter as Dsite (the dry site), and Wsite (the wet site). Both sites were flat or gently sloping areas with oligotrophic, acidic soils. The Dsite overlies tertiary sandy sediments whereas the Wsite overlies granite derived sands. The location and main climatic and edaphic characteristics of each site are shown in Table 1. Only Q. ilex was present as woody vegetation in both sites. Size of mature trees ranged from 7 to 12 m in canopy width and

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Table 1 Main climatic and edaphic characteristics in the sites studied Parameters

Sites Dry site (Dsite)

Localization Altitude (m a.s.l.) Slope (%) Mean annual rainfall (L m2) Mean summer rainfall (L m2) Mean annual temperature (8C) Mean of Tmina (8C) Mean of Tmaxa (8C) Soil type (FAO, 1998) Geology Soil depth (cm)

0

0

39842 N, 0814 W 380 0 506 50 16.2 3.4 35.6 Chromic Luvisol Sandstone >100

Wet site (Wsite) 40830 N, 0820 W 400 5 816 57 14.7 3 34.4 Distric Cambisol Granite >100

a

Tmin: daily minimum temperature in January. Tmax: daily maximum temperature in August.

5.9–10.5 m in height (ranging from 60 up to 150 years old). Each site included a stand density gradient ranging from very open (low density; LD) to dense (high density; HD) forest. The presence of juveniles increased in HD respect to LD (Fig. 1). Due to the presence of differently-sized trees, the stand density was defined as the percentage of ground covered by projected tree canopies (canopy cover). To determine canopy cover for each tree studied, the canopy width of every tree (mature or juvenile) located inside of a circle of 20-m radius around each target tree was measured. Expressed as canopy cover, stand density ranged from 10% (equivalent to 20 mature trees ha1; 8 m canopy width on average) to 100% (equivalent to 200 matures trees ha1) (Fig. 1).

Scholander chamber (Skye Instr., UK, model SKPM 1400). Each time, these measurements were made on two consecutives days, each site on a different day. Two measurements were taken each day: predawn potential before sunrise (Cd), and midday potential, which is assumed to be the minimal diurnal value (Cm). Eight trees per site, located along tree density gradients, were studied. Measurements were conducted on 3 current-year shoots per tree (around 1–2 mm in diameter and 2–4 leaves) immediately after excision. Sun-exposed shoots were excised from the outer portion of peripheral branches in the uppermost third of the canopy. 3.2. Net photosynthesis Net leaf photosynthesis (A, mmol CO2 m2 s1), leaf transpiration (Eleaf, mmol H2O m2 s1) and water use efficiency (WUE = A/Eleaf, mmol CO2 mmol1 H2O) of Q. ilex trees were measured by means of a portable differential infrared gas analyzer (IRGA model LCi, ADC BioScientific Ltd., UK) and a broadleaf chamber (area: 6.25 cm2). Measures were made on the same trees and days as C. Three sun-exposed current-year leaves per tree were measured from sunrise to sunset at intervals of around 2 h. Different leaves were selected during each sampling time. Approximately 4– 6 cm2 leaf area (1 leaf surface) was enclosed in the chamber. Leaves were harvested after each measurement to determine the leaf area, and then to recalculate A according to the amount of the leaf area that was included in the chamber. Measurements were conducted on clear or mostly clear days (PAR range = 104–2274 mmol m2 s1, with a mean value of 1593  420 S.D.).

3.1. Leaf water potential 3.3. Sap flow measurements Leaf water potential (C) was measured in Q. ilex trees bi-weekly, from May to September of 2004 by means of a

Fig. 1. Aerial view of one of the study sites (Dsite) showing the different measurement areas along a stand density gradient (from 10% to 100% of canopy cover).

Sap flow was measured at Dsite in 12 trees located in two stand density gradients, six trees per gradient. Sap flow density in individual trees was monitored at from the trunk at a height of 130-cm using radial sap flowmeters as described by Granier (1987). Each flowmeter comprised two cylindrical probes, 3 cm in length and 0.2 cm in diameter (each one containing a copper–constantan thermocouple) which were connected together in series. Both probes were surrounded by a constantan wire, but only the upper one was heated constantly by the Joule effect, while the lower one served as a thermal reference. Probes were inserted into the xylem, one above the other, at a distance of 10 cm (Granier, 1987). They were inserted radially into the stem after removing the bark to expose the outer surface of the sapwood. Probes were installed in March 2004, oriented to the north to avoid azimuthal effects, and probes and trunks were insulated with a 1-m2 piece of cork to minimize ambient temperature gradients. Temperature difference between both thermocouples was sampled every 5 s, and averaged and recorded every 30 min from March to September by two data loggers (model 21, Campbell Scientific Ltd., UK). The temperature difference between the probes was empirically related to sap flow density

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at time h (F d; L m2 s1), according to Granier (1987):  F d ¼ 0:119

DT m  DT h DT h

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independent of stand density (non-significant correlation; r = 0.28; p = 0.38; n = 12; Table 2). Tree transpiration (Etree) was considered equal to sap flow for daily totals, avoiding in such a way the complications introduced by tree capacitance (Oren and Pataki, 2001). Etree was expressed per unit of canopy-projected area (L m2 canopy d1). These values were converted to a ground area reference basis (Estand; L1 d1) when multiplied by the corresponding stand canopy cover.

1:231

where DTm = daily maximum thermal difference between the two probes and DTh = mean thermal difference measured at time h. This equation relates sap flow density to the thermal difference between the probes, corrected by the daily maximum difference. The correction allows us to take seasonal variation in the thermal properties of sapwood into account. With this approach zero flow is assumed at night, although this assumption is not always fulfilled, especially in some fast-growing species (e.g., Daley and Phillips, 2006). For Q. ilex, a very slow growing species, night-time transpiration has been shown negligible (Infante et al., 2001; Reichstein et al., 2003). Sap flow rate (L s1) was calculated for each measured tree as the product of F d (L m2 s1) and the sapwood crosssectional area (m2) at the measurement level. Sapwood crosssectional area was characterized qualitatively by dye perfusion. The dye safranine-O (5 g L1) was injected into the trunk (four orientations at 120 cm-height) on sunny days in spring 2007. Thirty to sixty minutes after injection, 5-cm-long cores were obtained with an increment corer 10, 20 and 50 cm over the point where the dye was injected, according to Infante et al. (2001). The radial extension of coloration (assumed as sapwood radius) was assessed visually in each core. Sapwood depth ranged between 27 and 38 mm (Table 2). For the three trees where sapwood depth was less than the 30-mm sensor length, F d was corrected according to Clearwater et al. (1999). When sapwood depth exceeded sensor length, F d was assumed to be uniform over the entire sapwood depth. This assumption was not assessed in this study, but it is based in the fact that diffuseporous species (the case of Q. ilex; Villar-Salvador et al., 1997) usually have an even or Gaussian radial pattern of F d (Clearwater et al., 1999). Indeed, David et al. (2004) comparing sap flow data for Q. ilex determined by the Granier and the heat pulse methods, showed that although the radial distribution of F d was non-uniform over the conductive area, the Granier method (probes 2-cm length with a sapwood radius of 4.8 cm) provided a good estimate for the average F d over the entire conductive area. Furthermore, the assumption was not critical for the objectives of our study because sapwood depth was

3.4. Soil moisture Time Domain Reflectometry (TDR) (Tektronic model 1502 C) was used to measure soil moisture (U) at the dry site within two contrasting areas: HD with a canopy cover close to 100%, and LD with a canopy cover close to 10%. In HD, 6 replicated profiles were monitored, while 12 replicated profiles were monitored in LD (6 profiles beneath the canopy, and 6 profiles at a 30-m distance from the tree). TDR-probes were installed at different depths, at intervals of 20 cm for the first meter and every 50 cm until a maximum depth of 250 cm (a total of 8 probes per profile). TDR-probes were installed during the spring of 2003 and probes were measured monthly from January 2004 to December 2005. TDR-probes were constructed manually according to Vicente et al. (2003). Each probe comprised two parallel rods made of stainless steel, 20 cm in length and sharpened at the tip to facilitate their introduction into the soil. Rod diameter was 0.6 cm and the separation between their axes was 3 cm. Probes (7-cm width) were placed vertically in the soil by drilling with a stainless steel soil column cylinder (10 cm diameter). During installation, efforts were made to ensure maximum contact between the rods and the undisturbed soil. After installing probes, the holes were carefully refilled with the original soil (layer by layer), trying to maintain the initial soil bulk density. The calibration curve of the TDR-probes was done in the laboratory with soil collected from the entire profiles of the experimental sites (Cubera et al., 2004). 3.5. Statistical analysis To conduct different comparisons of mean values by analysis of the variance (ANOVA), three levels of stand density were defined: low density (LD; 30% of canopy cover),

Table 2 Details of the trees used for sap flow measurements at dry site Tree

1 2 3 4 5 6

Gradient 1

Gradient 2

Cv (%)

DBH (cm)

Cw (m)

Sw depth (cm)

15 33 52 68 87 100

40 42 48 40 36 46

8.1 8.3 11.2 9.5 7.0 10.3

3.0 3.5 3.8 2.8 2.7 3

a

Cv (%)

DBH (cm)

Cw (m)

Sw depth (cm)a

10 25 54 66 83 100

52 41 45 38 46 47

12.0 9.3 10.1 7.4 11.2 10.4

3.4 2.9 3.2 3.7 3.1 3.0

Cv (canopy cover), DBH (diameter of trunk at 1.3 m height), Cw (canopy width), and Sw depth (sapwood depth). a Other authors have reported mean values of 2.0, 3.6 and 4.8 cm for Q. ilex (Infante et al., 2001, Martı´nez-Vilalta et al., 2003, and David et al., 2004, respectively).

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intermediate density (MD; 30–70%), and high density (HD; 70% of canopy cover). Comparison of mean values of C and A were conducted using two-way repeated-measures ANOVAs, with Cd, Cm and A, as dependent variables, site (Dsite and Wsite) and stand density (LD, MD and HD) as independent variables, and time (different days) as repeated measures. Daily mean values of WUE were compared by a three way ANOVA, including period (spring vs summer) as independent variable apart from site and stand density. Differences in soil water content (U) were tested with a twoway repeated-measures ANOVA, with U as dependent variable, and localisation (HD, LD beneath canopy, and LD out of the canopy) and soil depth, as independent variables. Data of different months were included as repeated measures. A two-way ANOVA was applied to detect significant variations of F d with season (spring (April–June) and summer (July–September)) and stand density (LD, MD, HD) as independent variables, and with monthly mean values of daily-integrated F d as dependent variable. In all cases, the data met the assumptions of normality and homoscedasticity, checked for by the Kolmogorov–Smirnov and Levene tests, respectively. The statistical difference between groups was determined by Tukey’s multiple-range test. Results were expressed in terms of the level of significance of the differences ( p < 0.05 indicating significance) and the degrees of freedom (d.f., hereafter). Additionally, to detect across-site differences in the relationships among Cd with A non-linear regressions were applied. Similarly, daily integrated Etree and Estand were regressed against canopy cover to determine the effects of stand density on tree water consumption at tree and stand levels, respectively. In all cases, results were expressed as coefficients of determination (R2), and sample size (n). For statistical analyses, the software STATISTICA v.6.0 (Statsoft, 2003) was used. 4. Results The seasonal trend of soil moisture (measured only n Dsite) was similar in HD to that in LD, both beneath and out of tree canopy (Fig. 2a). U decreased sharply during spring, and then decreased more slowly throughout the summer, when the herbaceous understory was already dry. Throughout the year, HD showed lower U values than LD ( p < 0.01; d.f. = 2 and 121). Differences in U values between these two areas were higher in the first 100 cm depth, although they were also significant throughout most of the soil profile (Fig. 2b). U also varied within LD area, with values slightly but significantly higher beyond than beneath tree canopy ( p = 0.013; Fig. 2a), especially in the deeper layers of the soil (significant Localisation  Depth interaction; p = 0.032; d.f. = 21 and 121; Fig. 2b). The shape and height of the daily sap flow density (F d) curves varied with stand density. LD trees showed a quasiGaussian curve throughout the summer, while HD trees showed quasi-plateau, even valley-shaped curves (Fig. 3a). Dailyintegrated F d values decreased during the drought period, but the decrease was more acute for HD than for LD trees Fig. 3b). Significant differences among LD and HD trees were found for

Fig. 2. (a) Seasonal evolution of average soil moisture (average values of the whole soil profile) in three different areas at dry site: dense forest (HD) and open woodland (LD) both beneath and out of the canopy cover. (b) Soil moisture profiles at the end of the dry season (October 2004) for the same three areas. Bars indicate standard deviation.

the summer period (mean values of 1250, 890 and 660 L m2 sapwood d1 in LD, MD and HD, respectively; p < 0.001; d.f. = 2 and 66), but not for the spring period (mean values of 1440, 1530 and 1410 L m2 sapwood d1 in LD, MD and HD, respectively; p > 0.17). At mid September, when U reached minimum values (Fig. 2), daily-integrated F d maintained very high values for trees located in very open areas (monthly mean value of 1290 L m2 sapwood d1 with 15% of canopy cover), while 3.5-fold lower values were found for trees located in dense areas (370 L m2 sapwood d1 with 100% of canopy cover; Fig. 3b). Differences among trees tended to disappear again with the first autumn rainfall. Mean daily transpiration of individual trees (Etree) measured by sap flowmeters decreased exponentially with increasing stand density. The decrease was more acute and significant in the summer period than in the spring (Fig. 4a). Conversely, tree transpiration of the whole stand (Estand) increased with increasing stand density (Fig. 4b). The increase was almost lineal in spring period, but in summer Estand tended to saturate at high stand density. From 0% to 50% of canopy cover, Estand increased in 5000 L ha1 d1, while from 50% to 100% of canopy cover, Estand increased only in 2300 L ha1 d1. Predawn leaf water potential (Cd) varied significantly across sites ( p < 0.001; d.f = 1 and 42), with lower values in Dsite than

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Fig. 3. Sap flow density of six Quercus ilex trees located along gradient 1 of stand density. Stand density varied from 15% to 100% of canopy cover. (a) Mean daily curve sap flow density of sunny days of dry season (21st June–21st September 2004). (b) Monthly variation of daily-integrated sap flow density from April to October 2004. Error bars indicate standard deviations.

Fig. 5. Leaf water potential predawn (a) and daily difference between Cd and Cm (b) as a function of both stand density (expressed as % of canopy cover) and site (wet site vs dry site). Values for early September (last days of the dry season) are shown (n = 12 in all cases).

in Wsite. Within-site variation of Cd resulted also significant ( p < 0.001; d.f. = 2 and 42), decreasing Cd with stand density. Nevertheless, the decrease was significant only in Dsite (significant Site  Stand density interaction; p < 0.001; d.f. = 2 and 42; Fig. 5a). An exponential decrease of Cd with stand density was found at Dsite but not at Wsite (R2 = 0.91 and 0.20, respectively; n = 8; Fig. 5a). Similar results were found when the daily differences between Cd and Cm were plotted, and showed a very significant relationship at Dsite and a poor relationship at Wsite (R2 = 0.94 and 0.09, respectively; n = 8; Fig. 5b).

Within-site differences of Cd increased during the summer (significant Stand density  Day interaction; p < 0.001; d.f. = 20 and 252; Fig. 6). At Wsite, within-site differences were significant only at the end of the dry season (Cd = 0.8 and 1.8 MPa in LD and HD, respectively). In contrast at Dsite, within-site differences were significant from the beginning of the dry season, reaching a difference in Cd of nearly 3 MPa between LD and HD trees at the end of the dry season. LD trees maintained high Cd throughout the summer drought (Cd > 1 MPa), while HD trees reached values of nearly 4 MPa (Fig. 6).

Fig. 4. Effect of stand density (% of canopy cover) on daily tree transpiration estimated from sap flow density measures. Data express daily mean values of two 3months periods (spring and summer) at both tree (a) and stand levels (b). All cases with n = 12.

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Fig. 6. Temporal changes in leaf water potential (C; left) and CO2 assimilation rates (A; right) of Quercus ilex. Trees were located at the two extremes of the stand density gradient (open woodlands (LD) vs dense forest (HD)) at two contrasting sites (Wet site vs Dry site). Error bars indicate standard deviations.

CO2 assimilation rate (A) mirrored the pattern described for C (Fig. 6), with significantly higher values in Wsite than in Dsite ( p < 0.001; d.f. = 1 and 282), and with much higher within-site differences in Dsite than in Wsite (significant Site  Stand density interaction; p = 0.012; d.f. = 2 and 282). A decreased significantly throughout the summer ( p < 0.01; d.f. = 6 and 1692). The decrease was more acute at Dsite than at Wsite, and for HD trees than for LD ones. A values remained high at the end of the dry season in LD trees, even at Dsite (Fig. 6). A significant relationships was found between Cd and A, although it changed across-sites, with a better relationships at Dsite than at Wsite (R2 = 0.60 and 0.45, respectively; Fig. 7). WUE (A/Eleaf) varied also across-sites (Table 3), with significantly higher values in Dsite than in Wsite (2.69 and 2.17 mmol CO2 mmol1 H2O, respectively; p = 0.001, d.f. = 1 and 282). WUE decreased significantly in summer respect to

spring period (2.82 and 2.03 mmol CO2 mmol1 H2O, respectively; p < 0.001, d.f. = 1 and 282), and across-sites differences resulted significant only in spring (significant Site  Period interaction; p = 0.001, d.f. < 1 and 282). By contrast, within-site differences were not-significant ( p = 0.86, d.f. = 2 and 282). The absence of significant differences was irrespective of the site and the period (non-significant interactions; p = 0.39 and p = 0.75, respectively; d.f. = 2 and 282). 5. Discussion 5.1. Tree transpiration Daily and seasonal sap flow patterns at Dsite showed that prolonged drought periods had little effect on the water status of LD trees in comparison with HD ones (Fig. 3). Moreover, in LD Table 3 Water use efficiency (mmol CO2 mmol1 H2O  S.D.) in Quercus ilex tree leaves comparing periods (Spring vs Summer), sites (dry site vs wet site) and stand densities (<30%, 30–70%, and >70% of canopy cover; named LD, MD and HD, respectively) Period

Site

Stand density LD

Fig. 7. Relationship between Cd and CO2 assimilation rate at midday in Quercus ilex located in two contrasting sites (Wet site vs Dry site) (n = 56).

Spring Spring Summer Summer

Dsite Wsite Dsite Wsite

3.50  0.08 2.04  0.14 2.14  0.24 2.01  0.16

MD a b b b

3.48  0.13 2.18  0.16 1.98  0.20 2.26  0.18

HD a b b b

Different letters indicate significant differences ( p < 0.05).

3.14  0.11 2.61  0.61 1.88  0.19 1.93  0.26

a ab b b

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trees the shape of the daily sap flow density (F d) curves was typical of non-water-stressed conditions (a bell-shaped curve). Estimated values of tree transpiration (Etree) from F d varied very little with stand density in spring period, but Etree decreased exponentially with stand density in summer period. In this latter period, mean values of daily Etree ranged from near 2 to 0.6 L m2 canopy d1 in LD and HD trees, respectively. Similar results of high tree transpiration estimated by sap flow probes have been reported before in Spanish and Portuguese open Q. ilex woodlands (Infante et al., 2003; David et al., 2004). The higher water use of individual trees in open areas could be explained by the increased water availability per tree, greater exposure to radiation, increased canopy roughness and ventilation in the thinned forest (Raper, 1998), or alternatively by a differential canopy development, with higher leaf areas in LD trees due to lack of competition with other crowding individuals. In our study, we have implicitly assumed that leaf density does not vary with canopy cover, but this is not necessarily true (Ansley et al., 1998). For instance, high Eleaf rates could be maintained when Etree and/or Estand decrease by reducing leaf density rate. This does not seem to be the case of trees studied here because c, A and Eleaf rates (measured at leaf level) decreased with tree density. However, future studies incorporating LAI values could improve the interpretation of canopy cover effects on soil water transpired by individual trees. It has been hypothesized that Q. ilex relies heavily on soil water in interspaces to maintain its physiological activity throughout the summer (Joffre and Rambal, 1993; Cubera and Moreno, 2007). Here, we have shown that U in Dsite increased in LD area with respect to HD one, which could benefit remaining trees in open woodlands through water uptake by lateral roots. Indeed, it has been reported that Q. ilex growing in open woodlands have a much more extended lateral root system (up to 6 times the canopy width; Moreno et al., 2005). Man has traditionally cleared forest in Mediterranean basin to increase pasture yield, and thus has indirectly favoured tree productivity, and potentially tree survival in some cases. As a result of the increased water availability for individual trees in dehesas (LD), Q. ilex exhibited high rates of water transpiration throughout the summer. By contrast, in dense forests (HD), soil water is more exhaustively consumed and an intense competition probably arises among the lateral roots of neighbouring trees, causing a significant decrease of Etree along the summer drought.

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more water per tree than HD trees, but tree transpiration ha1 was only slightly greater in HD than LD areas in spring, and did not differ from LD areas during summer. Although not so acute, we also found a certain trend to the saturation of Estand with the increase of stand density. In summer, a five-fold reduction of canopy cover (from 100% to 20%) caused only a 2.4-fold reduction of Estand (from 7300 to 2950 L ha1 d1). Similarly, Marshall and Chester (1992) found that a six-fold reduction of tree density by thinning caused only a three-fold reduction in water uptake in thinned stands compared to intact stands. This result implies that there was a limited pool of water available and intraspecific competition for soil water exists among adult trees, increasing this competition exponentially with stand density in semiarid dehesas. U in HD areas was even lower than beneath the canopy of LD trees, and U was lower beneath canopy than in interspaces in LD areas, indicating that in open woodlands available soil water was not completely depleted in the summer period. Indeed, in three adjacent farms (10–20% of canopy cover), U increased gradually and significantly with the distance from the tree trunk, and an important amount of soil water remains unused throughout the summer, especially below 1 m in depth (Cubera and Moreno, 2007). The increased capacity to explore and utilise the whole soil water in stands with increasing canopy cover explains why we found lower soil moisture in HD than in LD areas and why Estand increased with stand density. In light of our results, we can conclude that the present stand density of many dehesas could be slightly increased without provoking an acute water deficit to trees. Joffre et al. (1999), who analysed tree density along a gradient of rainfall at regional scales, arrived at the same conclusion. The saturation of the tree layer with respect to the transpiration rate may not be only to the limitation of soil resource but also to atmospheric limitations. In open Q. ilex stands, a near-zero decoupling coefficient between the canopy surface and the surrounding bulk air has been reported (Infante et al., 1997). Simioni et al. (2003) pointed out that transpiration of the tree layer increased with increasing tree density and decreased with increasing tree aggregation. Hence, a disadvantage of tree thinning is the increase of transpiration control by climatic conditions over the functioning of the isolated trees when compared with dense forests. This could make trees of open woodlands more vulnerable to prolonged soil drought.

5.2. Whole stand transpiration 5.3. Tree functional status in relation to tree density Contrarily to Etree, transpiration at stand level (Estand) increased with stand density, although the increase was more acute in spring period (quasi-lineal increment) than in summer period (Fig. 4). This increase is consistent with field measurements in tropical agroforestry systems and in arid dehesas of Southern Spain. These studies found that evapotranspiration of tree/grass stands was greater than that of pure grass stands by about 100% (Tournebize et al., 1996), and by 25–65% (depending on the year and site, Joffre and Rambal, 1993). Other authors have not found this trend; for instance Ansley et al. (1998) found that LD trees transpired from 2.5 to 4 times

Q. ilex has to cope with the high variability of the Mediterranean climate. Evergreen oak species have three main mechanisms to enable them to achieve this through drought resistance: stomatal control, deep rooting and reduced leaf area (Rambal, 1993). Among these mechanisms, man can easily modify leaf area index (LAI) through thinning and pruning trees in managed systems. Greenwood et al. (1985) found that LAI was the greatest single determinant of annual transpiration, and this argument has been well supported in the literature (Gupta et al., 1998; Raper, 1998).

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In the open areas of the dehesas studied, LAI had been heavily reduced through periodical thinning and pruning (Montero et al., 1998) and tree water status was found to be quite good throughout summer drought. All the monitored trees reduced leaf water potential (C), photosynthetic rate (A) and transpiration (Etree) throughout the summer drought, mirroring the increase in soil moisture deficit. However, the decrease was much more acute in HD trees than in LD, and in Dsite than in Wsite. Q. ilex growing in Dsite experienced severe water stress in summer, reaching mean values of Cd below 3 MPa at HD. By contrast, in LD trees maintained very favourable water conditions (mean value of Cd > 1 MPa), and hence high A, throughout the summer. Other authors have also reported high mean values of Cd during the summer for Q. ilex growing in open woodlands, with canopy cover between 20 and 40% (Cd = 1.9, 2.1 and 1.1 MPa reported by Infante et al. (1999), David et al. (2004) and Montero et al. (2004), respectively), while other authors measuring Cd in dense coppice of Q. ilex have reported much lower values for summer period (Cd < 3 MPa; Sala and Tenhunen, 1996; Save´ et al., 1999; Martı´nez-Vilalta et al., 2003). These authors also reported mean values of Cm below 4 MPa for summer period, similar to the value here recorded for HD trees at Dsite, while LD trees hardly surpassed 3 MPa. Daily variation of C (Cd  Cm) decreased along the summer, especially in HD trees at Dsite. This decrease has been commonly described for Q. ilex (e.g., Tognetti et al., 1998; Martı´nez-Vilalta et al., 2003; Mediavilla and Escudero, 2004), and it indicates that Q. ilex close stomata at relatively high water potential in comparison to other Mediterranean woody species (Ogaya and Pen˜uelas, 2003). However, Martı´nez-Vilalta et al. (2003) have shown that under very dry conditions, stomatal control in Q. ilex is insufficient to prevent extensive loss of hydraulic conductivity as a result of embolism. Results presented here show the importance of a reduced stand density in semiarid woodlands to prevent Q. ilex trees experiencing excessive water deficit, and hence a dangerous degree of xylem embolism (Tyree and Sperry, 1988). Variations of Cd and A with stand density were much less acute at Wsite, indicating that the reduction of stand density is not so critical in subhumid woodlands. The improved photosynthetic activity of trees in LD areas must have important consequences for tree productivity. Acorns that develop throughout the summer reach maturity in autumn. In dense forests, a massive fruit abortion usually takes place during summer, while in open woodlands, Q. ilex produces up to 13 times more acorns than in dense forests in semiarid areas (Pulido and Dı´az, 2005). Moreno et al. (2007) reported a very significant decrease in annual shoot elongation in mature trees with dehesa encroachment, also under semiarid conditions. 5.4. Water use efficiency A high WUE is usually expected under conditions of low water availability (Pen˜uelas et al., 2000). These authors pointed out that high WUE would be more beneficial on driest sites and years and less beneficial where and when water requirements of vegetation are exceeded by precipitation. According to this

hypothesis, we found significantly higher WUE values at Dsite than at Wsite (Table 3). This finding agrees with values reported by Hoff and Rambal (2003), who found a significant increase of WUE with the decrease of the mean annual precipitation. However, we failed to find any significant difference for WUE among stand densities (Table 3), despite the fact that trees experienced very different levels of water stress. Hence, we hypothesised that the across-differences of WUE found here, respond to more genetic differentiation than to acclimatization to dry conditions. This issue would require further studies given its potential interest for future aforestation in degraded semiarid lands in Mediterranean basin. 5.5. Final remarks The adjustment of stand density to soil water availability could be a mechanism to avoid tree dieback caused by severe drought. For instance, in an experimental thinning of a dense forest in NE Spain, no trees dried up after a severe summer drought, while in control plots (with no thinning) leaves of many trees were completely died by drought at mid summer (although most trees resprouted in wet season; Gracia et al., 1999). He and Duncan (2000) also found that Douglas fir survival was significantly higher in less dense patches of conspecifics, and that non-random tree death led to regularly spaced survivors, as expected from intraspecific competition. Our results indicate that the tree thinning traditionally practiced for dehesa creation is a useful mechanism for Q. ilex to cope with summer drought, especially at dry sites. Indeed, at geographical scale the distribution of man-made stand density of dehesas has been shown to be controlled to a large extent by water availability: as rainfall increased, mean stand density increased (Joffre et al., 1999). Hence, given the increase of aridity of the Mediterranean basin predicted by climate change models (IPCC, 2001), the traditional forest thinning practiced in Mediterranean countries for centuries should be maintained as a mechanism of water stress avoidance (LAI reduction) and control of severe tree dieback. However, dehesa encroachment is a currently generalized process in important areas of Spain (Garcı´a del Barrio et al., 2004), and tree encroachment in savannas is a worldwide phenomenon (Archer, 2003). Changes in the amount of soil water available for individual trees could be a key factor for tree survival and productivity in these systems, especially in semiarid regions. The long-term sustainability of these open woodlands may be further threatened in the future by regional climatic changes due to global warming. If an increase in tree cover occurs in dehesas or similar systems (e.g., savannas), our results suggest that changes in soil water content and tree transpiration can be expected. Potential impacts of such changes on hydrology and climate need to be explored. Acknowledgements We thank Dr. Jamie Quin for the English revision. This study was supported by The European Union (SAFE project,

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QLX-2001-0560), The Spanish Ministerio de Ciencia y Tecnologı´a (MICASA project, AGL-2001-0850) and the Consejerı´a de Educacio´n (Junta de Extremadura) (CASA project, 2PR02C012). Elena Cubera was awarded a grant by Consejerı´a de Educacio´n (Junta de Extremadura).

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