Forest Ecology and Management 258 (2009) 1677–1683

Growth response of mixed mediterranean oak coppices to rainfall reductionCould selective thinning have any influence on it?

Miriam Cotillas a, Santi Sabate a,b, Carlos Gracia a,b, Josep M Espelta a,*a CREAF, Centre for Ecological Research and Forestry Applications, Building C, Autonomous University of Barcelona, 08193 Bellaterra, Catalonia, Spainb UB, University of Barcelona, Department of Ecology. Diagonal 645, 08028 Barcelona, Catalonia, Spain

A R T I C L E I N F O

Article history:

Received 11 February 2009

Received in revised form 10 July 2009

Accepted 13 July 2009

Keywords:

Climate change

Drought

Forest management

Quercus ilex

Quercus cerrioides

Resprouting

A B S T R A C T

Climate change is one of the major challenges for ecosystem conservation. One of the most vulnerable

areas to climate change is the Mediterranean Basin which is expected to suffer important changes in

temperature and precipitation in the next few decades, leading to a warmer and dryer climate. Therefore,

it is necessary to determine species-specific responses to increased drought to predict possible future

changes in the structure and composition of Mediterranean forests, as well as to identify appropriate

management strategies to mitigate these effects.

The main aim of this study has been to experimentally simulate the effects of a 15% reduction in

annual rainfall on the survival and growth of two co-occurring Mediterranean oaks with contrasting leaf-

habit (the evergreen Quercus ilex spp. ilex and the winter-deciduous Quercus cerrioides) and, to assess

whether traditional selective thinning carried out in these mixed oak coppices (i.e. selection of one to few

stems per stump) can modify the consequences of rainfall reduction.

Soil moisture decreased under the rainfall reduction level while it increased in the thinned plots.

Reduced rainfall did not influence tree mortality, but did lead to species-specific effects on height

growth: no changes were observed in Q. ilex while height growth rate of Q. cerrioides decreased (c.a. 20%).

Selective thinning improved tree growth (c.a. 50%) in stands both under natural and, and to a lesser

extent, under reduced rainfall conditions. Nevertheless, the positive effects of thinning rapidly declined

during our three years experiment, probably because the vigorous resprouting of thinned stumps.

Our results show that the forecasted reduction in annual rainfall for the Western Mediterranean Basin

can constrain the growth of some deciduous oaks in mixed oak coppices. Traditional selective thinning

can increase soil moisture and encourage tree growth, thus partially mitigating this effect. However, the

transient results observed in this experiment suggest the need to reconsider the intensity and the

frequency of this traditional management practice in light of new climatic scenarios.

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1. Introduction

In a time of global change, one major challenge is to determinehow changes in environmental conditions will affect the structureand dynamics of ecosystems (Chapin et al., 2001). Mediterraneanareas are one of the most vulnerable biomes to global climatechange, because they represent a transition zone between arid andhumid regions (IPCC, 2007). Over the last century, temperatures inthe Mediterranean Basin have already shown an overall trendtowards warming, while precipitation has shown a remarkabledecrease (Pinol et al., 1998; De Luis et al., 2001). This situation mayturn out to be even more negative during the current century,when a rise in mean temperature of between 2.2 and 5.1 8C ispredicted (IPCC, 2007). Concerning precipitation, the forecasted

* Corresponding author. Tel.: +34 93 5814671.

E-mail address: Josep.Espelta@uab.cat (J.M. Espelta).

0378-1127/$ – see front matter � 2009 Elsevier B.V. All rights reserved.

doi:10.1016/j.foreco.2009.07.033

changes are more uncertain, with a potential decline of 4–27% ofrainfall and important changes in the precipitation patterns (IPCC,2007). A decrease or even maintenance of the current precipitationvalues, coupled with an increased evapotranspiration in spring andearly summer due to warming, is very likely to lead to reduced soilmoisture and to promote recurrent drought episodes (Douvilleet al., 2002; Wang, 2005). Since water stress is the mainconstraining factor in Mediterranean-type forests (Specht,1988), rainfall decline may intensify the current limiting condi-tions for plant development (Schroter et al., 2005; Resco et al.,2007). This may result in changes in growth and survival(Tenhunnen et al., 1990; Ogaya and Penuelas, 2007) and, in thelong term, changes in the distribution and abundance of treespecies (Thuiller et al., 2003; Benito Garzon et al., 2008).

The effects of increased drought may vary among species(Martinez-Vilalta et al., 2002; Ogaya et al., 2003; Lloret et al.,2004a; Quero et al., 2008). Leaf-habit could be an important factorinfluencing species-specific responses. Indeed, it has been argued

M. Cotillas et al. / Forest Ecology and Management 258 (2009) 1677–16831678

that evergreen habit can be an advantage in water-poor habitatsdue to its low resource-loss ratios in comparison to the higherlevels of water availability required by deciduous species (Aerts,1995; Villar and Merino, 2001; Quero et al., 2006). In fact, in theMediterranean Basin, evergreen Mediterranean trees tend tooccupy lowland areas, while deciduous ones are often located inmore humid sub-Mediterranean areas (Thuiller et al., 2003). If thecurrent trends of rising temperature and rainfall decline aremaintained over the coming decades, deciduous sub-Mediterra-nean species could be displaced by evergreen Mediterranean ones(Benito Garzon et al., 2008). In comparison to the large number ofstudies aimed at evaluating these potential effects of climatechange in the structure and functioning of Mediterranean forests,the subject of whether forest management can mitigate theconsequences of rainfall decline has seldom been explored. Yetmany authors have highlighted the priority of identifyingappropriate management procedures to maintain or enhancethe resilience of ecosystems to the projected impacts of climatechange (Lindner, 2000; Scarascia-Mugnozza et al., 2000; Wulls-chleger et al., 2000; Montoya-Oliver, 2001).

In the Mediterranean Basin, mixed oaks forests with contrastingleaf-habit are relatively abundant (Quezel and Medail, 2003). Due tothe recurrence of disturbances (e.g. wildfires, clear-cutting) mostMediterranean oak forests are ‘‘coppices’’ characterised by high-density stands of multi-stemmed stumps with slow vertical growthand low production rates (Terradas, 1999). To ameliorate thestructure of these forests and encourage their regeneration, theirmid-term gradual conversion into ‘‘stored coppices’’ (i.e. coppices inwhich there remains only one or two stems per stump) or in thelong-term to ‘‘high forests’’ has been suggested, through theelimination of the weakest resprouts (Serrada et al., 1996).Reduction in stand density may increase the availability of resourcessuch as water and nutrients for the remaining stems throughreducing competition (Canellas et al., 2004; Moreno et al., 2007).Unfortunately, the low economic revenue of the traditional productsobtained from Mediterranean oak coppices (charcoal, firewood) hasled to a dramatic decrease in their management in the second half ofthe 20th century (see among others, Terradas, 1999).

Although there are several studies reporting the response ofMediterranean oaks to drought (Lloret et al., 2004b; Ogaya et al.,2003; Ogaya and Penuelas, 2007), as well as the effects of selectivethinning (Retana et al., 1992; Mayor and Roda, 1993; Breda et al.,1995; Gracia et al., 1999; Rodriguez-Calcerrada et al., 2008), nostudies have attempted to integrate the response to increaseddrought with the effects of forest management. The main aims of thepresent study are: (i) to analyze the effect of reduced rainfall inmortality, growth and resprouting ability of two co-occurringMediterranean oaks with contrasting leaf-habit (the evergreenQuercus ilex ssp. ilex L. and the winter-deciduous Quercus cerrioides

Wilk. & Costa) and (ii) to assess whether traditional forestmanagement (selective thinning) conducted in these oak coppicescan mitigate the effects of reduced rainfall. To achieve theseobjectives, we carried out an experiment during 3 consecutive years(2006–2008) in a mixed oak forest where we factorially applied twolevels of rainfall (natural vs. reduced) and two levels of forestmanagement (thinning vs. no thinning). The results obtained mayimprove our knowledge about the response of Mediterranean oaksto rainfall decline, as well as help to develop management strategiesto mitigate the effects of climate change in Mediterranean forests.

2. Material and methods

2.1. Study area and species

The research was conducted in the region of Bages, Catalonia,NE Spain (418440N, 18390E), from January 2005 to February 2008.

The mean elevation in the study site is 800 m asl, facing south andwith 10% of slope. According to the data provided by the nearestMeteorological Station (8 km) the mean annual temperature is12 8C, with a range between 4 8C in January and 22 8C in July. Meanprecipitation, characterized by a seasonal irregular distributionand a high year-to-year variability, is 600 � 135 mm. The climate isdry-subhumid Mediterranean according to the Thornwaite index.The geological substrate is predominantly calcareous, surfacerockiness is high and soil is moderately well drained with a meandepth c.a. 25–50 cm. Forest structure corresponds to a mixed Q. ilex

and Q.cerrioides oak coppice with multi-stemmed stumps regener-ated by resprouting after a large wildfire in 1998 (Espelta et al.,2002).

2.2. Experimental design and sampling

In order to test for the effects of rainfall reduction and thinningon the survival, growth and resprouting patterns of Q.ilex and Q

.cerrioides trees, two levels of rainfall (natural rainfall vs. reducedrainfall) and thinning (thinning vs. no thinning) were factoriallycombined in four treatments with three replicates (plots). The 12plots (15 m � 20 m) were randomly distributed in the samplingarea with at least a 10 m buffer surrounding every plot. Reducedrainfall simulation was done through the installation of paralleldrainage channels suspended at c.a. 50 cm above the soil andcovering 15% of the ground surface on the selected plots (Fig. 1).This device was intended to intercept around 15% of rainfall,which, according to the IPCC (2007), is a reasonable predicteddecline in precipitation for the western rim of the MediterraneanBasin in the coming decades. To account for the effects of rainfalllevel and mthinning in soil moisture in the experimental plots, soilmoisture was monitored regularly (weekly or biweekly) throughten Time Domain Reflectometer (TDR) probes per plot. Probes were20 cm long and they were permanently installed in the same placethorough all the experiment. Values of soil moisture per plot wereseasonally averaged (winter, spring, summer and autumn) toresume the large amount of data obtained. Selective low-thinningwas done by hand and following the traditional criteria applied inthese young oak coppices (Espelta et al., 2002; Espelta et al., 2003):i.e. a reduction in 20–30% of total stump basal area per plot byleaving from one to three dominant stems per stump andeliminating the weakest ones.

To evaluate the effects of the experimental treatments in foreststructure we measured the mortality of stumps and stems, basaldiameter and height of all stems per plot, and the number andheight of the new wave of resprouts appeared in the stumps of thethinned plots. All these measures were recorded at the onset of theexperiment (2005) and every winter during the study (2006–2008). Due to the particular structure of oak coppices (multi-stemmed stumps) we calculated the basal area of each individualstem and the basal area per stump (i.e. the sum of the basal areas ofall stems per stump). The stem basal area may be an appropriatemeasure to evaluate biomass production in this forest-typestructure while the total stump basal area gives a more detailedview of the dynamics at the population level (Espelta et al., 2003).The main structural characteristics of the plots included in thisstudy are summarized in Table 1.

2.3. Data analysis

The effects of the experimental factors, season and year on soilmoisture were analyzed through a four-way repeated-measuresANOVA model with, rainfall, thinning level, species and season asbetween-subject factors and year (2006–2008) as a within-subjectfactor. In this analysis we used as the dependent variable the meansoil moisture per plot and per season.

Fig. 1. General view of the plots where the experimental treatments were combined: (A) no-thinning and natural rainfall, (B) no thinning and reduced rainfall, (C) thinning

and natural rainfall and (D) thinning and reduced rainfall.

Fig. 2. Soil moisture and precipitation trends in plots of the four experimental

treatments applied combining two levels of rainfall (natural vs. reduced) and two

levels of forest thinning (thinning vs. no thinning). Grey columns indicate monthly

values of precipitation.

M. Cotillas et al. / Forest Ecology and Management 258 (2009) 1677–1683 1679

At the onset of the study, we checked for potential structuraldifferences among the plots assigned to each experimentaltreatment, as well as between the two oak species. Differencesin stump basal area, stem basal area and stem height wereanalyzed by means of a three-way ANCOVA with species, rainfalllevel and thinning level as independent variables and stumpdensity as a covariate (due to differences in the number of stumpsamong plots, see Table 1). This analysis indicated that, Q. cerrioides

and Q .ilex differed in stem basal area and height (respectively,F = 9.3, p < 0.001 and F = 30.2, p < 0.0001). Q. cerrioides individualshad a larger mean stem basal area (12.4 � 0.8 cm2) and height(177 � 4 cm) than those of Q .ilex (9.7 � 0.8 cm2 and 144 � 4 cm,respectively). Since selective thinning involved a selection of thedominant resprouts, this resulted in technical differences in thestructural characteristics of thinned and non-thinned plots in stembasal area (F = 26.8, p < 0.0001), stump basal area (F = 11.0,p < 0.001) and height (F = 19.3, p < 0.0001). Thus, oaks in thinnedplots had a larger mean stem basal area and height (14.3 � 0.8 cm2,180 � 4 cm) than in no thinned plots (7.7 � 0.8 cm2, 146 � 4 cm).Concerning the two rainfall levels, no significant differences werefound in structural characteristics among plots that had beenassigned to the natural or to the reduced rainfall level.

Due to the abovementioned initial differences among speciesand thinning levels in structural characteristics, to calculategrowth in stem basal area, stump basal area and height duringthe experiment we used the relative growth rate (RGR):

RGRxi�xi�1¼ xi � xi�1

xi�1� 100

where xi is the value of the selected variable in year i, and xi�1 wasthe value in the previous year.

The effects of the experimental factors and time (year) on stumpmortality, stem mortality and growth were analyzed through athree-way repeated-measures ANCOVA model with, rainfall level,thinning level and species as between-subject factors, year (2006–2008) as a within-subject factor and stump density in the plots as a

covariate. The number of new resprouts produced in the stumps ofthinned plots and their mean height were analyzed through arepeated-measure two-way ANCOVA, where rainfall and specieswere included as between-subject factors, year as a within-subjectfactor, and stump density as a covariate. The thinning treatment wasnot included in this model because new resprouting only occurred inplots subjected to thinning.

3. Results

As expected under a Mediterranean-type climate, soil moisturein the experimental plots showed a highly variable temporal

Table 1Main structural characteristics of Q. ilex and Q. cerrioides at the onset of the experiment in the plots assigned to the different rainfall and thinning levels.

Plot Rainfall Thinning Species Stem density

(i ha�1)

Stump density

(i ha�1)

Mean stem

basal area (cm2)

Mean stump

basal area (cm2)

Mean stem

height (cm)

1 Natural rainfall No thinning Q.cerrioides 1583 495 8.0 29.6 151

Q.ilex 6630 2127 4.9 18.9 108

2 Natural rainfall No thinning Q.cerrioides 1877 691 8.2 25.7 178

Q.ilex 4307 1335 7.5 32.5 138

3 Natural rainfall No thinning Q.cerrioides 1120 345 13.8 62.6 198

Q.ilex 5661 1506 8.7 41.1 156

4 Natural rainfall Thinning Q.cerrioides 390 354 22.0 23.3 202

Q.ilex 3176 2175 15.0 23.3 164

5 Natural rainfall Thinning Q.cerrioides 529 378 15.1 23.7 180

Q.ilex 3746 2087 14.1 31.2 182

6 Natural rainfall Thinning Q.cerrioides 856 856 14.7 14.6 172

Q.ilex 2622 1766 10.6 19.5 141

7 Reduced rainfall No thinning Q.cerrioides 1744 511 13.9 43.0 183

Q.ilex 7597 2377 8.8 34.2 138

8 Reduced rainfall No thinning Q.cerrioides 1550 517 13.0 46.1 169

Q.ilex 5383 1630 6.6 26.2 125

9 Reduced rainfall No thinning Q.cerrioides 3333 959 7.6 29.2 150

Q.ilex 3188 1204 5.9 23.6 111

10 Reduced rainfall Thinning Q.cerrioides 1262 1131 5.4 5.5 121

Q.ilex 2499 1781 10.8 18.9 139

11 Reduced rainfall Thinning Q.cerrioides 574 488 15.5 17.4 192

Q.ilex 4197 2912 10.3 15.8 151

12 Reduced rainfall Thinning Q.cerrioides 399 354 19.5 24.0 219

Q.ilex 2436 1638 12.4 22.4 160

Table 2F values from the repeated-measures ANCOVA tests of effects of rainfall (R),

thinning (T), species (Sp), year (Y) and stump density per plot (covariate) on the

relative growth rate in stem basal area, stump basal area and height of Q. cerrioides

and Q. ilex.

Stem basal area Stump basal area Height

Rainfall (R) 1.86 1.81 6.67*Thinning (T) 30.17*** 34.35*** 55.41***Species (Sp) 0.70 0.48 0.80

Density 0.58 0.64 11.53**R � T 0.75 0.74 5.75*R � Sp 0.06 0.06 5.27*T � Sp 1.10 1.15 2.19

Year (Y) 0.00 0.01 0.02

Y � R 0.26 0.20 2.66

Y � T 5.55* 4.97* 8.05*Y � Sp 0.05 0.07 0.55

Y � R � T 1.69 1.56 4.42

Y � R � Sp 0.25 0.17 0.59

Y � T � Sp 2.22 1.82 0.72

Significant F coefficients are indicated in bold. Asterisks indicate the value of

significant p values: *p < 0.05, **p < 0.01, ***p < 0.001.

M. Cotillas et al. / Forest Ecology and Management 258 (2009) 1677–16831680

pattern, with an overall trend following the occurrence ofprecipitation events (Fig. 2). Soil moisture was significantlyinfluenced by year (F = 58.8, p < 0.0001) and season (F = 39.5,p < 0.0001). Soil moisture was higher in 2006 (13.7 � 0.3%), lowerin 2007 (10.6 � 0.3%) and intermediate in 2008 (12.0 � 0.2%).Regarding its seasonal pattern, it was higher in winter(15.3 � 0.4%), lower and very similar in spring and autumn(respectively, 12.4 � 0.4% and 11.6 � 0.4%), and it reached the lowestvalues during summer (9.1 � 0.4%). Besides these inter-annual andseasonal differences, the rainfall level assigned to the plots did alsoinfluence soil moisture (F = 18.9, p < 0.0001): mean soil moistureduring the 3 years experiment was lower in plots subjected toreduced rainfall (11.2 � 0.3%) than in those subjected to naturalconditions (13.1 � 0.3%). Interestingly, concerning the effects of thethinning treatment, the interaction between year and thinning level(F = 3.1, p < 0.041) indicated higher soil moisture values in thinned-than in non-thinned plots only for the first year after the experimentaltreatments were applied (respectively, 14.6 � 0.4% vs. 12.8 � 0.3% in2006), but no differences in 2007 (10.5 � 0.3% vs. 10.8 � 0.3%) or 2008(12.4 � 0.4% vs. 11.6 � 0.3%)

No stump mortality was reported during the experiment, whilesome stem mortality occurred. Thinning was a very importantfactor in reducing stem mortality (F = 4.57, p < 0.05): mortality ofstems was much lower in thinned plots in comparison to non-thinned ones (respectively, 5 � 1% vs. 26 � 1%). Concerning therainfall treatment, the two different levels of rainfall did not result insignificant differences in stem mortality.

Despite the lack of effects of reduced rainfall on survival,significant effects were observed in height growth with species-specific differences (interaction species � rainfall effect inTable 2): reduced rainfall did not modify height growth rates ofQ. ilex while it reduced that of Q. cerrioides by 20% (Fig. 3).Concerning the effects of selective thinning, the interactionrainfall � thinning level (Table 2) indicated that in the absence

of thinning, oaks had low and similar height growth rates, whilethinning increased growth rate both in natural and reduced rainfallplots (Fig. 4). However, growth in thinned plots subjected toreduced rainfall was lower than under natural conditions. In allcases, the positive effects of thinning decreased year by year(interaction thinning level � rainfall in Table 2, see also Fig. 5).

In contrast to the effects observed for height growth rate,neither stem nor stump basal area were significantly affected byreduced rainfall (Table 2).Thinning enhanced relative growth ratein stem basal area (23.4 � 1.1% in thinned plots vs. 14.6 � 1.4% innon-thinned ones) and in stump basal area (respectively, 23.3 � 1.0%vs. 14.4 � 1.0%). As previously mentioned for height growth rates,these effects decreased along the experiment (data not shown).

Fig. 3. Mean � SE relative height growth rate (RGR) of Q. ilex and Q. cerrioides in plots

with natural or reduced rainfall. Different letters indicate significant differences

between species per rainfall level (upper case letters) and between the two rainfall

levels per species (lower case letters), according to the LS means tests.

Fig. 5. Mean � SE relative height growth rate (RGR) of oaks in thinned or no thinned

plots during the three years monitored in this experiment (2006–2008). Different

letters indicate significant differences between the three years per thinning level

(upper case letters) and between the two thinning levels per year (lower case letters),

according to the LS means tests.

M. Cotillas et al. / Forest Ecology and Management 258 (2009) 1677–1683 1681

Both Q .ilex and Q. cerrioides resprouted vigorously after thethinning treatment, with a higher number of new resprouts in theformer species (F = 22.8, p < 0.001; respectively, 22.3 � 1.5 and12.5 � 1.5). There were no effects of the rainfall level on the numberof new resprouts but the interaction between this experimentaltreatment and year (F = 7.3, p < 0.05) indicated differences on theirheight growth. As shown in Fig. 6, the mean height of the new wave ofresprouts was initially higher in plots subjected to reduced rainfall,while these differences declined among years and disappeared at theend of the experiment, due to the mortality of some of these newresprouts.

At the end of the experiment (winter 2008) some of the initialstructural differences observed among species and among plotsassigned to the different experimental treatments (see Materialand methods) still persisted. Thus Q. cerrioides and Q. ilex

individuals differed in stem basal area and height (respectively,F = 7.2, p < 0.05 and F = 28.1, p < 0.0001). Q. cerrioides individualsstill maintained a larger stem basal area and height (19.5 � 1.1 cm2

Fig. 4. Mean � SE relative height growth rate (RGR) of oaks in thinned and no thinned

plots subjected to natural or reduced rainfall. Different letters indicate significant

differences between the two thinning levels per rainfall level (upper case letters) and

between the two rainfall levels per thinning level (lower case letters), according to the

LS means tests.

and 201 � 5 cm) than Q. ilex ones (15.4 � 1.1 cm2 and 164 � 5 cm).Similarly, there were differences among thinned and non-thinnedplots in stem and stump basal area (respectively, F = 59.7, p < 0.05and F = 33.8, p < 0.0001): thinned plots maintained a larger meanstem basal area and height than non-thinned ones (respectively,23.4 � 1.1 cm2 and 204 � 5 cm vs. 11.1 � 1.1 cm2 and 161 � 5 cm).However, the interaction between species and thinning treatmentrevealed a new trend in the stump basal area of individuals: Q.

cerrioides had a larger basal area than Q. ilex in non-thinned plots(respectively, 49.4 � 4.1 cm2 vs. 38.2 � 4.1 cm2) but the oppositepattern was observed in thinned ones (respectively, 31.1 � 4.1 cm2

vs. 39.5 � 4.1 cm2).

4. Discussion

Differences in soil moisture between natural and reducedrainfall plots ranged between 0% and 20% (Fig. 2), and they resultedin a significant lower mean soil moisture value in plots subjected to

Fig. 6. Mean � SE height of the new wave of resprouts appeared in the thinned oaks in

plots under natural or reduced rainfall during the three years monitored in this

experiment (2006–2008). Different letters indicate significant differences between the

three years per rainfall level (upper case letters) and between the two rainfall levels per

year (lower case letters), according to the LS means tests.

M. Cotillas et al. / Forest Ecology and Management 258 (2009) 1677–16831682

rainfall reduction. However, as shown in Fig. 2 seasonal differenceswere more important at high values of soil moisture (e.g. duringwinter) than in the driest season (e.g. summer). This suggests thatour experimental design may partly underestimate the effects ofincreased drought under the forecasted climate change scenario inthe Mediterranean Basin, as current prediction point to alengthening of the driest season and the occurrence of severedrought episodes in summer (IPCC, 2007). Interestingly, althoughit could be initially argued that the greater exposure of soil surfacein thinned plots could increase evaporation and contribute to soildesiccation, soil moisture was enhanced by thinning, probably by agreater effect of this treatment in diminishing rain interceptionand transpiration from the canopy of multi-stemmed stumps(Aussenac, 2000; Rodriguez-Calcerrada et al., 2008).

Rainfall reduction did not cause mortality of Q. ilex and Q.

cerrioides stumps or stems, as the experimental level applied wasmuch lower than the one experienced under severe droughtepisodes in the Mediterranean Basin, when oak mortality has beenrecorded (Montoya, 1995; Lloret et al., 2004b). Concerning forestmanagement, thinning enhanced stem survival, as a reduction ofcompetition within stumps increases nutrient and light avail-ability for the reserved stems (Canellas et al., 2004; Montes et al.,2004; Rodriguez-Calcerrada et al., 2008).

Despite the moderate differences in soil moisture amongtreatments, rainfall reduction decreased the height growth rate inQ. cerrioides while no significant effects were observed for Q. ilex.The different sensitivity of these oaks may be due to their differentleaf-lifespan. Sclerophyllous-evergreen oaks have lower waterlosses because an increased leaf hardening and stomatal regulation(Save et al., 1999; Infante et al., 2003) and thus they can maintainhigher values of leafiness even under intense radiation and inwater stressed situations (Hoff and Rambal, 2003). Conversely,deciduous oaks have higher specific leaf area (Damesin et al., 1998;Espelta et al., 2005; Quero et al., 2006, 2008) and higher stomatalconductance which results in higher water losses (Villar andMerino, 2001; Takashima et al., 2004; Wright et al., 2004, 2005).These traits make growth of deciduous oaks more sensitive todrought (Quero et al., 2006; Castro-Diez and Navarro, 2007, but seeDamesin et al., 1998). Although a 20% reduction in the heightgrowth rate of Q. cerrioides under the reduced rainfall level (Fig. 3)will not constrain, in the short term, the persistence of these oaks itmay have relevant consequences for their regeneration: i.e. wehave observed acorn production to be positively related with treegrowth and to decrease in oaks in the reduced rainfall plots(Sanchez-Humanes and Espelta, unpublished results).

Height growth in non-thinned plots was very low irrespectiveof the rainfall level applied, while it increased in thinned plots,both under natural and, to a lesser extent, under reduced rainfallconditions. This result indicates that, in the absence of thinning,resource availability may be so constrained by the high-density ofMediterranean oak coppices (see Table 1) that a 15% reduction inrainfall barely has any additional effect on growth. The effects ofthinning on growth quickly decreased after three years (Fig. 5, seealso Albeza, 2000). This can be attributed to: (iii) the fact that stemthinning does not involve the complete removal of stumps(individuals) which may still compete belowground for resources(Gracia et al., 1999) and (ii) the production of a new wave of basalresprouts in thinned individuals may compete with the reservedstems (Espelta et al., 2003). Clearly, the transient results observedfor the thinning treatment suggest the need to reconsider theintensity and the frequency of this management practice, if it isforeseen as a potential strategy to mitigate the effects of increasingdrought in Mediterranean oak coppices.

Interestingly, height growth in thinned plots was lower underthe reduced rainfall level than under natural conditions. This couldbe due to the fact that rainfall reduction initially encouraged the

growth of the new wave of resprouts after thinning (Fig. 6).Activation of dormant buds in the stump leading to new resprouts,as well as their initial growth, is a complex phenomenon controlledby the combined effect of changes in environmental factors (e.g.increasing light availability and temperature), changes in hormo-nal levels (Champagnat, 1989), and apical dominance (Espeltaet al., 2003, see also Montoya, 1996). Although we do not knowwhich physiological and hormonal changes lead to the initialhigher development of the new wave of resprouts in stumps of thethinned plots subjected to reduced rainfall, they probablycompeted with the reserved stems and partially constrained theirheight growth.

Under a global change scenario, forests in the MediterraneanBasin are expected to suffer increased drought which willstrengthen the already limiting conditions for tree growth. Infact, Siwecky and Ufnalski (1998) have recorded a sensitive declineof oak forests in Europe in the past decades, which has mainly beenattributed to the negative influence of long-lasting spring droughtson their growth and survival. According to our results, the effects ofincreased drought can be more important for deciduous Medi-terranean oaks. In the long term this could lead to their decline andtheir substitution by evergreen species, as has already beenpredicted in some models (Sabate et al., 2002). As shown by ourresults, traditional selective thinning may be an effective tool toincrease soil moisture. However, its moderate and transient effectson tree growth, suggests the need to re-examine the present-dayintensity and frequency of this practice in light of a climate changescenario.

Acknowledgements

We are grateful to Laura Albaladejo, Elisenda Sanchez, BelenSanchez-Humanes and Roger Sallas for their assistance during fieldwork. We also thank Roma Ogaya, Robert Save and threeanonymous reviewers for their valuable comments on an earlierdraft of this manuscript. This research has been funded by theprojects INIA (RTA04-028-02 and RTA2005-00100-CO2). MiriamCotillas is supported by a FPU grant from the Spanish Ministerio deCiencia e Innovacion.

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