soil water content at the catchment level and plant water status relationships in a mediterranean...

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Soil water content at the catchment level and plant water status relationships in a Mediterranean Quercus ilex forest J. Bellot a, * , J.M. Ortiz de Urbina b a Department of Ecology, Universidad de Alicante, Apartado de Correos 99, 03080, Alicante, Spain b LABAQUA SA, Torrent 41, Valencia, Spain Received 9 July 2007; received in revised form 18 April 2008; accepted 5 May 2008 KEYWORDS Quercus ilex; Predawn water poten- tial; Catchments; Water balance; Soil water; Plant water status Summary This paper presents an analysis of the forest hydrology and plant water status interaction, focusing on the relationship between the hydrological water balance at the catchment level and the predawn leaf water potential of the species Quercus ilex (holm oak). The catchment water balance approach was applied to a Mediterranean watershed forested with holm oak to evaluate the daily soil water reserve at the catchment level. After this, evapotranspiration and soil water content were combined to estimate the potential soil water reserve and evaluate plant water status at the catchment level. A close relationship was detected between leaf water potential and the soil water reserve, and was fitted to a negative exponential curve to estimate predawn leaf water potential from a hydrological database. The proposed equation can help to predict the frequency, intensity, and length of droughts potentially capable of causing structural damage to the forest, from the hydrological time series records. ª 2008 Elsevier B.V. All rights reserved. Introduction In the Mediterranean coastal of Spain, the evergreen holm oak (Quercus ilex L.) is the main oak tree species, and soil water conditions strongly determine its regional distribution and productivity, as well as the structural characteristics of the various communities that it belongs to (Gracia and Reta- na, 1996; Amorini and Fabbio, 1994). One reason for its wide distribution throughout the Mediterranean basin (Debazac, 1983) is that the species has low transpirational water requirements – only 440 mm per year (Terradas and Save ´, 1992) – and this permits its expansion even into semi-arid environments. This characteristic places holm oak between a typical Mediterranean dryland shrub such as Quercus coc- cifera, which has a transpirational water requirement of about 200 mm per year (Rambal and Debussche, 1995), and the species typical of closed forest communities in tem- 0022-1694/$ - see front matter ª 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jhydrol.2008.05.002 * Corresponding author. Tel.: +34 965 903555; fax: +34 965 909832. E-mail address: [email protected] (J. Bellot). Journal of Hydrology (2008) 357, 6775 available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/jhydrol

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Journal of Hydrology (2008) 357, 67–75

ava i lab le a t www.sc iencedi rec t . com

journal homepage: www.elsevier .com/ locate / jhydrol

Soil water content at the catchment level and plantwater status relationships in a MediterraneanQuercus ilex forest

J. Bellot a,*, J.M. Ortiz de Urbina b

a Department of Ecology, Universidad de Alicante, Apartado de Correos 99, 03080, Alicante, Spainb LABAQUA SA, Torrent 41, Valencia, Spain

Received 9 July 2007; received in revised form 18 April 2008; accepted 5 May 2008

00do

*

90

KEYWORDSQuercus ilex;Predawn water poten-tial;Catchments;Water balance;Soil water;Plant water status

22-1694/$ - see front mattei:10.1016/j.jhydrol.2008.05

Corresponding author. Te9832.E-mail address: juan.bello

r ª 200.002

l.: +34

[email protected]

Summary This paper presents an analysis of the forest hydrology and plant water statusinteraction, focusing on the relationship between the hydrological water balance at thecatchment level and the predawn leaf water potential of the species Quercus ilex (holmoak). The catchment water balance approach was applied to a Mediterranean watershedforested with holm oak to evaluate the daily soil water reserve at the catchment level.After this, evapotranspiration and soil water content were combined to estimate thepotential soil water reserve and evaluate plant water status at the catchment level. Aclose relationship was detected between leaf water potential and the soil water reserve,and was fitted to a negative exponential curve to estimate predawn leaf water potentialfrom a hydrological database. The proposed equation can help to predict the frequency,intensity, and length of droughts potentially capable of causing structural damage to theforest, from the hydrological time series records.ª 2008 Elsevier B.V. All rights reserved.

Introduction

In the Mediterranean coastal of Spain, the evergreen holmoak (Quercus ilex L.) is the main oak tree species, and soilwater conditions strongly determine its regional distributionand productivity, as well as the structural characteristics of

8 Elsevier B.V. All rights reserved

965 903555; fax: +34 965

(J. Bellot).

the various communities that it belongs to (Gracia and Reta-na, 1996; Amorini and Fabbio, 1994). One reason for its widedistribution throughout the Mediterranean basin (Debazac,1983) is that the species has low transpirational waterrequirements – only 440 mm per year (Terradas and Save,1992) – and this permits its expansion even into semi-aridenvironments. This characteristic places holm oak betweena typical Mediterranean dryland shrub such as Quercus coc-cifera, which has a transpirational water requirement ofabout 200 mm per year (Rambal and Debussche, 1995),and the species typical of closed forest communities in tem-

.

68 J. Bellot, J.M. Ortiz de Urbina

perate climates, such as Quercus petrea in France, whichrequire more than 640 mm per year of water to sustain tran-spiration (Nizinski and Saugier, 1989).

Transpiration of the holm oak forest may also affectwater fluxes within certain catchment areas, especiallywhere water supply is the limiting factor and evapotranspi-ration is the main output of water from the ecosystem(Landsberg and Gower, 1997; Roberts and Rosier, 2003;Ffolliott et al., 2003; Fair and Breshears, 2005). Bellot etal. (1992) confirmed that Q. ilex forests have a profoundinfluence on ecosystem water balance at the catchment le-vel. The annual water balance of such catchments revealsthat more than 80% of incoming water is transpired, that10% evaporates from the canopy after being interceptedby vegetation, and that the runoff, detected primarily dur-ing wet periods, is limited to 10% of the total (Bellot andEscarre, 1998). Moreover, periods of drought are notuncommon in the Mediterranean area, and during theseperiods, transpiration is often the only output flux of water.Thus, vegetation exerts nearly complete control over wateravailability in the soil, especially during dry years (Escarreet al., 1984; Zierl, 2007). However, although an obviousdependency between vegetation water status and soil watercontent can be assumed, differences in scale make it diffi-cult to quantify (Jarvis, 1995; Ramırez et al., 2007).

Despite that some studies suggest that this parameter isnot an efficient indicator of soil water availability (Franco-Vizcaino, 1994; Sellin, 1999; Donovan et al., 1999), it isbroadly accepted that predawn water potential (Wpd) re-flects the overall water status of a plant (Tyree, 1999; Mam-olos et al., 2001; Schwinning et al., 2005). Measurements ofplant water status using Wpd have been successfully used toreflect soil water availability and water stress (Kropfl et al.,2002), and the relationship has been particularly pro-nounced during periods of drought (Montana et al., 1995;Gonzalez-Rodriguez et al., 2004; Fair and Breshears, 2005;Schwinning et al., 2005). However, in arid and dry soilsWpd can differ among trees due soil heterogeneity, as hasbeen mentioned by Ameglio et al. (1998) and Ameglio andArcher (1996). As these authors indicated, under a hetero-geneous soil water content distribution, the assumption ofspatial homogeneity of Wpd is questionable, and this param-eter may not be the best indicator for plant water status.

The main purposes of the present study were to analyzevariations in the soil water reserve through the balance atthe catchment level in a Mediterranean ecosystem; investi-gate the relationship between the soil water reserve andplant water status, focusing on the predawn leaf water po-tential (Wpd) of Q. ilex as an indicator of plant water status;and propose a predictive equation for plant water status inQ. ilex based on the soil water reserve at the catchmentlevel.

Materials and methods

Study area and hydrological measurements

The study area is located at Monte Poblet in the Tarragonaprovince of northeastern Spain. The climate is typicallyMediterranean, and is characterised by an average annualrainfall of 596 mm and an average annual temperature of

13.8 �C (Bellot et al., 1992). The precipitation shows highinter-annual variability (ranging from 537 to 802 mm),which leads to varying lengths of an annual dry period thatcoincides with the summer season. A hydrological study ofthe forested Prades catchments in the study area com-menced in 1981 (Bellot et al., 1992), and the results havebeen reported in some depth by Roda et al. (1999). The firstmonitored catchment was the Avic, which has a surface of51.6 ha and a mean slope of 25.8�, and we used this catch-ment in the present study. This catchment lies at altitudesranging between 700 and 1018 m a.s.l., and is completelycovered by a continuous forest dominated by holm oak. Q.ilex accumulates a leaf area index (LAI) of 4 m2 m�2, and ac-counts for more than 80% of the forest’s total basal area of24 m2 ha�1. Other tree species present in the forest are, inorder of abundance, Phyllirea latifolia ssp. media, Arbutusunedo, and Pinus sylvestris.

The soil is a xerochrept, ranging in depth between 30 and80 cm, but reaching a depth of 150 cm at the valley bottomsites as a result of accumulated colluvial materials. It typi-cally has a water-holding capacity of between 70 and197 mm, depending on its depth, but has a maximum capac-ity of 247 mm at colluvial sites (Esclapes et al., 1998). Thepresence of slate bedrock (a Palaeozoic phyllite) belowthe soil layer guarantees that the bedrock below the catch-ment is essentially impermeable. During the study period,from 1992 to 1997, runoff water (R) was monitored at thebottom of the catchment area using an HS-flume gaugeequipped with a continuous water level recorder (OTT Mess-technik, Kempten, Germany). Precipitation (P) was re-corded using 12 rain gauges located at different placeswithin the forest, two of which were installed in the exper-imental plots and the rest of which were distributed 5 at topand 5 at bottom of the catchment. The water balance at thecatchment level was calculated using the water balance (in-put–output) approach (Likens et al., 1977):

P ¼ Rþ Etaþ DWr;

where Eta represents actual evapotranspiration and Wr rep-resents the soil water reserve (i.e., stored water). Thisequation assumes that the underlying bedrock is imperme-able (i.e., there is no loss of water to deep percolation)and that in the long term (at the end of the defined hydro-logical year), stored soil water remains relatively constant(i.e., DWr = 0). For any other period, the stored soil waterchanges, and DWr represents the changes in this parameter.This change is calculated as follows based on the water bal-ance approach:

DWr ¼ P � R� Eta:

Daily potential evapotranspiration (Eto) was calculatedusing data on net solar radiation and diurnal and nocturnaltemperatures provided by the weather stations in the studyarea, and by applying the method developed by Hargreavesand Samani (1982). Accounting for this output together withthe integrated daily measured runoff in the Avic catchment,we can calculate a potential maximum daily output fromthe soil. Once this value has been obtained, it can be com-pared with the total volume of water input through precip-itation, giving the theoretical water reserve of the soil (Wr)for a defined period.

Soil water content at the catchment level and plant water status relationships 69

The estimated theoretical water reserve (Wr), which canbe positive or negative, is used in this paper as a proxy forthe mean soil water content, and reflects the extent ofwater stress for vegetation in the catchment area as a wholeduring the selected period. In our study, we chose a periodof 90 days before each day, that was selected based on pre-vious analyses using periods of 30, 60, and 90 days before, inwhich the shorter periods showed a poorer correlation withthe measured hydrological variables (Ortiz de Urbina, 1999).

Experimental design for the measurements of plantwater status

To evaluate the plant water status we measured Wpd usingterminal twigs on branches of mature holm oak trees thatheld approximately 10 leaves. The evaluation was per-formed before sunrise to determine the water status ofthe excised twig in equilibrium with the soil water content.We used a Scholander pressure chamber (SoilmoistureEquipment Corp. USA). The experimental design involvedmonitoring of Wpd of six different trees in two plots (threetrees per plot) within the study area, on 10 separate datesbetween August 1994 and November 1996 (August andNovember 1994, February, May, August, and November1995, February, May, August, and November 1996), coveringa wide range of soil water conditions. On each measurementdate, we analysed six terminal twigs from each tree, distrib-uted between the upper and the middle part of the crown,in order to detect any differences in the vertical canopyprofile. The two plots were placed in the middle of a slopewith the same northwest orientation as the rest of thecatchment, and their locations were chosen to avoid ex-treme positions at the upper and bottom parts of the slope.We assumed that the predawn water pressures measured inthese trees are representative of the majority of the Q. ilex

1992 1993 1994 1995

Run

off (

mm

)

0

1

2

3

4

RunoffPrecipitation

Figure 1 Pattern of daily variation in precipitation and runoff mbetween 1992 and 1997.

trees in the forested catchment because the positions ofthese plots within the catchment represent the most domi-nant site type in the 51.6 ha.

Because the literature on plant water stress generallyagrees that wpd accurately reflects the overall water statusof a plant (Jones, 2007; Ladjal et al., 2007; Aranda et al.,2005; Kropfl et al., 2002; Donovan et al., 1999 and Nadezh-dina, 1999), we have assumed that this parameter can beused to represent both the ‘‘leaf water status’’ and the‘‘plant water status’’ of individual plants.

Statistical analysis

Seasonal changes in Wpd were compared by means of re-peated-measures ANOVA, using Proc GLM in the SAS 6.11software (SAS Institute Inc., 1995). Correlations betweenvariables were quantified using Pearson’s correlationcoefficient.

Results

Catchment water balances and soil water reserves

In order to understand the hydrological performance of for-ested Mediterranean Q. ilex ecosystems in response towater availability and to the duration of drought periods,we analysed daily rainfall (P) and runoff (R) over a 6-yearperiod (1992–1997), during which annual precipitation var-ied from well below average to more than twice the average(Fig. 1). Although rainfall often reached its maximum in au-tumn and its minimum in July, the only certainty was thepresence of a summer drought that revealed the presenceof a strong annual cycle. The maximum drought intensityoccurred in 1994, with poor rainfall during all 12 months

1996 1997 1998

Prec

ipita

tion

(mm

)0

25

50

75

100

125

150

175

200

easured in the Avic catchment (in the Prades area of Spain)

Table 1 Annual inputs (precipitation) and outputs (runoffand evapotranspiration) in the Avic catchment, in a set ofhydrological years (1992–1997)

Hydrologicalyear

Precipitation(mm)

Runoff(mm)

EctualEvapotranspirationEta ¼ P � R (mm)

91/92 929.49 72.68 856.8192/93 496.39 47.58 448.8193/94 499.05 24.09 474.9694/95 540.7 61.1 479.695/96 695.6 101.53 594.0796/97 727.4 101.61 625.79

70 J. Bellot, J.M. Ortiz de Urbina

and an absolute maximum 1-day rainfall of 174 mm re-corded in October 1994. The calculated annual water bal-ance shows actual evapotranspiration (Eta ¼ P � R) valuesranging from 448 to 856 mm (Table 1), indicating that theuse of available soil water by Q. ilex forest represents themost important impact on the soil water reserve.

This data permits calculation of the theoretical soilwater reserve (Wr). The daily pattern of theoretical Wrfrom 1992 to 1997 was calculated as the difference betweenthe accumulated input (P) and the maximum potential out-put (Eto + R) during the previous 90 days (Fig. 2). The dailyWr ranged between 300 and �550 mm, where zero meansa balanced equilibrium between water entering the catch-ment due to precipitation and maximum water leaving thecatchment as potential evapotranspiration (Eto) and runoff(R) during the previous 90 days. The negative values of Wrconfirm that the catchment experiences a water shortageevery year during the summer, when the cumulative poten-tial water demand in the catchment area surpasses thecumulative input of water. However, this picture changesduring the annual wet season, when precipitation exceedsthe maximum potential demand and outputs; Wr thus be-comes positive during this season. Fig. 2 also shows differ-

1992 1993 1994

Pote

ntia

l Soi

l Wat

er A

vaila

bilit

y (m

m)

(acc

umul

ated

90

days

bef

ore)

-600

-400

-200

0

200

400

Figure 2 Pattern of daily variation in potential soil water resecumulative input and output of water from the catchment during t

ences between 1993 and 1994, which were characterisedby a short wet period, and 1995, 1996, and 1997, whenthe wet periods were longer. The lowest Wr values calcu-lated during the study period occurred in August 1994 (al-most �550 mm) due to extremely low rainfall and high Etoduring this period.

The seasonal pattern of predawn water potential

The seasonal pattern of Wpd in the excised twig collected atthe upper and the middle part of the trees crown appears inFig. 3. There were no significant differences between thetwo levels in the trees (P > 0.05), which suggests homogene-ity in the water status of the whole crown. On the otherhand, significant seasonal differences (P < 0.0001) were de-tected. The pattern of Wpd during the study period showedthe lowest values (i.e., the highest level of water stress)during the summer and the maximum values during wetperiods, and these results paralleled the seasonal variabilityin precipitation, which determines the availability of soilwater. The severe drought during the summer of 1994 ledto a severe drop in Wpd, to values lower than we could mea-sure using the pressure chamber (�4.2 MPa). The highestvalues, which were greater than �1.0 MPa, were recordedin the autumn, winter, and spring.

Relationships between Wr and Wpd

Our results confirmed that the level of Wr (i.e., availabilityor deficit) significantly affected plant water status, as re-flected by Wpd. There was a clear and significant daily cor-relation (R2 = 0.855, P = 0.028) between Wr and mean Wpd

during each period that we analysed (Fig. 4). This relation-ship was expressed using the following negative exponentialequation:

Wpd ¼ a expð�bWrÞ:

1995 1996 1997 1998

rve (Wr), which was estimated as the balance between thehe 90-day period prior to each day.

Aug Nov Feb May Aug Nov Feb May Aug Nov

Pred

awn

Wat

er P

oten

tial (

MPa

)

-4

-3

-2

-1

0

Upper partMiddle part

1994 1995 1996

ns

--

ns

ns

ns ns

ns

ns nsns

Figure 3 Predawn water potential values (Wpd) in the upper and middle canopy. There were no significant differences betweenthe two levels within the canopy (Student–Newman–Keuls test, P>0.05). The results for August 1994 were more negative than thevalue (�4.2 MPa) that we could measure using the pressure chamber.

Potential Water Reserve (Wr)

P - Runoff - Eto (mm)

-500 -400 -300 -200 -100 0 100 200 300

Pred

awn

Wat

er P

oten

tial (

MPa

)

-5

-4

-3

-2

-1

0

R2 = 0.855

PWP = -0.148 * e-0.0073 * Wr

Figure 4 The relationship between the potential water reserve (Wr) and the predawn leaf water potential (Wpd) fitted to anegative exponential equation.

Soil water content at the catchment level and plant water status relationships 71

The parameter values were a = �0.148, which representsthe value of Wpd when Wr = 0, and the shape parameter forthe curve was b = �0.0073. The shape of the resulting curveshows two different regions for the evolution of Wpd versusWr (Fig. 4). For Wr values ranging between 300 and�200 mm, Wpd values remain moderate (from 0 to�1 MPa), reflecting a condition of adequate water availabil-ity for this species in this catchment. The main effect of thesoil water deficit on Wpd appears in the second region,where Wr declines below �200 mm. Beyond this point,Wpd decreases sharply in response to small changes in Wr.

We validated this relationship using a previously con-structed database of Wpd values for the same study areathat was not used to construct the proposed curve in thecurrent study. The temporal variation in Wpd values mea-sured by Sala (1992) in 1988 and 1989 is plotted alongsidevalues for the same catchment and period estimated usingthe proposed curve (Fig. 5); the predicted value was closeto the measured value in most cases. This data set was thenused to calculate the linear correlation between the mea-sured and estimated values; as Fig. 6 shows, the correlationwas strong and significant (r2 = 0.837, P < 0.001).

J F M A M J J A S O N D J F M A M J J A S O N D J

Estim

ated

Pre

daw

n W

ater

Pot

entia

l (M

Pa)

-5

-4

-3

-2

-1

0

1988 1989

measured Predawn water potentialEstimated Predawn water potential

Figure 5 Temporal variation in the predawn water potential (Wpd) measured by Sala (1992) in 1988 and 1989 in the holm oak forestin the Avic catchment (lines), and the corresponding values of Wpd (points) estimated using the negative exponential equationdeveloped in the present study and input–output balances for the 90 days preceding each point.

Calculated Predawn Water Potential (MPa)

-5-4-3-2-10

Estim

ated

Pre

daw

n W

ater

Pot

entia

l (M

Pa)

-5

-4

-3

-2

-1

0

Y= 0.873 X - 0.011R²=0.837P<0.001

Figure 6 The linear relationship between predawn leaf water potential (Wpd) estimated using the negative exponential equationdeveloped during the present study and values measured in the field by Sala (1992).

72 J. Bellot, J.M. Ortiz de Urbina

Applying the negative exponential equation to the dailycalculated potential soil water reserve for a 15-year period(1982–1997) produced the estimated Wpd daily values (Fig.7). The model outputs, thus accurately simulates a repeatedannual pattern that corresponds to the annual rainfall re-gime (Fig. 1). The simulated Wpd reflects an extreme valuenear �8 MPa during the severe drought period in 1994,which corresponds to the value of at least �4.2 MPa thatwe measured in August 1994. During the rest of the simu-lated period, the minimum (most negative) estimated Wpd

values dropped only slightly below �4 MPa during the sum-mer dry period, and this matches the pattern in our actualmeasured values (Fig. 3). The majority, (more than 95%)of the estimated Wpd values were above �1.0 MPa, and this

also matches the seasonal pattern in our measured values(Fig. 3).

Discussion

The annual water balances for the forested Mediterraneancatchment of Avic reflect the scarcity of precipitation andits frequent inability to satisfy the atmospheric water de-mand indicated by Eto. In these areas, the vegetation usesmost of the available soil water to satisfy the estimatedaverage atmospheric demand (Eto) of 1043 mm (Bellot etal., 1992), but this leaves an annual deficit of 500 to700 mm. We assume that the estimated demand is greater

-600.00

-400.00

-200.00

0.00

200.00

400.00

600.00

1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997Year

Pote

ntia

l Wat

er R

eser

ve-W

r (m

m)

-16.00

-14.00

-12.00

-10.00

-8.00

-6.00

-4.00

-2.00

0.00

Pred

awn

Wat

er P

oten

tial (

MPa

)

Wr Predawn Water Potential

Figure 7 The daily pattern of calculated water reserve (Wr, mm) and the predawn water potential (Wpd, MPa) from 1982 to 1997,estimated using our proposed equation.

Soil water content at the catchment level and plant water status relationships 73

than the actual demand, and that plants would transpiremore water if more were available. Instead, they close theirstomata earlier and do not transpire significant amounts ofwater when water is not easily available (i.e., during thesummer drought). The annual value for actual evapotranspi-ration (Eta), calculated using the water balance approach(Likens et al., 1977), is significantly correlated with precip-itation (Bellot et al., 2004), confirming the hypothesis thatin mediterranean region, the vegetation controls water bal-ance at the catchment (Escarre et al., 1984), and at the plotlevel (Cienciala et al., 1994).

Our results indicate that an extended soil water deficit isthe main characteristic of the study area. Fig. 2 shows thetemporal pattern of Wr, with negative values during mostof the year. The estimated values for the potential soil waterreserve (Wr) range from 300 to �550 mm, indicating the ex-treme severity of drought stress during the summer of 1994that resulted from the almost non-existent rainfall fromearly spring 1993 until September 1994, leading to a progres-sive reduction in runoff (R) and Eta during the period whenmaximum Eto values occurred. In fact, the 1994 droughtwas the most severe drought recorded in the study area since1926 (Bernabe, 1994), and by extension, since the beginningof hydrological studies that date back to 1981, as shown bythe potential water reserve (Fig. 7). The value estimatedduring the summer of 1994 (�550 mm) is 28% lower thanthe value that had been frequently reached during othersummers (around �430 mm). This difference representsthe distinction between a normal summer and an extremelydry summer, which is a rare occurrence even in the Mediter-ranean basin. These results indicate that severe droughtssuch as the one experienced during the summer of 1994may be promoted by a reduction in the total amount of pre-cipitation occurring during the summer, or as a consequenceof the lengthening of the summer dry period, for instancedue to a delay in the annual autumn storms.

The calculated Wr reached a minimum of approximately�550 mm, which corresponds to an estimated Wpd of around

�8 MPa based on our proposed equation. This value is belowthe most negative value previously reported for Q. ilex(Rambal and Debussche, 1995; Castell et al., 1994), and thiscould explain the extended LAI reduction (30%), detected inthe region’s forests in that year (Bernabe, 1994). Such areduction in the estimated Wpd value would be accompaniedby severe disruption of physiological activity of the tree,generalised cavitation within the water pathways, and ulti-mately, defoliation of the canopy. All of these processeswere observed in the study area during the 1994 drought(Lloret and Siscart, 1995; Bernabe, 1994). Similar resultswere obtained by Gonzalez-Rodriguez et al. (2004), whoestimated that during the driest periods, Wpd values below�7.3 MPa would be attained by shrub species facing a se-vere water deficit. These authors reported that average soilwater content explained 70 to 87% of the variation in Wpd.

The Wpd value measured during August 1994 was less than�4.2 MPa, and reflected a period of long-term droughtstress. Values this low are often recorded during severedrought in semiarid areas, in tall trees (Tognetti et al.,1996; Damesin and Rambal, 1995), in shrubs (Gonzalez-Rodriguez et al., 2004), as well as in seedlings (Vilagrosaet al., 2003). Wpd values of around �4.2 MPa are clearly be-low the turgor loss point in Q. ilex, which ranges between�2.5 and �3.5 MPa (Sala, 1992; Lo Gullo and Salleo,1993). Thus, these values probably represent exceptionalresults for a mature forest that has evolved over many yearsto reach equilibrium with its environment (Losch and Schu-lze, 1995). Our results also show that the trees included inour study had similar Wpd values among individuals andamong vertical positions within the canopy. This lack of sig-nificant differences between trees in different plots in ourstudy area could result from spatial homogeneity in soilwater content and the development of an extensive rootsystem capable of searching for water throughout the soilvolume (Djema, 1995). These results support the usefulnessof the Wpd to reflect plant water status, as was proposed byMamolos et al. (2001), Kropfl et al. (2002) and Schwinning et

74 J. Bellot, J.M. Ortiz de Urbina

al. (2005), and also suggests that plant water status is clo-sely related to potential soil water reserve (Tyree, 1999).

However, the Wpd approach of this plant physiologicalprocess could be improved with other techniques of mea-surement, as continuous sap flow records. The reason isthe difficulty of reaching an equilibrium between soil watercontent and plant water status in some areas with differentclimatic conditions. A cold and short nocturnal period wouldreduce the capacity of a plant to recharge its internal watersupply and reach hydrological equilibrium with the soil (Our-cival and Berger, 1995; Sellin, 1999; Donovan et al., 1999).Also in dry sites, Cermak et al. (2007) showed that plantwater storage is being permanently refilled by continuoussap flow over night and predawn, even under drought condi-tions. This lack of equilibrium could be better detected withthe sap flow technique than based on predawn water poten-tial measurements. A continuous record of sap flow mea-surement shows that it must not necessarily reflect astatic situation for this physiological process, due to thefact that the Wpd shoot water potential must not exactlyfit to soil water potential over most of the time (Stohr andLosch, 2004).

Conclusions

The equation for potential water reserve (Wr) that wasdeveloped in the present study shows that Wr appears tobe closely correlated with the predawn water potential(Wpd) of Q. ilex trees, and can thus provide a useful predic-tion tool based on hydrological parameters. This relation-ship could also be used to quantify the constraints thatwater deficits at the catchment level can exert on vegeta-tion, especially through their effect on Wpd. The capacityof the proposed negative exponential equation to predictpotential stress situations could be incorporated into futureforest drought management programmes, although addi-tional research will be required to confirm whether theequation must be calibrated for different species or regions.It is important to note that although our approach is validfor large-scale studies such as catchment-level studies, itmay be inappropriate when the goal is to obtain a more pre-cise understanding of microsite-level responses. In thatcase, more intensive sampling would be necessary to cap-ture the variation in soil properties and how this variationaffects the plant responses.

Acknowledgements

This research was supported by the Desertification RESELProgramme from the Ministerio de Medio Ambiente (SpanishGovernment), and by funds from the Spanish Plan NacionalI + D (Ministerio de Educacion y Ciencia), Programmes onHydrology and Water Resources (CGL2004-03627/HID). Thefirst author is part of the GRACCIE project of CONSOLIDER-INGENIO 2010 from MEC.

References

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