global$056
Global Change Biology (1997) 3, 107–118
Changes in carbon isotope discrimination in grain
cereals from different regions of the western
Mediterranean Basin during the past seven millennia.
Palaeoenvironmental evidence of a differential change in
aridity during the late Holocene
J . L . A R A U S , * A . F E B R E R O , * R . B U X O , † M . D . C A M A L I C H , ‡ D . M A R T I N , ‡ F. M O L I N A , §
M . O . R O D R I G U E Z - A R I Z A § and I . R O M A G O S A 1
*Unitat de Fisiologia Vegetal, Facultad de Biologia, Universitat de Barcelona, Avda. Diagonal 645, 08028 Barcelona, Spain,
†Centre d’Investigacions Arqueològiques, Generalitat de Catalunya, Girona, Spain, ‡Department of Prehistoria, Antropologı́a e
Historia Antigua, Universidad de la Laguna, Spain, §Department of Prehistoria y Arqueologia, Universidad de Granada, Spain
and 1Area de Conreus Extensius, Centre R 1 D UdL-IRTA, Lleida, Spain
Abstract
Carbon isotope discrimination (∆) was determined for kernels of six-row barley and
durum wheat cultivated in the western Mediterranean basin during the last seven
millennia. Samples came from different archaeological sites in Catalonia (north-east
Spain) and in the south-east of Spain (mainly eastern Andalusia). Samples from the
present were also analysed. Mean values of ∆ for barley and durum wheat grains
decreased slightly from Neolithic (7000–5000 BP) to Chalcolithic-Bronze (5000–3000
BP) and Iron ages (3000–2200 BP) both in Catalonia and in south-east (SE) Spain.
Values were consistently lower in SE Spain than in Catalonia throughout these five
millennia, which suggests that Catalonia was less arid than SE Spain in this period.
Within a given region, current discrimination values for kernels of the same cereal
species cultivated under rainfed conditions were lower than those of archaeological
grains, which implies more arid conditions at present. Furthermore, an empirical
relationship between ∆ of mature kernels and total precipitation (plus irrigation
where applicable) during grain filling (r2 5 0.73, N 5 25) was established for barley,
currently cultivated at different locations in the western Mediterranean basin in
Spain. The resulting relationship was applied to the ∆ data for barley kernels from
10 archaeological sites in Catalonia and 10 sites in SE Spain, to estimate the
precipitation during grain filling at the time the kernels were produced. For both
regions, current climatic conditions are consistently more arid than those inferred
for the Neolithic, Bronze and Iron ages. In addition, although Catalonia was estimated
to have had consistently wetter conditions (about 20% more precipitation) than SE
Spain throughout these millennia, differences in precipitation between these two
regions have recently increased, with 79% more precipitation in Catalonia. Results
indicate a more rapid increase in aridity in SE Spain than in Catalonia, probably
produced during the last few centuries, and due to anthropogenic causes.
Keywords: archaeological plant remains, cereals, palaeoclimate, precipitation, stable carbon
isotope discrimination, water use efficiency
Received 1 December 1995; revision received 24 April 1996; accepted 8 July 1996
Abbreviations
∆, stable carbon isotope discrimination; δ13C, ratio of
13C/12C relative to PeeDee belemnite standard; p /p
i a,
ratio of intercellular to atmospheric partial pressure of
© 1997 Blackwell Science Ltd.
CO2; WUE, water-use efficiency; BP, before present; NE
Spain, north-east of Spain; SE Spain, south-east of Spain;
VPD, vapour pressure deficit.
107
108
J . L . A R A U S et al.
Introduction
Analysis of the natural abundance of the stable isotopes
of the lighter elements in a variety of materials has
contributed substantially to understanding how these
elements pass through geochemical and biogeochemical
cycles. These studies form the basis of stable isotope
methods that have contributed much to the reconstruction
of past climates. The appropiate methods and the background information needed to use stable hydrogen
(D/H; Epstein et al. 1976) or oxygen (18O/16O; Gat
1980) ratios have been developed to reconstruct climate
(basically temperature and relative humidity) from plant
remains such as subfossil and fossil wood. Many climatic
reconstructions have been based on hydrogen and oxygen
isotopic analyses of cellulose derived from fossil wood
(see Marino & DeNiro 1987; Adams & Woodward 1992).
In contrast to D/H and 18O/16O ratios, the 13C/12C ratios
(of cellulose or other fractions) from terrestrial plants do
not directly reflect climatic conditions. To date, therefore,
the study of past climates based on the carbon isotope
composition of archaeobotanical specimens has not been
sufficiently developed, although the costs of carbon
isotope analyses of solid (organic) samples are substantially lower than those of hydrogen or oxygen. In fact,
stable carbon isotope methods have provided only
indirect information on palaeoclimates, based, for
example, on shifts in the ratio of C3 to C4 plant biomass
(see Goodfriend 1990; Nordt et al. 1994) or changes in
water-use efficiency (ratio of dry matter gained to water
lost: WUE) of cereal crops (Araus & Buxó 1993).
For C3 plants, discrimination against 13C (∆) in plant
material is an integrated indicator of the ratio of intercellular to atmospheric partial pressure of CO2 (pi/pa) and
thus, provided that vapour pressure deficit is constant,
of the WUE of these plants (Farquhar et al. 1982; Farquhar
et al. 1989; Hubick & Farquhar 1989). For C3 cereals, such
as wheat or barley, ∆ of mature kernels constitutes an
integrated measure of pi/pa and therefore of WUE of
photosynthetic organs during grain filling because most
of the carbon in mature grains is derived from photosynthesis during this period (Farquhar & Richards 1984;
Hubick & Farquhar 1989). WUE is strongly affected
by water status during growth. Both decreased water
availability and increased vapour pressure deficit cause
lower ∆ in plant material because of their effects on
stomatal conductance or photosynthetic capacity
(Condon et al. 1992). Therefore, from the analysis of ∆ in
kernels it should be possible to infer water status during
grain filling.
The south-east of the Iberian Peninsula (Spain) is now
one of the driest regions of western Europe. The reasons
for this may be anthropogenic climatic change, possibly
caused in recent decades by CO2 emissions, more local
changes resulting from agriculture and land use, or both
together. In addition, natural causes should be considered.
However, it is not known whether the present situation
has a modern cause or whether a progressive increase in
aridity starting in prehistorical times is involved (see
Puigdefábregas 1992). The present paper reports on the
carbon isotope discrimination (∆) in archaeological
kernels of cereals found in this the western Mediterranean
basin. We compare ∆-values of kernels from a set of
archaeological sites in the south-east of Spain (SE Spain),
as well as present-time kernels cultivated in the same
region, with those for Catalonia (north-east of Spain)
published in a previous paper (Araus & Buxó 1993). Both
regions are representative of two different biogeographic
categories of the western Mediterranean basin. Data
from the present work extend our earlier findings of a
progressive, albeit slight, decrease in ∆ of cereal crops
during the last seven Millennia to other areas of western
Mediterranean basin (Araus & Buxó 1993).
Regardless of differences in genotypes or in cultural
practices (see Discussion), any comparison of present and
past environmental conditions based on crop performance should consider the same area of cultivation. The
current exploitation of marginal land for rainfed agriculture may artefactually increase differences in ∆ between
past and present day crops. To avoid this, we compared
past and present rainfall at the same archaeological sites.
We have developed a procedure to reconstruct past
changes in precipitation at the time these kernels were
formed, and to compare the results with current records
of precipitation at the same archaeological sites where
kernels were found. We first established an empirical
relationship between carbon discrimination of mature
kernels and total precipitation (plus irrigation if applicable) during grain filling, for barley currently cultivated
at different locations of the western Mediterranean basin.
The relationship was applied to the ∆ of barley kernels
from 10 archaeological sites in Catalonia and 10 other
sites in SE Spain, and was used to estimate precipitation
at the time when kernels were produced. These values
were compared with current values of rainfall during
grain filling, estimated for the same archaeological sites.
Materials and methods
Plant material
Seeds of different cereals were analysed: durum wheat
[Triticum durum Desf., including T. aestivum/durum (after
van Zeist & Bakker-Heeres 1982)], hulled barley (Hordeum
vulgare L.) and naked barley (Hordeum vulgare var nudum
L.). When possible, samples from archaeological sites
consisted of at least five kernels per sample. They were
found in a carbonized state and were gathered in dispar© 1997 Blackwell Science Ltd., Global Change Biology, 3, 107–118
∆ O F K E R N E L S M O N I T O R E S R A I N FA L L D U R I N G H O L O C E N E
109
Fig. 1 Map of the location of the
archaeological sites from the two regions
of the western Mediterranean basin,
Catalonia (NE Spain) and the south-east
of Spain (SE Spain), used in this study.
ate fashion from dwelling places. Samples were cleaned
as reported elsewhere (Araus & Buxó 1993). The archaeological sites were located in SE Spain (Fig. 1). They
range from the origins of agriculture (Neolithic Age,
around 7000 BP) up to the Iron Ages (Iberian period,
around 2200 BP), coinciding with the turn of the Era
(Table 1). They are in the high plateaus or near the sea,
most of them in eastern Andalusia and two at La Mancha;
they are thus representative of Mediterranean climate in
the south-western Mediterranean basin. The latitude,
longitude and elevation above sea level of the archaeological sites studied are as follows: Campos (Cuevas
de Almanzora, Almerı́a), 37°189030N, 1°499150W, 110 m;
Castellón Alto (Galera, Granada), 37°449310N, 2°339520W,
900 m; Cerro de la Virgen (Orce, Granada), 37°439420N,
2°309460W, 920 m; Cuesta del Negro (Purullena, Granada),
37°209120N, 3°159300W, 950 m; Cueva del Toro (Torcal de
Antequera, Málaga), 36°579550N, 4°319410W, 1180 m; El
Malagón (Cúllar-Baza, Granada), 37°339330N, 2°259180W,
1100 m; Fuente Amarga (Galera, Granada), 37°459320N,
© 1997 Blackwell Science Ltd., Global Change Biology, 3, 107–118
2°369000W, 860 m;. Las Pilas Huerta Seca (Mojacar,
Almeria), 37°089550N, 1°509340W, 50 m; Los Millares (Sta.
Fé de Mondujar, Almeria), 36°589020N, 2°319050W, 240 m;
Los Palacios (Almagra, Ciudad Real), 38°599280N,
3°379580W, 640 m; Motilla del Azuer (Daimiel, Ciudad
Real), 39°029400N, 3°299390W, 640 m; Peñalosa (Baños de
la Encina, Jaen), 38°109190N, 3°479370W, 350 m; Puente
Tablas (Jaen, Jaen), 37°489480N, 3°449480W, 440 m. The
archaeological sites of Catalonia, used in this work to
establish comparisons (Fig. 1), are located in the plains
or near the sea coast and semi-mountainous areas (see
Araus & Buxó 1993). The chronology of archaeological
samples, in years before present (BP), was based on
stratigraphic dating and radiocarbon ages. Radiocarbon
determinations for the samples from SE Spain were
performed at Teledyne Isotopes (Westwood, NJ 07675,
USA), the Radiocarbon Laboratory of the University of
Claude-Bernard Lyon 1 (69622 Villeurbanne, France) and
the Laboratorium voor Natuurkunde (Groningen,
Netherlands). Determinations for the Catalonian samples
MiddleNeolithic
Late-Neolithic
Chalcolithic
Chalcolithic
Chalcolithic
Pre-campaniform
Campaniform
Chalcolithic
Early Bronze
Bronze
Bronze
Bronze
Bronze
Bronze
Bronze
Cueva del Toro
Cueva del Toro
Mean 6 SE
El Malagon
Los Millares
Campos
Cerro de la Virgen
Cerro de la Virgen
Las Pilas
Cerro de la Virgen
Fuente Amarga
Motilla del Azuer
Castellon Alto
Peñalosa
Los Palacios
Cuesta del Negro
Iron
Fuente Amarga
Mean 6 SE
Iron
Puente Tablas
Mean 6 SE
Age
Site
2400–2300
2500–2400
3190–3100
3520–3240
3570–3340
3720–3360
3700–3520
3730–3530
4100–4000
4040–3900
3940–3840
3840–3730
4165–3925
4240–3880
4265–3860
5700–5200
6500–5700
Calibrated
age
–23.01 6 0.72
–22.29
–23.72
–20.25
–21.51
–23.35
–22.26
–22.53 6 0.25
16.92 6 0.74
16.18
17.65
13.98
15.29
17.20
16.07
16.34 6 0.26
17.57
16.46
16.33
18.45
15.90
16.47
16.51
–22.10
–22.64
–22.68
–23.70
–22.63
–22.50
–24.54
15.54
14.35
16.84
17.45
15.70
16.61
17.31
16.84
15.23
17.25
16.02
16.34 6 0.45
–21.77
–20.61
–23.02
–23.59
–21.92
–22.79
–23.47
–23.05
–21.50
–23.43
–22.24
–22.55 6 0.44
–22.75
–21.95
–23.14
–20.71
–22.14 6 0.54
–22.96 6 0.29
–23.03
–22.63
–24.22
–23.92
–20.67
–22.14
–24.09
–23.49
–21.78
–23.34
–24.10
–22.90
–23.61
–21.53
–23.23 6 0.27
–23.49
–22.96
δ13C
δ13C
∆
H. vulgare
T. durum
16.64
15.80
17.06
14.55
16.01 6 0.55
16.80 6 0.30
16.88
16.46
18.12
17.81
14.42
15.95
17.97
17.36
15.58
17.17
17.97
16.72
17.49
15.30
17.02 6 0.28
17.29
16.74
∆
–22.08
–22.54 6 0.38
–25.09
–24.80
–23.39
–21.35
–21.82
–22.43
–21.49
–21.70
–20.99
–23.23
–22.73
–19.68
–22.28
–24.12
–23.02
–22.47
–22.77 6 0.46
–22.17
–23.67
δ13C
H. vulgare nudum
15.97
16.36 6 0.39
19.03
18.71
17.25
15.11
15.60
16.24
15.26
15.47
14.74
17.06
16.55
13.47
16.08
18.01
16.87
16.26
16.57 6 0.48
15.93
17.51
∆
Table 1 Ages, chronological date, δ13C and ∆ (‰) for the grain samples analysed from Triticum durum, Hordeum vulgare and Hordeum vulgare nudum. Calibrated data represents
the approximate age in years BP estimated from a combination of archaeological and 14C dating after Stuiver & Reimer (1986).
110
J . L . A R A U S et al.
© 1997 Blackwell Science Ltd., Global Change Biology, 3, 107–118
∆ O F K E R N E L S M O N I T O R E S R A I N FA L L D U R I N G H O L O C E N E
were performed as in Araus & Buxó (1993). Calibrated
ages were determined using the computer program CALIBTH (Stuiver & Reimer 1986). In addition, samples (at
least 5 g per sample) from present time (1994) were taken
from a set of 13 landraces of wheat and hulled barley
still cultivated in SE Spain.
Carbon-isotope analysis
13C/12C
ratios were determined by mass spectrometric
analysis at Isotope Services, Inc., Los Alamos, New
Mexico, USA. Results are expressed as δ13C values, where:
residual grain. In the same way, there was no correlation
between the variation in δ13C of kernels and those in
carbon content due to carbonization. Therefore, the δ13C
of these kernels was not corrected for carbonization on
further comparisons between regions. Similarly, Marino
& DeNiro (1987) reported that the carbon isotope ratio
was conserved during various food-processing steps,
carbonization included, and therefore it reflects values
in vivo.
Discrimination (∆) against 13C relative to air is calculated from δa and δp, where a and p refer to air and
plant, respectively; as follows (Farquhar et al. 1989):
δ13C(‰) 5 [(R sample/R standard)–1] 3 1000,
R being the 13C/12C ratio. A secondary standard calibrated against Peedee belemnite (PDB) carbonate was used
for comparison. Sample sizes of 5–10 mg were used.
The precision of analysis was better than 0.10‰. The
percentage of carbon in the set of samples was also
determined using a C/N analyser with atropine as the
standard.
Large variations in δ13C values between chemical constituents within the same plant have been reported. Thus,
isotopic fractionation must be considered if differential
preservation of cellular components is likely to occur, as
in archaeological specimens associated with food preparation practices (Marino & DeNiro 1987). In the case of
archaeological sites situated in moderately dry regions
(as in Catalonia or SE Spain), only carbonized plant
remains survive. Carbonized plant remains do not suffer
further damage from microorganisms, insects, rodents or
birds (Buxó 1993).
The effect of carbonization, if any, on δ13C of archaeological grains from SE Spain was evaluated. First, the δ13C
values of archaeological kernels showed no significant
correlation with their carbon content (%C) for the wheat
samples (r 5 0.04, N 5 26) and for hulled and naked
barley samples combined (r 5 0.17, N 5 40). This
lack of relationship contrasts with a previous report of
archaeological grains from Catalonia (Araus & Buxó
1993). The second approach was to evaluate experimentally the effect of carbonization on kernels from the
present. In this regard, present-time kernels of barley and
wheat were placed for 15, 30, 40, 60, 105 and 120 min in
an electric oven, attaining a maximum temperature of
400 °C. Grains were carbonized under two conditions:
they were either covered with aluminium foil or uncovered. They were then analysed for δ13C and total carbon
content. The corresponding intact kernels were also analysed. Different carbonization processes increased carbon
content from slightly over the value of intact kernels up
to values close to 80% of total dry weight. However,
we found no evidence that experimental carbonization
(under either condition) significantly affected the ∆ of the
© 1997 Blackwell Science Ltd., Global Change Biology, 3, 107–118
111
∆5
δ a – δp
.
1 1 δp
On the PDB scale, δa currently has a value of approximately – 8.00‰ (Farquhar et al. 1989; Keeling & Whorf
1992). Variations in the δa in the past are inferred from
ice-core records and Mauna Loa data (see references in
Mortlock et al. 1991; Leuenberger et al. 1992) and from leaf
material of Atriplex confertifolia recovered from packrat
middens (Marino et al. 1992). The overall shift in δa from
early (around 10,000 year BP) to late (pre-industrial time)
Holocene seems slight (Leuenberger et al. 1992; Marino
et al. 1992). However, assuming that ∆ for the C4 shrub
A. confertifolia was unchanged over time, Marino et al.
(1992) found a variation in δa from about – 6.1 to – 6.6‰
from 7000 to 1500 BP. Based on such values of δa as a
function of time (see Table 1 and Fig. 1 in Marino et al.
1992) we inferred δa for each calculation of ∆-values of
grain samples from the Neolithic to the Iron Ages.
Precipitation during the grain filling of archaeological
kernels
Precipitation accumulated during grain filling for the
archaeological kernels of hulled barley was estimated
from comparisons with present-day data. First we established, for barley currently cultivated in the Western
Mediterranean area, an empirical relationship between
carbon discrimination of mature kernels on the one hand
and total precipitation during grain filling on the other.
Secondly, the ∆ of barley kernels from archaeological
sites in Catalonia and SE Spain was used to calculate
precipitation at the time when these kernels were produced using the relationship derived from contemporary
data. For each region, hulled barley kernels from the
different archaeological sites ranging from Neolithic to
Iron ages were used. A total of 10 archaeological sites
from SE Spain and 10 from Catalonia (NE Spain) were
considered.
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J . L . A R A U S et al.
Table 2 Summary of carbon isotope discrimination (∆) in mature kernels of barley grown under rainfed and irrigated Mediterranean
conditions in Spain. Details of the mean and the range of values of ∆, the location and the climatic conditions of the cultures are
given. GF-water is the water apported (either rainfall alone or rainfall plus irrigation) during grain filling. SCSBC is the evapotranspiration
accumulated during grain filling calculated from the maximum and minimum daily temperatures during this period. Samples comes
from breeding trials, including old and new varieties. Discrimination values from some of the environments at Barcelona and Lleida
have been published previously (Romagosa & Araus 1991; Febrero 1994).
Location
Yearb
Aula Dei (Zaragoza) winterc
Barcelona (Barcelona) winter
Bell-lloc (Lleida) winter
Gimenells (Lleida) winter
Bell-lloc (Lleida) winter
Gimenells (Lleida) winter
Bell-lloc (Lleida) autumn
Bell-lloc (Lleida) winter
Gimenells (Lleida) autumn
Gimenells (Lleida) winter
Falces (Navarra) autumn
Falces (Navarra) winter
Solchaga (Navarra) winter
Solchaga (Navarra) autumn
Artesa (Lleida) autumn
Artesa (Lleida) winter
Aula Dei (Zargoza) autumn
Aula Dei (Zaragoza) winter
Gimenells (Lleida) autumn
Gimenells (Lleida) winter
Laventa (Navarra) autumn
Pueio (Navarra) autumn
Pueio (Navarra) winter
1990
1990
1990
1990
1991
1991
1992
1992
1992
1992
1992
1992
1992
1992
1993
1993
1993
1993
1993
1993
1993
1993
1993
Barcelona (Barcelona) winter
Barcelona (Barcelona) winter
Aula Dei (Zaragoza) winter
Aula Dei (Zaragoza) winter
1990
1991
1993
1994
GF-water
(mm)
Rainfed
51.8
86.9
48.5
63.3
11.8
14.0
158.1
156.6
70.6
101.7
16.3
37.0
33.6
42.0
62.5
88.7
92.3
111.3
88.8
97.9
89.2
99.8
139.4
Irrigated
186.9
243.9
262.0
252.0
SCSBC
(mm)
Mean ∆
(‰)
Range in ∆a
(‰)
159
186
200
206
166
171
185
203
183
204
180
203
171
148
165
177
–
–
171
184
155
132
144
12.51(14)
15.85(14)
16.26(14)
16.22(14)
14.46(14)
15.16(14)
18.22(10)
17.96(10)
16.96(10)
17.07(10)
12.72(10)
14.68(10)
16.12(10)
16.05(10)
16.13(10)
16.14(10)
15.03(10)
14.65(10)
17.24(10)
17.31(10)
16.10(10)
17.05(10)
17.12(10)
11.45–13.42
14.86–17.10
15.04–17.39
15.01–17.44
12.94–15.24
13.78–16.44
17.39–18.70
16.72–18.91
16.25–17.55
16.10–17.88
12.37–13.73
14.16–15.24
15.41–16.74
15.40–17.10
15.51–16.81
15.40–16.92
14.12–15.88
13.94–15.46
16.61–18.11
16.03–18.54
15.36–17.52
15.92–17.81
16.50–17.77
186
155
–
181
19.06(14)
19.57(10)
18.08(16)
18.07(17)
16.93–20.75
18.64–20.78
16.85–19.55
16.89–18.95
are given to indicate maximum range of ∆ observed in the study. The number of genotypes in the study is indicated in
parentheses. bYear of the grain filling cSowing season
aValues
Relationship between ∆ of kernels and water status
during grain filling
For a set of 25 different environments (locations and
sowing time or year of culture) in the Western
Mediterranean area of Spain (Table 2) the correlation
between ∆ of mature kernels of barley and the
precipitation (plus irrigation where applied) and the
evapotranspiration during grain filling was studied.
For each environment the ∆-value used in the correlation
was the mean of 10–14 barley varieties. The duration
and timing of barley grain filling under Mediterranean
conditions was based on the records from these trials,
as well as on information published for Catalonia and
SE Spain (Ramos et al. 1982; Romagosa & Araus 1991).
Thus, total precipitation during grain filling was
calculated as the total rainfall (plus irrigation) during
April plus the first half of May (autumn sowing) or
during the second half of April plus May (winter
sowing). Total evapotranspiration for the same period
was calculated from the average maximal and minimal
daily temperatures by using the computer-program
ETO (Snyder & Pruitt 1991; version 1.04 revised in
February 1994). The vapour pressure deficit (VPD)
between the photosynthetic organ and the air, rather
than evapotranspiration itself, may affect ∆. Thus, the
VPD of the air exerts a strong controlling influence
upon stomatal conductance, which in turn affects the
estimated intercellular CO2 partial pressure, pi, and
finally the ∆-value (Sharifi & Rundel 1993). However,
to calculate VPD it is necessary to measure the
atmospheric humidity in the regions studied. Since
© 1997 Blackwell Science Ltd., Global Change Biology, 3, 107–118
∆ O F K E R N E L S M O N I T O R E S R A I N FA L L D U R I N G H O L O C E N E
113
most of the meteorological stations close to the archaeological sites do not have continuous humidity records,
evapotranspiration could be considered as an alternative
to VPD, since it may be calculated with only maximum
and minimum air temperatures.
Current precipitation during the grain filling period at
the archaeological sites.
For the archaeological sites where barley grains were
found, current precipitation during grain filling of barley
was evaluated. Precipitation was estimated in each case
from historical (means of the 10–40 year before 1980)
records of rainfall from the meteorological station of the
Spanish National Network closest to each archaeological
site (Atlas Agroclimático Nacional de España 1986). Given
that old barley cultivars may have had a longer cycle
and, thus, later flowering, total precipitation during grain
filling was inferred as the overall rainfall during the
second half of April plus May.
Results and discussion
Patterns of changes in ∆ and WUE from Neolithic to
present time in the two western Mediterranean regions
The δ13C of the kernels collected in the set of archaeological sites of SE Spain along with the corresponding ∆values are detailed in Table 1. When all the samples
within a given period (Neolithic, Chalcolithic-Bronze or
Iron Ages) were considered, neither δ13C nor ∆ differed
significantly (P , 0.05) among cereal species (Table 1).
Lack of differences in ∆-values between barley and durum
wheat was also reported for samples from Catalonia
(Araus & Buxó 1993). Mean values of ∆ for all data of
both species combined within each period were then
calculated and compared with those of Catalonia (Araus
& Buxó 1993). Mean of ∆ decreased slightly from Neolithic
(7000–5000 BP) to Chalcolithic-Bronze (5000–3000 BP) and
Iron (3000–2200 BP) Ages both in Catalonia and in SE
Spain (Fig. 2). Values were consistently lower in SE Spain
than in Catalonia throughout these five millennia, which
suggests that more arid conditions in the former region
were already present at that time. Differences ranged
from 0.99‰ in the Neolithic to 0.59‰ in the Iron Ages.
Within a given region, current discrimination for kernels
of the same cereal species cultivated under rainfed conditions were more than 1‰ lower than those of archaeological grains, which suggests more arid conditions now
prevail. Results from SE Spain extend the earlier findings
of a slight progressive decrease in ∆ of cereal grains from
the Neolithic to the Iron Age period and a much steeper
increase during recent times (Araus & Buxó 1993) to
other areas of the western Mediterranean basin.
© 1997 Blackwell Science Ltd., Global Change Biology, 3, 107–118
Fig. 2 Changes in carbon isotope discrimination (∆) of kernels
of cereals cultivated in Catalonia (NE Spain: solid line and filled
circles) and eastern Andalusia and La Mancha regions (SE Spain:
broken line and open circles) during the last seven millennia.
Values from NE Spain represent overall means (6 SE), for sixrow hulled barley and durum wheat kernels together, from the
Neolithic (around 6500 BP), Chalcolithic-Bronze (4200 BP), Iron
(2500 BP) and Middle Ages (800 BP), 1910–20 period and 1990
and are based in a former work of Araus & Buxó (1993). Values
from SE Spain represent overall means (6 SE), for six-row hulled
and naked barley kernels, plus durum wheat kernels together,
from the Neolithic (6000 BP), Chalcolithic-Bronze (3800 BP), Iron
(2400 BP) and 1994.
Based on palaeoenvironmental evidence, the early and
mid-Holocene ‘warm’ phase, between 9000 and 6000
years ago, was associated with moister-than-present conditions over much of the northern hemisphere (Folland
et al. 1990; Street-Perrot et al. 1990). Since then, drought
developed progressively in Spain, interrupted only by
minor fluctuations of higher precipitation during the Iron
Age (third millennia BP) and more recently during the
Little Ice Age (XIV to XVIII centuries) (Burillo et al. 1981;
Creus & Puigdefábregas 1983; Montserrat 1992). Our data
show a progressive, although slight, decrease in values
of ∆ of archaeological kernels from both Catalonia and
SE Spain over the more than four millennia period of
time ranging from Neolithic (7000 BP) to Iron Ages (Fig. 2).
This agrees with a tendency towards aridity throughout
such period. The recent fast decrease in the ∆ of kernels
(Fig. 2) agrees with previous results in cereals (Araus &
Buxó 1993) and wild plants (Peñuelas & Azcón-Bieto
1992). From this recent decrease in ∆ it is possible to infer
a strong increase in WUE, provided constancy in VPD.
Beerling (1994) points out that this would arise largely
due to a decrease in stomatal conductance in response to
increases in atmospheric CO2 concentration since the preindustrial era. In addition, ∆ of cereals cultivated in SE
Spain was consistently lower than that of cereals in
Catalonia in former millennia. The lower ∆-values of
samples from SE Spain than in Catalonia is considered
to be a consequence of lower stomatal conductance in
cereal plants cultivated in SE Spain resulting from reduced
water supply and/or increased temperature.
114
J . L . A R A U S et al.
Fig. 3 Relationship between total precipitation (plus irrigation
if the case) during grain filling and carbon isotope discrimination
of mature barley kernels for a set of 25 trails detailed in Table 2.
For each trail, values of ∆ are means of 10–14 genotypes.
Relationship between ∆ of kernels and water status
Despite considerable taxonomic variability within a given
plant community, the stable carbon isotope signature for
that community gives a strong indication of moisture
availability (Stewart et al. 1995). For 12 plant communities
in Australia, the δ13C value averaged across 12–57 species
for each site was strongly correlated (r2 5 0.78) with
annual rainfall. The correlation was slightly lower when
δ13C was compared with moisture balance (rainfall –
evaporation). Under Mediterranean conditions, precipitation (on a logarithmic scale) explained 73% of the variability observed among environments in the ∆ of mature
kernels (Fig. 3). Differences in water status is by far the
strongest environmental factor affecting ∆ of C3 cereals
cultivated under Mediterranean conditions (Acevedo
1993). When the effect of evapotranspiration was added
to that of precipitation, by multiple regression, both
parameters combined explained only 74% of variability
in ∆ of kernels. The low variability among environments
for this parameter (Table 2) could explain its low additive
effect on ∆ of kernels. The fitted equation was used to
estimate the accumulated precipitation during grain filling for the set of archaeological sites where barley kernels
were found. For each archaeological site, the mean value
of ∆ for all hulled barley samples from Neolithic to Iron
ages (from around 7000 BP to 2300 BP) was used (Fig. 4)
due to the relative constancy, throughout this period, in ∆values (see Table 1 and Fig. 2). Catalonia was consistently
wetter than SE Spain during these millennia. Because
the mean value of ∆ for barley throughout the set of
archaeological sites was (in absolute terms) 0.41‰ higher
in Catalonia than in SE Spain (Fig. 4), the calculated
mean accumulated precipitation was about 20% higher
for Catalonia (Fig. 5).
There is a risk inherent in using present vegetation–
Fig. 4 Mean carbon discrimination values for barley, cultivated
from Neolithic to Iron Ages, at NE (circle) and SE (square) Spain,
plotted against their respective present-day precipitation (mean
for the 10–40-years period before 1980) accumulated during
grain filling (from half-April to the end of May). Carbon isotope
discrimination were the mean 6 SE of ∆-values reported through
this period for the set of 10 archaeological sites in NE Spain
(Araus & Buxó 1993) and 11 sites in SE Spain (Table 1),
from which hulled barley kernels were found. The scatter line
corresponds to the fitted relationship for barley between water
received during grain filling and ∆ of mature kernels (Fig. 3).
Fig. 5 Comparison between measured present-day precipitation
during barley grain filling and that calculated for the past (from
Neolithic to Iron Ages) using the mean ∆-value of kernels for
each archaeological site and the equation of Fig. 2. Values are
the means 6 SE of the 10 archaeological sites located in Catalonia
(NE Spain) and the 10 in SE Spain where kernels of hulled
barley were found.
climate relationships to reconstruct past climate (water
status) from the archaeological record. The direct effect
of changing CO2 partial pressure on the relationship
between precipitation and grain ∆ should be the main
factor to be considered. Beerling & Woodward (1993),
working with Salix herbacea leaves, suggested that the
© 1997 Blackwell Science Ltd., Global Change Biology, 3, 107–118
∆ O F K E R N E L S M O N I T O R E S R A I N FA L L D U R I N G H O L O C E N E
increase in CO2 from the Last Glacial Maximum (16500 BP)
to the present has caused ∆-values to decrease due to a
decrease in stomatal conductance. In fact, the available
data are scarce, and the regression of ∆ against stomatal
conductance explains less than 30% of variability of ∆
throughout this 16.5-kyr period. However, in long-term
experiments the pi/pa ratio (calculated from leaf δ13C
values) remained constant in oats grown at mean partial
pressure of CO2 from 160 to 330 µbar–1, and for wheat it
increased only slightly (about 4%) from 225 to 350
µbar–1 (Masle et al. 1990; Polley et al. 1993). In the same
way, a recent study using δ13C of woody plant remains
(Beerling 1996) reports that these plants probably maintained a nearly constant pi/pa ratio in response to the
increase in atmospheric CO2 concentrations since the
Pleistocene. In fact, coordination of stomatal and mesophyll functions minimizes variations in pi/pa in C3 species
(cf. Wong et al. 1979) grown over a range of CO2 characteristic of the Last Glacial Maximum-to-present atmospheric
partial pressure (Polley et al. 1993). Therefore, we concluded that changing pa values from 270 µbar–1 (prior to
industrialization) to the current value of 355 µbar–1 would
not affect the pi/pa ratio and therefore, based on the model
of Farquhar et al. (1989), the carbon isotope discrimination
would not change.
This model (Farquhar et al. 1989) predicts that changes
in ∆ are brought through changes in pi/pa (see above, in
Material & Methods). However, if pi/pa values from
barley plants of preindustrial times were slightly lower
than that from present time, the inferred (calculated)
precipitation from the archaeological sites may be slightly
underestimated. On the other hand, the lower pa during
past times would lead to a higher stomatal conductance
and therefore to higher transpiration rates. If it is accepted
that ∆ remains relatively steady within the range of
changes in pa monitored during the Holocene, it can be
concluded that a given ∆-value could not have been
attained in the past without greater amounts of water
than those needed today. In addition, if the ratio pi/pa
were somewhat lower during past times due to the lower
pa this would reinforce this tendency. In conclusion, in
the worst case our approach would provide a conservative
(low) estimate of the differences in water availability
from past to present time.
The influence of genotype on ∆ of kernels is much
lower than the effect of environmental factors, especially
water regime. For barley, whereas the range of mean
values across environments is 7‰, reflecting the strong
environmental effect, the genotypic differences within a
given environment are about 2‰ with a tendency to
decrease within the less productive environments (Table
2). For 144 genotypes of durum wheat (most of them
corresponding to the durum core collection of ICARDA)
the range of variation in ∆ was 2.5‰ in the best environ© 1997 Blackwell Science Ltd., Global Change Biology, 3, 107–118
115
ment and 2.2‰ in the less productive environment (Araus
& Nachit 1996). In addition, within a given environment,
the 95% confidence interval for the mean value across
genotypes was less than 0.1‰. A recent report on the
changes in the photosynthetic properties of Australian
wheat cultivars over the last century (Watanabe et al.
1994) concludes that there was no variation in the pi/pa
ratio. With regard to differences in phenology, trials with
old and new cultivars suggest that ‘older’ (or ancient)
genotypes may have flowered later (Davidson et al. 1985;
Cox et al. 1988; Austin et al. 1989; Slafer et al. 1993)
although duration of grain filling may have been almost
unchanged or even somewhat shortened (Austin et al.
1989). Indeed, it is well established that grain-filling
duration depends on the integral of the mean daily
temperature accumulated during this period and its
duration is reduced as temperature increases. Under
Mediterranean conditions there is a convergence in maturation time even comparing genotypes of contrasting
phenology (Perry & D’Antuono 1989; Siddique et al.
1989). If so, the climate in April and May in former
millennia should be probably wetter (because grain filling
would occur somewhat late in the season and would be
even shorter) than that calculated and thus we would
have a conservative estimate of precipitation in the past.
The (scarce) available data suggest that other physiological differences related to plant performance and water
status between ancient and new cultivars could be small
(Amir & Sinclair 1994). In addition, if some effect of
carbonization (slightly increasing the ∆-values) were considered (Araus & Buxó 1993), calculated precipitation
during former millennia should be around 10% greater.
Therefore, data would still indicate a drying of the climate.
Regarding the cultural conditions, there is no archaeological evidence of irrigation in the western Mediterranean basin until the Roman settlement (2200 year BP), even
when earlier irrigation practices have been proposed for
a few archaeological sites in SE Spain (Chapman 1978;
Gilman & Thornes 1985). The effect of nitrogen fertilization on ∆ seems incostintent (see Condon et al. 1992),
whereas competition against weeds for light should not
have significant effects on ∆ because photosynthetic
organs contributing to the growth of kernels are located
in the upper part of the stem, and are thus unlikely to
be shaded by weeds.
Differential decreases in present-time precipitation
between the regions studied
Precipitation during grain filling at the time when archaeological kernels formed was compared with present-day
precipitation estimated for the same archaeological sites
(Fig. 5). Because the 1980–95 period has been particularly
dry and warm, present-day precipitation was calculated
116
J . L . A R A U S et al.
from meteorological records from before 1980. For both
regions, current climatic conditions are consistently more
arid (with a lower precipitation) than those inferred for
the Neolithic, Bronze and Iron ages, particularly in SE
Spain. Thus, whereas in Catalonia precipitation has
decreased about 12%, in SE Spain it has decreased 41%.
This range of decrease in precipitation in SE Spain from
past to present agrees with the information inferred from
palaeobotanical (charcoal analysis) data (Rodrı́guez-Ariza
1992). In addition, although Catalonia had wetter conditions (about 20% more precipitation) than SE Spain
through these ages, differences in precipitation between
the two regions have increased recently, with about 79%
more precipitation in Catalonia (Fig. 5), evidence of a
faster shift towards aridity in SE Spain. For each of the two
regions considered, the mean present-day precipitation
across the set of archaeological sites where hulled barley
was found was plotted against the mean value of ∆ of
these barley kernels (Fig. 4). Whereas for Catalonia this
value fitted well within the relationship between ∆ and
precipitation established for barley cultivated in the present, for SE Spain this value appeared clearly beyond,
showing a very high ∆ of archaeological kernels in relation
to current precipitation (Fig. 4).
Palaeobotanical data from archaeological sites of south
of France and Catalonia (Vernet & Thiébault 1987; Vernet
1990), and Spanish Levante (Vernet et al. 1983; Badal
1990) suggest that large-scale degradation of vegetation
began in the Middle Neolithic Age (seventh millennium
BP). Along the coast of SE Spain the process may also
have begun in the Middle Neolithic Age, whereas in the
interior it may have begun early in the fourth millennium
BP (Rodrı́guez-Ariza 1992) or even later (Rodrı́guez-Ariza
et al. 1992). Therefore, the available palaeobotanical
information from the western Mediterranean basin could
be interpreted in terms of a progressive shift to aridity
from early times associated with an anthropogenic effect
on climate. However, in their studies of erosion around
archaeological sites comprising from Late Neolithic
(between 5700–5200 BP) to Early Bronze Age (3800–3500
BP) in SE Spain, Wise et al. (1982) and Gilman & Thornes
(1985) concluded that accelerated erosion tends to be
very localized and not due to large-scale climatic factors
(see also Wainwright 1994).
In fact, the strong shift to more arid conditions in SE
Spain seems to be recent. Precipitation decreased steadily
in the south of Spain from the late 1800s to the 1940s
(Thornes 1991). This shift to arid conditions has been
particularly evident in SE Spain; for example, the Murcia
region has suffered an overall decrease in annual precipitation of about 150 mm during the same period (Brandt
et al. 1991). During the past decade, and especially the
last four years, dryness has become dramatic in this area.
The shift towards drought in SE Spain during the last
centuries could be anthropogenic. Indeed, the dramatic
increase in the sediment deposition rate in most Mediterranean deltas of SE Spain during the second half of the
XVIII century, can not be explained by climatic changes
(Hoffmann 1988). Expansion of new crops such as the
grapevine and related cultural practices (XVI century AD)
after the end of Moorish rule (Puigdefábregas 1992), as
well as the strong development of a national policy,
favouring extensive sheep pastoralism (Thirgood 1981),
could be responsible for this increase in erosive processes.
Whereas for SE Spain such degradation has continued
up to the present (see Rodrı́guez-Ariza 1992), vegetation
in Catalonia has suffered much less degradation and
during this century the forest area has even increased
(Thirgood 1981).
Summarizing, our data suggest a gradual slight
increase in aridity along the Western Mediterranean Basin
during the last five millennia before our Era, and a more
rapid increase in aridity in recent times. Differential
evolution between regions towards current, more arid
conditions, may have occurred during the last few centuries, coinciding with a rapid development of arid conditions in SE Spain, and perhaps related to the effect of
man on the biosphere.
Acknowledgements
We thank Drs R. Austin and K. Hogan for their valuable
comments on the manuscript and Dr R.L. Snyder for his generous
help with calculations of evapotranspiration. We are also grateful
to Dr R. Aragües and R. Isla for providing data of ∆ and
precipitation from Aula Dei. This work was supported in part
by the Research Projects of CICYT AGF93–0755-CO2 and AGF95–
1008-C05–03, Spain.
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