Crop Water Availability in Early Agriculture: Evidence from Carbon Isotope Discrimination of Seeds from a Tenth Millennium BP Site on the Euphrates

Global Change Biology (1999) 5, 201–212

Crop water availability in early agriculture: evidence from carbon isotope discrimination of seeds from a tenth millennium BP site on the Euphrates

J. L. ARAUS,* A. FEBRERO,* M. CATALA,† M. MOLIST,‡ J. VOLTAS§ and I. ROMAGOSA§ *Unitat de Fisiologia Vegetal, Universitat de Barcelona, Spain, †Facultat de Humanitats. Universitat Pompeu Fabra. Spain, ‡Departament d’Antropologia Social i Prehistoria, Universitat Autonoma de Barcelona, Spain §Area de Conreus Extensius, Centre R ϩ D UdL – IRTA, Lleida, Spain

Abstract The analysis of carbon isotope discrimination (∆) in crop plant remains from archaeological sites may help to assess water availability for early agriculture. This study presents the analysis of ∆ in seeds of naked wheat (Triticum aestivum/durum), lentil (Lens orientalis/ culinaris), and flax (Linum sp.) found at the archaeological site of Tell Halula in the valley of the Middle Euphrates (Syria). This Neolithic site is the oldest in this region of the Fertile Crescent where the cultivation of domesticated plants has been reported, with seed remains ranging from 9550 to 8465 BP. Most of the seeds analysed showed ∆ values greater than 16 ‰, reaching 20 ‰ for some samples of flax. For wheat, ∆ values were much higher than those reported in present-day (1996) durum wheat crops cultivated under rainfed conditions in north-west Syria under environments with somewhat higher rainfall than Tell Halula. Similarly, grains of present-day (1997) barley cultivated in the archaeological site also showed lower values than those found in archaeological kernels. An empirical relationship between ∆ of mature kernels and total precipitation (plus irrigation where applicable) from heading to maturity (r2 ⍧ 0.82, n ⍧ 11) was established for durum wheat, currently cultivated in different environments of the Mediterranean basin. The resulting relationship was applied to the data on ∆ of wheat fossil kernels from Tell Halula to estimate the accumulated water inputs during the time (about 6 weeks) the kernels were produced. Calculated water inputs for wheat during early agriculture were (over 110 mm) at least 5 times higher than current-day rainfall accumulated in Tell Halula during the same phenological period. These results strongly suggest that early agriculture wheat was cultivated at Tell Halula under much wetter conditions than are currently to be found in the area. The presence of flax and its very high ∆ values also support this conclusion. Whether such humid conditions during cultivation were due to moister conditions prevailing at this time, by planting in alluvial areas or by irrigation works is discussed.

Keywords: early agriculture, ﬂax, lentil, stable carbon isotope discrimination, water regime, wheat Abbreviations: ∆, stable carbon isotope discrimination; δ13C, ratio of 13C/12C relative to PeeDee belemnite standard; pi/pa, ratio of intercellular to atmospheric partial pressure of CO2; BP, before present; L-N, Late Neolithic; M- and L-PPNB, Middle and Late Pre-Pottery Neolithic B (PPNB). Received 11 November 1997; revised version received and accepted 9 February 1998

Introduction The environmental conditions prevailing when agricul- somewhat obscure. Although recent data on climatic ture was ﬁrst developed in the Old World, in the Fertile conditions are more precise chronologically (Baruch & Crescent (Near East) during the early Holocene, are Bottema 1991; Moore & Hillman 1992) and can explain the environmental setting during the transition from hunter/gathering to farming communities, such data Correspondence: J.L. Araus, e-mail [email protected], are indirect and not conclusive (Hillman 1996). Present fax ϩ34/ 34112842 palaeoclimatological and archaeological ﬁndings suggest

© 1999 Blackwell Science Ltd. 201 202 J.L.ARAUS et al. a more humid environment was to be found in the Near just a short time later (Zohary 1996), although in the East at the birth of agriculture. However, much remains to domestication of lentils this may have occurred before be discovered about the precise nature of the environment any other crop (Ladizinsky 1987, 1989). Whatever the and crop growing conditions (Zohary 1996; Hole 1998), case, as with cereals, domestication of grain legumes not least the crop water status during these crucial would have only been possible only under cultivation millennia for which no reliable (direct) data are available (Zohary 1989). (Rossignol-Strick 1993). In the present study the ∆ of fossil seeds is analysed For C3 plants, such as those crops first cultivated in to determine the water status of several crops (wheat, the Fertile Crescent, carbon isotope discrimination (∆)in lentil and flax) of the Neolithic crop assemblage grown crop grains constitutes an integrated record of the ratio in Tell Halula. The period studied corresponds to the of intercellular to atmospheric partial pressure of CO2 first millennium of cultivation of domesticated plants at (pi/pa) and, thus, of the water status during the growth this archaeological site. of these grains (Farquhar & Richards 1984). For example, in cereals the water regime during grain filling strongly Materials and methods affects the ∆ values of mature kernels (Condon et al. 1987, 1992; Craufurd et al. 1991; Romagosa & Araus 1991; Araus Archaeological site and plant material et al. 1997a,b). Decreased water availability (and increased evapotranspiration) causes lower pi/pa and, thus, a Tell Halula is an archaeological site situated in the Middle decrease in ∆ of grains because of the effects of these Euphrates region, Syria (Fig. 1). This site comprises (to environmental factors on stomatal conductance or photo- date) three periods: Middle and Late PPNB, and Late synthetic capacity (Farquhar & Richards 1984; Condon Neolithic (pre-Halaf) and is being excavated by the et al. 1992). Therefore, from the analysis of ∆ of grains it Universitat Autonoma de Barcelona. The present-day should be possible, in principle, to infer water status natural vegetation in the region is a degraded steppe, during grain filling (Araus et al. 1997a). In this context, with a total annual rainfall of about 250 mm (see legend the measurement of ∆ from seeds found in archaeological in Fig. 1). At present the land above the valley floor is sites has been proposed as a method for evaluating the extensively used for lamb and goat grazing and rainfed water status at the time these crops were grown (Araus cultivation of barley, whereas durum wheat and horticul- & Buxo´ 1993; Araus et al. 1997a,b). tural crops are only cultivated where supplementary The beginnings of agriculture in the Old World seems irrigation is available. to be associated with cereal domestication. Indeed, even Samples of seeds of durum wheat [Triticum durum in those case where experimental studies provide a Desf., including T. aestivum/durum (after van Zeist & theoretical indication of a time-lapse between the first Bakker-Heeres 1982), lentil (Lens orientalis/culinaris), and attempts of cultivation and morphological domestication flax (Linum sp.) were used for stable carbon isotope (Hillman & Davies 1990; Willcox 1991, 1992), to date analysis (Fig. 2). They were found in a carbonized state cultivation occurring prior to morphological domestica- and were gathered in disparate fashion from domestic tion has not been identified from plant remains found in fires, cooking ovens and room floors. Six stratigraphic the Near East (Willcox 1996). In this context, the Pre- levels from the archaeological site were studied. Soil Pottery (aceramic) Neolithic B (PPNB) site of Tell Halula samples were treated using a standard flotation tank is the oldest archaeological site on the Euphrates at which in the field with 0.3 mm (flotation) and 2.5 mm (wet) domesticated crops have been reported (Molist et al. 1995; sieves. Plant remains were then dried slowly before Willcox 1996; see also Hole 1998). Thus, levels from transport and sorting of seeds. Material was compared the tenth millennium before the present (BP) at Halula with modern reference material which had been indicate the appearance of domesticated crops such as gathered from various locations in the Near East. In naked wheat (Triticum aestivum/durum), emmer (Triticum order to characterize the weed plant assemblage associ- dicoccum) and barley (Hordeum vulgare) (Molist et al. 1995; ated with these early crops, determination of weed Willcox & Catala` 1996; Willcox 1996). The domestication plants from carbonized material recovered in the of these cereals went hand-in-hand with the introduction same stratigraphic sequences was also performed. of several companion plants to the Fertile Crescent. Most Palaeobotanical determinations were performed at the common in the PPNB farming villages are the remains Institut de Prehistorie Orientale of the CNRS Jale`s, of two pulses: lentil (Lens culinaris) and pea (Pisum France (Willcox 1996; Willcox & Catala` 1996). The three sativum); and of a single oil-and-fibre crop, flax (Linum crop species considered in this study were present usitatissimum) (Zohary 1996). These companion crops throughout the stratigraphic sequence studied. The were very probably domesticated (and taken into cultiva- chronology of archaeological samples, in years before tion) simultaneously with the wheats and barley, or the present (BP), was based on stratigraphic dating and

Fig. 1 Map of the location of the archaeological site studied. Tell Halula is situated in the Euphrates region (Syria) about 100 km east of Aleppo and 30 km south-east of Memjib (a). This site is on the west river bank, 4 km from the main Euphrates valley, on a side valley (b). The site is an artiﬁcial mound (i.e. Tell) 8 m in height, roughly circular (360 ϫ 300 m) in shape, delimited to the south and east by two rivers (wadis) tributaries of the Euphrates (c). Latitude and longitude above sea level of the archaeological site are 35°55ЈN, 38°30ЈE, 300 m. Average (1978–94) annual precipitation of the closest meteorological stations around Halula are: Membij (36°01ЈN, 38°05ЈE) 282 mm, El Khafseh (36°12ЈN, 38°04ЈE) 211 mm and Mazra et Maskaneh (36°32ЈN, 37°57ЈE) 206 mm. Accumulated precipitation during April, the ﬁrst half of May, corresponding, in the case of winter barley, with the period from heading- anthesis to maturity (Araus et al. 1997a), was 33, 28 and 27 mm, respectively. Precipitation during the second half of April plus May, corresponding, in the case of durum wheat, with the period between heading and maturity was 23, 18 and 18 mm, respectively.

Fig. 2 Scanning electron micrographs of charred seeds of durum wheat (a), lentil (b) and ﬂax (c) found in the site of Tell Halula and used for stable carbon isotope analysis. radiocarbon ages. Radiocarbon determinations were program CALIBTH3. After calibration the range of performed at Beta Analytic Inc. (Miami, FL, USA). dates for the material studied was 9550–8465 BP.In Dating ranged from c. 8700 Ϯ 60 BP to 7690 Ϯ 130 BP addition samples from the present day (1997) were uncalibrated. Calibrated ages were determined accord- taken from hulled barley cultivated in the same Tell ing to Stuiver & Reimer (1986) by using the computer and its surroundings.

Carbon-isotope analysis carbon discrimination of mature kernels on the one hand and total water inputs accumulated from heading to crop Prior to stable carbon isotope analysis, seed samples were maturity on the other. The ∆ of wheat kernels from Tell cleaned as reported elsewhere (Araus & Buxo´ 1993). Halula was then used to calculate precipitation at the time The isotopic 13C/12C ratios were determined by mass when these kernels were produced using the relationship spectrometric analysis at Isotope Services, Inc., Los derived from contemporary data. Alamos, New Mexico, USA and the ‘Serveis Cientific For a set of 11 environments (locations and/or years Te` cnics de l’Universitat de Barcelona’, Barcelona, Spain. of culture), mostly in the Mediterranean areas of Spain Results are expressed as δ13C values, where: and Syria, the correlation between ∆ of mature kernels δ13C(∆) ϭ [(R sample/R standard) – 1] ϫ 1000, (1) of durum wheat and the precipitation (plus irrigation where applied) from heading to maturity was studied. R being the 13C/12C ratio. A secondary standard calib- The timing of wheat heading and the further duration of rated against Peedee belemnite (PDB) carbonate was used grain filling under Mediterranean conditions was based for comparison. Sample sizes of 5–10 mg were used. The on the records from these trials. Thus, total precipitation precision of analysis was less than 0.10 ‰. The percentage during grain filling was calculated as the total rainfall of carbon in the set of samples was also determined (plus irrigation) during April plus the first half of May using a C/N analyser with atropine as the standard. The (autumn sowing) or during the second half of April plus potential effect of carbonization on δ13C of archaeological May (winter sowing). Current precipitation at Tell Halula grains was ignored based on findings reported elsewhere was estimated from historical (means from 1978 to 1994) (Marino & DeNiro 1987; Araus et al. 1997a,b). Moreover, records of rainfall from the three meteorological stations the total carbon content of fossil seeds was closely similar closest to this site (see Table 1). Given that old wheat to those of intact present-day seeds, which suggests a cultivars may have had a longer cycle and, thus, later slight carbonization process (Table 1). The stable nitrogen flowering, total precipitation during grain filling was composition (δ15N) of present-day barley seeds collected inferred as the overall rainfall during the second half of at the archaeological site was also analysed by mass April plus May. spectrometric analysis. The ratio 15N/14N in the samples was measured in relation to that of a standard of atmospheric N2, according to the same expression than (1) Results and discussion (Amaro et al. 1995). Discrimination against 13C relative to air (∆) was calcu- ∆ of fossil seeds vs. those of current-day seeds lated from δ and δ , where a and p refer to air and a p The carbon isotope composition (δ13C) of the fossil seeds plant, respectively; as follows (Farquhar et al. 1989): collected in the six stratigraphic levels studied at Tell δ – δ Halula, along with their total carbon content values are ∆ ϭ a p . (2) δ detailed in Table 1. After calculation of carbon isotope 1 ϩ p discrimination (∆), and except for two samples (one of δ On the PDB scale, a currently has a value of approxi- wheat and the other of lentil), all the seeds studied mately – 8.00 ‰. For calculation of ∆ values of grain showed ∆ values greater than 16 ‰, reaching 20 ‰ for δ samples from the archaeological site, a values were some samples of flax (Fig. 3a). Mean ∆ values of the seeds inferred from the work of Marino et al. (1992) as reported found in the different stratigraphic levels corresponding δ elsewhere (Araus & Buxo´ 1993). Thus the a values used to either Middle Pre-Pottery Neolithic B (PPNB), Late for the samples of the six different stratigraphic levels PPNB and Late Neolithic were plotted for each plant were as follows: first level (c. 8700 BP uncalibrated) species (Fig. 3b). Flax seeds showed higher values of ∆ – 6.609 ‰, second (c. 8350 BP) – 6.583 ‰, third than wheat kernels and lentil seeds during the Middle (c. 8300 BP) – 6.580 ‰, fourth (c. 8100 BP) – 6.568 ‰, and Late PPNB. Lentil seeds also tended to show higher fifth (c. 7880 BP) – 6.552 ‰, and sixth (c. 7690 BP) – 6.539 ‰. ∆ values than wheat during the Late Neolithic, although differences were not significant. This could reflect the growth habit (Araus et al. 1997c), lentils being less deter- Water inputs during grain filling of archaeological minate plants than wheat. kernels Values of ∆ of fossil seeds are very high for rainfed Precipitation accumulated during grain filling for the crops growing under Mediterranean conditions. Thus, archaeological kernels of durum wheat was estimated for example, ∆ of wheat kernels were much higher than from comparisons with present-day data. First we estab- those reported in present-day (1996) durum wheat crops lished an empirical relationship, for the durum wheat cultivated under rainfed conditions in north-west Syria currently cultivated in the Mediterranean area, between (Araus et al. 1997b) in environments with somewhat

Table 1 Carbon isotope composition (δ13C, ‰) and carbon content (C, %) of fossil seeds of durum wheat, lentil and ﬂax found in Middle Pre-Pottery Neolithic B (M-PPNB), Late Pre-Neolithic B (L-PPNB) and Late Neolithic (L-N) levels at Tell Halula. All the dates are in uncalibrated radiocarbon years BP. Means with different letters are signiﬁcantly different (Pഛ0.05) by Duncan’s comparison test

Archaeological Radiocarbon Durum wheat Lentil Flax levels ages δ13CC δ13CC δ13CC

M-PPNB 8800–8600 –23.17 41.48 –23.27 27.34 –26.10 41.39 –25.34 46.19 –22.45 19.62 –25.34 51.99 –23.26 37.27 –23.70 39.40 –23.68 46.04 –24.24 40.59 –23.64 46.50 –23.43 47.91 –22.28 41.66 –24.14 43.86 –23.87 43.28 –23.54 47.01 –22.35 45.12 –22.37 40.39

Mean Ϯ SE –23.42 Ϯ 0.24bcd 43.89 Ϯ 0.88bc –22.42 Ϯ 0.33bcd 31.74 Ϯ 4.35a –25.72 Ϯ 0.27ab 46.69 Ϯ 3.75bc

L-PPNB 8500–8200 –23.07 49.06 –23.85 45.20 –24.46 46.72 –24.59 44.40 –24.17 34.74 –25.75 47.22 –23.41 41.83 –23.04 42.81 –23.97 53.86 –24.25 46.15 –23.12 45.85 –23.08 47.08 –22.90 49.10 –22.45 46.10 –21.62 30.37 –24.70 48.36 –25.09 41.58 –22.41 46.86

Mean Ϯ SE –23.50 Ϯ 0.33bcd 46.23 Ϯ 0.75bc –23.40 Ϯ 0.39bcd 41.38 Ϯ 2.31bc –24.73 Ϯ 0.43abc 49.27 Ϯ 1.88bc

L-N (Pre-Halaf) 8200–7600 –22.00 45.17 –22.79 42.31 –22.57 44.40 –23.30 46.44 –22.89 41.60 –24.23 55.38 –23.56 44.98 –24.39 37.38

Mean Ϯ SE –22.95 Ϯ 0.39cd 45.53 Ϯ 0.37bc –23.36 Ϯ 0.42bcd 40.79 Ϯ 0.97b –23.40 Ϯ 0.59bcd 49.89 Ϯ 3.88c

higher rainfall (Breda and Tel Hadya with about 340 and situation resembling that expected in ancient agriculture. 390 mm per year, respectively) than Tell Halula (about Mean Ϯ SE of ∆ was 16.02 Ϯ 0.19 ‰. This value is again 250 mm). Thus, ∆ values (mean Ϯ SD) for Breda and Tel clearly below ∆ values for all the archaeological seeds. Hadya were 14.0 Ϯ 0.4 ‰ and 15.3 Ϯ 0.4 ‰, respectively, Moreover, under present-day conditions, ∆ of durum which is clearly below those measured in the fossil wheat would be even lower. Considering that barley kernels (around 17 ‰) during the three periods studied attains maturity at least 2–3 weeks earlier than durum (Fig. 3a). Indeed fossil kernels showed ∆ values even wheat, under Mediterranean conditions the grain ﬁlling slightly higher than those (16.7 Ϯ 0.5 ‰) of durum wheat period for barley usually takes place under wetter condi- cultivated with supplementary irrigation at Tel Hadya tions than those expected for durum wheat. Indeed this (Araus et al. 1997b) and not much different from those difference in phenology between both crops is one of the typical (over 17.5 ‰) of durum wheat grown in absence main reasons why barley is the only cereal cultivated of water stress (e.g. full irrigation, Araus et al. 1997c). under rainfed conditions in areas with rainfall regimes Samples from present-day (1997) hulled barley kernels comparable to that which currently predominates in cultivated at seven points of the archaeological site were Tel Halula. also analysed. These barley crops grow without signiﬁc- ant (if any) amounts of chemical nitrogen fertilization, as Water status during wheat cultivation inferred from the low total nitrogen content (mean Ϯ SE ϭ 1.52 Ϯ 0.11%) and the high 15N isotopic composition The relationship between ∆ of wheat kernels and water (δ15N ϭ 6.63 Ϯ 1.26 ‰) of kernels (Amaro et al. 1995), a status from heading to maturity under Mediterranean

© 1999 Blackwell Science Ltd., Global Change Biology, 5, 201–212 WATER AVAILABILITY IN EARLY AGRICULTURE AND ∆ 207 conditions was studied in more detail. Carbon isotope which would agree with a higher vapor pressure deficit discrimination of kernels from recent years was positively during the latter part of wheat growth than that of barley correlated with accumulated precipitation (plus irrigation and might be explained by the later occurrence of this where applied). Differences in water inputs (on a logarith- period in the former crop. Thus, for barley cultivated mic scale) between environments explained 82% (∆ ϭ under Mediterranean conditions differences between 6.95 ϩ 4.85 log [water received from heading to maturity]; environments in terms of water status (also on a logarith- r2 ϭ 0.82) of the variability observed in the ∆ of mature mic scale) from heading to maturity explained 73% of kernels (Fig. 4). The fitting line showed the same slope the variability in ∆ of mature kernels (Araus et al. 1997a). (P ഛ 0.05) as that previously reported for barley (see The results of Stewart et al. (1995) which show a strong dotted line in Fig. 4) cultivated under mediterranean (linear) correlation (r2 ϭ 0.78) between annual rainfall conditions (Araus et al. 1997a). However, fitting line for and δ13C across a set of 12 plant communities also support wheat was shifted somewhat to the right (P ϭ 0.077), the usefulness of δ13C (or ∆) as an indicator of moisture availability. In both cases, the further addition (by mul- tiple regression) of the effect of evapotranspiration to that of water inputs did not explain a higher percentage of variability in ∆ of kernels. ∆ in photosynthate may also respond to mean relative humidity in the atmosphere by itself as derived from the model of Ball et al. (1987). Thus, for example, a shift in relative humidity in the canopy, ignoring offsets between leaf temperature and air temperature, from 50% to 60% would account for a 1.2 ‰ increase in ∆. However, the variations in ∆ in response to shifts in relative humidity seems to be substantially lower than those predicted by the model at low relative humidities. Actually whereas a relative humidity of 30% would lead to a ∆ of about 15 ‰, relative humidities of 20% and 10% would produce ∆ values of 9 ‰ and – 9 ‰, respectively, which are far outside the range of experimental data. From the equation fitted above (Fig. 4), and by using the corresponding ∆ values of fossil kernels, the amount of water received (from heading to maturity) by the Neolithic wheat crops of Tel Halula could be calculated (Fig. 5). Thus, for the durum wheat kernels found in the Medium and Late PPNB sequences the total water received by the crop from heading to maturity surpassed

Fig. 3 Upper (a). Carbon isotope discrimination of durum wheat, lentil and flax seeds found in 6 different stratigraphic levels of Tell Halula. All dates are in uncalibrated radiocarbon years BP. Samples ranged from Middle PPNB (c. 8700 Ϯ 60 BP uncalibrated and 9550 BP calibrated) to late Neolithic (c. 7690 Ϯ 130 BP uncalibrated and 8465 BP calibrated). ∆ values from present-day (1996) mature kernels of durum wheat cultivated under rainfed conditions in north-west Syria in environments with similar (Breda) or somewhat higher (Tel Hadya) precipitation than Tell Halula are presented for comparison. For these two environments values presented are means Ϯ SD of 144 genotypes and two plots each (Araus et al. 1997b). Lower (b). Mean Ϯ SE of ∆ values of durum wheat, lentil and flax seeds found in stratigraphic levels of Tell Halula corresponding to Middle PPNB (M-PPNB), Late PPNB (L-PPNB) and Late Neolithic (L-N). Means with different letters are significantly different (P ഛ 0.05) by Duncan’s comparison test.

Fig. 4 Relationship between total precipitation (plus irrigation if the case) from heading to maturity and carbon isotope discrimination of mature durum wheat kernels (continuous fitting line) for a set of 11 trials developed in Spain, Syria and Mexico. For each Spanish and Mexican trial, the ∆ value used in the correlation was the mean of 6–10 varieties, whereas for Syrian trials ∆ was the mean of 125 genotypes. Samples are Fig. 5 Comparison between present-day precipitation drawn from breeding trials, including old and new varieties. accumulated from heading to maturity of durum wheat and Discrimination values from Syrian trials (Araus et al. 1997b) and barley and water inputs calculated for durum wheat from the from some of the Spanish trials (Araus et al. 1993) have been Neolithic at Tell Halula. Present-day data correspond to mean published previously. Broken line represents the relationship values (1978–94) of accumulated precipitation during April plus between water inputs and ∆ for barley currently cultivated the first half of May (barley: open bars) and the second half of under mediterranean conditions as reported elsewhere (Araus April plus May (durum wheat: filled bars) recorded in the three et al. 1997a). Fitting equation for data of barley and wheat meteorological stations closest to Tell Halula (see legend of combined was ∆ ϭ 9.58 ϩ 3.65 (log. mm water), r2 ϭ 0.72, Fig. 1). Mean water inputs Ϯ SE from heading to maturity of C.V. ϭ 4.75) durum wheat (filled bars) during the three Neolithic periods were calculated using the ∆ values of fossil kernels for each 130 mm and fell to below 110 mm during late Neolithic period and the equation in Fig. 4. In addition, precipitation from (Fig. 5). These amounts of water are much greater than heading to maturity of barley (April and first half of May) ∆ the present-day precipitation for the Middle Euphrates cultivated at Tell Halula in 1996 was also estimated using the region during this period (of almost 7 weeks) of spring. values of kernels and the relationship between total water inputs during grain filling and ∆ of mature barley reported elsewhere Thus, the average (1978–94) precipitation in the three (Araus et al. 1997a). meteorological stations closest to Halula (Fig. 1) accumulated during the second half of April plus May was around 20 mm (Fig. 5). Whereas this period of time would With regard to the risk inherent in using present probably fit better with the phenology of old (or ancient) vegetation–climate relationships to reconstruct past water wheat genotypes which may have been later heading status from the archaeological record, the direct effect and flowering (see references in Slafer et al. 1993; Araus of changing atmospheric CO2 level on the relationship et al. 1997a), were we to consider the period from April between water inputs and grain ∆ should be considered. to the first half of May the accumulated precipitation In long-term experiments, it has been reported that the δ13 would only increase by about 30 mm. In any case, these pi/pa ratio (calculated from leaf C values) remained values differ greatly from those calculated in fossil wheat constant in a cereal plant such as oats grown at mean µ –1 kernels. Additionally, the accumulated precipitation at CO2 partial pressure from 160 to 330 bar bar , and for Tell Halula during April plus the first half of May of wheat it increased only slightly (about 4%) from 225 to 1997 was also estimated, from the ∆ of barley kernels 350 µbar bar–1 (Masle et al. 1990; Polley et al. 1993). A collected around the archaeological site, using the recent study using δ13C of woody plant remains (Beerling reported relationship for this crop between ∆ of kernels 1996) reports that these plants probably maintained a and water status (Araus et al. 1997a). Although the nearly constant pi/pa ratio in response to the increase in estimated precipitation was 56 mm (which agrees with atmospheric CO2 concentrations over the Pleistocene. the wet spring season of this particular year), this value In fact, the coordination of stomatal and mesophyll was still less than half of the estimated water inputs for functions minimizes variations in pi/pa in C3 species the fossil wheat kernels (Fig. 5). (Wong et al. 1979) grown over a range of CO2 partial

© 1999 Blackwell Science Ltd., Global Change Biology, 5, 201–212 WATER AVAILABILITY IN EARLY AGRICULTURE AND ∆ 209 pressures characteristic of the Last Glacial Maximum-to- much better water status than could be expected from present atmospheric concentrations (Polley et al. 1993). present-day (rainfed) conditions. Whereas the presence In fact Ehleringer (1995) postulates that pi can be consid- of flax also supports this conclusion, its very high ∆ ered as a ‘set point’ for photosynthetic activity, providing values should be interpreted with caution due to the fact integrated information about photosynthetic metabolism. that flax is a oil-rich seed and there is a discrimination 13 Thus pi as a ‘set point’ would be more stable than the against C during the process of oil biosynthesis. Cultiva- absolute flux rates of CO2 uptake. That is, whereas tion under moister conditions could have been possible absolute flux rates will vary greatly in response to as a result of more humid environmental conditions resource levels over the short-term or due to stress levels prevailing at this time or by planting in alluvial areas over the long-term, changes in the set point may be (Bar-Yosef & Kislev 1989). Another possibility lies in substantially less. Therefore, here we considered that the existence of irrigation practices. Indeed one indirect µ –1 changing pa values from about 270 bar bar , prior to method to assess the presence of ancient irrigation takes µ industrialization, to a present-day value of 355 bar into consideration the development of weeds; for –1 bar would not affect the pi/pa ratio and therefore based example, from the size of charred flax seeds which ∆ on the model of Farquhar et al. (1989), the would not occurred in an archaeological assemblage of plant remains change. Furthermore, even if p /p values from durum i a (Helbaek 1960). However, even when flax seeds were wheat plants of Neolithic were slightly lower than those of very abundant in all the stratigraphic levels studied at the present day, the inferred (i.e. calculated) precipitation Tell Halula their length did not reach 3.0 mm, which has from the archaeological sites may be slightly underesti- been proposed as the minimum size for domesticated mated. On the other hand, the lower p during Neolithic a plants. As yet, there is no archaeological evidence for would lead to a higher stomatal conductance and there- irrigation in the early (i.e. Neolithic) agricultural sites of fore to higher transpiration rates. If it is accepted that ∆ the Near East (Bar-Yosef & Kislev 1989; Hillman & Davies remains relatively steady within the range of changes in 1990), including Tell Halula (Molist et al. 1995). For durum p monitored during the Holocene, it can be concluded a ∆ that a given ∆ value could not have been attained in the wheat, although of fossil kernels were higher than past without greater amounts of water than those needed those of present-day kernels cultivated under rainfed today. Therefore, in the worst case our approach would conditions, due to the existence of a better water status provide a conservative (low) estimate of the differences in the past, they were still lower than the typical values in water availability from past to present time. of full irrigation crops under Mediterranean conditions ∆ Although other sources of uncertainty associated with (Araus et al. 1997c). Because appears to show a logarith- carbonization of grains (Araus & Buxo´ 1993), differences mic dependence on water accumulated from heading to in the rate of decline of soil water content, or changes in maturity (Fig. 4), ∆ values fall slightly below those conditions of cultivation and in genotype are difficult to reported for full irrigated wheat crops (over 17.5 ‰) and evaluate, the data available do not seem to invalidate would seem to be associated with considerably lower this approach and it might even suggest (for some of amounts of water being available during grain filling. these parameters) that a conservative estimate of water Thus, an ∆ value 0.5 ‰ lower than 17.5 ‰ would status in the past is obtained (see discussion in Araus correspond to the availability of an amount of water et al. 1997a,b). There is for example the possibility of during grain filling which was about 30% lower than systematic differences between ancient (i.e. cultivated that for irrigated crops (Araus et al. 1997a). Of course, several millennia ago) and new wheats in ∆. The available any condition in between rainfed and full irrigation (i.e. information suggests that modern varieties show consist- supplementary irrigation) would produce kernels with ∆ ently higher ∆ values than the old (i.e. reported to be values below those of full irrigation. cultivated during the last century) varieties and landraces The existence of a wetter climate in the past is a more not only in wheat (see references in Blum 1996) but plausible possibility. Archaeobotanical evidence supports also in barley (Munõz et al. 1998). A higher stomatal the possibility that environmental conditions during early conductance and earlier flowering and maturation in agriculture were more favourable (i.e. cooler and moister) new genotypes could be responsible (see Richards 1996). in this region than nowadays. The presence of charcoal ∆ Whatever the cause a systematically lower in older from ash, vine, maple, plane, alder and elm from the wheats would again lead to a conservative estimate of gallery forest, and wild rye, wild einkorn, deciduous oak, water status in the past. wild almond, Pistacia, and Pyrus, from the hinterland, suggests moister and cooler conditions in the Middle Further insights into the palaeoenvironmental context Euphrates region during early agriculture (Harlan 1995; of early agriculture Willcox 1996; see also Hillman et al. 1993). Particularly in The above results clearly indicate that during the period Tell Halula, although no evidence of rye appears, the studied wheat was cultivated at Tell Halula under a presence of Quercus, Amygdalus, Pistacia, Celtis and Olea

© 1999 Blackwell Science Ltd., Global Change Biology, 5, 201–212 210 J.L.ARAUS et al. europaea during the PPNB suggests that climate was much Table 2 Identified carbonized weed plant remains more humid than during the present-day (Willcox & Taxa Middle Late Late Catala` 1996; Willcox 1996). In addition, not only a higher PPNB PPNB Neolithic overall precipitation (and perhaps a lower evapotranspiration), but also the seasonal distribution of precipitations, Agrostemma githago —P— would be involved. Thus at the site of Mureybet (on the Androsace —P— Middle Euphrates), it has been reported that about 8000 BP Arnebia PPP Poaceae pollen was, in relative terms, much more abund- Astralagus PP— ant than today (El-Moslimany 1994). This suggests that Atriplex ——P Avena ——P summer precipitation was much higher than today. A Bromus PP— shift throughout the region to dominant winter rainfall Capparis PP— (typical of Mediterranean climates) would have occurred Carex —P— after 6000 BP (El-Moslimany 1994). Because durum wheat Centaurea P—P and flax seed formation takes place in late spring (and Chenopodium —PP also early summer for flax), any increase in precipitation Fumaria P—— during this period, even when total accumulated rainfall Galium PPP Glaucium P—P during cultivation does not vary, would eventually lead ∆ Heliotropium —P— to higher in seeds. Isatis P—— However, palaeoenvironmental reconstruction through Lithospermum P—P the archaeobotanical study of past vegetation has serious Lolium temulentun PPP limitations. Present-day natural vegetation corresponding Malva —P— to a degraded steppe could be the consequence of man’s Polygonum —PP effect on the landscape. Expansion of cultivation as well Reseda —P— Rumex PP— as the (early) great development of extensive goat and Setaria —P— sheep pastoralism (Thirgood 1981), rather than climatic Sherardia ——P change, could be responsible (at least in part) for this Teucrium PP— increase in landscape degradation (Willcox 1996) and Trifolium —P— the contemporary steppe/weed vegetation. Thus, annual weed plants, usually associated with open landscape and P indicates a record the cultivation of cereals, among other crops, were already present in the early crop assemblage of Tell Halula. values for almost all the samples analysed in the three Among these annual plants it is worth mentioning the crops (Fig. 3) and the strong (more than five times) genera Agrostemma, Arnebia, Astragalus, Centaurea, difference in water inputs estimated from past to present Fumaria, Glaucium, Lithospermum, Rumex, Sherardia and conditions at Tell Halula (Fig. 5) seems difficult to explain Teucrium (Table 2). The weeds more specifically associated just by a wetter climate prevailing in the past. In this with legume cultivation are Astralagus, Atriplex, Chenopod- context a wetter climate combined with planting in ium, Galium and Trifolium; whereas Lolium would more naturally wet soils may have been typical. In fact one likely be associated with flax cultures. way of stabilizing yields that were fluctuating because Nevertheless the pattern of changes with time in ∆ of water stress is small-scale cultivation in areas fed would agree with a gradual change in climate to drier by permanent springs and duly cleared of competing conditions. Thus, as a general trend mean ∆ values vegetation (Hillman 1996). The exploitation of naturally tended to decrease throughout the millennium comprised wet soils is widely found in early agricultural sites of the between M-PPNB to Late Neolithic. This was particularly Near East (Bar-Yosef & Kislev 1989). It has been argued evident for flax (Fig. 3b), which is reported to be an that cultivation, whenever possible, was based on the indicator of high water availability (Hole 1998). Indeed sowing of alluvial fans and terraces as well as the edges the first evidence of domesticated plants (cereals) at Tell of freshwater swamps where the water table was always Halula appear a full thousand years after the Climatic high and the soil was fertilized by silt deposited by Optimum was already established (Becker et al. 1991). periodic floods. As Hillman (1996) point out, once native Therefore, the progressive attenuation of the Climatic vegetation was eliminated the moisture-enhanced soils Optimum until around 6000 years ago, when climatic on small terraces and other breaks of slope around the conditions were not very different from those prevailing lower reaches of wadi systems which characterized most at present were established throughout the Near East of the favoured (i.e. oasis) locations (see Fig. 1c for Tell (Roberts & Wright 1993), would agree with this tendency Halula) would have made possible yields far greater than to lower ∆ values. However, the constancy in high ∆ in the drier locations where the cereals grew naturally.

Conclusions millennia. Palaeoenvironmental evidence of a differential change in aridity during the late Holocene. Global Change To summarize, our results provide direct evidence that Biology, 3, 107–118. during early agriculture wheat crops were cultivated Ball JT, Woodrow IE, Berry JA (1987) A model predicting at Tell Halula under much wetter conditions than are stomatal conductance and its contribution to the control of suggested by present-day conditions. The presence of photosynthesis under different environmental conditions. In: ﬂax and its very high ∆ values also support this conclu- Progress in Photosynthesis Research (ed. Biggins J), pp. 221– sion. Higher water availability may have been attained 224. M. Nijhoff Publishers, Dordrecht. exclusively from the moister (and cooler) conditions Baruch U, Bottema S (1991) Palynological evidence for climatic prevailing at this time. However, the large differences changes in the Levant ca. 17,000–9,000 B.P. In: The Natuﬁan between present-day rainfall and the estimated water culture in the Levant. International Monographs in Prehistory (eds Bar–Yosef O, Valla F), pp. 11–20. Ann Arbor, Michigan. input in the past, also suggest the possibility that cultural Bar-Yosef O, Kislev M (1989) Early farming communities in the practices such as planting in naturally wet soils were Jordan Valley. In: Foraging and Farming. The evolution of plant followed. exploitation (eds Harris DR, Hillman GC), pp. 633–642. Unwin Hyman, London. Becker BB, Kromer B, Trimborn P (1991) A stable isotope tree- Acknowledgements ring time scale on the Late Glacial/Holocene boundary. We wish to thank Dr C. Royo and J. Zarco from UdL-IRTA, Nature, 353, 647–649. Lleida (Spain), and E. Sanchez-Monge from SENASA, Pamplona Beerling DJ (1996) Ecophysiological responses of woody plants

(Spain), for supplying accumulated water inputs, together with to past CO2 concentrations. Tree Physiology, 16, 389–396. ∆ kernel samples and values from several durum wheat trials. Blum A (1996) Yield potential and drought resistance: are they We are also very grateful to G. Willcox, from the CNRS of Jale`s mutually exclusive? In: Increasing Yield Potential in Wheat: (France) for supervising palaeobotanical determinations and to Breaking the Barriers (eds Reynolds MP, Rajaram S, McNab A), Annette Oberle, from ICARDA, Aleppo (Syria) for providing pp. 76–89. CIMMYT, Me´xico. the climatic data close to the Halula site. The constructive comments raised on the manuscript by Drs T. Dawson, V.P. Condon AG, Richards RA, Farquhar GD (1987) Carbon isotope Gutschick, and Roger B. Austin as well as those of the anonymous discrimination is positively correlated with grain yield and referee also are appreciated. This work was supported in part dry matter production in field-grown wheat. Crop Science, by the grants CICYT AGF95-1008-C05-03 and A6F96-1137-C02- 27, 996–1001. 03 and a fellowship DGICYT PR95-006 (Spain) Condon AG, Richards RA, Farquhar GD (1992) The effect of variation in soil water availability, vapor pressure deficit and nitrogen nutrition on carbon isotope discrimination in wheat. References Australian Journal of Agricultural Research, 43, 935–947. Craufurd PQ, Austin RB, Acevedo E, Hall MA (1991) Carbon Amaro T, Bergareche C, Araus JL (1995) Changes in Stable C isotope discrimination and grain yield in barley. Field Crop and N isotope composition in barley as affected by N Research, 27, 301–313. nutrition and sowing date. Interdrought. In: Proceedings of Ehleringer JR (1995) Variation in gas exchange characteristics the International Congress on Integrated Studies on Drought The Ecophysiology of Photosynthesis. Tolerance of Higher Plants, pp. III- 1. Montpellier. among desert plants. In: Araus JL, Amaro T, Zuhair Y, Nachit MM (1997b) Effect of leaf Ecological Studies, Vol. 100 (eds Schulze ED, Caldwell MM), structure and water status on carbon isotope discrimination pp. 361–392. Springer, Berlin. in field grown durum wheat. Plant, Cell and Environment, 20, El-Moslimany AP (1994) Evidence of early Holocene summer 1484–1494. precipitation in the continental Middle East. In: Late Araus JL, Brown HR, Febrero A, Bort J, Serret MD (1993) Quaternary Chronology and Paleoclimates of the Eastern Ear photosynthesis, carbon isotope discrimination and the Mediterranean (eds Bar-Yosef O, Kra RS), pp. 121–130. Radiocarbon, Department of Geosciences, The University of contribution of respiratory CO2 to differences in grain mass in durum wheat. Plant, Cell and Environment, 16, 383–392. Arizona, Tucson. Araus JL, Buxo´ R (1993) Changes in carbon isotope Farquhar GD, Ehleringer JR, Hubick KT (1989) Carbon isotope discrimination in grain cereals from the north-western discrimination and photosynthesis. Annual Review of Plant mediterranean basin during the past seven millennia. Physiology and Plant Molecular Biology, 40, 503–537. Australian Journal of Plant Physiology, 20, 117–128. Farquhar GD, Richards RA (1984) Isotopic composition of plant Araus JL, Buxo´ R, Febrero A et al. (1997c) Identification of carbon correlates with water-use efficiency of wheat Ancient Irrigation Practices based on the Carbon Isotope genotypes. Australian Journal of Plant Physiology, 11, 539–552. Discrimination of Plant Seeds: a Case Study from the Harlan JR (1995) The Living Fields: Our Agricultural Heritage. Southeast Iberian Peninsula. Journal of Archaeological Science, Cambridge University Press, Cambridge. 24, 729–740. Helbaek H (1960) Cereals and weed grasses in Phase A. In: Araus JL, Febrero A, Buxo´ R et al. (1997a) Changes in carbon Excavations in the Plain of Antioch I (eds Braidwood RJ, isotope discrimination in grain cereals from different regions Braidwood LS), pp. 540–543. University of Chicago Press, of the western Mediterranean Basin during the past seven Chicago.

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