Most of the water in the River Nile originates from monsoonal rainfall over the Ethiopian Highlands. Despite warnings of future climate change, little is known about the historical variability in this supply, particularly at annual resolution. Development of tree-ring records in this region has been limited by the occurrence of bi- or multimodal rainfall regimes, causing the development of multiple growth rings that cannot be dated with confidence. In this study, we identified annual rings in 30 Juniperus procera trees from northwest Ethiopia by dendrochronology and AMS radiocarbon dating. Carbon isotope ratios (4 series) and ring widths (73 series) were measured.The carbon isotope series did not contain strong trends possibly attributable to increased anthropogenic atmospheric CO2 concentrations or the juvenile effect. Both carbon isotope values and ring widths were strongly correlated with Blue Nile baseflow, and from composite chronology indices (r=0.75, p < 0.01), a preliminary reconstruction of Blue Nile baseflow back to AD 1836 was developed. Subsample signal strength remained above 0.85 for most of the reconstruction. Uncertainty bands were relatively narrow and the reliability of the preliminary reconstruction was confirmed by correspondence with reported years of drought and famine. The preliminary reconstruction is characterized by an exceptional decline in baseflow during the late AD 1960s. Flows recovered during the late 1990s. Additional sampling is advised to increase replication, spatial coverage and length of the preliminary reconstruction.
Borella S., Leuenberger M., Saurer M. and Siegwolf R. ( 1998) Reducing uncertainties in δ13C analysis of tree rings: Pooling, milling, and cellulose extraction. Journal of Geophysical Research103: 19 519-19 526.
2.
Bräuning A. ( 2001) Climate history of the Tibetan Plateau during the last 1000 years derived from a network of juniper chronologies. Dendrochronologia19: 127-137.
3.
Bräuning A., Mantwill B. ( 2004) Summer temperature and summer monsoon history on the Tibetan plateau during the last 400 years recorded by tree rings. Geophysical Research Letters31: L24205, doi:10.1029/2004GL020793.
4.
Bréda N., Huc R., Granier A. and Dreyer E. ( 2006) Temperate forest trees and stands under severe drought: A review of ecophysiological responses, adaptation processes and long-term consequences. Annals of Forest Science63: 625-644.
5.
Brienen RJW, Zuidema PA ( 2005) Relating tree growth to rainfall in Bolivian rain forests: A test for six species using tree ring analysis. Oecologia146: 1-12.
6.
Briffa KR, Jones PD ( 1990) Basic chronology statistics and assessment. In: Cook ER, Kairiukstis LA (eds) Methods of Dendrochronology, Applications in the Environmental Sciences. Kluwer Academic Publishers , 137-152.
7.
Broadmeadow MSJ, Griffiths H. ( 1993) Carbon isotope discrimination and the coupling of CO2 fluxes within forest canopies. In: Ehleringer JR, Hall AE and Farquhar GD (eds) Stable Isotopes and Plant Carbon-Water Relations. Academic Press, 109-129.
8.
Cernusak LA, Marshall JD, Comstock JP and Balster NJ ( 2001) Carbon isotope discrimination in photosynthetic bark. Oecologia128: 24-35.
9.
Cherubini P., Gartner BL, Tognetti R., Bräker OU, Schoch W. and Innes JL ( 2003) Identification, measurement and interpretation of tree rings in woody species from mediterranean climates. Biological Reviews78: 119-148.
10.
Conway D. ( 1997) A water balance model of the Upper Blue Nile in Ethiopia . Hydrological Sciences Journal42: 265-286.
11.
Conway D., Brooks N., Briffa KR, Desta S., Merrin PD and Jones PD ( 1997) Exploring the Potential for Dendroclimatic Analysis in Northern Ethiopia. University of East Anglia.
12.
Conway D., Brooks N., Briffa KR and Merrin PD ( 1998) Historical climatology and dendroclimatology in the Blue Nile River basin, northern Ethiopia. In: Servat E, Hughes D, Fritsch JM and Hulme M (eds) Water Resources Variability in Africa During the XXth Century. IAHS Publications252: 243-251.
13.
Conway D., Schipper L., Yesuf M., Kassie M., Persechino A. and Kebede B. ( 2007) Reducing Vulnerability in Ethiopia: Addressing the Implications of Climate Change: Integration of Results from Phase I. University of East Anglia.
14.
Cook ER ( 1990) A conceptual linear aggregate model for tree rings . In: Cook ER, Kairiukstis LA (eds) Methods of Dendrochronology, Applications in the Environmental Sciences. Kluwer Academic Publishers , 98-104.
15.
Cook ER, Holmes RL ( 1992-1997) ARSTAN, A Computer Program for Chronology Development. University of Arizona.
16.
Cook ER, Holmes RL ( 1999) User Manual for Program ARSTAN. University of Arizona.
17.
Cook ER, Briffa KR, Shiyatov S. and Mazepa VS ( 1990) Tree-ring standardization and growth-trend estimation . In: Cook ER, Kairiukstis LA (eds) Methods of Dendrochronology, Applications in the Environmental Sciences. Kluwer Academic Publishers , 104-123.
18.
Coplen TB ( 1995) Discontinuance of SMOW and PDB. Nature375: 285.
19.
Coplen TB, Brand WA, Gehre M., Gröning M., Meijer HAJ, Toman B. et al. (2006) After two decades a second anchor for the VPDB δ13C scale. Rapid Communications in Mass Spectrometry20: 3165-3166.
20.
Couralet C., Sass-Klaassen U., Sterck F., Bekele T. and Zuidema PA ( 2005) Combining dendrochronology and matrix modelling in demographic studies: An evaluation for Juniperus procera in Ethiopia. Forest Ecology and Management216: 317-330.
21.
Couralet C., Sass-Klaassen U., Sahle Y., Sterck F., Bekele T. and Bongers F. ( 2007) Dendrochronological investigations on Juniperus procera from Ethiopian Dry Afromontane Forests. TRACE Dendrosymposium 2006, Forschungszentrum Jülich, 73-79.
22.
D’Arrigo RD, Wilson R., Liepert B. and Cherubini P. ( 2008) On the ‘"Divergence Problem" in Northern Forests’: A review of the tree-ring evidence and possible causes. Global and Planetary Change60: 289-305.
23.
Dai K.-M, Fan CY ( 1986) Bomb-produced 14C content in tree rings grown at different latitudes. Radiocarbon28: 346-349.
24.
Degefu W. ( 1988) Some aspects of meteorological drought in Ethiopia . In: Glantz MH (ed.) Drought and Hunger in Africa: Denying Famine a Future. Cambridge University Press, 23-36.
25.
Duursma RA, Marshall JD ( 2006) Vertical canopy gradients in δ13C correspond with leaf nitrogen content in a mixed-species conifer forest. Trees - Structure and Function20: 496-506.
26.
Ehleringer JR, Hall AE and Farquhar GD ( 1993) Stable Isotopes and Plant Carbon-Water Relations . Academic Press.
27.
Esper J., Frank D., Büntgen U., Verstege A., Luterbacher J. and Xoplaki E. ( 2007) Long-term drought severity variations in Morocco . Geophysical Research Letters34: L17702, doi:10.1029/2007GL030844.
28.
Ethiopian Mapping Authority (1988) National Atlas of Ethiopia. Ethiopian Mapping Authority .
29.
Farmer JG, Baxter MS ( 1974) Reply to Freyer and Wiesberg (1974 ). Nature252: 757.
30.
Farquhar GD, O’Leary MH and Berry JA ( 1982) On the relationship between carbon isotope discrimination and the intercellular carbon dioxide concentration in leaves. Australian Journal of Plant Physiology9: 121-137.
31.
Fetene M., Feleke Y. ( 2001) Growth and photosynthesis of seedlings of four tree species from a dry tropical afromontane forest. Journal of Tropical Ecology17: 269-283.
32.
Francey RJ, Allison CE, Etheridge DM, Trudinger CM, Enting IG, Leuenberger M. et al. (1999) A 1000-year high precision record of δ13C in atmospheric CO2. Tellus51B: 170-193.
33.
Freyer HD, Wiesberg L. ( 1974) Dendrochronology and 13C content in atmospheric CO2. Nature252: 757.
34.
Friis I. ( 1992) Forests and Forest Trees of Northeast Tropical Africa . HMSO.
35.
Fritts HC ( 1976) Tree Rings and Climate. Academic Press .
36.
Gagen M., McCarroll D., Loader NJ, Robertson I., Jalkanen R. and Anchukaitis KJ ( 2007) Exorcising the ‘segment length curse’: Summer temperature reconstruction since AD 1640 using nondetrended stable carbon isotope ratios from pine trees in northern Finland. The Holocene17: 435-446.
37.
Gagen M., McCarroll D., Robertson I., Loader NJ and Jalkanen R. ( 2008) Do tree ring δ13C series from Pinus sylvestris in northern Fennoscandia contain long-term non-climatic trends?Chemical Geology252: 42-51.
38.
Gasse F. ( 2000) Hydrological changes in the African tropics since the Last Glacial Maximum. Quaternary Science Reviews19: 189-211.
39.
Gebrekirstos A., Mitlöhner R., Teketay D. and Worbes M. ( 2008) Climate- growth relationships of the dominant tree species from semi-arid savanna woodland in Ethiopia. Trees - Structure and Function22: 631-641.
40.
Gebrekirstos A., Worbes M., Teketay D., Fetene M and Mitlöhner R (2009) Stable carbon isotope ratios in tree rings of co-occurring species from semi-arid tropics in Africa: Patterns and climatic signals. Global and Planetary Change66: 253-260.
41.
Gray J., Thompson P. ( 1977) Climatic information from 18O/16O analysis of cellulose, lignin and whole wood from tree rings. Nature270: 708-709.
42.
Grießinger J., Bräuning A., Thomas A. and Schleser GH ( 2008) Stable oxygen isotopes in juniper trees from the Tibetan plateau as a proxy for monsoonal activity, TRACE Dendrosymposium 2007, Forschungszentrum Jülich, 110-114.
43.
Grissino-Mayer HD ( 2001) Evaluating crossdating accuracy: A manual and tutorial for the computer program COFECHA. Tree-Ring Research57: 205-221.
44.
Guiot J. ( 1990) Methods of calibration. In: Cook ER, Kairiukstis LA (eds) Methods of Dendrochronology, Applications in the Environmental Sciences. Kluwer Academic Publishers, 165-178.
45.
Heaton THE ( 1999) Spatial, species and temporal variations in the 13C/12C ratios of C3 plants: Implications for paleodiet studies. Journal of Archaeological Science26: 637-649.
46.
Holmes RL ( 1983) Computer-assisted quality control in tree-ring dating and measurement. Tree-Ring Bulletin43: 69-78.
47.
Hoper ST, McCormac FG, Hogg AG, Higham TFG and Head MJ ( 1998) Evaluation of wood pretreatments on oak and cedar . Radiocarbon40: 45-50.
48.
Hua Q., Barbetti M. ( 2004) Review of tropospheric bomb 14C data for carbon cycle modeling and age calibration purposes. Radiocarbon46: 1273-1298.
49.
Hua Q., Jacobsen GE, Zoppi U., Lawson EM, Williams AA, Smith AM et al. (2001) Progress in radiocarbon target preparation at the ANTARES AMS Centre. Radiocarbon43: 275-282.
50.
Hurst HE, Philips P. ( 1932 et seq.) The Nile Basin. Ministry of Public Works, Egypt.
51.
Intergovernmental Panel on Climate Change ( 2007) Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Solomon S, Qin D, Manning M, Chen Z, Marquis M, Averyt KB et al. (eds). Cambridge University Press.
52.
Jacoby GC ( 1989) Overview of tree-ring analysis is tropical regions . IAWA Bulletin10: 99-108.
53.
Juckes MN, Allen MR, Briffa KR, Esper J., Hegerl GC, Moberg A. et al. (2006) Millennial temperture reconstruction intercomparison and evaluation. Climate of the Past Discussions2: 1001-1049.
54.
Köppen WP ( 1918) Klassifikation der Klimate nach Temperatur, Niederschlag und Jahreslauf. Petermanns Geographische Mitteilungen.
55.
Krishnamurthy RV, Epstein S. ( 1985) Tree ring D/H ratio from Kenya, East Africa, and its palaeoclimatic significance. Nature317: 160-162.
56.
Lamb HF, Leng MJ, Telford RJ, Ayenew T. and Umer M. ( 2007) Oxygen and carbon isotope composition of authigenic carbonate from an Ethiopian lake: A climate record of the last 2000 years . The Holocene17: 517-526.
57.
Legesse D., Gasse F., Radakovitch O., Vallet-Coulomb C., Bonnefille R., Verschuren D. et al. (2002) Environmental changes in a tropical lake (Lake Abiyata, Ethiopia) during recent centuries. Palaeogeography, Palaeoclimatology, Palaeoecology187: 233-258.
58.
Loader NJ, Robertson I. and McCarroll D. ( 2003) Comparison of stable carbon isotope ratios in the whole wood, cellulose and lignin of oak tree-rings. Palaeogeography, Palaeoclimatology, Palaeoecology196: 395-407.
59.
Lucy D., Robertson I., Aykroyd RG and Pollard AM ( 2008) Estimates of uncertainty in the prediction of past climatic variables. Applied Geochemistry23: 2961-2965.
60.
McCarroll D., Loader NJ ( 2004) Stable isotopes in tree rings. Quaternary Science Reviews23: 771-801.
61.
McCarroll D., Loader NJ ( 2005) Isotopes in tree rings. In: Leng MJ (ed.) Isotopes in Palaeoenvironmental Research. Springer, 67-116.
62.
McCarroll D., Pawellek F. ( 1998) Stable carbon isotope ratios of late-wood cellulose in Pinus sylvestris from northern Finland: Variability and signal strength . The Holocene8: 675-684.
63.
McCarroll D., Gagen MH, Loader NJ, Robertson I., Anchukaitis KJ, Los S. et al. (2009) Correction of tree ring stable carbon isotope chronologies for changes in the carbon dioxide content of the atmosphere. Geochimica et Cosmochimica Acta73: 1539-1547.
64.
McCormac FG, Kalin RM and Long A. ( 1993) Radiocarbon dating beyond 50,000 years by liquid scintillation counting. In: Noakes JE , Schönhofer F and Polach HA (eds) Liquid Scintillation Spectrometry . Radiocarbon, 125-133.
65.
McDowell NG, Phillips N., Lunch C., Bond BJ and Ryan MG ( 2002) An investigation of hydraulic limitation and compensation in large, old Douglas-fir trees. Tree Physiology22: 763-774.
66.
McDowell N., Pockman WT, Allen CD, Breshears DD, Cobb N., Kolb T. et al. (2008) Mechanisms of plant survival and mortality during drought: Why do some plants survive while others succumb to drought ? New Phytologist178: 719-739.
67.
Meko DM, Therrell MD, Baisan CH and Hughes MK ( 2001) Sacramento River flow reconstructed to A.D. 869 from tree rings. Journal of the American Water Resources Association37: 1029-1039.
68.
Mozeto AA, Fritz P., Moreira MZ, Vetter E., Aravena R., Salati E. et al. (1988) Growth rates of natural Amazonian forest trees based on radiocarbon measurements. Radiocarbon30: 1-6.
69.
Mueller K., Muzikar P. ( 2002) Correcting for contamination in AMS 14C dating . Radiocarbon44: 591-595.
70.
National Research Council (2006) Surface Temperature Reconstructions for the Last 2,000 Years. The National Academic Press.
71.
Nicholson SE ( 2001) Climatic and environmental change in Africa during the last two centuries. Climate Research17: 123-144.
72.
Nydal R., Lövseth K. ( 1996) Carbon-14 Measurements in Atmospheric CO2 from Northern and Southern Hemisphere Sites, 1962-1993. Oak Ridge National Laboratory.
73.
Olago DO, Odada EO ( 2004) Palaeo-research in Africa: Relevance to sustainable environmental management and significance for the future. In: Battarbee RW, Gasse F and Stickley CE (eds) Past Climate Variability Through Europe and Africa. Springer , 551-565.
74.
Orvis KH, Grissino-Mayer HD ( 2002) Standardizing the reporting of abrasive papers used to surface tree-ring samples. Tree-Ring Research58: 47-50.
75.
Polge H. ( 1966) Etablissement des courbes de variation de la densité du bois par exploration densitométrique de radiographies d’échantillons prélevés à la tarière sur des arbres vivants. Application dans les domaines téchnologiques et physiologique. Annales des Sciences Forestières23: 1-206.
76.
Pourtahmasi K., Parsapjouh D., Bräuning A., Esper J. and Schweingruber FH ( 2007) Climatic analysis of pointer years in tree-ring chronologies from northern Iran and neighboring high mountain areas. Geoökodynamik28: 27-42.
77.
Reimer PJ, Brown TA and Reimer RW ( 2004) Discussion: Reporting and calibration of post-bomb 14C data. Radiocarbon46: 1299-1304.
78.
Rinn F. ( 2003) TSAP-Win, Software for Tree-ring Measurement, Analysis and Presentation. Rinntech.
79.
Robertson I., Rolfe J., Switsur VR, Carter AHC, Hall MA, Barker AC et al. (1997a) Signal strength and climate relationships in 13C/12C ratios of tree-ring cellulose from oak in southwest Finland. Geophysical Research Letters24: 1487-1490.
80.
Robertson I., Switsur VR, Carter AHC, Barker AC, Waterhouse JS, Briffa KR et al. (1997b) Signal strength and climate relationships in 13C/12C ratios of tree-ring cellulose from oak in east England. Journal of Geophysical Research102: 19507-19519.
81.
Robertson I., Leavitt SW, Loader NJ and Buhay W. ( 2008) Progress in isotope dendroclimatology. Chemical Geology252: EX1-EX4.
82.
Ryan MG, Yoder BJ ( 1997) Hydraulic limits to tree height and tree growth . Bioscience47: 235-242.
83.
Sass-Klaassen U., Couralet C., Sahle Y. and Sterck FJ ( 2008) Juniper from Ethiopia contains a large-scale precipitation signal. International Journal of Plant Sciences169: 1057-1065.
84.
Saurer M., Siegwolf RTW and Schweingruber FH ( 2004) Carbon isotope discrimination indicates improving water-use efficiency of trees in northern Eurasia over the last 100 years . Global Change Biology10: 2109-2120.
85.
Schäfer KVR, Oren R. and Tenhunen JD ( 2000) The effect of tree height on crown level stomatal conductance. Plant, Cell and Environment23: 365-375.
86.
Schweingruber FH, Fritts HC, Bräker OU, Drew LG and Schär E. ( 1978) The X-ray technique as applied to dendroclimatology . Tree-Ring Bulletin38: 61-91.
87.
Seibt U., Rajabi A., Griffiths H. and Berry JA ( 2008) Carbon isotopes and water use efficiency: Sense and sensitivity . Oecologia155: 441-454.
88.
Shah SK, Bhattacharyya A. and Chaudhary V. ( 2007) Reconstruction of June-September precipitation based on tree-ring data of teak (Tectona grandis L.) from Hoshangabad, Madhya Pradesh, India. Dendrochronologia25: 57-64.
89.
Slota PJ, Jull AJT, Linick TW and Toolin LJ ( 1987) Preparation of small samples for 14C accelerator targets by catalytic reduction of CO. Radiocarbon29: 303-306.
90.
Stokes MA, Smiley TL ( 1968) An Introduction to Tree-ring Dating. University of Chicago Press.
91.
Stuiver M., Polach HA ( 1977) Discussion; reporting of 14C data. Radiocarbon19: 355-363.
Till C., Guiot J. ( 1990) Reconstruction of precipitation in Morocco since 1100 A.D. based on Cedrus atlantica tree-ring widths. Quaternary Research33: 337-351.
94.
Touchan R., Hughes MK ( 1999) Dendrochronology in Jordan. Journal of Arid Environments42: 291-303.
95.
Touchan R., Meko DM and Hughes MK ( 1999) A 396-year reconstruction of precipitation in southern Jordan. Journal of American Water Resources35: 45-55.
96.
Touchan R., Xoplaki E., Funkhouser G., Luterbacher J., Hughes MK, Erkan N. et al. (2005) Reconstructions of spring/summer precipitation for the Eastern Mediterranean from tree-ring widths and its connection to large-scale atmospheric circulation. Climate Dynamics25: 75-98.
97.
Touchan R., Akkemik Ü., Hughes MK and Erkan N. ( 2007) May-June precipitation reconstruction of southwestern Anatolia, Turkey during the last 900 years from tree rings. Quaternary Research68: 196-202.
98.
Touchan R., Anchukaitis KJ, Meko DM, Attalah S., Baisan C. and Aloui A. ( 2008a) Long term context for recent drought in northwestern Africa. Geophysical Research Letters35: L13705, doi:10.1029/2008GL034264.
99.
Touchan R., Meko DM and Aloui A. ( 2008b) Precipitation reconstruction for Northwestern Tunisia from tree rings. Journal of Arid Environments72: 1887-1896.
100.
Treydte KS, Schleser GH, Helle G., Frank DC, Winiger M., Haug GH et al. (2006) The twentieth century was the wettest period in northern Pakistan over the past millennium. Nature440: 1179-1182.
101.
Trouet V., Esper J., Graham NE, Baker A., Scourse JD and Frank DC (2009) Persistent positive North Atlantic Oscillation mode dominated the Medieval Climate Anomaly. Science324: 78-80.
102.
Umer M., Legesse D., Gasse F., Bonnefille R., Lamb HF, Leng MJ et al. (2004) Late Quaternary climate changes in the Horn of Africa. In: Battarbee RW, Gasse F and Stickley CE (eds) Past Climate Variability Through Europe and Africa . Springer, 159-180.
103.
Urey HC ( 1947) The thermodynamic properties of isotopic substances . Journal of the Chemical Society of London85: 562-581.
104.
Verschuren D. ( 2004) Decadal and century-scale climate variability in tropical Africa during the past 2000 years. In: Battarbee RW, Gasse F and Stickley CE (eds) Past Climate Variability Through Europe and Africa. Springer, 139-158.
105.
Verschuren D., Laird KR and Cumming BF ( 2000) Rainfall and drought in equatorial east Africa during the past 1,100 years. Nature403: 410-414.
106.
Warren CR, McGrath JF and Adams MA ( 2001) Water availability and carbon isotope discrimination in conifers. Oecologia127: 476-486.
107.
Waterhouse JS, Barker AC, Carter AHC, Agafonov LI and Loader NJ ( 2000) Stable carbon isotopes in Scots pine tree rings preserve a record of flow of the river Ob. Geophysical Research Letters27: 3529-3532.
108.
Wigley TML, Briffa KR and Jones PD ( 1984) On the average value of correlated time series, with applications in dendroclimatology and hydrometeorology. Journal of Climate and Applied Meteorology23: 201-213.
109.
Wils THG ( 2007) Dendroclimatology on Juniperus procera in the upper Blue Nile and Tekeze basin. Quaternary Newsletter113: 55-58.
110.
Wils THG ( 2009) Isotope dendroclimatological studies on Juniperus procera from Ethiopia: towards a reconstruction of Blue Nile baseflow. PhD Thesis, Swansea University.
111.
Wils THG, Eshetu Z. ( 2007) Reconstructing the flow of the River Nile from Juniperus procera and Prunus africana tree rings (Ethiopia) - an explorative study on cross-dating and climate signal. TRACE Dendrosymposium 2006, Forschungszentrum Jülich, 277-284.
112.
Wils THG, Eshetu Z. and Koprowski M. ( 2008) Reconstructing the Flow of the River Nile from Juniperus procera Tree Rings and Historical Sources: Dating the Tree Rings . Royal Geographical Society.
113.
Wils THG, Robertson I., Eshetu Z., Sass-Klaassen UGW and Koprowski M (2009) Periodicity of growth rings in Juniperus procera from Ethiopia inferred from crossdating and radiocarbon dating. Dendrochronologia27: 45-58.
114.
Worbes M. ( 2002) One hundred years of tree-ring research in the tropics - a brief history and an outlook to future challenges. Dendrochronologia20: 217-231.
