Periodicity of growth rings in Juniperus procera from Ethiopia inferred from crossdating and radiocarbon dating

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Dendrochronologia 27 (2009) 45–58 www.elsevier.de/dendro

ORIGINAL ARTICLE

Periodicity of growth rings in Juniperus procera from Ethiopia inferred from crossdating and radiocarbon dating Tommy H.G. Wilsa,b,, Iain Robertsona, Zewdu Eshetuc, Ute G.W. Sass-Klaassend, Marcin Koprowskie a

School of the Environment and Society, Swansea University, Singleton Park, Swansea SA2 8PP, UK Department of Geography, Rotterdam University, Museumpark 40, 3015 CX Rotterdam, The Netherlands c Forestry Research Centre, Ethiopian Institute of Agricultural Research (EIAR), P.O. Box 30708, Addis Ababa, Ethiopia d Forest Ecology and Forest Management Group, Centre of Ecosystem Studies, Wageningen University, P.O. Box 47, 6700 AA Wageningen, The Netherlands e Laboratory of Dendrochronology, Institute of Ecology and Environment Protection, Faculty of Biology and Earth Science, Nicolaus Copernicus University, Gagarina Street 9, 87-100 Torun´, Poland b

Received 2 February 2007; accepted 12 August 2008

Abstract African pencil cedar (Juniperus procera Hochst. ex Endlicher 1847) is a tropical, irregularly growing species that can produce annual growth rings in response to an annual cycle of wet and dry seasons. In this paper, we assess the periodicity of growth-ring formation for 13 stem discs from a site in Central-Northern Ethiopia by crossdating and radiocarbon dating. The crossdating process is described more transparently than usual to allow open discussion of the methodology employed. Although the ring-width series could be tentatively matched, radiocarbon dating revealed that the growth rings of the junipers from the studied site are neither annual nor represent a common periodicity. It was found that the trees are exceptionally sensitive and respond individually to the complex local climate. For future research, it is recommended to select more mesic sites with an unambiguously unimodal rainfall regime and to gain external evidence to support assumptions about the periodicity of growth-ring formation in Juniperus procera. r 2008 Elsevier GmbH. All rights reserved. Keywords: Tropical dendrochronology; Mediterranean dendrochronology; Dry afromontane forest; Wood anatomy; COFECHA; Climate–growth relationship

Introduction Growth rings observed in tropical trees can be annual in nature if cambial dormancy is induced by an annually recurrent event such as drought or flooding (Worbes, Corresponding author at: School of the Environment and Society, Swansea University, Singleton Park, Swansea SA2 8PP, UK. Tel.: +44 1792 513065; fax: +44 1792 295955. E-mail addresses: [email protected], [email protected] (T.H.G. Wils).

1125-7865/$ - see front matter r 2008 Elsevier GmbH. All rights reserved. doi:10.1016/j.dendro.2008.08.002

1995, 2002; Stahle, 1999). According to Stahle (1999), the annual nature of tropical growth rings can be proven by crossdating alone, arguing that growth rings will not crossdate if they do not reflect an annual cycle. An additional test can be provided by radiocarbon dating (Worbes, 1995). Baillie (1995, p. 20) described crossdating as ‘the art of dendrochronology’. It entails matching of ring-width series in order to synchronise them, assuming that yearto-year variability in ring width is conditioned by

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environmental factors that affect all trees in a similar way. It is a process of trial and error (Stokes and Smiley, 1968), that philosophically resembles the hermeneutic circle (Gadamer, 1960) applied to the natural sciences (Eger, 1997). The hermeneutic circle is a circular movement of thought between ‘an individual part’ and ‘the whole’ in order to deepen the understanding. In crossdating this movement of thought occurs between an individual ring-width series and all ring-width series, as well as between evidence (matching, wood anatomy) and theory (causes of observed features). From a Popperian point of view (Popper, 1980), postulating that hypothesises should be tested independently by challenging them (falsificationism), crossdating does not constitute a test. It rather improves understanding in order to obtain a work of craftsmanship (or art): a dated chronology. Numerous methodologies and statistics have been developed to facilitate the crossdating process. Methodologies include inter alia skeleton plotting (Stokes and Smiley, 1968), visual comparison of samples or ringwidth curves (Pilcher, 1990), and the detection of pointer years (Schweingruber et al., 1990). Statistical evaluation tools include inter alia the Gleichla¨ufigkeitswert (Eckstein and Bauch, 1969), t-values (Baillie and Pilcher, 1973; Hollstein, 1980; Munro, 1984; Wigley et al., 1987) and the computer program COFECHA (Holmes, 1983; Grissino-Mayer, 2001). COFECHA standardises the raw ring-width series and subsequently ‘tests’ the correlation of each whole series and series segment (by default 50-ring) with a master series, which is either a series entered into the program separately or the mean series of all other series under consideration (GrissinoMayer, 2001). Default standardisation involves fitting of a spline (rigidity 32 rings, frequency response 50% at a wavelength of 32 rings), autoregressive modelling and logarithmic transformation. Due to its transparency and sophistication, COFECHA can be used as a benchmark for comparing different matches. Dendrochronological work on Juniperus procera from Eastern Africa has been promising but challenging due to a lack of knowledge on the response of tree growth to climate conditions in the dry tropics as well as irregular and often slow tree growth (Krishnamurthy and Epstein, 1985; Jacoby, 1989; Conway et al., 1997, 1998; Couralet et al., 2005, 2007; Eshetu, 2006; Wils and Eshetu, 2007; Sass-Klaassen et al., 2008b). Examples of crossdating tropical trees from SubSaharan Africa stress the importance of using stem discs rather than cores (e.g. Conway et al., 1997, 1998; Eshete and Sta( hl, 1999; Stahle et al., 1999; Trouet et al., 2001, 2006; Worbes et al., 2003; Fichtler et al., 2004; Couralet et al., 2005, 2007; Gebrekirstos, 2006; Scho¨ngart et al., 2006; Therrell et al., 2006, 2007; Sass-Klaassen et al., 2008b). Reported mean series inter-correlations vary from 0.24 (Trouet et al., 2006) to 0.63 (Therrell et al.,

2006). Unfortunately, most authors do not discuss the crossdating process extensively, which makes it difficult to learn from and assess their work. A study concerned with crossdating highly irregularly growing dwarf shrubs from Norway emphasised the importance of detecting all rings (Ba¨r et al., 2006). The authors accomplished this by analysing a high number of cross-sections along the entire plant (serial sectioning). Radiocarbon dating can be used to test whether crossdated growth rings are annual. However, from around AD 1650 to AD 1955 there is a ‘plateau’ in the radiocarbon calibration curve, making radiocarbon dating almost impossible in this period (Stuiver et al., 1998). Atmospheric nuclear bomb tests from AD 1945 to AD 1963 elevated the atmospheric radiocarbon concentration dramatically (Dai and Fan, 1986). Thereafter the concentration dropped heavily, essentially due to the incorporation of radiocarbon in other reservoirs of the carbon cycle (Nydal et al., 1979). The resulting radiocarbon bomb peak varies with latitude, because the heaviest nuclear bomb tests were performed at the Northern Hemispheric test site of Novaya Zemlya (721N, 531E) (Dai and Fan, 1986). A local or regional dataset of atmospheric radiocarbon concentration measurements allows highly accurate dating after AD 1955. The periodicity of growth rings in tropical trees has been studied with radiocarbon dating before (e.g. Mozeto et al., 1988; Worbes and Junk, 1989; Dezzeo et al., 2003; Robertson et al., 2004, 2006; Norstro¨m et al., 2005). In this paper we try to conceptualise crossdating of J. procera trees from a dry tropical site in Ethiopia in order to make the process more transparent and open for discussion. We test two hypotheses based on bomb radiocarbon dating: (1) that the growth rings are annual and (2) that the constructed match is a synchronous match. The aim of this study is to contribute to the further development of dendrochronology in East Africa.

Materials and methods Study area The study site, Doba forest (Fig. 1, 101480 N, 39190 E), is located in South Wollo, an administrative zone of the Amhara region in the Ethiopian Highlands, near the Blue Nile River and about 20 km west of the town of Akesta (Fig. 2A). It is a dry afromontane forest on a mountain ridge at 2950–3000 m a.s.l. The prevailing climate is characterised by one monsoonal wet season from June to September, with some additional rainfall from January to May. Temperature is relatively constant over the year, but varies strongly between day and night. Fig. 2B illustrates climate and rainfall patterns in Ethiopia and includes

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Fig. 1. Photographs of the study site. (A) Doba forest and (B) one of the sampled trees (tree 10).

climate diagrams of nearby stations in Kombolcha (11150 N, 391440 E) and Debre Markos (101200 N, 371440 E). Average annual precipitation in Kombolcha is 1050 mm. The substrate consists of Tertiary volcanic basalts of the Trap series in which highly fertile soils have been developed (EMA, 1988). Due to its location on a welldrained divide, the forest is dry. It consists of many species, but is dominated by J. procera and undergrowth is sparse. On the top of the ridge an old forest is present, whereas younger junipers occupy adjacent slopes. Several paths, a 30-year-old road and a road under construction intersect the old forest. It has been used for wood harvesting and in part as a graveyard. The interest of the local community to use it in a sustainable way may have been undermined by nationalisation of forests after the communist revolution in 1974 (Demeke, 2004).

Study species J. procera (Fig. 1B), an evergreen conifer, belongs to the family of the Cupressaceae (Friis, 1992). It is the only Juniperus species occurring in Africa and the largest of the world, attaining heights of 40 m. It grows predominantly between 2000 and 3000 m a.s.l., but may occur at altitudes up to 3500 m a.s.l. (Friis, 1992). Due to the alternation of wet and dry seasons J. procera forms growth rings (Conway et al., 1997, 1998). It is hypothesised that in Doba forest the growth rings are annual in nature, as the prevailing climate is characterised by only one wet season. Additional rains preceding the main wet season are herein assumed to advance the onset of the growing season. Double rings may be formed when two wet seasons occur (Stokes and Smiley, 1968; Couralet et al., 2005), but regarding the

weak separation of the additional rains from the main wet season, it is assumed that the boundaries within double rings are indistinct in Doba forest. Growth rings in J. procera can be identified by the alternation of small, flattened and thick-walled tracheids in the darker latewood and large, round and thin-walled tracheids in the lighter earlywood. The abruptness of the change between latewood and earlywood, i.e., ring distinctness (Fig. 3A), is assumed to depend on the strength of the seasonality (based on Baisan and Swetnam, 1994). Ring variables, such as ring width and isotopic signature, can be related to moisture availability (Krishnamurthy and Epstein, 1985; Couralet et al., 2005). Growth-ring formation in juniper is very irregular and asymmetric, and partial rings are frequent. Partial rings occur as either wedging or partially indistinct rings (terminology after Worbes, 1995) (Fig. 3B). Ring boundaries tend to be particularly indistinct at those parts along the stem where the cambium is most active. A possible explanation is that growth does not cease in these cambial areas, but continues year-round. J. procera was chosen as the study species, because Conway et al. (1997, 1998) identified it as the most suitable tree species for dendrochronology in Ethiopia.

Sampling, ring boundary detection and ring-width measurement In May 2006, stem discs were collected in Doba forest from one young and 12 old forest juniper trees. The old forest trees grew on top of the ridge and on the adjacent upper slopes and differed in diameters (approximately 21–61 cm) and heights (approximately 4–16 m). The stem discs were mostly taken from one of the stems of

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Fig. 2. Maps of Ethiopia. (A) Location of study site (Doba Forest) and major cities. (B) Climate and rainfall patterns in Ethiopia and Eritrea (after EMA, 1988). Sampling sites of various authors are indicated: Doba forest (A), Menagesha-Suba forest (G), Adaba-Dodola forest (O), Shasheme plantation (V), Semien Mountains (S) and Taragadem forest (T). Couralet et al. (2005, 2007) and Sass-Klaassen et al. (2008b) sampled trees of known age at site V and trees of unknown age at sites G and O. Conway et al. (1997, 1998) sampled trees at sites G, S and T.

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Fig. 2. (Continued)

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1

2

3

Fig. 3. Ring distinctness in Juniperus procera. (A) Growth-ring 7–9 in tree 2. Growth is from left to right (marked by an arrow) and measured ring boundaries are indicated. Boundary 3 is clear and continuous, whereas boundaries 1 and 2 are weaker and partial. During crossdating boundary 2 was discarded. Magnification is 4  and scale bar is 500 mm. (B) Partially indistinct growth ring in tree 8, indicated by arrows. The ring boundary changes from distinct (bottom right) to less distinct (top right), to indistinct (top left).

multi-stem trees and at the lowest height where trunk irregularity was minor (between approximately 0.4 and 1.6 m above ground level). The relatively low sample size is related to ethical considerations. The stem discs were sanded using sandpaper with progressively finer grit sizes (ISO 40-grit (425–500 mm) to ISO 800-grit (20.8–22.8 mm); Orvis and GrissinoMayer, 2002). During macroscopic inspection, if necessary alternated with microscopic inspection, a number of radii, considered to contain all growth rings, were scored into the wood with a scalpel (Fig. 4) and linked with each other by marking distinct growth rings. One path, changing from one radius to another, that contained all rings was selected. Ring widths were measured along this path with a precision of 0.01 mm

using a Velmex measuring stage and the computer program TSAP-Win (Rinn, 2003), and with continuous comparison to parallel radii, effectively scrutinising the whole disc for partial rings. At transition points from one radius to another, attention was paid to retain the high-frequency variance in ring width. If there was doubt whether the path contained all rings, relevant parallel radii were also measured. Reliable parallel radii were averaged. Weakly defined ring boundaries were judged from their continuity and change in tracheid size, morphology and cell-wall thickness. If doubt remained, a comment was added to the ring measurement including an expression of the degree of doubt. A comment was also added to relatively small partial rings and to partial

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9

8

7 4

6 5

2 1

3

Fig. 4. Stem disc (tree 2) showing the radii that were scored into it. Numbers 1–9 next to radius sections indicate the path along which ring width was measured. Dashed lines indicate parallel radii.

rings that were missing along the measured radius. Other wood-anatomical anomalies were also recorded, such as deformed tracheids and tracheids filled with coloured, probably phenolic substances. There was no evidence for reaction wood in the samples. Growth rings with a weak variation in tracheid characteristics between exceptional, bright white, large-celled and more common, darker and denser earlywood were regarded as double rings, and hence measured as one ring.

Crossdating Scanning for synchronies between the available ringwidth series through visual comparison of the series was the first step in the crossdating process. The series were shifted back and forth in time, particularly when shifts were suggested by wood-anatomical remarks. The series were ordered according to their suspected crossdating potential. Crossdating proceeded in this order. The subsequent crossdating process is schematised in Fig. 5. Visual comparison of the ring-width series, during which exceptionally narrow rings were regarded as pointer rings (after Schweingruber et al., 1990), was performed to yield hypothetical adjustments. The like-

lihood of those adjustments was assessed by examining wood-anatomical remarks and subsequent re-inspection of the sample. If the examination confirmed the possibility of the adjustments, the series was adjusted, and the computer program COFECHA (Holmes, 1983) was run to examine the strength of the association within the match. The main statistic used was the t-test significance level of the correlation coefficient between the individual series and the master series of the other matched series. In case of negative results the whole process was restarted. The crossdating process as described and visualised should be regarded as its general structure. In reality the process was highly dynamic and without strict rules, which is illustrated by the four bypasses in Fig. 5. Series segments flagged by COFECHA as potentially misdated were re-examined.

Radiocarbon dating Radiocarbon activities were measured in seven samples from three trees at the School of the Environment and Society of Swansea University. Wood shavings (approximately 4 g) were macerated in a blender, then suspended in water (300 ml) containing sodium

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Visual comparison

Hypothetical adjustments

Wood anatomy

No adjustment

Adjustment

COFECHA

Accept match

Next series

Do not accept match

Series laid aside

Fig. 5. Flow chart summarising the crossdating process. Black arrows indicate the ‘normal’ procedure, whereas dashed grey arrows represent ‘bypasses’. See text for explanation.

chlorite (4 g) and concentrated hydrochloric acid (1 ml) at 70 1C for several hours, until the wood was light straw coloured. The suspended wood pulp was filtered through a glass sinter, washed and dried overnight at 60 1C. Those samples were burned in a combustion tube under a pure oxygen atmosphere, and the resultant carbon dioxide was collected by freezing it in a glass trap cooled with liquid nitrogen. The carbon dioxide was then converted to methane by adding hydrogen and passing the gas mixture over a ruthenium catalyst at 520 1C (Smith et al., 1970). The methane was collected by trapping with liquid nitrogen. The radioactivity of the methane was measured in one of several gas proportional counters located within a low background radiation shield for a minimum of 2 weeks (Geyh and Schleicher, 1990). Carbon isotope ratios were measured on sub-samples of carbon dioxide, using a PDZ Europe 20-20 mass spectrometer interfaced to an ANCA elemental analyser. In addition to the sample activity, the background activity was measured using methane derived from anthracite, and

the modern activity (AD 1950) was measured by using methane derived from a sample of NBS oxalic acid. Values were expressed in fraction modern (F14C) (Stuiver and Polach, 1977; Reimer et al., 2004). Two atmospheric radiocarbon concentration datasets were used for calibration. For the period AD 1963–1969, at the height of the bomb peak, a dataset from Debre Zeit (40 km southeast of Addis Ababa) was available (Nydal and Lo¨vseth, 1996), but outside this period a regional dataset had to be used (Northern Hemisphere zone 3; Hua and Barbetti, 2004).

Results Crossdating The 13 available ring-width series were numbered according to their crossdating potential from 1 (highest potential) to 13 (lowest potential). Tree 13 was discarded because most ring boundaries were indistinct. Statistics

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of series 1–12 are listed in Table 1. Differences in mean ring width can be explained by differences in vitality, age and habitat (wind exposure, competition). Wide rings are associated with many wood-anatomical anomalies (particularly variations in tracheid size, morphology and cell-wall thickness) and a high standard deviation in ring width. Crossdating of the first four series is visualised in Fig. 6. After making three wood-anatomically justifiable adjustments in the first two ring-width series, the match was statistically significant according to COFECHA (r ¼ 0.12, Po0.05), Gleichla¨ufigkeitswert (W ¼ 58, Po0.01) and Baillie/Pilcher t-value (tB/P ¼ 4.1, Po0.01). Ring-width series 3 could not be matched unambiguously to series 1 and 2, because a suspected false ring near ring 80 was not supported by the woodTable 1.

anatomical characteristics of the sample and the exact position of a false ring between ring 1 and 50 was unclear. Series 4 posed the same problem near ring 80, so that instead the adjustments were made in series 1 and 2, where they could be justified from a woodanatomical point of view. Series 4 and subsequently series 3 could then be added to the match. The remaining series could now be added in the order of crossdating potential, although the last two series were laid aside. In the final stage the weakly replicated section beyond ring 230 was re-examined and tentatively adjusted. The final match, the associated adjustments and the correlation between each individual series and the master series of the other matched series are visualised in Fig. 7. Although whole series correlations are significant (Po0.02), most of the 50-ring segment correlations (77%) are flagged by COFECHA as not significant (P40.01). No improvement is possible, however: the suggested improvements either result in a minimal increase in correlation or are unrealistic because proposed shifts are large and opposite.

Statistics of ring-width series 1–12.

Tree

Rings

Mean RW

St. dev. RW

Anomalies (%)

1 2 3 4 5 6 7 8 9 10 11 12

303 289 178 99 189 241 175 141 155 136 156 68

0.88 1.13 1.06 2.53 1.11 2.28 1.01 2.12 2.20 2.52 1.06 1.88

0.92 1.03 1.32 2.71 0.85 2.32 0.99 1.98 1.78 1.81 1.12 1.94

33 29 29 43 38 45 21 34 36 54 26 41

53

Radiocarbon dating Dating of two samples with high radiocarbon contents is illustrated in Fig. 8A. The samples unambiguously date from the 1960s. Rings 76–78 from tree 1 were formed in 1964 and 1965. Rings 76–79 from tree 2 were formed between 1963 and 1968, which is a wide range that cannot be narrowed due to the high standard error. Dating of five samples outside the period 1963–1969 is illustrated in Fig. 8B. Rings 44–46 from tree 1 and rings 28–30 from tree 2 were formed after rings 76–79, thus unambiguously date from after the 1960s. This elim-

Rings – number of rings, Mean – mean ring width, St. dev. RW – standard deviation of ring widths, Anomalies – percentage of rings with an annotated wood-anatomical anomaly.

2 1 3 4

1

1

2

2

300 Pith

250

200

150 Ring number

100

50

0 Bark

Fig. 6. Crossdating of the first four ring-width series. Series were standardised using a spline (rigidity 32 rings, frequency response 50% at a wavelength of 32 rings) and autoregressive modelling. Filled circles indicate adjustments and adjustments in consecutive rings indicate a split ring. The crossdating process proceeds from bottom to top. Series 1 and 2 are matched by three adjustments. Two more adjustments were made to match them with series 4 and 3. The adjustment indicated in ring 3 reflects its final position.

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10 (0.29***) 9 (0.21*) 8 (0.20*) 7 (0.20**) 6 (0.23***) 5 (0.29***) 3 (0.30***) 4 (0.33***) 2 (0.24***) 1 (0.32***)

300 Pith

250

200

150

100

50

0 Bark

Ring number

Fig. 7. Final match of ten ring-width series. Series were standardised using a spline (rigidity 32 rings, frequency response 50% at a wavelength of 32 rings) and autoregressive modelling. Filled circles indicate adjustments and adjustments in consecutive rings indicate a split ring. Correlation coefficients between each individual series and the master series of the other matched series are given in brackets behind the tree number. Stars indicate level of significance: three stars Po0.005, two stars Po0.01, one star Po0.02. Dashed vertical lines indicate pointer rings.

1.9

Atmospheric F14C

1.8

1 (76-78)

1.7

18 15

2 (76-79)

1.6

12

1.5

9

1.4

6

1.3

3

1.2 1963

1964

1965

1966 1967 Year (AD)

1968

1969

Monthly Precipitation (dm)

21

0 1970

1.8

Atmospheric F14C

1.7 1.6 1.5 1.4

1 (44-46)

1.3 1.2 1.1

4 (50) 2 (28-30) 4 (42) 4 (32)

1.0 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 Year (AD)

Fig. 8. Atmospheric radiocarbon concentration compared to values measured in the samples. Diamonds indicate atmospheric F14C, black lines indicate sample F14C, error bars indicate 71s. Sample and (in brackets) ring numbers are given next to the black lines. Year tick marks indicate the start (1 January) of the particular year. Columns in (A) represent monthly precipitation in Kombolcha, indicating potential growing seasons. Atmospheric radiocarbon concentration values are from Debre Zeit, Ethiopia (Nydal and Lo¨vseth, 1996) (A) and the Northern Hemisphere zone 3 dataset (Hua and Barbetti, 2004) (B).

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Table 2.

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Details on the radiocarbon dates and deduced mean number of rings formed per year.

Tree

Lab ref.

Ring no.

Hypo. age (AD)

d13C (%)

F14C

Error

Approx.

14

1

SWAN-1025 SWAN-1024

76–78 44–46

1929–1931 1961–1963

18.4 19.9

1.7356 1.4057

0.011 0.009

1964–1965 1974–1975

3.00 1.43

2

SWAN-1027 SWAN-1026

76–79 28–30

1928–1931 1977–1979

18.2 18.5

1.6646 1.1771

0.11 0.008

1963–1968 1987–1989

2.11 1.61

4

SWAN-1004 SWAN-1005 SWAN-1006

50 42 32

1957 1965 1975

21.4 20.7 21.8

1.2136 1.1600 1.1300

0.0043 0.0043 0.0047

1984 1989/1990 1993

1.38 2.44 2.46

C date

Rings/yr

Lab ref. – laboratory reference number, Ring no. – ring number, Hypo. age – age if rings were annual, d13C – d13CVPDB, F14C – sample radiocarbon content in fraction modern, Error – standard error of F14C, Rings/yr – mean number of rings formed per year (between respective and next date).

inates the intercept of the sample radiocarbon content with the increasing slope of the bomb peak. Rings 44–46 from tree 1 were thus formed in 1974 and 1975 and rings 28–30 from tree 2 in 1987–1989. A similar argument applies to rings 32 and 42 from tree 4, which can therefore be dated at 1993 and 1989/1990, respectively. Ring 50 from tree 4 was probably formed in 1984, because the alternative date, 1959–1961, would imply the unlikely event that only eight rings were formed in 30 years. Full details of the radiocarbon dating are given in Table 2. It is clear from the radiocarbon dates that the growth rings of the J. procera trees in Doba forest are not annual in nature and that the match is not a synchronous match, although the match between tree 1 and 2 may be approximately synchronous. The trees produce one to three or even more rings per year.

Discussion The crossdating process has produced a match with a statistically significant mean series inter-correlation of 0.26 (individual correlations Po0.02), which is similar to the value of 0.24 reported by Trouet et al. (2006), but which occurred almost entirely by chance. The growth rings are neither annual nor do the trees produce growth rings with a similar periodicity. These results contradict Stahle’s (1999) suggestion that the annual nature of tropical growth rings can be proven by crossdating alone, although it might be argued that the mean seriescorrelation is too low to accept the match as dated. Strictly, the correlation coefficients are of limited value, as they are the result of a circular process, in which series are edited to increase the coefficients. A demand for higher series inter-correlations must therefore be accompanied by a maintained high degree of confidence in the adjustments. The high number of insignificant (P40.01) 50-ring segment correlations is a consequence

of a similar correlation with a lower sample size and can be regarded as a warning. The crossdating process may work if trees form growth rings synchronously, but the fact that it produced an erroneous match, emphasises its weaknesses. Visual comparison of ring-width series may mislead the researcher, as there will always be some matching, whereas skeleton plotting may provide a more difficult and robust way of crossdating. The studied J. procera trees possessed a highly ambiguous wood anatomy, leading to wood-anatomical remarks attached to up to 54% of the growth rings. Although woodanatomical characteristics did still render some of the potential adjustments improbable, this effect was too minor to yield a trustworthy match. The J. procera trees in Doba forest produce one to three (or more) rings per year (Table 2). Although the rainfall pattern in nearby Kombolcha is on average unimodal (Fig. 2B), short rains in the period January– May can be separated from the main wet season and even from each other in such a way that cambial activity in trees stops or slows down, hence potentially producing even more than three rings per year. In Fig. 8A this is particularly evident in the years 1963 and 1966–1969. Besides, mist can serve as a source of moisture (Kerkfoot, 1964). As J. procera is an evergreen conifer, it seems to be able to respond quickly to short rains or mist and, worse, to short droughts. Similar problems have been reported from Mediterranean areas. Cherubini et al. (2003) classify dry afromontane forests as Mediterranean, rather than as tropical. Where growth-ring formation in Mediterranean areas can be induced by summer drought and winter cold, this happens in dry afromontane forests through multiple droughts and potentially frost at night. Frost damage adversely affects the hydraulic conductivity of the xylem, which can induce growth-ring formation in water-limited trees (Cherubini et al., 2003). Tracheids damaged by frost were observed in several

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samples, but they did not seem to relate to ring boundaries. Wide rings are often associated with many woodanatomical anomalies, a high standard deviation in ring width (Table 1) and multiple growth rings per year (comparing tree 4 to trees 1 and 2 in Table 2). Similarly, in Pinus species from Eastern Spain, Martı´n-Benito et al. (2008) observed lower growth inertia in dominant as opposed to suppressed trees, and De Luis et al. (2007) observed false rings only in wide rings. This suggests that healthy and/or young trees are more capable of responding to short periods of rainfall and only slow down growth during the periods of minor drought. The response of trees therefore varies between trees and over time in relation to, e.g. vitality, growth history, age and habitat. This is also stressed by Cherubini et al. (2003), emphasising that evergreen species are particularly problematic, because they are capable of cambial growth at any time during the year, but will only do so if not limited by other factors. In that respect, it is important to consider that these trees may not experience true dormancy, but rather varying degrees of quiescence, in which tracheid size, morphology and cell-wall thickness depend on the degree of quiescence, that is, cambial activity (Cherubini et al., 2003). As a consequence, matching cannot date ring-width series at this site and it is impossible to link growth rings with potential growing seasons. These results are in stark contrast to the work of Couralet et al. (2005, 2007) and Sass-Klaassen et al. (2008b). Those authors concluded to have found annual rings, based on ring counts in trees of known age, higher series inter-correlations and significant (Po0.05) correlations between chronologies and instrumental climate data. Fig. 2B may explain this discrepancy. All study sites reporting annual rings in J. procera (Couralet et al., 2005, 2007; Sass-Klaassen et al., 2008b) are located south of Addis Ababa, in areas experiencing a more continuous wet season than Doba forest. The separation between summer and spring rains is weak, to such an extent that part of the area is classified as experiencing ‘summer rain’ or ‘rain most of the year’. In the latter case, a short dry season appears to be long enough to induce the formation of a ring boundary (comparable to the results from Doba forest). The continuous nature of the growing season may also cause the formation of annual rings at the Conway et al. (1997, 1998) sites located centrally in the area experiencing ‘summer rain’. The location of Doba forest at the border of two relatively dry rainfall regimes may have complicated the quasi-bimodal pattern further, that is, the forest is located in a marginal area of the short rains, subjecting it to low but highly erratic winter rainfall. Besides, it is situated on top of a dry divide, which enhances the effect of short droughts, rendering the site too sensitive for dendrochronology. Similarly, erratic rainfall in an

extremely dry area (Oman) has been reported to cause multiple missing rings in Juniperus excelsa, rendering crossdating impossible (Sass-Klaassen et al., 2008a). Future site selection should take place at mesic locations in areas with unambiguously unimodal rainfall patterns, e.g. the areas experiencing ‘summer rain’, ‘winter rain’ and ‘rain most of the year’ (Fig. 2B). Relatively dry border areas are particularly problematic (see also Filfil, Fig. 2B), and temporal changes in rainfall patterns may complicate work on old trees. It would be worth to subject assumed successful samples to radiocarbon dating to test whether the growth rings really are annual. However, crucial for future research is monitoring of tree growth at different locations across Ethiopia by, e.g. cambial sampling or pinning, dendrometer measurements, growth experiments, quantitative woodanatomical analysis, etc. (e.g. Baisan and Swetnam, 1994; Worbes, 1995; Verheyden et al., 2004; Heinrich and Banks, 2006; De Luis et al., 2007).

Conclusion Growth rings of Juniperus procera trees in Doba forest are not annual in nature. The trees grow under similar environmental conditions, but their response depends on individual characteristics. Growth-ring formation is related to quiescence rather than dormancy, and the degree of quiescence varies between trees. Growth rings can be matched, but the match does not constitute a match in time. This observation appears to be a local phenomenon caused by a complex rainfall regime and site characteristics that render the growth behaviour of the trees too sensitive. Careful site selection, radiocarbon dating and particularly monitoring tree growth are advised for future research.

Acknowledgements The research work is funded by Swansea University and co-funded by the Royal Geographical Society, the Dudley Stamp Memorial Trust (Royal Society) and the Quaternary Research Association. Iain Robertson thanks the European Union for financial assistance provided through the Millennium project (017008-2). Ute Sass-Klaassen is funded by The Netherlands Organisation for Scientific Research (NWO) (MEERVOUD program). The Forestry Research Centre (FRC) of the Ethiopian Institute of Agricultural Research (EIAR) in Addis Ababa provided vehicle and other logistic support. South Wollo, Debre Sina and Legambo governmental offices for agriculture and rural development are acknowledged for support and kind permission to work in Doba forest. The National Meteorological Agency (NMA) of Ethiopia (Addis Ababa) is acknowl-

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edged for provision of meteorological data. We thank Dr. Alemu Gezahegn and Mr. Negash Mammo (FRC directors) for dedicated support, Mr. Tesfaye Ayalew for excellent driving to the research site, Dr. Quentin Dresser for radiocarbon dating, Mrs. Nicola Jones and Ms. Anna Ratcliffe for drawing maps, and Dr. Declan Conway (University of East Anglia) for fruitful discussions and help. We thank two anonymous reviewers for extensive feedback. Special thanks we owe to the people of Ethiopia for their hospitality, help, love and courage.

References Baillie, M.G.L., 1995. A Slice Through Time: Dendrochronology and Precision Dating. The Bath Press, Bath, (176pp.). Baillie, M.G.L., Pilcher, J.R., 1973. A simple cross-dating program for tree-ring research. Tree-Ring Bulletin 33, 7–14. Baisan, C.H., Swetnam, T.W., 1994. Assessment of Phenological Growth Patterns in Four Coniferous Species, Rhyolite Canyon, Chiricahua National Monument. University of Arizona, Tucson, (23pp.). Ba¨r, A., Bra¨uning, A., Lo¨ffler, J., 2006. Dendroecology of dwarf shrubs in the high mountains of Norway – a methodological approach. Dendrochronologia 24, 17–27. Cherubini, P., Gartner, B.L., Tognetti, R., Bra¨ker, O.U., Schoch, W., Innes, J.L., 2003. Identification, measurement and interpretation of tree rings in woody species from Mediterranean climates. Biological Reviews 78, 119–148. Conway, D., Brooks, N., Briffa, K.R., Desta, Sherrew, Merrin, P.D., Jones, P.D., 1997. Exploring the Potential for Dendroclimatic Analysis in Northern Ethiopia. University of East Anglia, Norwich, (38pp.). Conway, D., Brooks, N., Briffa, K.R., Merrin, P.D., 1998. Historical climatology and dendroclimatology in the Blue Nile River basin, Northern Ethiopia. In: Servat, E., Hughes, D., Fritsch, J.M., Hulme, M. (Eds.), Water Resources Variability in Africa during the XXth Century. Proceedings of the Abidjan ’98 Conference. IAHS Publications 252, pp. 243–251. Couralet, C., Sass-Klaassen, U., Sterck, F., Bekele, T., Zuidema, P.A., 2005. Combining dendrochronology and matrix modelling in demographic studies: an evaluation for Juniperus procera in Ethiopia. Forest Ecology and Management 216, 317–330. Couralet, C., Sass-Klaassen, U., Sahle, Y., Sterck, F., Bekele, T., Bongers, F., 2007. Dendrochronological investigations on Juniperus procera from Ethiopian dry afromontane forests. In: Haneca, K., Verheyden, A., Beeckman, H., Ga¨rtner, H., Helle, G., Schleser, G.H. (Eds.), Tree Rings in Archaeology, Climatology and Ecology (TRACE), vol. 5. Proceedings of the Dendrosymposium 2006. Royal Museum of Central Africa, Tervuren, pp. 73–79. Dai, K.-M., Fan, C.Y., 1986. Bomb produced 14C content in tree rings grown at different latitudes. Radiocarbon 28, 346–349. De Luis, M., Gricˇar, J., Cˇufar, K., Ravento´s, J., 2007. Seasonal dynamics of wood formation in Pinus halepensis from dry and semi-arid ecosystems in Spain. IAWA Journal 28, 389–404.

57

Demeke, F., 2004. Socio-economics (PRA) report of Denkoro Forest. Institution of Biodiversity Conservation and Research (IBCR), Addis Ababa (70pp.). Dezzeo, N., Worbes, M., Ishii, I., Herrera, R., 2003. Annual tree rings revealed by radiocarbon dating in seasonally flooded forest of the Mapire River, a tributary of the lower Orinoco River, Venezuela. Plant Ecology 168, 165–175. Eckstein, D., Bauch, J., 1969. Beitrag zur Rationalisierung eines dendrochronologischen Verfahrens und zur Analyse seiner Aussagesicherheit. Forstwissenschaftliches Centralblatt 88, 230–250. Eger, M., 1997. Achievements of the hermeneutic-phenomenological approach to natural science. Man and World 30, 343–367. EMA, 1988. National Atlas of Ethiopia. Ethiopian Mapping Authority, Addis Ababa. Eshete, G., St(ahl, G., 1999. Tree rings as indicators of growth periodicity of acacias in the Rift Valley of Ethiopia. Forest Ecology and Management 116, 107–117. Eshetu, Z., 2006. Dendroclimatological Research in Ethiopia. Ethiopian Institute of Agricultural Research, Forestry Research Centre, Addis Ababa, (19pp.). Fichtler, E., Trouet, V., Beeckman, H., Coppin, P., Worbes, M., 2004. Climatic signals in tree rings of Burkea africana and Pterocarpus angolensis from semiarid forests in Namibia. Trees 18, 442–451. Friis, I., 1992. Forests and Forest Trees of Northeast Tropical Africa. Kew Bulletin Additional Series XV. Her Majesty’s Stationary Office (HMSO), London, (396pp.). Gadamer, H.-G., 1960. Wahrheit und Methode: Grundzu¨ge einer philosophischen Hermeneutik. Mohr, Tu¨bingen, (486pp.). Gebrekirstos, A., 2006. Stable Carbon Isotopes and Plant Water Relations in the Acacia Savanna Woodlands of Ethiopia: Implications for Reforestation and Paleoclimatic Reconstructions. Cuvillier Verlag, Go¨ttingen, (143pp.). Geyh, M.A., Schleicher, H., 1990. Absolute Age Determination: Physical and Chemical Dating Methods and their Application. Springer, Berlin, New York, (503pp.). Grissino-Mayer, H.D., 2001. Evaluating crossdating accuracy: a manual and tutorial for the computer program COFECHA. Tree-Ring Research 57, 205–221. Heinrich, I., Banks, J.C.G., 2006. Tree-ring anomalies in Toona ciliata. IAWA Journal 27, 213–231. Hollstein, E., 1980. Mitteleuropa¨ische Eichenchronologie. Verlag Philipp von Zabern, Mainz am Rhein, (273pp.). Holmes, R.L., 1983. Computer-assisted quality control in treering dating and measurement. Tree-Ring Bulletin 43, 69–78. Hua, Q., Barbetti, M., 2004. Review of tropospheric bomb 14C data for carbon cycle modeling and age calibration purposes. Radiocarbon 46, 1273–1298. Jacoby, G.C., 1989. Overview of tree-ring analysis in tropical regions. IAWA Bulletin 10, 99–108. Kerkfoot, O., 1964. The distribution and ecology of Juniperus procera Endl. in East Central Africa, and its relationship to the genus Widdringtonia Endl. Kirkia 4, 75–87. Krishnamurthy, R.V., Epstein, S., 1985. Tree ring D/H ratio from Kenya, East Africa, and its palaeoclimatic significance. Nature 317, 160–162.

ARTICLE IN PRESS 58

T.H.G. Wils et al. / Dendrochronologia 27 (2009) 45–58

Martı´n-Benito, D., Cherubini, P., Del Rı´o, M., Can˜ellas, I., 2008. Growth response to climate and drought in Pinus nigra Arn. trees of different crown classes. Trees 22, 363–373. Mozeto, A.A., Fritz, P., Moreira, M.Z., Vetter, E., Aravena, R., Salati, E., Drimmie, R.J., 1988. Growth rates of natural Amazonian forest trees based on radiocarbon measurements. Radiocarbon 30, 1–6. Munro, M.A.R., 1984. An improved algorithm for crossdating tree-ring series. Tree-Ring Bulletin 44, 17–27. Norstro¨m, E., Holmgren, K., Mo¨rth, C.-M., 2005. Rainfalldriven variations in d13C composition and wood anatomy of Breonadia salicina trees from South Africa between AD 1375 and 1995. South African Journal of Science 101, 162–168. Nydal, R., Lo¨vseth, K., 1996. Carbon-14 Measurements in Atmospheric CO2 from Northern and Southern Hemisphere sites, 1962–1993. Oak Ridge National Laboratory, Oak Ridge, (48pp.) /http://cdiac.ornl.gov/epubs/ndp/ ndp057/ndp057.htmS Last accessed: 19 November 2008. Nydal, R., Lo¨vseth, K., Gullicksen, S., 1979. A survey of radiocarbon variation in nature since the test ban treaty. In: Berger, R., Suess, H.E. (Eds.), Radiocarbon Dating. Proceedings of the Ninth International Conference. University of California, pp. 313–323. Orvis, K.H., Grissino-Mayer, H.D., 2002. Standardizing the reporting of abrasive papers used to surface tree-ring samples. Tree-Ring Research 58, 47–50. Pilcher, J.R., 1990. Sample preparation, cross-dating and measurement. In: Cook, E.R., Kairiukstis, L.A. (Eds.), Methods of Dendrochronology, Applications in the Environmental Sciences. Kuwer Academic Publishers, Dordrecht, pp. 40–51. Popper, K.R., 1980. The Logic of Scientific Discovery. Hutchinson, London, (479pp.). Reimer, P.J., Brown, T.A., Reimer, R.W., 2004. Discussion: reporting and calibration of post-bomb 14C data. Radiocarbon 46, 1299–1304. Rinn, F., 2003. TSAP-Win, Software for Tree-Ring Measurement, Analysis and Presentation. Rinntech, Heidelberg. Robertson, I., Froyd, C.A., Walsh, R.P.D., Newbery, D.M., Woodborne, S., Ong, R.C., 2004. The dating of dipterocarp tree rings: establishing a record of carbon cycling and climatic change in the tropics. Journal of Quaternary Science 19, 657–664. Robertson, I., Loader, N.J., Froyd, C.A., Zambatis, N., Whyte, I., Woodborne, S., 2006. The potential of the baobab (Adansonia digitata L.) as a proxy climate archive. Applied Geochemistry 21, 1674–1680. Sass-Klaassen, U., Leuschner, H.H., Buerkert, A., Helle, G., 2008a. Tree-ring analysis of Juniperus excelsa from the northern Oman mountains. In: Elferts, D., Brumelis, G., Ga¨rtner, H., Helle, G., Schleser, G.H. (Eds.), Tree Rings in Archaeology, Climatology and Ecology (TRACE), vol. 6. Proceedings of the Dendrosymposium 2007. University of Latvia, pp. 99–108. Sass-Klaassen, U., Couralet, C., Sahle, Y., Sterck, F.J., 2008b. Juniper from Ethiopia contains a large-scale precipitation signal. International Journal of Plant Sciences 169, 1057–1065. Scho¨ngart, J., Orthmann, B., Hennenberg, K.J., Porembski, S., Worbes, M., 2006. Climate-growth relationships of tropical tree species in West Africa and their potential for climate reconstruction. Global Change Biology 12, 1139–1150.

Schweingruber, F.H., Eckstein, D., Serre-Bachet, F., Bra¨ker, O.U., 1990. Identification, presentation and interpretation of event years and pointer years in dendrochronology. Dendrochronologia 8, 15–38. Smith, A.G., Pearson, G.W., Pilcher, J.R., 1970. Belfast radiocarbon dates I. Radiocarbon 12, 285–290. Stahle, D.W., 1999. Useful strategies for the development of tropical tree-ring chronologies. IAWA Journal 20, 249–253. Stahle, D.W., Mushove, P.T., Cleaveland, M.K., Roig, F., Haynes, G.A., 1999. Management implications of annual growth rings in Pterocarpus angolensis from Zimbabwe. Forest Ecology and Management 124, 217–229. Stokes, M.A., Smiley, T.L., 1968. An Introduction to Tree-Ring Dating. University of Chicago Press, Chicago, (73pp.). Stuiver, M., Polach, H.A., 1977. Discussion; reporting of 14C data. Radiocarbon 19, 355–363. Stuiver, M., Reimer, P.J., Bard, E., Beck, J.W., Burr, G.S., Hughen, K.A., Kromer, B., McCormac, G., Van der Plicht, J., Spurk, M., 1998. INTCAL98 radiocarbon age calibration, 24,000-0 cal BP. Radiocarbon 40, 1041–1083. Therrell, M.D., Stahle, D.W., Ries, L.P., Shugart, H.H., 2006. Tree-ring reconstructed rainfall variability in Zimbabwe. Climate Dynamics 26, 677–685. Therrell, M.D., Stahle, D.W., Mukelabai, M.M., Shugart, H.H., 2007. Age, and radial growth dynamics of Pterocarpus angolensis in southern Africa. Forest Ecology and Management 244, 24–31. Trouet, V., Haneca, K., Coppin, P., Beeckman, H., 2001. Tree ring analysis of Brachystegia spiciformis and Isoberlinia tomentosa: evaluation of the ENSO-signal in the Miombo woodland of Eastern Africa. IAWA Journal 22, 385–399. Trouet, V., Coppin, P., Beeckman, H., 2006. Annual growth ring patterns in Brachystegia spiciformis reveal influence of precipitation on tree growth. Biotropica 38, 375–382. Verheyden, A., Kairo, J.G., Beeckman, H., Koedam, N., 2004. Growth rings, growth ring formation and age determination in the mangrove Rhizophora mucronata Lam. Annals of Botany 94, 59–66. Wigley, T.M.L., Jones, P.D., Briffa, K.R., 1987. Cross-dating methods in dendrochronology. Journal of Archaeological Science 14, 51–64. Wils, T.H.G., 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. In: Haneca, K., Verheyden, A., Beeckman, H., Ga¨rtner, H., Helle, G., Schleser, G.H. (Eds.), Tree Rings in Archaeology, Climatology and Ecology (TRACE), vol. 5. Proceedings of the Dendrosymposium 2006. Royal Musem of Central Africa, Tervuren, pp. 277–284. Worbes, M., 1995. How to measure growth dynamics in tropical trees, a review. IAWA Journal 16, 337–351. Worbes, M., 2002. One hundred years of tree-ring research in the tropics-a brief history and an outlook to future challenges. Dendrochronologia 20, 217–231. Worbes, M., Junk, W.J., 1989. Dating tropical trees by means of 14C from bomb tests. Ecology 70, 503–507. Worbes, M., Staschel, R., Roloff, A., Junk, W.J., 2003. Tree ring analysis reveals age structure, dynamics and wood production of a natural forest stand in Cameroon. Forest Ecology and Management 173, 105–123.