Tree-ring analysis of kauri (Agathis australis) timbers from a colonial-era villa in Birkenhead, North Shore City

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1 New Zealand Tree-Ring Site Reports Tree-ring analysis of kauri (Agathis australis) timbers from a colonial-era villa in Birkenhead, North Shore City Jan Wunder, Gretel Boswijk & Peter Crossley NZTRSR School of Environment 1 Working Paper 38 (ISSN: ISBN: ) The University of Auckland Tree-Ring Laboratory, School of Environment, The University of Auckland, Private Bag 92019, Auckland, New Zealand 1 formerly School of Geography, Geology and Environmental Science (ISSN: )

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3 Tree-ring analysis of kauri (Agathis australis) timbers from a colonial-era villa in Birkenhead, North Shore City This is a technical archive report describing recent dendrochronological analysis of kauri timbers from a colonial-era villa in Birkenhead district, North Shore City. The report describes the calendar dating of kauri timbers from a veranda of the villa and considers aspects of chronology building using low sample sizes. It should be noted that although the tree-ring dates will not change, interpretation based on these dates may change as new information comes to light. Summary Tree rings are an important archive of past climate phenomena such as the El Niño-Southern Oscillation (ENSO). The ENSO reconstruction based on tree rings of kauri (Agathis australis) is currently being extended from the last ca. 400 years to the last Millennium. In this study, we used tree-ring information from kauri timbers of a colonial-era villa built in the 1870s or 1880s in Birkenhead, North Shore City, to improve the quality of the current master chronology by increasing its sample depth (tree-ring information per calendar year) during the data scarce time window of AD 1000 AD Analysis of 149 samples from these kauri timbers resulted in a 249-year site chronology spanning from AD 1063 to AD 1311 (BIRKa) and a 116-year five-timber sequence spanning from AD 1360 to AD 1475 (BIRKb). Many samples were characterised by very narrow rings, many resin bands and wedging rings suggesting that they originated from old trees (800+ years) with long periods of suppressed growth. Both, the site chronology and the five-timber sequence, are important contributions to a 1000 year-high quality master chronology of kauri since they will significantly increase the sample depth during the first half of the last Millennium. 1

4 Tree-ring analysis of kauri (Agathis australis) timbers from a colonial-era villa in Birkenhead, North Shore City Introduction Tree-ring based reconstructions of climate phenomena such as the El Niño-Southern Oscillation (ENSO) require master chronologies with high sample depth, i.e. tree-ring information per calendar year. High sample depth increases the likelihood of capturing a common signal in the analysed tree-ring sequences, and well replicated chronologies assure an accurate crossdating process. However, current ENSO reconstructions using tree rings of kauri (Agathis australis) are limited to the last 400 to 500 years (e.g. Fowler 2008) since the sample depth of the existing long-term kauri chronology (1724 BC AD 2002, Boswijk et al. 2006) is relatively low prior to AD Recent attempts to increase the sample depth of the kauri master chronology focus on analysing archaeological timbers and subfossil kauri since most old living kauri trees are strictly protected and the Department of Conservation permits coring only in very rare cases. In this study, we analysed kauri timber from the veranda of a colonial-era villa (Fig. 1) in Birkenhead, North Shore City, to increase the sample depth of the master chronology from AD 1000 to AD 1500 in order to achieve a high-quality millennial length chronology suitable for climate reconstructions. Fig. 1: The veranda of the analysed kauri villa (Birkenhead Villa), 16 Maritime Terrace, North Shore City. The veranda boarding was replaced in February 2007 and the old kauri timber analysed in this study, Photo: J. Wunder, Nov

5 Material & Methods Birkenhead Villa The kauri villa, referred to here as Birkenhead Villa, is situated at 16 Maritime Terrace, Birkenhead District, North Shore City ( S, E). The city belongs to the Auckland region and is located north of the Waitemata Harbour and Auckland City (Fig. 2). The first houses of European settlers in the region of North Shore City were built in the 1840s (McClure 1987), and today the city s 223,000 inhabitants make it the 4 th largest city in New Zealand (Statistics New Zealand 2008). Most of the villas in the early years were built using timber from kauri. Tree-ring dating of timbers from other buildings constructed in the 1870s to 1900s indicates that trees > 500 years old (e.g., Lorrey et al. 2004, Boswijk 2007) were being felled to supply the timber industry. The exact construction date of the villa is unknown, however, it is estimated that the villa was built either in the 1870s or 1880s; a small extension to the sitting room was made in the 1920s, and a section of a porch was enclosed to make a laundry in the mid 1970s (Leonard Bell 1, personal communication). The veranda was modified over time, in the 1920s some of it was enclosed to make a gallery space, which was reversed in the mid 1970s. Furthermore, a few boards were replaced at various times in the 1970s and 1980s, and the original wooden steps were replaced with concrete steps. In February 2007, the veranda timber was replaced, providing an opportunity to sample the old boarding for tree-ring analysis. North Shore City 16 Maritime Terrace, Birkenhead North Shore City Birkenhead North Shore City, Birkenhead Auckland City Fig. 2: Location of the analysed kauri villa (Birkenhead Villa) at 16 Maritime Terrace, North Shore City (left), the Waitemata Harbour (bottom right) and the relative position of North Shore City with New Zealand (top right), Satellite photos: Google Earth 2009, Blank map of New Zealand: geography.about.com. 1 Associate Professor at the Department of Art History, University of Auckland and owner of Birkenhead Villa. 3

6 Sampling Cross sections of the veranda boards were taken using a chainsaw. The resulting 169 pieces of timber were macroscopically identified: 149 were derived from kauri (Agathis australis, Table A1 and A2), 18 from another endemic conifer species, possibly Totara (Podocarpus totara, Table A3) and two pieces from (exotic) Monterey Pine (Pinus radiata, Table A3). In this report, we focus on kauri timber. Out of the 149 kauri samples, 130 were derived from large boards (cross section of ca. 138 x 20 mm), the remaining 19 from smaller boards (ca. 105 x 25 mm, Fig. 3). For nearly all kauri samples, the type of board was classified as tongue & groove (145); the remaining four samples were from boards with two grooves on each side (referred to as groove & groove boards). Sample preparation & pre-screening The site was assigned a four letter code, BIRK, and the samples were labelled with a unique three-letter three-number code (e.g. BIR001). After collection, the cross-sectional surface of each sample was sanded to a fine polish using increasingly fine grades of sand paper (up to grit 1200). The clarity of the ring pattern was noted as well as special or unusual features (e.g. resin bands). From the 149 kauri samples, 75 (ca. 50%) were assumed to be suitable for tree-ring analysis, i.e. they contained at least ca. 40 to 50 consecutive rings. Shorter sequences are assumed to be too short to contain a unique growth pattern and may lead to spurious results during crossdating (Cook & Kairiukstis 1990, English Heritage 2004). Measurement & crossdating The width of the annual rings was measured using a binocular microscope and a travelling stage fitted with a linear encoder which is connected to a computer (Fig. 3). Ring width data were recorded using an input program (Input for 32-bit Windows, Tyers 2004). Once a set of suitable series had been measured, the series were compared to each other to identify those that crossmatched. Crossmatched series were then averaged together to construct a working site chronology. Averaging the ring width series can reduce the noise of endogenous effects on tree-ring formation (e.g. tree competition) and enhance the exogenous climate effect on ring formation on which crossdating is dependent (Baillie 1982). The working site chronology was then compared to master and independent site chronologies / dated sequences (modern sites, archaeological sites, subfossil sites) to identify a calendar date for the chronology. Further samples from the Birkenhead Villa assemblage were measured and crossdated to the working site chronology, which was incrementally updated to include all newly dated series. Fig. 3: Binocular microscope and travelling stage (left) used to analyse timber of Birkenhead Villa (right), the crosssections are ca. 138 x 20 mm (large pieces) and ca. 105 x 25 mm (small pieces, e.g. top left), Photos: J. Wunder, Sep and Feb

7 Statistical crossdating was performed using the crossdating programs CROS73 (Baillie & Pilcher 1973) and Cross84 (Munro 1984) included in the Dendro for Windows suite (version , Tyers 2004). These programs compare pairs of tree-ring series and calculate the correlation coefficient (r) for every position of overlap of the previously standardised series. A Student s t-statistic is calculated to provide a measure of the probability of the r-value arising by chance (Baillie 1982). Earlier work on kauri adopted a criterion of identifying t-values > 6.00 as highly significant, provided that the visual match is acceptable (Boswijk et al. 2000). The crossdating results suggested by Dendro for Windows were verified using the tree-ring crossdating program COFECHA (Holmes 1983, Grissino-Mayer 2001). For each detrended tree-ring series, COFECHA creates a master chronology from every other series, and then calculates the correlation coefficient between every 50-year segment of that series and the master chronology (NOAA Paleoclimatology Program 2005). The resulting correlation matrix can be used to examine flagged (problem) segments, changes in sample depth over time, and changes in average correlation over time (NOAA Paleoclimatology Program 2005). If not stated differently, all crossdating statistics (t- and r-values) in this report are based on Cross84 (Munro 1984). All matches suggested by the crossdating programs were checked visually using line plots, overlaid on a light box. Occasionally very high t-values may be obtained between series. If supported by close visual agreement of ring-width plots and macroscopic features of the wood samples (e.g. colour), this may indicate that these samples were obtained from the same original length of timber or were cut from the same tree. In this case, such same-tree series were averaged to form a timber-sequence which was subsequently used for chronology building. This procedure reduces the weight of assumed same-tree -material during the chronology building (English Heritage 2004). False and missing rings Generally, a key issue during the chronology building process is the secure identification of false rings. False rings can occur when the growing season is interrupted by a period of inferior growth (e.g. during a drought period in spring). During this period the stressed tree may form new tissue with thick cell walls that look similar to latewood cells, and the so formed ring boundary looks similar to the boundary of a real ring. False rings can usually be identified by a slow decrease of cell wall thickness between false latewood cells and later formed earlywood cells, whereas real ring boundaries are usually characterised by a sharp border between latewood and earlywood (Stokes & Smiley 1968). Missing or locally absent rings are a more extreme expression of stress, they indicate years in which none or only a partial ring was formed. For kauri, the formation of false and missing rings is characteristic for many sites (e.g., Boswijk et al. 2006). Crossdating within and between trees of one particular site and against other chronologies usually helps to resolve problems with individual series. If not resolvable, these series are discarded from further analysis. 5

8 Results The assemblage collected from Birkenhead Villa comprised 149 kauri samples, 75 of those had more than ca. 40 to 50 rings that are assumed to be required for the crossdating process (Cook & Kairiukstis 1990, English Heritage 2004). Crossdating within the individual tree-ring series of the assemblage resulted in twelve groups that are referred to as timber sequences BIRS1 to BIRS12. Each timber sequence consists of two to five tree-ring series (Table A1, Appendix). The high mean t-values for each sequence (up to t = 49.09, Table A1) and similar macroscopic appearance of the timber (colour, resin bands, etc.) suggest that the series within each sequence are same-tree -material. Of the twelve timber sequences, three were calendar dated by comparison to master chronologies (AGAUl07r and AGAUc07r 1, TRL unpubl.): BIRS1 (four series: BIR035, BIR38, BIR045, BIR088) & BIRS2 (two series: BIR153, BIR166) are the basis of the small site chronology BIRKa (AD 1063 AD 1311) and BIRS3 (five series: BIR033, BIR052, BIR057, BIR137, BIR138) represents a five-timber sequence labelled BIRKb (AD 1360 AD 1475, Fig. 4). Thus, the three sequences are based on 11 series (all tongue & groove large) whose length ranged from 65 to 213 rings, with an average length of 117 rings. AD 1063 AD 1475 BIR 045 BIR 035 BIR 088 BIR 038 BIR153 BIR 166 BIRKa BIRKb BIR 138 BIR 052 BIR 033 BIR137 BIR 057 Fig. 4: Composition of the site chronology BIRKa (BIRS1: horizontal hatching, BIRS2: vertical hatching) and fivetimber sequence BIRKb (BIRS3: white). Each bar represents one series, with all bars arranged by location and aligned by end date. Site chronology BIRKa The site chronology, BIRKa, spans 249 years from AD 1063 to AD 1311 (Fig. 4). BIRKa was built using two dated timber sequences: the four-timber sequence BIRS1 (BIR035, BIR038, BIR045, BIR088) and the two-timber sequence BIRS2 (BIR153, BIR166, see Table 1, Table A4, Table A6). For BIRS1, the (very) high t-values between the series BIR035, BIR038 and BIR088 and similar colour and structure of these timbers strongly suggests that the three series are same-tree -material. The crossdating statistics favour a similar classification of the remaining series BIR045 (Table 1), even though its colour and structure deviates slightly from the other series. This variation indicates that BIR045 either originated from a different part of the trunk of the same tree or a different tree with very similar growth patterns. The two series of BIRS2 could be also same-tree material, even though the t-values are con- 1 AGAUl07r is comprised of raw modern and archaeological data and spans AD 911 AD AGAUc07r is comprised of raw modern, archaeological and subfossil data and spans 1724 BC AD

9 siderably lower as within BIRS1. Therefore, the site chronology BIRKa may consist of two to three trees only. Table 1: Triangular t-value matrix of cross-dating values for all individual series in the chronology BIRKa. Symbols: - = t-values less than 3.00, * = empty triangle. Average series length: 146 rings (max: 213, min: 65). Filenames BIR035 BIR038 BIR045 BIR088 BIR153 BIR166 - start AD1063 AD1099 AD1073 AD1129 AD1163 AD end AD1264 AD1311 AD1251 AD1279 AD1227 AD1231 BIR035 AD1063 AD1264 * BIR038 AD1099 AD1311 * * BIR045 AD1073 AD1251 * * * BIR088 AD1129 AD1279 * * * * BIR153 AD1163 AD1227 * * * * * 9.50 BIR166 AD1167 AD1231 * * * * * * n = 15, min t = 1.88, max t = 35.50, mean t = 12.39, s.d. = The signal strength of the site chronology BIRKa is consistently very high across different time windows (Table 2): for all nine time windows, the average segment correlation never falls below 0.80 (Table 2). Table 2: Quality control and dating check of tree-ring measurements of BIRKa (COFECHA): correlation of series by segments (for more details see section Material & Methods, p.5) Series Time span (all AD) BIR035 AD1063 AD BIR038 AD1099 AD BIR045 AD1073 AD BIR088 AD1129 AD BIR153 AD1163 AD BIR166 AD1167 AD Av. segment correlation Five-timber sequence BIRKb The five-timber sequence BIRKb, spans 116 years from AD 1360 to AD 1475 (Fig. 3). BIRKb represents the dated five-timber sequence BIRS3 (BIR033, BIR052, BIR057, BIR137, BIR138). High t-values of the crossdating table (Table 3) and similar colour and structure of the timber suggest that the five series were derived from the same tree. The signal strength of the five-timber sequence BIRKb is also consistently very high across different time windows (Table 4, Table A5, Table A7). Table 3: Triangular t-value matrix of cross-dating values for all individual samples in the five-timber sequence BIRKb. Symbols: * = empty triangle. Average series length: 88 rings (max: 95, min: 79). Filenames BIR033 BIR052 BIR057 BIR137 BIR138 - start AD1382 AD1361 AD1397 AD1393 AD end AD1469 AD1454 AD1475 AD1474 AD1451 BIR033 AD1382 AD1469 * BIR052 AD1361 AD1454 * * BIR057 AD1397 AD1475 * * * BIR137 AD1393 AD1474 * * * * 9.00 BIR138 AD1360 AD1454 * * * * * n = 10, min t = 8.20, max t = 22.51, mean t = 12.38, s.d. =

10 Table 4: Quality control and dating check of tree-ring measurements of BIRKb (COFECHA): correlation of series by segments (for more details see section Material & Methods, p.5) Series Time span (all AD) BIR033 AD1382 AD BIR052 AD1361 AD BIR057 AD1397 AD BIR137 AD1393 AD BIR138 AD1360 AD Av. segment correlation Comparison with other chronologies and dated sequences A comparison of the site chronology BIRKa and the five-timber sequence BIRKb against independent master and site chronologies and dated sequences revealed very good crossdating results with the master chronologies, i.e. the modern & archaeological sites chronology and the long chronology (modern + archaeological + subfossil sites, see Table 5). The crossdating results against the independent site chronologies and dated sequences have to be treated carefully since BIRKa & BIRKb as well as most site chronologies are characterised by a (very) low sample depth during the overlap period. For example, a comparison of BIRKb (one tree) against Yakas1 (one tree) is a comparison of two individual trees from different sites, for which low t-values could occur. Nevertheless, BIRKa and BIRKb crossdate well with most house site chronologies and the Display piece, a logging relic of unknown origin. However, the crossdating with material from the two subfossil sites (Hoa001 and Yakas1) resulted in considerably lower t-values (Table 5). Note that the term site chronology is used for both chronologies of modern (ecological) sites and archaeological sites: Modern site chronologies are characterised by a common signal derived from increment cores taken at one forest site, i.e. the tree-ring formation of different trees at that site occurs under similar abiotic conditions (soil, climate, etc.). For archaeological site chronologies, the term site chronology refers to a set of all timbers used to build a house or a wooden structure, irrespective of the origin of the kauri trees that were used during the building process. Hence, the common signal of archaeological site chronologies may be based on trees that originate from different forest sites characterised by a different set of abiotic factors that influenced the formation of the tree rings. 8

11 Table 5: Comparison of BIRKa and BIRKb to the current (working) kauri master chronologies (AGAUl07r = master modern sites + archaeological sites, AGAUc07r = master long chronology = modern sites + archaeological sites + subfossil sites), to archaeological site chronologies (Wynd28a = Wynyard 28a, SPchurch = St. Paul s church, George1 = Georgina 1), to logging relics (Dsp001 = Display piece), to modern site chronologies (Manaia and Mt. Moehau) and to subfossil timber sequences (Hoa001 = Hoanga 001 and Yakkas 001). Date span refers to the start and end dates of each chronology or sequence. Overlap refers to the length of the common period between the reference chronologies / dated sequences and BIRKa / BIRKb. Type of chronology / dated sequence (region) Date span chronology/ dated sequence BIRKa AD AD 1311 BIRKb AD AD1475 Filename (Reference) start end Overlap t-value r Overlap t-value r dates dates (years) (years) Master chronology AGAUl07r AGAUc07r (TRL unpubl.) (TRL unpubl.) Archaeological sites (Auckland, Northland) Wynd28a (Lorrey et al. 2004) SPchurch (Boswijk 2007) George1 (Bridge unpubl.) Logging relic (origin unknown) Dsp001 (TRL unpubl.) Modern sites (Coromandel) Manaia Mt. Moehau (Boswijk et al. 2000) (TRL unpubl.) AD911 AD BC AD AD940 AD AD1125 AD AD1207 AD AD911 AD AD1269 AD AD1360 AD Subfossil sites (Northland) Hoa001 (Boswijk AD1093 AD ) Yakas1 (Boswijk & AD304 AD Palmer 2004) Note: no crossdating statistic was calculated for overlaps < 50 years (years in italics). 9

12 New Zealand Tree-Ring Site Report 31 Undated material The remaining nine timber sequences (BIRS4 to BIRS12) could not be dated against master chronologies (AGAUl07r and AGAUc07r, TRL unpubl., Table A1). Most of this material is characterised by very narrow rings and many resin bands. Hence it seems plausible that some of the material originates from old trees (800+ years) that experienced very long periods of suppressed growth (Fig. 5), which may have affected their cross-dating potential. The rest of the 38 measured series remain undated (Table A1, Appendix) since they do not crossdate to any of the sequences from the site (BIRS1 to BIRS12) or to any master chronologies. Again, the main reasons for this are probably suppressed growth patterns with many very narrow, wedging and locally absent tree rings and areas with resin filled cells that affected both the crossdating potential and also the clarity of the ring boundaries (Fig. 5). For the short series (ca. 50 rings) without suppressed growth pattern, the overlap to potentially crossdating series within the Birkenhead assemblage may be not long enough to calculate reliable crossdating statistics. Resin bands Ray 20 mm Ring boundaries Fig. 5: Left image: Examples of resin bands and narrow wedging rings (undated series BIR155, BIR043, BIR164 from top to bottom). Right image: Resin bands of BIR067 (not dated). These bands and the suppressed growth pattern with narrow wedging rings were typical features of the measured Birkenhead samples, Photos: J. Wunder, Feb. 2009, right image: Olympus U-TV0.5XC-3 colour view imaging system, magnification 5x. False rings The Birkenhead samples contained some false rings. Their clear identification was sometimes difficult due to very narrow bands of latewood cells (only two to three cells wide), i.e. not enough to observe the above mentioned transition zone between false latewood cells and later formed earlywood cells (Stokes & Smiley 1968). One of the samples, (BIR038, Fig. 6) of the BIRKa chronology, contained such a ring that was treated as false ring formed in AD 1201, mainly due to the fact, that none of the other BIRKa samples contained signs of wedging rings in that particular calendar year. AD 1201 AD AD 1201

13 AD1201 AD1200 Fig. 6: Assumed false ring formed during AD 1201 (BIR038), Photo: J. Wunder, Feb. 2009, Olympus U-TV0.5XC-3 colour view imaging system, magnification 5x. A verification of this false ring with other series of the kauri master chronology at around AD 1200 is difficult since the master consists of only five to six trees at this time period, depending on the treatment of same-tree material. A comparison of the AD 1201 ring across all samples that contribute to the master chronology, i.e. series from the sites Wynyard Street and St. Paul s Church (archaeological timbers), Hoanga 1 (subfossil sample) and the Display piece (logging relic of unknown origin) revealed that ring-like structures were formed at two samples from St. Paul s Church (SPC 141 & SPC 142) immediately after the establishment of the AD 1200-tree ring (Fig. 7). These had been treated as false rings during the development of the St. Paul s Church site chronology. AD1201 AD1201 AD1201 AD1200 AD1200 AD1200 Fig. 7: Ring-like structures formed after AD 1200 at STP141 (left and middle: assumed wedging ring between AD 1200 and AD 1201) and STP 142 (right: assumed false ring during AD 1201), Photos: J. Wunder, Feb. 2009, Olympus U-TV0.5XC-3 colour view imaging system, magnification 5x. 11

14 Discussion From the Birkenhead Villa material, a site chronology BIRKa (BIRS1 + BIRS2: AD 1063 AD 1311) and a five-timber-sequence BIRKb (BIRS3: AD 1360 AD 1475) could be developed. Both fall into a time period where replication of tree-ring information is highly desired since the sample depth of the kauri master chronology between AD 1000 and AD 1500 is particularly low (Fig. 8). This time window falls into the overlap phase between modern/archaeological material and subfossil material of kauri, i.e. a time period where the sample depth of subfossil timbers is thinning out and the oldest modern and archaeological timbers become available (Boswijk et al. 2006). Therefore, the eleven dated series from BIRS1 (one two trees), BIRS2 (one tree) and BIRS3 (one tree) will considerably improve the current kauri master chronology (e.g. AGAUl07r, TRL unpubl., Fig. 8) and will contribute to an updated high-quality millennial master chronology suitable for paleoclimate reconstructions. Sample depth (AD AD 2002) Frequency of tree-ring series AGAUl07r (master chronology) BIRKa (site chronology) BIRKb (five-timber mean) Time (calendar years) Frequency of tree-ring series Sample depth (AD AD 1500) Time (calendar years) Fig. 8: Sample depth of the individual tree-ring series ( trees) for the 1092-year-master chronology AGAUl07r (archaeological + modern sites) and the contribution of the site chronology BIRKa and the five-timber mean BIRKb. BIRKa consists of 6 tree-ring series (2-3 trees), BIRKb of 5 tree-ring series (1 tree). Graph: R for Windows version 2.9.1, R Development Core Team For the site chronology BIRKa, the crossdating results against independent master and site chronologies are generally very good, with exceptions of the house site chronology Georgina St (George1), and the subfossil timber series Hoanga1 and Yakkas1. However, the overlap between BIRKa and George1 is across a period when George1 has only one series and it may be possible that such low t-values occur when comparing one tree with a site chronology consisting of two to three trees. This could also explain the low t-values of BIRKa derived for the comparison against the two subfossil sites that both represent only one individual tree. 12

15 For the five-timber sequence BIRKb, all crossdating results were fairly good except for the two sites George1 and Mt. Moehau. For George1, the inferior crossdating statistic may be again a result of the low sample depth across the overlap period with BIRKb: across the time window AD 1360 to AD 1475, the George1 site chronology is a mean of two to four tree-ring sequences. In addition, the relatively short overlap period (116 years) may lead to low t-values. The weak crossdating results between BIRKb and Mt. Moehau are in line with previous observations suggesting that Mt. Moehau shows weaker crossdating results against all other kauri sites (Fowler et al. 2004) which may be caused by the potentially different growth reactions of kauri growing at their altitudinal limit. Since the Birkenhead timber did not include any dated tree-ring information from around AD 1900, it was not possible to identify the construction date of the veranda. However, the calendar dates of some series, as well as other characteristics such as series length, indicates that some of the timber was derived from trees that were > 800 years old. Same-tree & Same-site material Probably each of the timber sequences BIRS1 to BIRS12 contains (sometimes exclusively) same-tree material. However, even though high t-values of the crossdating statistic and similar colour and structure of the cross-sections may support that assumption, the variability of tree-ring series within one individual tree remains unknown. For example, no statistical analysis of the tree-ring patterns at different trunk heights of kauri has been undertaken, i.e. possible positions of origin for any particular piece of building timber. Such knowledge would be highly useful to better classify same-tree material in building timber, and to get a better estimate of the total sample depth of trees in archaeological site - and composite chronologies (combined archaeological site - and modern site chronology). For the Birkenhead assemblage, as for many other house sites, it is not possible to decide whether all dated series originated from trees of the same forest site. One potential criterion indicating same-site material may be similar growth patterns of trees at one particular site characterised by a certain regional climate. This climate forcing influences the trees of one forest site in a similar way - and hence the intrasite correlation of these trees would be higher as compared to inter-site correlations between different sites. This difference would be reflected in crossdating statistics, i.e. t- and r-values. However, for such a test of same-site material, a higher sample depth of trees would be needed: the three dated sequences from Birkenhead Villa are clearly not enough to perform such a test. Furthermore, the dated sequences of Birkenhead Villa only partly overlap (BIRS1 & BIRS2, with a 49 year gap to BIRS3) therefore it is not possible to gain any information concerning whether BIRS1/BIRS2 was derived from the same site as BIRS3 (see Fig. 4). Sample size & false vs. missing rings Small sample sizes can affect the clear identification of false rings and the building of a site chronology independent of a master chronology: For example, out of the 6 series of BIRKa, one had an AD 1201-ring that was divided by an apparent boundary such that it looked like two annual rings (Fig. 6). Further analysis with a microscope did not allow for a clear conclusion, even though it favours the classification as false ring formed in AD In this case, assuming an independent chronology without reference to a master chronology, two assumptions are possible: the narrow ring is real and a locally absent ring has to be added to the five other samples - or the narrow ring is a false ring. Unfortunately, the sample depth of the master chronology equals only five to six trees around AD 1200 (depending on the classification of same-tree material) given this sample depth it cannot be falsified that all these trees have an undis- 13

16 covered locally absent ring in the same year, even though the likelihood for such an event appears to be relatively low. Nevertheless, such an error would offset the chronology by one year, and given low sample sizes, the probability for those offsets increases when going back in time, especially with regard to millennial chronologies. The question arises how likely such errors are given a certain small sample size of trees. Therefore, the proportion of locally absent rings in the kauri master chronology across different time windows should be analysed to estimate a minimum number of trees that is necessary to ensure the correct detection of all locally absent rings during the process of chronology building. Interestingly, multi-proxy summer temperature reconstructions based on tree rings and ice cores show a negative anomaly in the year AD 1201/1202 for both hemispheres, possibly associated with volcanic activity (Jones et al. 1998, Oppenheimer 2003). Future kauri samples dated to around AD 1200 may show whether kauri populations more than 800 years ago were showing any reaction to this anomaly, i.e. whether the tree-ring series at that point of time are characterised by locally absent rings or by false rings. Conclusion The analysis of Birkenhead assemblage resulted in the development of a site chronology and a five-timber sequence, both will significantly increase the relatively low sample depth of the current kauri master chronology before AD Furthermore, the assemblage is an excellent example of how low sample depth may influence the process of chronology building and what directions of future research may be useful to facilitate the building of high-quality chronologies suitable for the reconstruction of climate phenomena such as ENSO. Acknowledgements We would like to thank Leonard Bell for the opportunity to use old building timber from his villa in Birkenhead for research purposes, Anthony Fowler for helpful comments on an earlier version of the manuscript and Shane McCloskey for valuable advice regarding the crossdating program COFECHA. JW acknowledges funding by the Swiss National Science Foundation SNF (post-doctoral fellowship PBEZ ). Appendix Table A1: Details of all measured kauri samples obtained from Birkenhead Villa. Table A2: Details of all unmeasured kauri samples obtained from Birkenhead Villa. Table A3: Details of all unmeasured non-kauri samples obtained from Birkenhead Villa. Table A4: Inter-tree comparisons of all trees of the site chronology BIRKa (COFECHA). Table A5: Inter-tree comparisons of all trees of the five-timber sequence BIRKb (COFECHA). Table A6: Descriptive statistics for each individual series of the site chronology BIRKa (COFECHA). Table A7: Descriptive statistics for each individual series of the five-timber sequence BIRKb (COFECHA). 14

17 References Baillie, M.G.L. & Pilcher, J.R. (1973): A simple crossdating program for tree-ring research. Tree-Ring Bulletin 33: 7-14 Baillie, M.G.L. (1982) Tree-ring dating and Archaeology. University of Chicago Press, Chicago, 274 p. Boswijk, G., Fowler, A., and Ogden, J., (2000): Tree-ring analysis of kauri (Agathis australis) from Manaia Sanctuary, Coromandel Peninsula, New Zealand Tree-ring Site Report 1. Department of Geography Working Paper 9, University of Auckland, Auckland. Boswijk, G. & Palmer, J. (2004): Tree-ring analysis of sub-fossil kauri (Agathis australis) from Yakas' Farm, Babylon Coast Road, near Dargaville, Northland, New Zealand Tree-ring Site Report 13. School of Geography and Environmental Science Working Paper 22, University of Auckland, Auckland. Boswijk, G. (2004): Tree-ring analysis of buried kauri (Agathis australis) from Hoanga Road, Dargaville, and Pouto, north Kaipara Peninsula, New Zealand Tree-ring Site Report 14. School of Geography and Environmental Science Working Paper 24, University of Auckland, Auckland. Boswijk, G., Fowler, A., Lorrey, A., Palmer, J. & Ogden, J. (2006): Extension of the New Zealand kauri (Agathis australis) chronology to 1724 BC. The Holocene 16(2): Boswijk, G. (2007): Tree-ring analysis of kauri (Agathis australis) timbers from St Paul s Anglican Church, Kawakawa, New Zealand Tree-ring Site Report 28. School of Geography and Environmental Science Working Paper 36, University of Auckland, Auckland. Cook, E., & Kairiukstis, L. (eds.) (1990): Methods of Dendrochronology. Applications in the Environmental Sciences. Kluwer Academic Publishers, Dordrecht, 394 p. English Heritage (2004): Dendrochronology: Guidelines on producing and interpreting dendrochronological dates. London, 39 p. ( March 23, 2009). Fowler, A., Boswijk, G., Ogden, J. (2004): Tree-ring studies on Agathis australis (kauri): a synthesis of development work on late Holocene chronologies. Tree-Ring Research 60(1): Fowler, A.M. (2008): ENSO history recorded in Agathis australis (kauri) tree rings. Part B: 423 years of ENSO robustness. International Journal of Climatology 28: Grissino-Mayer, H. D. (2001): Evaluating crossdating accuracy: A manual and tutorial for the computer program COFECHA. Tree- Ring Research 57(2): Holmes, R. L. (1983): Computer-assisted quality control in tree-ring dating and measurement. Tree-Ring Bulletin 43: Jones, P.D., Briffa, K.R., Barnett, T.P. & Tett, S.F.B. (1998): High-resolution palaeoclimatic records for the last millennium: interpretation, integration and comparison with General Circulation Model control-run temperatures. The Holocene 8: Lorrey A., Lux. J, Boswijk. G, and Crossley, P. (2004): Dendrochronological analysis of salvaged kauri timber from 26 and 28 Wynyard Street, The University of Auckland. New Zealand Tree-ring Site Report 12. School of Geography and Environmental Science Working Paper 20, University of Auckland, Auckland. McClure, M. (1987): The story of Birkenhead. Shoal Bay Press, Christchurch, 223 p. Munro, M.A.R. (1984): An improved algorithm for crossdating tree-ring series. Tree-Ring Bulletin 44: NOAA Paleoclimatology Program (2005): User Guide to COFECHA output files. ( March 23, 2009). Oppenheimer, C. (2003): Ice core and palaeoclimatic evidence for the timing and nature of the great mid-13 th century volcanic eruption. International Journal of Climatology 23: R Development Core Team (2009) R: a Language and Environment for Statistical Computing. R Foundation for Statistical Computing, Vienna, Austria. Statistics New Zealand (2008): Subnational Population Estimates: At 30 June p. ( July 6, 2009). Stokes, M.A. & Smiley, T.L. (1968): An Introduction to Tree-Ring Dating. University of Chicago Press, Chicago, 73 p. Tyers, I. (2004): Dendro for Windows Program Guide 3 rd edition. Project Report 500b. ARCUS Dendrochronology Laboratory, University of Sheffield, Sheffield. 15

18 Table A1: Details of all measured kauri samples obtained from Birkenhead Villa. Key: sample = number of sample/sequence, timber type: T & G = tongue & groove board (used for flooring of the veranda), dim. = dimension of sample cross-section, AGR = average growth rate for the measured series, date span = calendar years, or relative years if undated, ring orientation (comments column) = oblique, radial, tangential, dist. pith (comments column) = rough estimate of the distance between the pith and the first measured ring of the series (using a template of concentric rings - only for dated samples), t = mean t-value for sequence (measured using Cross84, for short series BIRS9 BIRS11: CROS73). Sample Timber type Dim. (mm) No. of rings AGR Date span Comments BIRKa BIRS1 = dated four-timber sequence (t = 24.81) BIR035 T & G -large 138 x AD 1063-AD 1264 oblique, dist. pith > ca. 8 cm BIR038 T & G -large 139 x AD 1099-AD 1311 oblique, dist. pith > ca. 8 cm BIR045 T & G -large 131 x AD 1073-AD 1251 broken (tongue), oblique, dist. pith > ca. 8 cm BIR088 T & G -large 138 x AD 1129-AD 1279 oblique, dist. pith > ca. 8 mm BIRS2 = dated two-timber sequence (t = 9.50) BIR153 T & G -large 137 x AD 1163-AD 1227 oblique, dist. pith > ca. 8 cm BIR166 T & G -large 138 x AD 1167-AD 1231 oblique, dist. pith > ca. 8 cm BIRKb BIRS3 = dated five-timber sequence (t = 12.38) BIR033 T & G -large 140 x AD 1382-AD 1469 broken (groove), oblique, dist. pith ca. 8 cm BIR052 T & G -large 131 x AD 1361-AD 1454 broken (tongue), oblique, dist. pith ca. 8 cm BIR057 T & G -large 137 x AD 1397-AD 1475 broken (groove), oblique, dist. pith ca. 8 cm BIR137 T & G -large 137 x AD 1393-AD 1474 oblique, dist. pith ca. 8 cm BIR138 T & G -large 140 x AD 1360-AD 1451 oblique, dist. pith ca. 8 cm BIRS4 = undated two-timber sequence (t = 29.91) BIR030 T & G -large 136 x broken (groove), oblique BIR049 T & G -large 136 x oblique BIRS5 = undated five-timber sequence (t = 11.95) BIR037 T & G -large 138 x oblique, many wedging rings BIR087 T & G -large 136 x oblique, many wedging rings BIR136 T & G -large 138 x oblique BIR139 T & G -large 137 x oblique BIR148 T & G -large 130 x h broken (tongue), oblique, 10 last rings not measured, probably many missing rings BIRS6 = undated two-timber sequence (t = 49.09) BIR031 T & G -large 131 x broken (tongue), oblique BIR162 T & G -large 133 x broken (tongue), oblique BIRS7 = undated two-timber sequence (t = 33.29) BIR140 T & G -large 140 x radial BIR167 T & G -large 140 x radial BIRS8 = undated three-timber sequence (t = 16.98) BIR001 T & G -large 138 x oblique BIR135 T & G -large 139 x h oblique, last 8 rings not measureable BIR145 T & G -large 121 x oblique, broken (groove), groove side: ca. 2 cm are missing, fungi (inner part difficult to measure) BIRS9 = undated three-timber sequence (t = 8.60) BIR002 T & G -small 106 x oblique BIR009 T & G -small 107 x oblique BIR015 T & G -small 107 x oblique BIRS10 = undated two-timber sequence (t = 7.65) BIR005 T & G -small 105 x oblique BIR007 T & G -small 107 x oblique BIRS11 = undated three-timber sequence (t = 8.13) BIR013 T & G -small 106 x oblique BIR018 T & G -small 105 x oblique BIR019 T & G -small 105 x oblique BIRS12 = undated four-timber sequence (t = 13.51) BIR023 T & G -large 140 x radial BIR029 T & G -large 140 x broken (groove), radial BIR047 T & G -large 139 x radial BIR157 T & G -large 139 x radial, blue coloured rings 213 &

19 Table A1 continued. Sample Timber type Dim. (mm) No. of rings AGR Date span Comments Undated series (measured) BIR003 T & G -small 105 x oblique BIR017 T & G -small 104 x broken (groove), oblique BIR021 T & G -large 139 x oblique BIR024 T & G -large 139 x oblique BIR026 T & G -large 137 x broken (tongue), oblique BIR027 T & G -large 139 x tangential BIR028 T & G -large 139 x broken (groove), oblique, very narrow, many wedging rings, abrupt growth changes BIR032 T & G -large 138 x broken (tongue& groove), oblique BIR034 T & G -large 137 x oblique BIR036 T & G -large 139 x radial BIR042 T & G -large 138 x broken (groove), oblique BIR043 T & G -large 131 x broken (tongue), oblique, many wedging rings, wavelike structures BIR050 T & G -large 135 x broken (groove), oblique BIR053 T & G -large 140 x oblique, growth rate changes, very narrow rings BIR054 T & G -large 135 x broken, groove, tangential BIR062 T & G -large 138 x oblique BIR064 T & G -large 138 x broken (tongue), oblique, very wide rings BIR067 T & G -large 138 x oblique BIR141 T & G -large 135 x broken (groove), tangential BIR142 T & G -large 139 x h oblique, last 60 rings not measured (resin ducts) BIR143 T & G -large 137 x oblique BIR144 T & G -large 139 x broken (groove), rotted (tongue), oblique BIR146 T & G -large 140 x broken (groove), oblique BIR147 T & G -large 135 x broken (groove), oblique BIR149 T & G -large 134 x broken (tongue + groove), oblique BIR150 T & G -large 141 x radial BIR151 T & G -large 139 x broken (groove), radial, surface damage (tongue), outer 6 rings not measured (on tongue) BIR152 T & G -large 129 x broken (tongue& groove), oblique, wedging rings BIR154 T & G -large 134 x broken (part of tongue), oblique, similar colour as BIR153 BIR155 T & G -large 137 x broken (groove), oblique, many resin bands BIR156 T & G -large 140 x radial BIR158 T & G -large 136 x tangential BIR159 T & G -large 140 x broken (groove), oblique BIR160 T & G -large 140 x broken (groove), oblique BIR161 T & G -large 136 x broken (groove), oblique BIR163 T & G -large 135 x broken (tongue), oblique BIR164 T & G -large 138 x h broken (groove), oblique, last 105 rings not measured (rings of resin ducts) BIR165 T & G -large 138 x h oblique, last 68 rings not measured (very narrow, wedging rings) 17

20 Table A2: Details of all unmeasured kauri samples obtained from Birkenhead Villa. Key: sample = number of sample, timber type T & G = tongue & groove board, G & G = groove & groove board (used for flooring of the veranda), dim. = dimension of sample cross-section, ring orientation (comments column) = oblique, radial, tangential. Sample Timber type Dim. (mm) Comments Sample Timber type Dim. (mm) Comments Samples with broken tongue Not broken (intact tongue & groove) BIR012 T & G -small 105 x 25 tangential BIR010 T & G -small 105 x 25 tangential BIR014 T & G -small 105 x 25 tangential BIR011 T & G -small 105 x 25 tangential BIR048 T & G -large 138 x 20 tangential BIR016 T & G -small 105 x 25 tangential BIR055 T & G -large 138 x 20 oblique BIR022 T & G -large 140 x 20 oblique BIR106 T & G -large 138 x 20 oblique BIR025 T & G -large 138 x 20 tangential BIR108 T & G -large 138 x 20 oblique BIR039 T & G -large 138 x 20 tangential BIR114 T & G -large 138 x 20 oblique BIR059 T & G -large 139 x 20 oblique BIR115 T & G -large 138 x 20 tangential BIR061 T & G -large 138 x 20 tangential BIR117 T & G -large 138 x 20 tangential, very BIR063 T & G -large 139 x 20 tangential 0 strong mark rays BIR065 T & G -large 138 x 20 tangential BIR124 T & G -large 138 x 20 tangential BIR066 T & G -large 138 x 20 oblique BIR068 T & G -large 138 x 20 oblique Samples with broken groove(s) BIR089 T & G -large 138 x 20 oblique BIR004 T & G -small 105 x 25 oblique BIR090 T & G -large 138 x 20 oblique BIR006 T & G -small 105 x 25 tangential BIR091 T & G -large 138 x 20 oblique BIR008 T & G -small 105 x 25 oblique BIR092 T & G -large 138 x 20 oblique BIR020 T & G -small 105 x 25 oblique BIR093 T & G -large 138 x 20 oblique BIR041 T & G -large 138 x 20 oblique BIR094 T & G -large 138 x 20 tangential BIR051 G & G -large 138 x 20 oblique BIR096 T & G -large 138 x 20 tangential BIR056 T & G -large 138 x 20 oblique BIR097 T & G -large 138 x 20 tangential BIR058 T & G -large 138 x 20 oblique BIR101 T & G -large 138 x 20 oblique BIR060 T & G -large 138 x 20 oblique BIR102 T & G -large 138 x 20 tangential BIR095 T & G -large 126 x 20 oblique BIR103 T & G -large 138 x 20 tangential BIR098 T & G -large 123 x 20 tangential BIR104 T & G -large 138 x 20 tangential BIR099 T & G -large 73 x 20 oblique, broken in half BIR105 T & G -large 138 x 20 oblique BIR100 T & G -large 138 x 20 oblique BIR110 T & G -large 138 x 20 oblique BIR107 G & G -large 138 x 20 oblique BIR113 T & G -large 138 x 20 oblique BIR111 T & G -large 138 x 20 tangential BIR118 T & G -large 138 x 20 oblique BIR116 T & G -large 138 x 20 tangential BIR119 T & G -large 138 x 20 tangential BIR120 G & G -large 138 x 20 tangential BIR127 T & G -large 138 x 20 oblique BIR123 G & G -large 138 x 20 oblique BIR128 T & G -large 138 x 20 tangential BIR130 T & G -large 138 x 20 oblique BIR129 T & G -large 138 x 20 tangential BIR133 T & G -large 138 x 20 oblique BIR131 T & G -large 138 x 20 tangential BIR132 T & G -large 138 x 20 tangential Samples with broken tongue & groove BIR134 T & G -large 138 x 20 tangential BIR040 T & G -large 138 x 20 tangential BIR134 T & G -large 138 x 20 tangential BIR044 T & G -large 138 x 20 oblique BIR046 T & G -large 138 x 20 radial BIR109 T & G -large 138 x 20 tangential BIR112 T & G -large 138 x 20 oblique BIR121 T & G -large 138 x 20 oblique BIR122 T & G -large 138 x 20 oblique BIR125 T & G -large 138 x 20 tangential BIR126 T & G -large 138 x 20 oblique 18

21 Table A3: Details of all unmeasured non-kauri samples obtained from Birkenhead Villa. Key: sample = number of sample, timber type T & G = tongue & groove board, ST = structural studs (used for flooring of the veranda), ring orientation (comments column) = oblique, radial, tangential. Sample Timber type Dim. (mm) Comments Podocarpus totara* BIR069 ST 100 x 51 pith included BIR070 ST 90 x 44 tangential BIR071 ST 90 x 44 tangential BIR072 ST 90 x 44 oblique BIR073 ST 90 x 44 tangential BIR074 ST 90 x 44 broken, oblique BIR075 ST 92 x 43 oblique BIR076 ST 92 x 43 oblique BIR077 ST 90 x 44 tangential BIR078 ST 91 x 49 broken, oblique BIR079 ST 91 x 49 broken, oblique BIR080 ST 90 x 44 broken, oblique BIR081 ST 90 x 44 tangential BIR082 ST 92 x 44 oblique BIR083 ST 90 x 44 tangential BIR084 ST 90 x 48 oblique BIR085 ST 93 x 49 oblique BIR086 ST 100 x 73 broken, oblique Pinus radiata BIR168 T & G -large 137 x 18 oblique BIR169 T & G -large 137 x 18 broken (tongue), oblique *species classification to be verified. 19