- Research article
- Open Access
The chemistry and bioactivity of various heartwood extracts from redwood (Sequoia sempervirens) against two species of fungi
© Davies et al.; licensee Springer 2014
- Received: 8 May 2013
- Accepted: 12 June 2014
- Published: 15 July 2014
Sequoia sempervirens (D. Don) Endl.) (redwood) has the potential to be grown in New Zealand in commercial forestry operations and is valued for its naturally durable heartwood. A viable redwood industry based on planted forests can only be achieved if the timber produced meets quality expectations, in particular durability. Natural durability is highly variable among trees. Also, a within-tree pattern of low durability close to the pith has been observed. Natural durability is preliminarily caused by secondary metabolites deposited into the cell walls during heartwood formation. The exact nature of the compounds responsible for natural durability in redwood is unknown.
Samples of heartwood from 22 different trees were obtained, ground and extracted using a range of solvents. The ability of some of these extracts to reduce the growth of two fungi (Gloeophyllum trabeum and Trametes versicolor) was tested in vitro. Information on the composition of the extracts was obtained using infrared spectroscopy and gas chromatography.
Fungicidal properties were found in solvent extracts of ground S. sempervirens heartwood samples at concentrations comparable to those known to be present in intact wood. The entire acetone-soluble extracts and ethyl-acetate-soluble fraction of the ethanol extracts caused the greatest reduction in the growth of both fungi tested. Large variations in acetone-soluble or ethanol-soluble extract content and fungicidal activity among trees were found. Agatharesinol and sequerin-C appear to be trace compounds in the dried extracts of S. sempervirens.
Further work is needed to identify the key compounds contributing to the natural durability of S. sempervirens.
- Sequoia sempervirens
- Natural durability
- Gloeophyllum trabeum
- Trametes versicolor
The timber of Sequoia sempervirens (D. Don) Endl. (redwood) is widely used and highly valued for its natural durability, attractive colour, dimensional stability and low density (Cornell ; Cown et al. ). Supply of redwood timber from natural forests is dwindling and classed vulnerable by the International Union for Conservation of Nature (ICUN) (Farjon et al. ). Growing redwoods in planted forest to substitute this resource has the potential to be a highly profitable business in New Zealand, which is helped by the fast growth rates of this species (Cornell ; Palmer et al. ). A key requirement for establishing a redwood industry based on planted forests is to ensure a consistent high quality of the produced timber (Cown ; Cown et al. ). This is especially true for wood from young trees as the desired properties usually improve with tree age (Walker ). The natural durability of redwood timber has been reported to be highly variable, ranging from very durable to moderately/non-durable (Clark and Scheffer ; Scheffer and Morell ; Jones et al. ). Variation in natural durability exists among trees (due to genetic and environmental factors) as well as within trees (mainly associated with cambial age).
The natural durability of timber has been largely attributed to the deposition of low molecular weight secondary metabolites into the wood during heartwood formation (Hillis ; Taylor et al. ). The deposited secondary metabolites are also known as heartwood extractives as they can be extracted by various solvents from the heartwood. Extractives are comprised of numerous organic compounds of which some possess fungicidal, bactericidal or insecticidal properties (Rowe ).
Despite the importance of natural durability to the product and the size of the redwood industry surprisingly little is known of the molecular basis for its natural durability, i.e. the extractive compounds responsible.
This article (a) reviews the existing literature regarding the natural durability of S. sempervirens heartwood and the extractives compounds in this material and (b) provides some experimental data on the quantity and bioactivity of various S. sempervirens extracts and their variability.
Published studies assessing the natural durability of Sequoia sempervirens timber
Test type (number and name of species in brackets, if known)
Origin of timber (type of wood in brackets)
Hedley and Foster 
Laboratory test (ASTM ) (5 brown-rot fungi (Gloeophyllum trabeum, Coniophora olivacea (Fr.) P. Karst., Postia placenta and 2 unidentified isolates) and 5 white-rot fungi (Fuscoporia gilva (Schwein.) T. Wagner & M. Fisch., Pycnoporus coccineus (Fr.) Bondartsev & Singer, Trametes versicolor and 2 unidentified isolates))
Hedley and Foster 
Modified laboratory test (buried) (5 brown-rot fungi (Gloeophyllum trabeum, Coniophora olivacea, Postia placenta and 2 unidentified isolates) and 5 white-rot fungi (Fuscoporia gilva, Pycnoporus versicolor, Trametes versicolor and 2 unidentified isolates))
Hedley and Foster 
Laboratory (2 brown-rot fungi (Gloeophyllum trabeum, Postia placenta))
Weight-loss of 18-45%
unknown origin (sawmill boards of widest possible natural range)
Wilcox and Piirto 
Laboratory (16 soft-, brown- and white-rot fungi)
Very resistant against soft-rot, resistant against white-rots and moderately resistant against brown-rots
unknown origin (sapwood)
Eslyn and Highley 
Laboratory (2 brown-rot fungi (Gloeophyllum trabeum and Postia placenta))
Very to moderately resistant
California (outer third heartwood of the top of the butt-log)
Clark and Scheffer 
Field tests (above ground)
Sapwood: 15 years in moist and 25 years in dry climates; heartwood: >20 years in moist and 30 years in dry climates
unknown origin (sapwood and heartwood)
Laboratory test (11 soft-rot fungi)
Little weight loss compared to Pinus ponderosa
unknown origin (heartwood)
Morrell and Smith 
Laboratory using termites (Coptotermes formosanus Shiraki)
California and Oregon (Top-grade glulam stock)
Grace and Yamamoto 
Durable in dry site; Non-durable in wet-sites
Johnson et al. 
Laboratory (2 brown-rot fungi (Coniophora puteana (Schumach.) P. Karst., Gloeophyllum trabeum) and 1 white-rot fungus (Trametes versicolor))
Large variation from very-durable (1) to not-durable (5). Site averages (2)-(4) (increasing resistance with age)
New Zealand (22, 38 and 71 years old)
Jones et al. 
Variability among trees
Wilcox and Piirto () found a 15-fold variation in weight loss among redwood heartwood samples of the widest possible naturally occurring range of colour intensity. These samples contained wood from old-growth trees and second-growth trees. Their findings were consistent with the large variability in natural durability reported for plantation-grown redwood from New Zealand (Jones et al. , ). Heartwood durability according to Standard EN350-1 (CEN ) varied from ‘not-durable’ to ‘very-durable’, with the majority of samples being ‘durable’ or ‘very-durable’. Also, variable resistance against termite attack has been reported for redwood heartwood sourced from different regions in the USA (Grace and Yamamoto ).
Variability within trees
Sherrard and Kurth ([1933a]) analysed the variation in natural durability within a single redwood stem using a range of decay tests conducted either under field or laboratory conditions. Natural durability significantly increased from pith to bark and slightly from bottom to top in the heartwood of the stem. These radial and axial gradients in natural durability were confirmed for old-growth and second-growth trees (<100 years) (Clark and Scheffer ), and are consistent with the data reported for plantation-grown redwood from New Zealand (Jones et al. , ). Gradients in natural durability have also been observed in other species (Taylor et al. ) and highlight the general inferior wood quality of corewood, the wood formed by cambium of young age (Burdon et al. ).
Published data on the yield of various types of extracts from Sequoia sempervirens heartwood
Extractive content (%)
Hot water extract
Hawley et al. 
Sherrard and Kurth [1933b]
Resch and Arganbright 
Wilcox and Piirto 
Kuo and Arganbright 
Cold water extract
Hawley et al. 
Anderson et al. 
Anderson et al. 
Demaree and Erickson 
Hot ethanol (after hot water)
Wilcox and Piirto 
Resch and Arganbright 
Kuo and Arganbright 
Hot ethanol (after cold water)
Anderson et al. 
Anderson et al. 
Buchanan et al. 
Anderson et al. 
The amount of heartwood extract varies greatly among trees, with younger trees generally having lower quantities than older trees (Sherrard and Kurth [1933a]; Anderson ; Resch and Arganbright ; Kuo and Arganbright ). Within stems radial and axial gradients in heartwood extracts follow the natural durability pattern indicating that young trees are less active in metabolising extractives. Additionally, reports of extremely low extractive contents next to the pith for very old S. sempervirens trees (600+ years) suggest a slow degradation of extracts (Resch and Arganbright ).
Bioactivity of S. sempervirens heartwood extracts
Although extractive compounds are likely to be responsible for improving natural durability, it is unclear how active individual compounds of S. sempervirens heartwood are against individual wood decaying fungi. Some work in the past century has attempted to identify various active compounds by investigating the toxicity of fractions of the heartwood extractives against various fungi.
The fungicidal activity of water extracts has been found to vary with fungal species. Cold-water extracts were found to retard the growth of the white-rot fungus Heterobasidion annosum (Fr.) Bref. and hot-water extracts were even more inhibiting (Hawley et al. ; Sherrard and Kurth [1933a]). Cold-water extracts also inhibited the growth of the brown-rot fungi Postia placenta (Fr.) M. J. Larsen & Lombard, Neolentinus lepideus (Fr.) Redhead & Ginns and Gloeophyllum trabeum (Pers.) Murrill although hot-water extracts did not (Anderson et al. ). Interestingly, water extracts from sapwood showed some activity against Heterobasidion annosum (Hawley et al. ). The water-insoluble compounds from an acetone extract (largely phlobaphenes) did not affect growth of Postia placenta, Neolentinus lepideus and Gloeophyllum trabeum (Anderson et al. ). An ethyl acetate extract did not show any activity against Phytophthora ramorum Werres, De Cock & Man in 't Veld (Manter et al. ). Cabrera () found no bioactivity from hexane, dichloromethane or ethanol extracts on Postia placenta and Trametes versicolor (L.) Lloyd when tested at concentrations as high as 24,000 ppm. Wood extracted with hot-water lost most of its resistance against decay by Postia placenta and also exhibited partly reduced resistance against decay by Neolentinus lepideus and Gloeophyllum trabeum (Anderson et al. ). Acetone extraction slightly reduced the natural durability against Postia placenta but this was not attributed to the removed extractives but rather the higher temperatures to which the wood was exposed for solvent removal (Anderson et al. ). The durability of S. sempervirens heartwood against Postia placenta and Gloeophyllum trabeum was correlated with the amount of matter present in the ethanol extracts after hot-water extraction while no correlation was found for the hot-water extractable material (Wilcox and Piirto ).
Chemical nature of heartwood extracts in S. sempervirens
The structures of individual compounds found in heartwood of S. sempervirens trees are largely unknown. Some studies report the structures of compounds isolated from small twigs that likely do not contain heartwood but a mixture of leaves, bark and sapwood (Gadek and Quinn ; Zhang et al. ; Zhang et al. ). Others analysed isolated leaves (as summarised by Erdtman and Norin ), cones (Kritchevsky and Anderson ) or bark (Lewis et al. ). The extracts in these plant tissues are known to differ greatly (Anderson ). Therefore, it is unclear if any of these reported compounds are present in heartwood.
Early analyses of S. sempervirens heartwood extracts quantified several broad classes of compounds. The composition of a typical water extract of green S. sempervirens heartwood was summarised by Anderson (). The major compounds were condensed tannins (Buchanan et al. ), several cyclitols (Anderson et al. ) and unidentified polyphenolics. Other unspecified components, carbohydrates (mainly arabinose) (Smith and Zavarin ) and colouring matter (Sherrard and Kurth [1933b]) are present as minor compounds. The composition of a typical water-insoluble heartwood extract was reported to consist of about three quarters phlobaphenes and the remaining quarter contained similar amounts of native lignin, phenolics, fatty acids, waxes and neutrals (Anderson ).
Several norlignans have been isolated from S. sempervirens heartwood (as summarised by Rowe ). These include sugiresinol (also known as sequerin-A) (Sherrard and Kurth [1933b]; Balogh and Anderson ), hydroxysugiresinol (also known as sequerin-B), sequerin-C (also known as sequerin) (Hatam and Whiting ; Riffer and Anderson ), sequerin-D (Begley et al. ), yateresinol (Erdtman and Harmatha ) and probably agatharesinol (Henley-Smith and Whiting ; Rowe ; Castro et al. ). Methyl anisate, anisaldehyde, and p-dimethoxybenzene were also listed by Henley-Smith and Whiting ().
Chemical reactions of S. sempervirens heartwood extracts
Biosynthesis of norlignans differs from that of lignans (Suzuki and Umezawa ; Yoshida et al. ). There is evidence that the conversion of agatharesinol into sequerin-C involves enzymes (Imai et al. ) but an abiotic conversion mechanism has also been proposed (Erdtman and Harmatha ). These norlignans have also been proposed to be a precursor of tannins/phlobaphenes in S. sempervirens which polymerise by enzyme- or acid-catalysis (Erdtman and Harmatha ). Norlignans, in particular agatharesinol and sequerin-C, have been connected to the natural durability of Cryptomeria japonica (L. f.) D. Don heartwood (Ohtani et al. ). C. japonica is a species related to S. sempervirens (Gadek et al. ; Christenhusz et al. ). The role of norlignans in the natural durability of S. sempervirens has been suggested by Balogh and Anderson () but not considered in subsequent studies (e.g. Piirto and Wilcox ). The bioactivities of norlignans (Suzuki and Umezawa ) and lignans (MacRae and Towers ) have been reviewed.
The influence of drying on the natural durability of redwood has been investigated in various studies (Anderson et al. ; Scheffer and Eslyn ; Anderson et al. ). Temperatures above 77°C (as well as pre-steaming and solvent drying) reduced the natural durability of redwood timber. The oxidation of heartwood compounds was found to be a contributing factor. Reduced resistance against the brown-rot fungus Fomitopsis palustris (Berk. & M. A. Curtis) Gilb. & Ryvarden and termites following high temperature drying were also reported for C. japonica. The decrease in termite resistance was attributed to a loss in agatharesinol and sequerin-C (summarised by Matsushita et al. ).
Redwood heartwood is pale in the freshly felled green state and changes colour when exposed to the atmosphere (e.g. Sherrard and Kurth [1933b]; Balogh and Anderson ). A similar phenomenon is observed in the heartwood of C. japonica where the observed colour change has been related to the present norlignans, which change colour upon air-oxidation (with the exception of agatharesinol and sugiresinol) (Takahashi and Mori ). Discolouration of the heartwood of S. sempervirens can also occur (Ellwood et al. ). It has been related to the presence of sequerin-C (Balogh and Anderson ) and is facilitated by partial oxygen pressure, (low) pH, temperature and heavy metals (Zavarin and Smith ).
Samples for assessing the effects of different solvents on extractive recovery and their composition were obtained from the inner five annual rings of heartwood from a single 17-year-old Sequoia sempervirens tree (Tree A). This tree was located on the North Island, New Zealand and sampled at breast height.
Samples for assessing the variability of heartwood extracts were obtained from a further 21 different US or New Zealand grown S. sempervirens trees (Trees B–V). Heartwood was collected at breast height from the inner seven year rings.
All samples were milled to pass a 2 mm screen and stored in a desiccator over dry silica gel prior to use.
The brown-rot fungus Gloeophyllum trabeum (ICMP 13887) and the white-rot fungus Trametes versicolor (ICMP 18215) were obtained from the International Collection of Micro-organisms from Plants (Landcare Research, New Zealand). These fungi are used in international standards (e.g. CEN , Australian Wood Preservation Committee ) for assessing the natural durability of wood. Solvents (water, ethanol, acetone, ethyl acetate or dichloromethane (DCM)) used for the extraction of wood were of HPLC grade. Fungal growth was assessed using a base of malt-extract agar (Merck) containing 30.0 g L−1 malt extract, 3.0 g L−1 soymeal peptone and 15 g L−1 agar.
Extractions: S. sempervirens heartwood was extracted using an Accelerated Solvent Extractor (Thermo) equipped with 33 mL cells. In each run ~7.2 g of dry milled heartwood (accurately weighed) was extracted. The extraction conditions were two cycles at 70°C for 15 min (static time) followed by a rinse using 50% of the cell volume, resulting in approx. 70 mL of extract. Extractions for the bioactivity of the extracts were performed in duplicate. The limited amount of material from Trees B-V did not allow replication of the extractions of the 21 samples used for the variability tests. Extractive content was measured gravimetrically after drying at 105°C using a third of each extract. The ethyl acetate soluble fraction of the ethanol extract (EtOH → EtAc) was obtained by removing the ethanol in a vacuum rotary evaporator then re-dissolving the residue in ethyl acetate and shaking overnight; the non-soluble fraction was removed by filtration. The remaining two thirds of each extract were used directly for the fungal assay described below.
Fourier Transform-Infrared (FT-IR) spectroscopy: Spectra of the dried portion of each extract were taken in duplicate with a Tensor 37 spectrometer (Bruker) using the Attenuated Total Reflectance (ATR) sampling technique. Each spectrum was composed of 32 scans. No qualitative difference between the replicates of each extract was found so the four spectra (two replicate FT-IR measurements of two extractions per solvent) were averaged. Spectra were normalised to the aromatic signal at 1510 cm-1.
Gas chromatography (GC): Air dried extracts were dissolved in pyridine at a concentration of approximately 100 g L−1. A 15 μL aliquot of each solution was trimethylsilylated at room temperature using 50 μL of N,O-bis(trimethylsilyl)-trifluoroacetamide (BSTFA, Sigma-Aldrich) in a septum-sealed vial for 20 min according to the supplier's recommendations. The trimethylsilyl derivatives were analysed by GC on a CP-3800 (Varian) chromatograph fitted with a fused-silica capillary column (30 m × 0.25 mm/Equity®-1) using helium as the carrier gas (1.2 mL min−1) and FID detection at 300°C. The initial oven temperature was set to 116°C, ramped up to 280°C at 10°C min−1 and held for 40 min. Sequerin-C and agatharesinol (isolated from Metasequoia glyptostroboides Hu & Cheng by ChemFaces Biochemical Co., Wuhan, P.R.C) were used as reference materials.
Fungal assays: Malt agar gels (4.8 % w/v) containing the remaining extract (i.e. from 4.8 g of dry wood) were prepared so that the amount of extract in each gel corresponded to the extractive content of the wood sample. Each extract solution (~50 mL) was added to 100 mL of distilled water. Each solution was placed in a water bath (80°C) until no solvent odour was noticeable to remove fungicidal effects of the extraction solvent. This step took approximately 6 h. Care was taken to prevent the extracts from drying. Any loss of water due to evaporation was corrected by making the final volume up to 100 mL with distilled water. Malt agar (4.8 g) was added and the solution was autoclaved (121°C, 10 min) and poured into 50 mm diameter petri dishes (five petri dishes for each extract and each fungus). Solvent controls containing the equivalent amount of solvent but no extractives underwent the same procedures as the malt agars containing wood extracts. Plates were allowed to cool. After gelation, a small amount of fungus was placed in the centre of each petri dish. Fungi were grown at 25°C. Two perpendicular diameters of the fungus in each petri dish were measured approximately every 24 h over the course of a week using a pair of digital callipers. The absolute growth rate (mm h−1) was calculated by fitting a linear regression for the averaged diameter against time for each petri dish. The experiment was duplicated for testing different solvent extracts but only a single extraction was possible due to the small sample size when assessing the variability of the extracts in 21 separate tree samples.
Statistics: Solvent controls showed that potentially remaining solvent traces had no significant effect on the growth of the individual fungi so the control data were pooled (n = 30) to calculate the reference growth rate. As no statistically significant differences were found between the duplicate extractions using a F-test, the respective data were pooled (n = 10) to calculate the average absolute growth rate for each fungus and solvent extract. The relative growth rates were calculated as the ratio between the reference growth rate for the individual fungi and the different solvent extracts. Standard errors of the ratios were estimated (Kendall et al. ).
Yields of S. sempervirens heartwood extracts
Yield of extract from the inner five annual heartwood rings from Tree A and the inner seven annual heartwood rings from Trees B-V. Given values are the averages of duplicate extractions
Extract (%) of dry wood
Inner heartwood of Tree A
Heartwood of Trees B to V; min – max. (mean in brackets)
5.4 – 12.8 (8.9)
Ethanol → Ethyl acetate1
1.4 - 5.1 (2.6)
Bioactivity of extracts
Effect of extracts from the inner five annual rings of Sequoia sempervirens heartwood (Tree A) on the growth of Gloeophyllum trabeum (Gt) and Trametes versicolor (Tv)
Absolute growth rate (mm h−1)
Standard error of absolute growth rate (mm h−1)
Relative growth rate1(%)
Standard error of relative growth rate (%)
Ethanol → Ethyl acetate3
Ethanol → Ethyl acetate3
Acetone extracts were found to retard growth of both fungi tested to a similar extent as the ethanol extracts (Table 4) although they comprised only roughly half the weight (Table 3) so were roughly twice as potent (Table 4). This is somewhat at odds with observations by Anderson et al. () who reported that acetone extraction of solid wood (in contrast to water extraction) only slightly reduced decay resistance against G. trabeum. To verify the findings of the current study, another sample from Tree A was extracted with acetone in a Soxhlet apparatus for 8h. The extraction liquor was collected and all the solvent evaporated (which facilitated chemical changes). The remaining solid was bioactive as it slowed the growth of G. trabeum to 76% (data not shown). This result showed that that compounds with fungicidal properties were present in acetone extracts independent of extraction method and that they are reasonably stable against chemical degradation.
Ethyl acetate extracts contained compounds capable of retarding fungal growth of both T. versicolor and G. trabeum (Table 4); however the inability of the solvent to swell the cell wall due to its low polarity only partially removes the active compounds. Manter et al. () tested the activity of ethyl acetate extracts from S. sempervirens heartwood against Phytophthora ramorum (sudden oak death) but could not detect any activity.
The compounds in the ethanol extract were found to have a high contribution to the fungicidal activity of S. sempervirens heartwood (Table 4). The mass loss of solid S. sempervirens heartwood blocks after incubation with G. trabeum was reported to correlate with the amount of ethanol extractable matter (Wilcox and Piirto ). The bioactivity of the ethanol extract of S. sempervirens heartwood obtained in the current study was contained in the ethyl acetate soluble fraction (Table 4). This was consistent with the fact that the resistance of sugi (Cryptomeria japonica) against butt-rot was related to compounds in the ethyl-acetate-soluble fraction of ethanol heartwood extracts (Ohtani et al. ).
In summary, none of the extracts completely inhibited fungal growth in vitro.
Chemical features of the extracts
The norlignans sequerin-C and agatharesinol have been associated with the natural durability of S. sempervirens and C. japonica heartwood (Ohtani et al. ; Balogh and Anderson ). These and several other norlignans have been identified in S. sempervirens wood (e.g. Rowe ). However, based on a comparison of GC retention times, these two compounds appeared to be present only in trace amounts in the extracts of dried S. sempervirens heartwood tested here. Norlignans have been described as unstable i.e. ‘being easily oxidised to amorphous substances’ and very acid sensitive and therefore cumbersome to isolate from seasoned wood (Erdtman and Harmatha ). This could be a reason why larger quantities of those compounds were not detected here. The low levels of sequerin-C and agatharesinol could explain why characteristic FT-IR features, like a double peak ~1500 cm−1, of such compounds (Balogh and Anderson ) were not observed in IR spectra of the extracts tested here (Figure 1).
Variability of S. sempervirens heartwood extracts
Effect of acetone-soluble extracts from the inner seven annual heartwood rings from Trees B-V on the growth of Gloeophyllum trabeum (Gt) and Trametes versicolor (Tv)
Absolute growth rate (mm h−1)
Relative growth rate1(%)
min. – max. (mean in brackets)
min. – max. (mean in brackets)
0.213 – 0.371 (0.293)
65 – 112 (89)
0.329 – 0.544 (0.426)
64 – 106 (83)
The quantity of ethanol-soluble material extracted from the inner seven annual rings of heartwood from Trees B-V ranged from 5.4% to 12.8% (average of 8.9%) (Table 3). The amount extracted from heartwood less than five years old from Tree A was within this range (6.4%, Table 3). Wilcox and Piirto () found that the mass loss of solid S. sempervirens heartwood blocks after incubation with G. trabeum was correlated to the amount of ethanol-soluble material in the wood (R2 = 0.69).
The entire acetone-soluble extracts and ethyl-acetate-soluble fraction of the ethanol extracts caused the greatest reduction in the growth of both fungi tested. The norlignans sequerin-C and agatharesinol were only present in trace amounts within the bioactive extracts of dried S. sempervirens heartwood, indicating that other compounds contribute to the natural durability.
Large variation in the amount of acetone-soluble material as well as the bioactivity of those extracts against the fungi G. trabeum and T. versicolor was found among 21 redwood samples. The variability of the growth rate of the fungi was only partially explained by the quantity of the acetone-soluble material. Natural durability is an essential feature of S. sempervirens heartwood and the large variability in extract content and in vitro antifungal activity demonstrated here has implications on the quality S. sempervirens timber.
The samples were kindly provided by Wade Cornell. We would like to thank Richard Woollons (New Zealand School of Forestry) for advice regarding the statistical analysis of the data. The work was financially supported by the University of Canterbury Summer Scholarship Program.
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