Picconia Azorica (Oleaceae)

IAWA Journal, Vol. 33 (4), 2012: 375–390

Wood anatomy and technological properties of an endangered species: Picconia azorica (Oleaceae)

Raquel Caetano Ferreira2, Angela Lo Monaco1, Rodolfo Picchio1, Avra Schirone1, Federico Vessella1 and Bartolomeo Schirone1,*

Summary Picconia azorica (Tutin) Knobl. is an Azorean endemic species of the Oleaceae, exploited over centuries and recently classified as endangered. It suffers from reduction and fragmentation of its habitat, and from competition with exotic species. Wood anatomy was examined and compared with Picconia excelsa, enhancing our knowledge about the genus Picconia which contains only these two species. Macroscopic and technological characterizations by colour, pH, wood density, compression and bending strengths, shrinkages, static quality factor, ash content and Higher Heating Value were investigated and compared with other Oleaceae and other hardwoods. At the anatomical level, P. azorica does not differ from P. excelsa, except for the number of vessels and rays per mm2 and the ray type. The technological features support the profitable use of P. azorica for the furniture industry instead of biomass production. Because of its valuable wood, P. azorica might be reconsidered and its properties emphasized to combat the species’ decline and to encourage the restoration of its habitat. Key words: Picconia azorica, wood anatomy, physico-mechanical properties, wood utilisation.

Introduction

The genus Picconia belongs to the Oleaceae and it is endemic to the Macaronesian Region with Picconia excelsa (Ait.) DC., occurring in Madeira and the Canary archipelago, and Picconia azorica (Tutin) Knobl. in the Azores. Picconia azorica is an evergreen tree with smooth, pale bark. The Portuguese ver- nacular name of the species is “pau-branco”, meaning that its wood colour is white to pinkish, though it becomes darker over time. It can grow up to 15 m in height and the maximum diameter registered is 53 cm for a tree of undetermined age (Dias 2001),

1) Department of Agriculture, Forests, Nature and Energy (DAFNE), Università degli Studi della Tuscia, Via S. Camillo de Lellis, 01100 Viterbo, Italy. 2) Azores Regional Coordinator for Nature Conservation, Azorina, S.A. - Sociedade de Gestão Ambiental e Conservação da Natureza, Rua do Galo 118, 9700-091 Angra do Heroísmo, Azores, Portugal. *) Corresponding author [E-mail: [email protected]]. Associate Editor: Steven Jansen

Downloaded from Brill.com09/28/2021 04:43:02PM via free access 376 IAWA Journal, Vol. 33 (4), 2012 but for a specimen with a diameter of 22 cm, an age exceeding 64 years was inferred (sample held at the Museu Carlos Machado, Ponta Delgada, São Miguel Island). The species is xerophytic, apt to colonize dry environments and is resistant to sea spray (Dias 2001). The first references toP. azorica in the Azores were written over a century after the Portuguese settlement in 1440 by Frutuoso (1583), who reported the occurrence of dense stands of this species on all nine islands. Only in 1844 Seubert and Hochstetter provided the first botanical description of the plant under the denomination ofP. excelsa, considering it identical to the Picconia species found in Madeira and Canary Islands. Later, the species was revised by Tutin (1953) and placed within another genus, with the denomination of Notelaea azorica Tutin, presently recognized as Picconia azorica (Tutin) Knobl. (1934). This taxonomic delimitation has recently been confirmed by Ferreira et al. (2011) after genetic study. The species is currently present in all the Azorean islands, with the exception of Graciosa (Schaefer 2002), widespread in small populations of coastal and marginal sites. More than one thousand reproductive individuals occur in the archipelago, even if populations have been declining over the last three decades because of harvesting without restocking, new plantations with exotic species, utilization as wood fuel, clearing land for agricultural purposes, and because of its valuable wood (Martín et al. 2008; International Union for Conservation of Nature, IUCN 2011). Indeed, P. azorica was of important social value in the past, being used to build wooden wagons, agricultural machinery and tools, house beams, as well as luxury furniture, especially in the period 1580–1640 (Frutuoso 1583; Martins 1981; Dias et al. 2007). Nowadays, P. azorica is classified as “Endangered” (IUCN 2011) and protected under the Habitat Directive (Annexes II and IV) (EC 1992) and Bern Convention (Annex I) (Council of Europe 1993). The fragile status of P. azorica led it to be included in the 100 taxa (endemic and non-endemic) list of Macaronesian Flora priority species, for which management and conservation actions are crucial for their survival and population increment (Martín et al. 2008). These authors classified the species with an ecological value of “structuring species”. Additionally, it plays an important ecological role as part of the diet of two protected Azorean endemic bird species: Columba palumbus azorica and Pyrrhula murina (Dias et al. 2007; SPEA 2008). In 1988, Baas et al. investigated the wood anatomical features of the Oleaceae, including P. excelsa, enabling wood identification of specimens belonging to this family. This study represents the first wood anatomical and technological investigation of P. azorica based on both micro- and macroscopic analyses. Here we investigated the number of vessels, tangential and radial diameters, wall thickness, vessel, tracheid, libriform fibre and parenchyma strand lengths. Ray features and density were measured and compared with P. excelsa. At the same time, colour, pH, wood density, compression and bending strengths, shrinkage, static quality factor, ash content and HHV were investigated and compared with other Oleaceae and other hardwoods.

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Materials and Methods Plant material and macroscopic analyses Since Picconia azorica is under a high level of protection, it is impossible to collect wood by felling trees. Therefore, wood samples (disk and board) were collected from three individuals blown over by wind some years ago in Terceira Island on a stony site characterized by recently developed lava soils rich in potassium, i.e., an Andosol peculiar to the Azorean formation and constituents. The trees were growing in a stand with 15 plant/ha (the typical P. azorica population density ranges from 4–5 up to 30 plant/ha). The diameter at breast height (DBH) of the sampled trees ranged from 10 to 32 cm, while the height ranged from about 4 to 7 m. The trees were healthy before being windblown, and the preservation status of the samples was good, without signs of decay and microcracks. Samples were macroscopically described by sapwood and heartwood differentiation, odour, and structure; the main physical features were checked by eye and with a magnifier (5–20× resolution) under natural or UV light.

Microscopic investigations Wood samples were investigated using both light (LM) and scanning electron microscopy (SEM). Transverse, radial and tangential sections of 20–25 µm thick of each tree were prepared from cubic samples of 5–15 mm wide, with the aid of a sliding microtome (Reichert Jung 1150 Autocut). Sections were then cut, and stained with safranine (1% in 50% ethanol). After staining, sections were washed with distilled water, dehydrated with ethanol and mounted on slides with Canadian Balsam. Observations were first carried out with a Wild M420 stereomicroscope, then with LM using a Reichert-Jung Polivar 100. Pictures were taken with a Moticam 2500 – 5.0 M Pixel digital camera and analyzed using Motic Image PLUS 2.0 ML software package. Other samples were fixed with 4% paraformaldehyde + 5% glutaraldehyde, pH 7.2 in 0.1 M cacodylate buffer for 1 h at 4 °C (Karnovsky 1965) to be scanned by SEM. After rinsing overnight in the same buffer, the sections were post-fixed in cacodylate- buffered 1% osmium tetroxide for 1 h, and sections were dehydrated in a graded acetone series, dried by the critical point method using CO2 in a Balzers Union CPD 020, sputter-coated with gold in a Balzers MED 0.10 unit, and observed with a JEOL JSM 5200 scanning electron microscope. Cellular morphology was investigated also by microscopic observations on macerated wood from 8 samples reduced to slivers about 1 mm thick and 2 cm long, and placed in vials with 1:1 mixture of acetic acid and hydrogen peroxide (30 vol.) for 48 h. The vials were then placed in a boiling water bath for 48 h, the fluid was gently removed and the slivers were rinsed several times in distilled water, and a saturated sodium bicarbonate solution was added to buffer the acidity excess. After some rinsing, distilled water was added again, the tubes were shaken vigorously to obtain xylem cell separation, and some suspension drops were placed in microscope slide covered with slips. The identification of the cellular elements was conducted using a Zeiss Axioskop

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Physical and mechanical characterization The colour was quantitatively characterized by an X-Rite CA 22 reflectance spectrophotometer, generally employed in wood technology and cultural heritage studies to evaluate the colour changes after conservation and treatment operations (Della Patria & Omarini 2009; Lo Monaco et al. 2011a). The standard illuminant D65 and the standard observer 10° were used according to the European recommendation (EN15886), adopting the L*a*b* colour space system measurement defined by the Commission Internationale de l’Éclairage (CIE) in 1986. L* indicates lightness with values ranging from 0 (black) to 100 (white); a* indicates the direction of the colour ranging from red (positive a*) to green (negative a*), while b* indicates the direction of the colour ranging from yellow (positive b*) to blue (negative b*). Since the measurement of wood colour is not easy, due to the surface variability (Butler et al. 2001; Della Patria & Omarini 2009; Lukmandaru et al. 2009; Schnabel et al. 2009; Lo Monaco et al. 2011a), five points were chosen along the radial direction on the disk and three measurements per point were performed, then the average was assumed as the point value, according to the UNI NorMaL 14/83 (UNI 1983); the same method was adopted on 7 points of the board’s tangential surface (Genco et al. 2011). Wood powder suspended in distilled water with a 1:5 ratio (Stamm 1961) was used for pH determination: a total of 6 measurements was taken from a disk and a longitudinal section (Fig. 1), plus 2 measurements from a specimen collected close to the pith (transverse and tangential sections; picture of the specimen not shown). The suspensions were kept at 20 °C and stirred with a glass rod, and the pH measurements were determined after 24 h by means of an Hanna Hi pH-meter equipped with a Hi1413 glass electrode, adequately calibrated.

Figure 1. Disk (a) and tangential surface (b) of the Picconia azorica sample used in this study. White solid circles indicate pH determination measurements with the respective values. White solid squares marked with d and b and subscript numbers show the measurement performed for defining the colour of the disk and the board respectively.

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Wood density (g cm-3) at 12% MC (moisture content) and at oven-dry conditions (MC = 0%) was determined on a set of 94 specimens (20×20×30 mm), representing both heartwood and sapwood, according to the UNI ISO 3131 standard (UNI 1985). The Higher Heating Value (HHV) was determined (CEN/TS 14918) on 13 random samples, by the means of an adiabatic calorimeter Parr, Model 6200 (Canagaratna & Witt 1988; Picchio et al. 2009), and the ash content in terms of dry weight percentage was also calculated. Radial, axial, tangential and volumetric total shrinkage were ascertained on a set of 26 specimens (20×20×30 mm) following Lo Monaco et al. (2011b), dissected according to the UNI ISO 4469 and the UNI ISO 4858 (UNI 1985d, 1988). The compression strength (σ y) was determined on a set of 94 specimens (20×20×30 mm at 12% of moisture content) according to UNI ISO 3787 (UNI 1985c), while the bending strength (σ b) was obtained on a set of 31 specimens (20×20×300 mm at 12% moisture content) according to UNI ISO 3133 (UNI 1985b). The ratio of static quality was obtained by calculating the ratio between the compression strength parallel to the grain and wood air density by 100 (kg cm-2). This coef- ficient has the dimensions of a length in km, and it indicates the hypothetical length of an element in a constant section that, if put vertically, will be broken by the effect of its own weight (Giordano 1999). Statistical analysis of the collected data was performed using Statistica (StatSoft 2007) software package. The ANOVA non-parametric Kruskal Wallis (Sprent & Smee- ton 2001) was used because the data were not normally distributed and showed a lack of homogeneity of variance, as showed by applying the test of Bartlett and Levene (Zar 1999).

Results Macroscopic features Seasoned (air-dried) wood colour is yellowish verging on pale brown, and no differentiation, checked by eye and under UV light, has been observed between heartwood and sapwood (Fig. 1a). The odour was not appreciable on freshly exposed surfaces. Annual growth rings are evident but sometimes false or incomplete, and a moderately fine texture was observed.

Microscopic features Picconia azorica growth rings are delimited by marginal parenchyma bands and by medium thick-walled latewood fibres (Fig. 3b). The species has diffuse-porous wood with grouped vessels, arranged in oblique to dendritic pattern (Y or X shapes) (Fig. 3a). Vessels are rounded to quadrangular in cross section (Fig. 3b, c), 47–106 (21–116) per mm2; confidence limits at α level < 0.05, calculated on N=30 random vessels with minimum and maximum values given in parentheses, are 42–74 (28–85) µm for tangential diameter and 18–55 (15–87) µm for radial diameter, while the walls are 2–5 µm thick. A low percentage of solitary vessels was recorded, with vascular tracheids and paratracheal parenchyma (occasionally apotracheally diffuse). Vessel member length

Downloaded from Brill.com09/28/2021 04:43:02PM via free access 380 IAWA Journal, Vol. 33 (4), 2012 is 184–431 (103–592) µm (N=107); perforations are simple, and generally in oblique position. Vessel elements exhibit well developed helical thickenings in their walls (Fig. 3f). Intervessel pits are nonvestured, alternate, round or elliptic; vessel-ray and vessel-axial parenchyma pits are similar, even if fairly different in a few cases. Torus- bearing pit membranes were observed between vessel members, vessel members and narrow tracheary elements, and between narrow tracheids, in agreement with the results of Dute et al. (2008) and Rabaey et al. (2008). On the other hand, the distinction between tori and pseudo-tori is not easy to make in some cases (Fig. 2).

Figure 2. SEM illustration of pit membrane in radial section (a) bearing a torus (T) surrounded by a margo (M) with the annulus (A) at the outline of the membrane; overview of pit membrane in radial section (b) with tori (T) and hypothetical pseudo-tori (P). — Scale bars: a = 1 μm; b = 5 μm.

→ Figure 3. Microscopic features of Picconia azorica as observed at light and scanning electron microscope. – a: Grouped vessels in cross section arranged in oblique to dendritic pattern. – b: Mar- ginal parenchyma and medium thick-walled latewood fibres in cross section (arrow). – c: Rounded to quadrangular vessels in cross section. – d: Bi-seriate and tri-seriate rays in tangential section. – e: Procumbent and erect body ray cells with square marginal cells in radial section. –

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f: Helical thickenings in a vessel element in tangential section. – g: Diamond and cubic shaped crystals in ray cells. – h: Libriform fibre in macerated wood. – i: Crystals of various shapes and spherical bodies. — Scale bars for c: 500 μm; a: 200 μm; b, d, e: 100 μm; f: 50 μm; i: 5 μm.

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Vascular tracheids (N=270), 108–383 (42–565) µm long, are associated with vessel groups and normal narrow vessel elements. Libriform fibres (N=175) are 620–921 (422–1060) µm long and very thick-walled (Fig. 3h). Fibre to vessel length ratio (F/V) ranges between 2.1 and 3.3. Axial parenchyma forms complete to incomplete sheaths (1–4 cells of adjoining parenchyma cells sheathing part of the vessels) all around the vessel/tracheid groups, in 1–6-seriate marginal bands. The axial parenchyma cells are square with a length ranging from 41 µm to 245 µm (N=91). Parenchyma strand length varies from 2–5 up to 6 cells long. Rays 11 up to 13 per mm (8–16), 7–11 (4–12) cells high, uniseriate or bi-(tri-)seriate (Fig. 3d), including heterogeneous II, occasionally III, and less frequently homogeneous types (cf. Kribs 1935; Carlquist 2001). Body ray cells are procumbent or rarely erect, with one row upright of square marginal cells (Fig. 3e). Spherical starch grains were observed by SEM analysis together with crystals of various shapes (cubic, diamond) and sizes in ray and axial parenchyma cells (Fig. 3g, i). In comparison with P. excelsa described by Baas et al. (1988), the anatomical features of P. azorica wood show differences in terms of dimension ranges of some elements (e.g. vessel number and libriform fibre length), but not significantly so as to provide a relevant distinction between these species at the anatomical level. Conversely, some characteristics, such as the number of vessels per mm2, and eventually the type of ray’s heterogeneity might be worthy of consideration in order to differentiate the two Pic- conia species (see Table 1 for descriptive statistics, including mean values and standard deviation of the measured microscopic features).

Table 1. Comparison of Picconia azorica and Picconia excelsa anatomical elements. Data of P. azorica with mean ± standard deviation (SD) and confidence limits at α level < 0.05 (UCL: Upper Confidence Limit; LCL: Lower Confidence Limit); if available, minimum and maximum values are given in parentheses. Data of P. excelsa are retrieved from Baas et al. (1988).

P. azorica P. excelsa Mean ± SD LCL-UCL (min-max) LCL-UCL (min-max)

Number of vessels/mm2 76 ± 30 47–106 (21–116) 35–80 Tangential diameter (μm) 58 ± 16 42–74 (28–85) 45–55 (25–85) Radial diameter (μm) 36.3 ± 18.5 18–55 (15–87) > 90 Wall thickness (μm) – 2–5 2–5 Vessel element length (μm) 308 ± 123 184–431 (103–592) 360–420 (280–690) Tracheid length (μm) 246 ± 137 108–383 (42–565) – Libriform fibre length μ( m) 771 ± 151 620–921 (422–1060) 990–1190 (650–1460) Parenchyma cell length (μm) 90 ± 35 55–125 (41–245) – F/V ratio – 2.1–3.3 2.6–2.8 Ray width (no. of cells) – 1–2 1–2 (rarely 3) Ray height (no. of cells) 9 ± 2 7–11 (4–12) 6–10 (1–15) Number of rays/mm 12 ± 2 11–13 (8–16) 10–12 (9–14) Ray type Heterogeneous II Heterogeneous III (rarely III) (rarely II)

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Table 2. Colour characterization by rectangular opponent scale system (CIElab 1986) on different points of a disk and board of Picconia azorica. Standard deviation (SD) of each parameter is given in brackets; point labels are retrieved from Figure 1a and 1b.

L* (± SD) a* (± SD) b* (± SD) P%

Disk d0 65.2 (± 0.03) 6.9 24.5 (± 0.01) 34.3 d1 66.4 (± 0.01) 5.4 (± 0.01) 21.0 (± 0.03) 29.0 d2 65.4 6.4 (± 0.02) 25.2 (± 0.01) 35.1 d3 66.3 (± 0.01) 5.9 (± 0.03) 29.5 (± 0.03) 40.0 d4 64.4 (± 0.03) 6.3 (± 0.02) 30.3 (± 0.01) 42.0

Board b0 68.2 (± 0.1) 5.6 (± 0.02) 17.6 (± 0.04) 24.2 b1 70.0 (± 0.03) 6.3 (± 0.01) 20.7 (± 0.01) 27.6 b2 66.1 (± 0.03) 7.9 (± 0.01) 21.7 (± 0.01) 30.6 b3 69.4 (± 0.3) 5.2 (± 0.02) 20.3 (± 0.1) 27.0 b4 72.0 (± 0.2) 6.3 (± 0.03) 23.3 (± 0.04) 30.2 b5 78.3 (± 0.05) 4.7 (± 0.01) 21.7 (± 0.01) 26.1 b6 71.4 (± 0.05) 5.7 (± 0.01) 21.9 (± 0.02) 28.6

Physical and mechanical characterization The colour characterization by reflectance spectrophotometer on the disk and board revealed CIElab parameters with insignificant variations along the sampled points. In particular, L* ranges between 66.1 and 78.3, a* varies from 4.7 to 7.9, and b* from 17.6 to 23.3 (see details in Table 2). Although few references are available about the quantitative colour properties of the woods, and few samples of P. azorica were examined, comparisons were made with two broadleaved species, Juglans regia L. and Prunus avium L. commonly employed and appreciated for furniture making and for

Table 3. Comparison of colorimetric characteristics of Picconia azorica with other selected hardwoods. Picconia Picconia Juglans Juglans Prunus (disk) (board) regia* regia* avium** mean mean mean Mean Mean (± SD) (± SD) (± SD) (min–max) (min–max)

L* 65.58 70.78 59.77 56.15 54.45 (± 0.78) (± 3,65) (± 7.06) (52.51–62.27) (51.24–58.91)

a * 6.17 5.96 4.96 9.26 14.70 (± 0.53) (± 0,99) (± 1.21) (8.150–10.1) (11.00–15.8)

b * 26.10 21.01 17.49 20.63 28.13 (± 3,53) (± 1.70) (± 2.66) (17.86–22.71) (20.34–30.02)

Note: * Genco et al. (2011); ** Della Patria & Omarini (2009).

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Table 4. Mean values for each parameter examined for the physical and mechanical characterization and p-values from the analysis of variance at α < 0.05 probability level (ANOVA Kruskal Wallis non-parametric test). LCL and UCL refer to upper and lower confidence limits stated at 95% confidence level.

Parameter N Average Min. Max. LCL UCL p-value (± SD) Density (g cm-3; MC = 12%) 94 0.82 (±0.05) 0.55 0.92 0.81 0.83 0.19 Density (g cm-3; MC = 0%) 94 0.77 (±0.05) 0.48 0.84 0.76 0.78 0.16 Axial shrinkage (%) 26 1.36 (±0.75) 0.51 3.01 1.06 1.66 0.23 Radial shrinkage (%) 26 6.29 (±1.22) 4.60 9.99 5.08 6.79 0.52 Tangential shrinkage (%) 26 8.12 (±0.66) 6.48 9.13 7.36 8.34 0.94 Volumetric shrinkage (%) 26 13.90 (±1.45) 11.32 18.18 13.32 14.49 0.72 HHV (MJ kg-1) 13 18.66 (±1.20) 16.35 20.01 17.93 19.38 0.90 Ash content (% on weight) 13 1.34 (±0.25) 1.00 1.18 1.19 1.49 0.79 σ y (MPa; MC = 12%) 94 61.85 (±6.68) 42.98 79.42 60.48 63.22 0.21 σ b (MPa; MC = 12%) 31 92.18 (±21.97) 55.42 120.11 84.12 100.24 0.85 Static quality factor (km) 94 7.46 (±0.46) 6.50 8.90 7.56 7.75 0.21 which data are available in the literature (Della Patria & Omarini 2009; Genco et al. 2011). Results indicate that a* and b* values of P. azorica fall within the range of both broadleaves, while L* is highest (Table 3). Results from pH analysis showed homogeneous conditions between the sampled disk and board with values from 4.63 to 5.02 on the first, 4.62–5.30 on the second (Fig. 1a, b), and 4.80–5.20 on the specimens’ sections (not shown). The density of the study samples allowed classifying P. azorica wood as “hard”, with an average of 0.77± 0.05 g cm-3 and 0.82 ± 0.05 g cm-3 at 0% and 12% of moisture content, respectively (Table 4). Radial, axial, tangential, and volumetric total shrinkage tests r were 6.29 ±1.22%, 1.36 ± 0.75%, 8.12 ± 0.66%, and 13.9 ±1.45% respectively. -1 The determined HHV mean value was 18.66 ± 1.20 MJ kgdw , and the ash content, in percentage on dry weight, resulted of 1.34 ± 0.25% (Table 4). Concerning the compression (σ y) and the bending strengths (σ b), average values of 61.85 ± 6.68 MPa and 92.18 ± 21.97 MPa were calculated, and the mean static quality factor was 7.46 ± 0.46 km (Table 4). The comparison of physical and mechanical features among P. azorica samples showed no statistical differences at α < 0.05 probability level as assessed by p-values attained by the ANOVA non-parametric Kruskal Wallis tests reported for each variable in Table 4. Conversely, the comparison with other species occurring in Europe and retrieved from the literature (Table 5) showed that P. azorica wood density is similar to Olea europaea L. among the Oleaceae, while it is higher than almost any other hardwoods except for Quercus ilex L. and Quercus pubescens Willd. (Table 5).

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Table 5. Comparison of physical and mechanical characteristics of Picconia azorica with other hardwoods, including some Oleaceae. Standard error is given in brackets where available from literature, otherwise confidence interval with mean value in brackets is reported.

Radial Tangential Density σ y (MPa) σ b (MPa) HHV shrinkage shrinkage TG/RAD (g cm-3) (MC 12%) (MC 12%) (MJ kg-1) RAD (%) TG (%)

Picconia azorica Tutin 0.82 6.29 8.12 1.29 61.85 92.18 18.66 (± 0.05) (± 1.22) (± 0.66) (± 6.68) (± 21.97) (± 1.20) Fraxinus excelsior L. 0.52–0.87b 4.90a 9.20a 1.88a 31–67 (51)b 86–127.5 18.24f (106)b Fraxinus americana L. 0.6e 4.80e 7.80e 1.62e – – – Olea europaea L. 0.801 2.91 5.42 1.97 47–64 118–160 (± 0.048)a (± 0.73)a (± 1.05)a (± 0.61)a (55)a (135)a 18.60e Carpinus betulus L. 0.70–0.88 6.40b 11b 1.70b 37–57 102–147 (0.78)b (47)b (118)b 18.10f Acer pseudoplatanus L. 0.66a – – – 48.10a 93.20a 18.64 (± 0.15)g Quercus pubescens L. 0.88d 7d 13.4d 1.90d 61.40d 129d 19.39d Castanea sativa Mill. 0.58d 4.10d 7.80d 1.90d 54d 102d 19.26d Quercus petraea Liebl. 0.58–0.97 5–8b 9–14b 1.40–2.30b 60.80a 107.80a – (0.74)b Quercus cerris L. 0.805 6.60 11.10 1.68i 66.50–73d 140–150d 19.46d (± 0.08)i (± 3.2)i (± 3.2)i Juglans regia L. 0.63–0.75b 5.20b 7.20b 1.40b 48–85 80–149 – (64)b (98)b Fagus sylvatica L. 0.52–0.93b 2–9b 9–20b 2.30–2.90b 38–78.50 68–149 19.13 (61)b (118)b (± 0.23)g Prunus avium L. 0.50–0.70 4.50b 7.50b 1.70b 44–61 88–126 18.26 (0.60)b (53)b (106)b (± 0.12)g Quercus ilex L. 0.80–1.10 – – – 36–70 69–132 18–80h (0.94)b (50)b (100)b Ostrya carpinifolia L. 0.75–0.88 6.90d 11.70d 1.70d 31.50–63 97–159 19.43d (0.82)b (48)b (133)b Robinia pseudoacacia L. 0.75a 6.80a – – 61.80a 147.10a 19.70c

Note: a Berti et al. (1979); b Giordano (1981); c Kitani & Hall (1989); d Berti et al. (1991); e Tsoumis (1991); f Hellrigl (2006); g Telmo & Lousada (2011); h Zamorano et al. (2011); i Lo Monaco et al. (2011b). MC = Moisture content.

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Picconia azorica wood presents a medium/high volumetric shrinkage, and suffers limited deformation (resulting from tangential and radial shrinkage ratio), being more stable than any other studied species. The mean value of the resistance to compression falls within the range of Fraxinus excelsior L. and Olea europaea, while the resistance to bending is comparable to F. excelsior, but is lower than O. europaea. A similar comparison with the other hardwood species reported in Table 5 shows that σ y value of P. azorica is slightly higher than Carpinus betulus L., Acer pseudoplatanus L., Castanea sativa Mill., Prunus avium L., Quercus petraea Liebl., and Q. pubescens Willd. On the other hand, σ b is generally lower than almost any species, falling within the range of J. regia, Fagus sylvatica L., P. avium, and Q. ilex. The static quality factor is low if compared to other broadleaves commonly used for structural applications with values ranging from 8 to 9 km (Giordano 1999). Concerning the HHV, the mean value found is lower than all softwoods (20–22 MJkg-1; Wegner et al. 1989) and other hardwood species such as C. sativa, F. sylvatica, Ostrya carpinifolia L., Q. pubescens, Q. cerris, Robinia pseudoacacia L., Quercus rubra L., Ulmus americana L., Carya ovata (Mill.) K.Koch, or seven Populus hybrids (19.49 MJkg-1; Bowersox et al. 1979; Slusher 1995).

Discussion

Generally, the observed anatomical features of Picconia azorica are similar to P. excelsa, with some exceptions regarding the number of vessels per mm, and the ray type. To explain these overall similarities we might hypothesize the recent geological events of the Macaronesia Archipelagos causing a differentiation process between P. excelsa and P. azorica not sufficient to have an effect on the wood anatomy (Schirone et al. 2010; Fernandez-Palacios et al. 2011; Ferreira et al. 2011; Schaefer et al. 2011; Triantis et al. 2011). Overall, the characteristics that describe P. azorica wood anatomy are in agreement with the group V defined by Baas et al. (1988). To this group belong the genera Phillyrea, Picconia, Nestegis, Notelaea, Osmanthus, and Olea C, which are characterized by having “nonseptate libriform fibres, marginal parenchyma, vessels in oblique to dendritic pattern, usually associated with vascular tracheids in a vasicentric position, vessels with well-developed spiral thickenings, intervessel pits more than 6 µm in diameter” (Baas et al. 1988). Within the same group, the genera Picconia and Osmanthus are distinguished by the occurrence of inter-vessel and inter-tracheary pits with tori (Dute et al. 2008). At the same time, the opportunity to discriminate between P. azorica and P. excelsa by investigating the taxonomic value of the torus’ dimensions is still not confirmed (cf. Rabaeyet al. 2008). Concerning the macroscopic features of P. azorica, colour, grain and texture are crucial characteristics for the aesthetic evaluation of its wood. The wood is aesthetically pleasing and odourless, and thus suitable for furniture making and decorative applications. The slightly acid wood pH values, above the lower limit of metal corrosion of 4.0– 4.3 found by Farmer (1967), do not differ from those in most of the studied woods (Xing et al. 2004; He & Yan 2005). Thus the bonding capacity and the corrosiveness, in particular under humid conditions, are similar to other species used for furniture making (Packman 1960; Giordano 1981; Landi & Staccioli 1992).

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Despite the investigations performed in the last 30 years regarding the physical and mechanical properties of several Oleaceae (cf. Berti et al. 1979; Giordano 1981, 1988; Tsoumis 1991), no information about the genus Picconia was available before this study. Historical references attest to the wood’s hardness and its difficulty to work (Dias et al. 2007). Indeed, Picconia azorica wood was observed to be heavy, falling within those species with high density, and classified as hard according to Berti (1985). Volumetric shrinkage, tangential/radial shrinkage ratio, resistance to compression and bending, as well as static quality factor confirm the effectiveness of the wood in furniture and pavement. For such purposes, P. azorica shows a performance more comparable to Juglans regia than to other Oleaceae. On the other hand, the high density, the considerable ash content and the calorific potential retrieved from P. azorica samples classify the species as not recommendable for biomass, as also confirmed by the comparison with other Portuguese species (Telmo & Lousada 2011). The history of P. azorica utilization demonstrated that this species was appreciated by the inhabitants of the Azorean archipelago as an income resource since the first settlements in the XV century (Dias et al. 2007). Indeed, Picconia azorica, together with other endemic species as Juniperus brevifolia (Seub.) Antoine and Frangula azorica Tutin, was recognized to play an essential role in making highly valuable furniture. Unfortunately, five centuries of exploitation and the recent invasion by exotic plant species in the Archipelago have led to a continuous reduction and fragmentation of P. azorica’s habitat on each island. However, the assumptions presented in this study could be improved by further investigation and experiments about the species biology (e.g. growth performance) and technological features (e.g. response of the wood to polishing and varnishing). Other applications taking into account its properties could then be explored, namely utilization for cabinets, interior panelling, veneer, handles, toys, etc. Such novel uses could boost a niche market and move the public awareness and the forest owners towards restoring the ecosystems where P. azorica occurs in the wild.

Acknowledgements

The authors are indebted to Dr. Anna Rita Taddei of CIME (Centro Interdipartimentale di Microscopia Elettronica), University of Tuscia, for the valuable support in providing SEM pictures of Picconia azorica.

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