Wood Anatomy of the Mascarene Dombeyoideae: Systematic and Ecological Implications

Home , Dombeya acutangula, Dombeyoideae, Nesogordonia, Pterospermum, Sterculiaceae, Trochetia

IAWA Journal, Vol. 32 (4), 2011: 493–519

Wood anatomy of the Mascarene Dombeyoideae: Systematic and ecological implications

Anaïs Boura1, Timothée Le Péchon2 and Romain Thomas1

SUMMARY The Dombeyoideae (Malvaceae) are one of the most diversified groups of plants in the Mascarene Islands. Species of Dombeya Cav., Ruizia Cav. and Trochetia DC. are distributed in almost all parts of the archipelago and show a wide diversity in their growth forms. This study provides the first wood anatomical descriptions of 17 out of the 22 Mascarene species of Dombeyoideae. Their wood anatomy is similar to that of previously described species: wide vessels, presence of both apotracheal and paratracheal parenchyma, and storied structure. In addition, we also found a second wood anatomical pattern with narrower vessels, high vessel frequency and thick-walled fibres. The two aforementioned wood patterns are considered in a phylogenetic context and used to trace the evolutionary history of several wood anatomical features. For example, the pseudo- scalariform pit arrangement supports a sister group relationship between Trochetia granulata Cordem. and T. blackburniana Bojer ex Baker and may be a new synapomorphy of the genus Trochetia. Finally, wood variability is evaluated in relation to geographic, climatic and biological data. Despite the juvenile nature of some of the specimens studied, we discuss how the habit, but also factors related to humidity, influence the variability observed in the Mascarene Dombeyoideae wood structure. Key words: Dombeya Cav., Ruizia Cav., Trochetia DC., Mascarene Archipelago, wood anatomy, habit, systematics, ecology.

INTRODUCTION The subfamily Dombeyoideae (Malvaceae, including the former Sterculiaceae) are well represented throughout the Mascarene Archipelago. From the sub-xerophilous megathermal sector to the high altitude mesothermal forest of Réunion, species of Dombeya Cav., Ruizia Cav. and Trochetia DC. are present in almost all parts of the archipelago (Friedmann 1987). The Mascarene Islands lie to the east of Madagascar in the south-west of the Indian Ocean, between 19° 40'–21° 07' S lat. and 55° 13'–61° 10' E long. This archipelago is composed of three volcanic islands: Mauritius, Rodrigues and La Réunion. Mauritius and La Réunion are 150 km apart and Rodrigues is 574 km east of Mauritius (Fig. 1–4).

1) UMR 7207 CR2P Centre de recherche sur la paléobiodiversité et les paléoenvironnements, Université Pierre et Marie Curie (UPMC)/Muséum national d’Histoire naturelle (MNHN), CP 38, 57 rue Cuvier, 75231 Paris cedex 5 [E-mail: [email protected]]. 2) Laboratoire LIM-IREMIA, Parc Technologique Universitaire, Bat. 2, 2 rue Joseph Wetzell, 97490 Sainte Clotilde, La Réunion, France.

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Figure 1–4. Mascarene Islands. – 1: Position of the Mascarene Archipelago. – 2–4: Sampling areas in La Réunion, Rodrigues and Mauritius. Numbers refer to sample numbers; see Table 1. Map (http://d-maps.com /).

The Dombeyoideae are one of the most diversified groups in the archipelago (Le Péchon et al. 2009, 2010). Friedmann (1987) lists 22 species representing four genera (Astiria Lindl., Dombeya, Ruizia, and Trochetia). Three genera, Astiria, Ruizia and Trochetia, are endemic to the archipelago and one, Dombeya, is native. Species of Dombeya, Ruizia and Trochetia are distributed from the western dry stations (Mean Annual Precipitation, MAP: 500 mm/year) to the perhumid stations (MAP: 6000 mm /

Downloaded from Brill.com10/03/2021 11:11:59PM via free access Boura, Le Péchon & Thomas — Mascarene Dombeyoideae 495 year and more), and from sea level to 2,000 m in altitude. The highest variety of species is found in the cloud forests (Dombeya ferruginea Cav., D. ficulnea Baill., D. pilosa Cordem., D. punctata Cav., and D. reclinata Cordem.). Several species are restricted to wet and warm habitats (Dombeya blattiolens Frapp. ex Cordem.); others to drier areas (Dombeya populnea (Cav.) Baker, D. rodriguesiana Friedmann, Ruizia cordata Cav., Trochetia boutoniana Friedmann). In contrast, a few species have a broad ecological amplitude and occur in several types of environment (Dombeya ciliata Cordem., D. elegans Cordem.). Dombeyoideae are only absent from high mountain areas (over 2,000 m). Nevertheless, anthropological pressures resulted in the progressive disappearance of the natural vegetation of the Mascarene Archipelago. The dry and low-altitude areas

Nesogordonia crassipes (Md) Nesogordonia suzannae (Md) Dombeya acutangula (Md) Dombeya acutangula subsp. acutangula var. acutangula (R) Dombeya acutangula subsp. acutangula var. acutangula (Ro) Dombeya acutangula subsp. rosea aff. var. acutangula (M) Dombeya acutangula subsp. rosea aff. var. palmata (M) Dombeya amaniensis (Md) Dombeya burgessiae (Afr) Dombeya tiliacea (Afr) Dombeya lucida (Md) Dombeya sp 310 (Md) Dombeya blattiolens (R) Dombeya punctata (R) Dombeya umbellata (R) Dombeya delislei (R) Dombeya elegans var. elegans (R) Dombeya elegans var. virescens (R) Dombeya ciliata (R) Dombeya ficulnea (R) Dombeya pilosa (R) Dombeya reclinata (R) Dombeya brevistyla (Md) Dombeya cacuminum (Md) Dombeya sp 277 (Md) Dombeya rottleroides (Md) Dombeya viburnifolia (Md) Dombeya farafanganica subsp. endrina (Md) Dombeya 252 (Md) Dombeya ferruginea subsp. borbonica (R) Dombeya ferruginea subsp. ferruginea (M) Trochetia blackburniana (M) Trochetia granulata (M) Trochetia boutoniana (M) Trochetia parviflora (M) Trochetia triflora (M) Dombeya mauritiana (M) Dombeya populnea (M) Dombeya populnea (R) Dombeyasevathianii (M) Ruizia cordata (R) Dombeya tremula (Md) Dombeya macrantha (Md) Helmiopsis bernieri (Md) Helmiopsis pseudopopulus (Md) Dombeya rodriguesiana (Ro) Dombeya superba (Md) Trochetiopsis erythroxylon (H) Pterospermum (A) Figure 5. Bayesian tree generated by four molecular markers (ITS, psbM-trnD, trnQ-rps16, rpl16 intron). Bootstrap values and posterior probabilities are above and below the branches, respectively. Clades A–C correspond to the ones discussed in the text. Species, subspecies or variety included in the present study appear with a grey background. Md, Madagascar; R, Réunion; Ro, Rodrigues; M, Mauritius; A, Asia; Afr, Africa; H, Saint Helena (from Le Péchon et al. 2010; modified).

Downloaded from Brill.com10/03/2021 11:11:59PM via free access 496 IAWA Journal, Vol. 32 (4), 2011 are the most affected: all the taxa associated with this environment are endangered (Ruizia cordata, Dombeya mauritiana Friedmann, D. populnea, D. rodriguesiana). In spite of their importance for Mascarene ecosystems and natural heritage, as well as their endangered status, few studies have focussed on the systematics, the anatomy and the ecology of Mascarene Dombeyoideae. With the exception of studies on the wood of the “Sterculiaceae” and Dombeya (Chattaway 1932; Metcalfe & Chalk 1950; Seyani 1991) and a survey of woods from La Réunion (Détienne & Jacquet 1993), the wood of Dombeyoideae is largely unknown. It is described as relatively light (with a specific gravity range between 0.35 and 0.45) (Friedmann 1987; Détienne & Jacquet 1993). The wood anatomy of the endemic genera Trochetia and Ruizia and of ten Dombeya species endemic to the Mascarene Islands has never been described. Besides, Mascarene ecosystems represent interesting environments to study ecological and biological trends in xylem anatomy given that they have specific climatic conditions and support a wide diversity of growth forms in the Dombeyoideae. Recently, the phylogeny of the Mascarene Dombeyoideae has been reconstructed by a molecular study (Le Péchon et al. 2010), with special attention to the relationships between the Mascarene Dombeyoideae and other species from Madagascar, Africa, and Asia. The Mascarene taxa are included in four independent lineages, within which two radiation events occurred (Fig. 5). The aforementioned study also concludes that the genera Dombeya, Ruizia, and Trochetia are nested within the same clade and that Dombeya is paraphyletic. However, morphological and molecular data give conflicting results and several molecular clades are not supported by morphological synapomor- phies. Several studies indicate that wood anatomy can provide interesting characters for phylogenetic systematics (Malécot et al. 2004; Lens et al. 2007, 2008). The main aims of this study are: 1) to provide the first wood anatomical descriptions of 19 Dombeyoideae species from Mascarenes; 2) to trace the evolutionary history of selected wood anatomical features, and to determine the significance of these characters in a phylogenetic context; 3) to analyse the variability of the Dombeyoideae at the intra- and interspecific level in association with ecological and biological parameters.

MATERIAL AND METHODS

Fifty-three representative trees, subtrees and shrubs belonging to 17 of the 22 species of Mascarene Dombeyoideae were chosen and collected during the years 2006 and 2007 by one of us (TLP) in various places of the archipelago (Fig. 2–4). The sampled species, the collection numbers, localities, and habit types are given in Table 1. The monotypic species Astiria rosea Lindley from Mauritius has not been encountered since 1960 (Friedmann 1987) and the species Dombeya mauritiana, Trochetia boutoniana, T. parvifloraBojer, and T. unifloraDC. are very rare and thus were not sampled. When it was possible, wood was sampled from several trees of the same species in contrasting environmental localities. The descriptions of the wood as well as the measurements are based on samples taken at breast height in the trunk of the largest specimens (when the specimen was tall enough) with a 12 mm diameter punch (Boura & De Franceschi 2008) or from

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Table 1. Mascarene Dombeyoideae wood samples. Nb: number of specimens per species, O = island of origin (R = La Réunion, M = Mauritius, Ro = Rodrigues), Al = altitude (in m), VT= vegetation type (M = megathermal, m = mesothermal, H = hygrophilous, X = semi-xeric), HT = habit type (SH = shrub, ST = subtree, T = tree, * indicates stem wood).

Species Sample number Nb O Al VT HT

Dombeya acutangula Cav. subsp. acutangula var. acutangula Arènes 252 1 R 87 MX SH D. blattiolens Frapp ex Cordem. 120 / 291 / 292 3 R 663–670 MH T D. ciliata Cordem. 10 / 186 / 201 / 210 / 284 / 301 6 R 504–1526 MH/mH T D. delislei Arènes 174 / 198 / 278 3 R 1163–1211 mH/mX SH*-ST D. elegans Cordem. var. elegans 17 / 172 / 207 / 208 / 220 5 R 640–1394 mH/MH SH*-ST D. elegans Cordem. var. virescens 272 1 R 1402 mX ST D. ferruginea Cav. subsp. ferruginea 155 1 M 687 MH ST D. ferruginea Cav. subsp. borbonica Friedmann 197 / 249 2 R 1230–1320 mX SH* D. ficulnea Baill. 7 / 84 / 216 / 218 4 R 760–2110 MH/mH/mX T D. pilosa Cordem. 59 / 65 / 231 / 240 / 241 / 259 6 R 1236–1785 mH T D. populnea (Cav.) Baker 251 1 R 87 MX T D. punctata Cav. 68 / 88 / 175 / 178 / 180 / 238 / 246 / 266 8 R 983–1451 mH/mX ST-T D. reclinata Cordem. 6 / 176 / 179 / 227 4 R 837–1230 mH T D. rodriguesiana Friedmann 158 1 Ro 100 MX ST* D. sevathianii Le Péchon 143 1 M 608 MH T D. umbellata Cav. 47 1 R 660–1119 mX T Ruizia cordata Cav. 81 / 137 1 R 87 MX T Trochetia blackburniana Bojer ex Baker 144 1 M 672 MH SH* Trochetia granulata Cordem. 308 1 R 1291 mX SH*

Downloaded from Brill.com10/03/2021 11:11:59PM via free access 498 IAWA Journal, Vol. 32 (4), 2011 smaller stems of the other individuals. For these latter samples, measurements should be considered with great caution because of the juvenile nature of the specimens (Bhat et al. 1989; Falcon-Lang 2005). Samples were preserved in ethanol (70%). Prior to sectioning, they were boiled in a 10% glycerine solution in water (Ives 2001). Standard 15–30 µm thick transverse, tangential longitudinal and radial longitudinal sections were cut using a sliding micro- tome. Sections were stained using iodine-green (1% in 70% ethanol) (adapted from Chamberlain 1901). Wood samples and sections are kept in the UPMC xylarium under collection numbers 1941 to 1994. Numbers given in the following text refer to the sample numbers (Table 1). Micro-photos were taken with a Nikon D300 digital camera.

Microscopic wood description All of the features listed in the IAWA List of microscopic features for hardwood identification (IAWA Committee 1989) were examined. Features from the list not mentioned specifically in the description are absent. Eight quantitative wood anatomical features were measured for both anatomical description and evaluation of intra- and interspecific wood variations. Vessel density (VD) is the number of vessels per square millimetre. Mean diameter (MD) is the mean vessel diameter measured in transverse section. Minimal diameter (MIND) is the diameter of the narrowest vessel measured. Maximal diameter (MAXD) is the diameter of the largest vessel measured. All features related to diameter (MD, MIND and MAXD) were estimated from the vessel lumens areas. VD, MD, MIND MAXD were measured and averaged from an average 45 mm² area (from 7.65 mm² to 126.98 mm²), which represents between 80 to 1500 vessels measured (average: 450). Vessel element length (VEL) was approximated as the average of at least 30 vessel elements measured with an optical microscope on tangential longitudinal section. Percentage of solitary vessels (PSV) is the proportion of solitary vessel in relation to the total number of vessels. Ray density (RD) is the number of rays per square millimetre. Mean multiseriate ray height (MRH) (Carlquist 1966; Noshiro & Suzuki 1995) is the average height of multiseriate rays. RD and MRH were measured and averaged from 2 × 4 mm² areas. Measurements and counting were done through observations and digital analysis with “ImageJ” (Abramoff et al. 2004). In addition to these quantitative features, we also considered the presence of storied structure (SS), the arrangement of intervessel pits as defined by the IAWA Committee (1989) and the fibre wall thickness (FWT). FWT was defined as the qualitative appreciation of the wall thickness. Fibre walls were considered thick when the diameter of the fibre lumen was less than half of the fibre diameter. In quantitative terms, fibre walls defined as thin are equal or less than 3 µm thick, fibre walls defined as thick are more than 3 µm thick.

Evolution of selected characters in a phylogenetic context In phylogenetic reconstruction, continuous characters should be coded into character states (Farris 1990; Rae 1998). Within our sampling, several threatened or uncommon species are represented by only one specimen (e.g., Dombeya acutangula Cav., D. populnea, D. rodriguesiana, D. umbellata and Ruizia cordata). This is enough to establish wood anatomical descriptions, but the variability of the quantitative characters

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(e.g. VD or MD) at the intraspecific level cannot be assessed for these species. The definition of character states is therefore artificial and, to avoid these difficulties, we chose to remove all the continuous features. According to Herendeen and Miller (2000), characters of the axial parenchyma are the most difficult to analyse and to classify into discrete character states. The strong polymorphism of these features does not allow the definition of unambiguous character states, and consequently we ignored them in our analyses. Finally, we selected three characters: the fibre wall thickness, the presence of storied structure and the intervessel pit arrangement. The states of these characters were defined according to Herendeen and Miller (2000). To infer the evolutionary history of these characters, we used a molecular phylogenetic tree published in a previous study (Fig. 5, Le Péchon et al. 2010). Wood anatomical information was extracted from the present study and from the literature (Chattaway 1932; Barnett 1988; Seyani 1991; Détienne & Jacquet 1993; InsideWood 2004). The data matrix is provided in Table 2. The wood anatomical description was not available for certain species (e.g. Dombeya viburniflora Bojer, Trochetia triflora DC.); consequently, we scored their character states as missing data. Evolution of the three selected characters was inferred using MacClade (Maddison & Maddison 2000).

Geographical, ecological and biological influences on wood anatomy We considered the following factors: altitude (AL); coldest month mean temperature (CMMT); mean annual precipitation (MAP); driest month precipitation (DMP); forest type (FT); site humidity (H); diameter of the sampled trunk or stem (D) and habit type (HT). All specimens defined as shrubs were multi-stemmed. The links between quantitative wood anatomical traits and geographic parameters were examined by Spearmann non parametric rank correlations. Due to the low number of observations, we used the Kruskal-Wallis non parametric test – the non parametric analogue to a one-way analysis of variance – for studying the connections between anatomical quantitative characters and qualitative environmental and biological variables. χ² tests were performed to test for associations between qualitative wood anatomical characters (FWT and SS) and environmental and biological parameters. Statistical relationships were considered significant at p < 0.05. To detect broad trends in variation among multiple anatomical traits, we performed a principal component analysis (PCA). Statistical analyses were performed with the R software (R development Core team 2011).

RESULTS Microscopic wood description Summaries for each species are presented in Tables 3–5. In all specimens, the wood structure is not homogeneous through the entire transverse section and displays various indications of slowing down of the wood formation during certain periods of the year (Fig. 6–24). Nevertheless, growth rings are usually faint and irregularly spaced or absent, and defined by: (i) a change in the vessel diameter (Dombeya acutangula, D. blattiolens, D. elegans, D. reclinata, D. umbellata Cav.; Fig. 6–10); (ii) a change in the vessel density or in the fibre wall thickness (D. blattiolens, D. elegans, D. ferruginea,

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Table 2. Anatomical matrix used for the evolutionary inferences. Characters and character state coding. WS = wood structure (NS = not storied, T = tendency to storied structure, SS = storied); FWT = fibre wall thickness (TNTK = presence of only thin-walled fibres or both thin- and thick-walled fibres, TK = presence of only thick-walled fibres); IPT = intervessel pit arrangement (A = alternate, PSS = pseudo-scalariform). WS FWT IPT Dombeya acutangula Cav. NS TK A D. acutangula subsp. acutangula var. acutangula NS TK A D. acutangula subsp. rosea Friedmann aff. var. acutangula ? ? ? D. acutangula subsp. rosea Friedmann aff. var. palmata ? ? ? D. amaniensis Engl. NS TNTK A D. blattiolens Frapp. ex Cordem. NS TNTK A D. brevistyla Arènes ? ? ? D. burgessiae Gerr. ex Harv. & Sond. NS TNTK A D. cacuminum Hochr. ? ? ? D. ciliata Cordem. NS-T-SS TK-TNTK A D. delislei Arènes NS TNTK A D. elegans Cordem. var. elegans NS TNTK A D. elegans Cordem. var. virescens NS TNTK A D. farafanganica Arènes subsp. endrina ? ? ? D. ferruginea Cav. subsp. borbonica NS-SS TK A D. ferruginea Cav. subsp. ferruginea NS TK A D. ficulnea Baill. NS TNTK A D. lucida Baill. SS TNTK A D. macrantha Baker ? ? ? D. mauritiana Friedmann ? ? ? D. pilosa Cordem. NS-T-SS TNTK A D. populnea (Cav.) Baker (Mauritius) ? ? ? D. populnea (Cav.) Baker (Réunion) SS TK A D. punctata Cav. NS TNTK A D. reclinata Cordem. NS-T TNTK A D. rodriguesiana Friedmann SS TK A D. rottleroides Baill. ? ? ? D. sevathianii Le Péchon & Baider NS TK D. superba Arènes ? ? ? D. tiliacea Planch. NS TNTK A D. tremula Hochr. ? ? ? D. umbellata Cav. NS-T-SS TNTK A D. viburnifolia Bojer ? ? ? Helmiopsis bernieri (Baill.) Arènes ? ? ? H. pseudopopulus (Baill.) Capuron SS TNTK A Nesogordonia crassipes (Baill.) Capuron NS TK A N. suzannae Labat, Munzinger & O. Pascal ? ? ? Pterospermum Schreb. SS TNTK A Ruizia cordata Cav. SS TK A Trochetia blackburniana Bojer NS TK A-PSS T. boutoniana Friedmann ? ? ? T. granulata Cordem. NS TNTK A-PSS T. parviflora Bojer ? ? ? T. triflora DC. ? ? ? Trochetiopsis erythroxylon (G. Forst.) W.Marais ? ? ?

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Table 3. Qualitative wood anatomical features (in Dombeya, Ruizia and Trochetia). D = wood diffuse-porous, SP = semi-ring-porous wood, TK = thick-walled fibres, TN = thin- walled fibres. Between parentheses: maximum values. A = absent, T = tendency, P = present. Porosity arrangement, soli- Vessel tary or grouped vessels Fibre wall thickness Uniseriate ray height in cells Multiseriate ray width* and height in cells Storied structure Crystals

D. acutangula D-SP 1 (2-3) TK <10 3-4* 12 A P (20) D. blattiolens D 1 (2-3) TN-TK <20 4-5* 20 A P (50) D. ciliata D 1-3 (6) TN <5 4-5* 40 A-T-P P (TK D186) D. delislei D 2-3 (10) TN-TK <6 4-5* 50 A P D. elegans D-SP 1-2 (7) TN-TK <15 4-5* 40 A P D. ferruginea D (1) 2-3 TK <5 4-5* 15 A-P P D. ficulnea D 1-2 (7) TN-TK <15 4-5* 30 A P D. pilosa D (1) 2-3 (7) TN-TK <5 4-5* 15 A-T-P P (20) D. populnea D 1(2-4) TK <5 3-4* 15 P A D. punctata D 1-3 (10) TN-TK <20 3-6* 40 A P D. reclinata D 1 (2-3) TN-TK <10 2-5* 40 A-T P D. rodriguesiana D 1-3 (7) TK <5 2-3* 25 P P D. sevathianii D-SP 1 (2-3) TK <5 3-4* 20 A P (50) D. umbellata D 1-3 (7) TN-TK <10 4-5* 15 A-T-P P (20) R. cordata D 1 (2-4) TK <5 3-4* 10 P A T. blackburniana D 1 (2-4) TK <10 3-10* 60 A A T. granulata D 1 (2-6) TN-TK <10 3* 20 A P

D. reclinata, D. umbellata; Fig. 8, 10–12, 22); (iii) the presence of flattened thick-walled latewood fibres (D. delislei Arènes, D. ferruginea, D. rodriguesiana; Fig. 13–14, 23); (iv) the presence of marginal parenchyma bands (D. blattiolens, D. ciliata, D. pilosa, D. populnea, D. rodriguesiana, D. sevathianii Le Péchon & Baider, Ruizia cordata, Trochetia blackburniana Bojer; Fig. 14–20, 24); or a combination of all or some of the previous characters. Within a species, changes in the structure vary according to individuals and to their growth locality. For example, one Dombeya ciliata specimen

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Table 4. Occurrence of parenchyma types. P = present, A = absent.

Paratracheal Apotracheal Banded ‑––––––––––––––– –––––––––– –––––––––––––– Species Scanty Vasicentric Diffuse in Narrow Wide aggregates bands bands

Dombeya acutangula P A P A A D. blattiolens P P P P A D. ciliata P P P P A D. delislei P P P P A D. elegans P P P P A D. ferruginea P P P P A D. ficulnea A P P P A D. pilosa P P P A P D. populnea P A P P P D. punctata A P P A P D. reclinata A P P A P D. rodriguesiana P A P P P D. sevathianii P A P P A D. umbellata P A P P A Ruizia cordata P A P P P Trochetia blackburniana P A P P P T. granulata P P P A P

(186) shows marked fibre and parenchyma bands, while the other specimens (201/284) have a more homogeneous wood (Fig. 20 & 21). The same phenomenon occurs in D. pilosa. Most of the species are diffuse porous, nevertheless some of them tend to- ward a semi-ring porous pattern (D. acutangula, D. blattiolens, D. elegans; Fig. 8–9, Table 3). Between 17% (D. umbellata and D. sevathianii) and 58% (D. rodriguesiana) of the total number of vessels are solitary (Table 5). The others are grouped radially in multiples of two, three, four or more (Table 3), or tangentially (D. ciliata, D. ficulnea, D. pilosa, D. reclinata; Fig. 25). Perforation plates are always simple (Fig. 26). The intervascular pits are small and alternate, with a rounded to a polygonal shape (Fig. 26), except on the narrowest vessels of Trochetia blackburniana and T. granulata in which they are elongated and pseudo-scalariform (Fig. 27). The ray parenchyma pits are apparently simple and elliptic with reduced borders in all species. Several species show exclusively thick-walled fibres (Dombeya acutangula, D. ferruginea, D. populnea, D. rodriguesiana; Table 3) while other species have thin or both thin and thick fibre walls. Most of the D. ciliata samples have thin or both thin and thick fibre walls (10, 201, 210, 284, 301), whereas the one from the highest locality (186) only has thick-walled fibres.

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Table 5. Quantitative wood anatomical features. nb = number of samples studied, VEL = vessel element length (mm), PSV = percentage of solitary vessels, RD = ray density (number/mm²), MRH = multiseriate ray height (mm). M = mean, σ = standard deviation.

nb VEL PSV RD MRH M σ M σ M σ M σ

Dombeya D. acutangula 1 0.155 – 56 29 – 0.265 – D. blattiolens 3 0.333 0.057 37 9 21 3 0.406 0.093 D. ciliata 6 0.325 0.069 37 17 30 5 0.368 0.056 D. delislei 3 0.273 0.029 24 9 28 11 0.352 0.101 D. elegans 6 0.279 0.044 39 23 23 3 0.422 0.139 D. ferruginea 3 0.182 0.032 32 17 44 6 0.280 0.076 D. ficulnea 4 0.270 0.054 24 10 32 11 0.352 0.072 D. pilosa 6 0.276 0.028 34 18 30 5 0.296 0.058 D. populnea 1 0.198 – 45 – 16 – 0.315 – D. punctata 8 0.257 0.062 26 16 36 6 0.286 0.057 D. reclinata 4 0.305 0.045 27 5 33 9 0.292 0.035 D. rodriguesiana 1 0.232 – 58 – 38 – 0.203 – D. sevathianii 1 0.254 – 17 – 18 – 0.567 – D. umbellata 2 0.322 0.042 17 2 34 7 0.358 0.027 Ruizia R. cordata 1 0.199 – 39 – 43 – 0.150 – Trochetia T. blackburniana 1 0.199 – 30 – 29 – 0.535 – T. granulata 1 0.189 – 43 – 55 – 0.287 –

Axial parenchyma is very abundant in most species (Fig. 28–32, Table 4). Indeed, we find: (i) paratracheal parenchyma in more or less complete sheaths around solitary or grouped vessels (Fig. 30); (ii) banded parenchyma, in narrow (up to three cells wide; Fig. 16, 31) or wider bands (more than three cells wide; Fig. 19) or in both forms (Fig. 32); (iii) apotracheal parenchyma with short discontinuous tangential lines (Fig. 29). Uni- and multiseriate rays are abundant and relatively low (< 600 µm). They are usually 10–20 cells high, up to 40 to 50 cells high for the highest ones (Table 3). In all the species, the largest rays are 2- to 6-seriate whereas in Trochetia blackburniana they are broader and higher, sometimes more than twelve cells wide and up to sixty cells high (Fig. 33, Table 3). Rays are always heterocellular: body ray cells are usually procumbent, with 1–4 rows of marginal square and upright cells and often sheath cells. Rays and axial elements (fibres and vessel elements) are storied (Dombeya ciliata, D. pilosa, D. populnea), irregularly storied (D. ferruginea) or not storied (D. acutangula; Fig. 34 & 35, Table 3). In D. ciliata, this feature varies within the different samples: none of the elements is storied in samples 10 and 186; rays, fibres and vessel elements

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6 7 8

Figure 6–12. Transverse sections of the wood of Mascarene Dombeyoideae 1. – 6: Dombeya umbellata 195. – 7: D. acutangula 252. – 8: D. elegans 17. – 9: D. blattiolens 292. – 10: D. blattiolens 291. – 11: D. ferruginea 197. – 12: D. reclinata 176. — Scale bars 2 mm.

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13 14 15

19 16

Figure 13–21. Transverse sections of the wood of Mascarene Dombeyoideae 2. – 13: Dombeya delislei 278. – 14: D. rodriguesiana 158. – 15: D. sevathianii 143. – 16: Trochetia blackburniana 144. – 17: Ruizia cordata 137. – 18: D. populnea 251. – 19: D. pilosa 241. – 20: D. ciliata 186, arrowheads indicate parenchyma bands. – 21: D. ciliata 201. — Scale bars 2 mm.

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22 23

24 26

Figure 22–26. Wood anatomy of Mascarene Dombeyoideae 1, transverse (22–25) and radial longitudinal (26) sections. – 22: Abrupt transition between thick-walled and thin-walled fibres in Dombeya elegans 172. – 23: Band of flattened fibres inD. ferruginea 197. – 24: Parenchyma band in D. blattiolens 291. – 25: Radial and tangential vessel groupings in D. ficulnea 218. – 26: Simple perforation plate in D. ciliata 301. — Scale bars in 22: 0.5 mm; 23 & 24: 0.25 mm; 25: 1 mm; 26: 0.1 mm.

are irregularly storied in samples 201 and 210; rays, fibres and vessel elements are regularly storied in samples 284 and 301. The same phenomenon occurs in the different D. pilosa samples. Prismatic calcium oxalate crystals in chambered axial parenchyma cells are observed in almost all species (Fig. 36 & 37, Table 3). Crystals are sometimes present in all

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27 28 29

30 31 32

Figure 27–32: Wood anatomy of Mascarene Dombeyoideae 2, tangential longitudinal (27 & 28) and transverse (29–32) sections. – 27: Horizontally fused, pseudo-scalariform intervascular pits in Trochetia blackburniana 144. – 28: Paratracheal parenchyma in Dombeya punctata 88. – 29: Apotracheal scattered parenchyma and vasicentric parenchyma in D. acutangula 252. – 30: Vasicentric parenchyma in D. ciliata 284. – 31: Diffuse parenchyma and parenchyma bands in D. ferruginea 197. – 32: Parenchyma bands in D. rodriguesiana 158. — Scale bars in 27 & 28: 0.25 mm; 29–32: 0.5 mm. the specimens within a species (D. elegans) but may be present in some and absent in others (D. punctata). Mucilage canals occur in the pith of all the species in which complete stems were studied (D. acutangula, D. delislei, D. elegans, D. ferruginea, D. rodriguesiana, and Trochetia blackburniana; Fig. 38).

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33 34

35 36

37 38

Figure 33–38. Wood anatomy of Mascarene Dombeyoideae 3, tangential longitudinal (33–37) and transverse (38) sections. – 33: Pluriseriate rays in Trochetia blackburniana 144. – 34: Storied structure in Ruizia cordata 81. – 35: Storied structure in Dombeya ciliata 284. – 36: Crystals in chambered parenchyma cells in D. umbellata 47. – 37: Crystals in chambered parenchyma cells in D. blattiolens 291. – 38: Mucilage cavity in the pith of D. elegans 17. — Scale bars in 33, 35 & 38: 1 mm; 34: 2 mm; 36: 0.5 mm; 37: 0.1 mm.

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Vessel density ranges from 7/mm² (Dombeya umbellata) to 47/mm² (D. sevathianii; Table 5). Sometimes, this feature varies significantly between individuals of the same species (D. elegans, D. ferruginea) (Fig. 39) and between several parts of the wood transverse section in a single individual (D. blattiolens, D. delislei) (Fig. 10). Mean vessel diameter varies from 30 µm (Trochetia blackburniana) to 125 µm (Dom- beya pilosa; Table 5, Fig. 40). Mean vessel element length is short and ranges from 180 µm (D. ferruginea subsp. ferruginea) to 350 µm (D. ciliata, D. umbellata; Table 5).

Figure 39 & 40. Boxplots of quantitative wood anatomical features measured on Mascarene Dombeyoideae. – 39: Vessel density (number/mm²). – 40: Mean vessel diameter (µm). Notches correspond to the median standard deviation.

Downloaded from Brill.com10/03/2021 11:11:59PM via free access 510 IAWA Journal, Vol. 32 (4), 2011 ld ld Clade A Clade B C ld ld Clade A Clade B C ld ld Clade A Clade B C Clade A Clade A Clade A 2 2 2 Clade A Clade A Clade A 1 1 1

Wood structure Intervessel pit arrangement Fibre wall thickness

non-storied structure alternate only thick-walled structure tendency to storied structure presence of both alternate and presence of both thick- and pseudo-scalariform arrangement thin-walled fibres storied structure polymorphic polymorphic equivocal equivocal equivocal

Figure 41. Evolutionary reconstruction of selected features of wood anatomy inferred from the phylogenetic relationships as reconstructed from molecular data. – A: Wood structure. – B: Intervessel pit arrangement. – C: Fibre wall thickness.

Evolution of selected characters Inferred hypotheses for the evolution of storied wood structures imply 13 evolutionary steps (Fig. 41). For the Mascarene Dombeyoideae, acquisition of storied structures would have occurred independently several times: once in Dombeya rodriguesiana, at least once in Clade A1, once in D. ferruginea subsp. borbonica (Clade A2) and four times in Clade C for the polymorphic species, D. ciliata, D. pilosa, D. reclinata, and D. umbellata. The evolutionary hypotheses for the fibre wall thickness imply five evolutionary steps. Thick-walled fibres appear as the ancestral state. At least two independent acquisitions of the presence of both thick-walled and thin-walled fibres would have occurred for the Mascarene Dombeyoideae: once in Trochetia granulata and once in Clade C. One step is needed to infer the evolution of the intervessel pit arrangement. The acquisition of elongated pseudo-scalariform pits is a synapomorphy of the monophyletic clade including T. blackburniana and T. granulata.

Biological, ecological and geographic influences on wood anatomy With the exception of the presence of “elongated pseudo-scalariform pits” and “large rays (up to 10 cells wide)” occurring only in the two Trochetia species and in

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-1 Second PCA axis -2

-3

-4

-5 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 First PCA axis Figure 42. Principal component axes score plots. Loadings plot of specimens. NB: ellipses are not statistically significant. Dacu =Dombeya acutangula; Dbla = D. blattiolens; Dcil = D. ciliata; Ddel = D. delislei; Dele = D. elegans; Dfer = D. ferruginea; Dfic =D. ficulnea; Dpil = D. pilosa; Dpop = D. populnea; Dpun = D. punctata; Drecl = D. reclinata; Drod = D. rodriguesiana; Dse = D. sevathianii; Dumb = D. umbellata; Rcor = Ruizia cordata; Tbla = Trochetia blackburniana; Tgran = T. granulata.

T. blackburniana respectively, none of the observed wood anatomical characters is diagnostic at the species level. First, second, and third PCA axes explained 42%, 20% and 14% of the variation, respectively. In the first principal component axis,MD , MIND, MAXD and VEL have high loadings, whereas FWT and VD have low loadings. RD and MRH are important in the second principal component, while, PSV and SS have high loadings in the third principal component (data not shown). The projection on the first two components (Fig. 42) allows to distinguish two groups of species. The first one consisting of species with only thin-walled or both thin- and thick-walled fibres, with wide vessels and low vessel frequency is composed of tree species more or less typical of tropical humid forests (D. blattiolens, D. ciliata, D. elegans, D. ficulnea, D. pilosa, D. punctata, D. reclinata, D. umbellata). The second one is composed of species with exclusively thick-walled fibres, narrow vessels and high vessel frequency (D. acutangula, D. delislei, D. ferruginea, D. populnea, D. rodriguesiana, D. sevathianii, Ruizia cordata, Trochetia blackburniana, and T. granulata) and is rather characteristic of shrubs and subtrees from dry tropical forests. It also includes some specimens studied from small stems. It is then interesting to note that two of the species (Dombeya ciliata and D. elegans), which have specimens in the two groups, are the ones with the broadest ecological amplitude.

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Table 6. Spearman’s rank correlation coefficient between quantitative wood anatomical characters and geographic parameters. VD = vessel density, MD = mean vessel diameter, MIND = minimal vessel diameter, MAXD = maximum vessel diameter, VEL = vessel element length, RD = ray density, MRH = multiseriate ray height, * = (p < 0.05). Non significant results in grey.

VD MD MIND MAXD VEL RD MRH VD 1 -0.81* -0.74* -0.79* -0.64* 0.13 -0.04 MD -0.81* 1 0.76* 0.94* 0.58* -0.15 0.02 MIND -0.74* 0.76* 1 0.68* 0.55* -0.15 -0.01 MAXD -0.79* 0.94* 0.68* 1 0.56* -0.10 0.05 VEL -0.64* 0.58* 0.55* 0.56* 1 -0.37* 0.47* RD 0.13 -0.15 -0.15 -0.10 -0.37* 1 -0.59* MRH -0.04 0.02 -0.01 0.05 0.47* -0.59* 1

Table 7. Results of Kruskal-Wallis tests. FWT = fibre wall thickness, SS = storied structure, AL = altitude, MAP = mean annual temperature, DMP = dryest month precipitation, FH = forest humidity, FT = forest type, HT = habit type, D = diameter of the sampled trunk or branch. H = Kruskal-Wallis rank sum statistic, df = degree of freedom. Statistically significant values in black. * = (p< 0.05); ** = (p < 0.001). Non significant results in grey.

Vessel Mean Minimal Maximal density diameter diameter diameter ––––––––––– ––––––––––– ––––––––––– ––––––––––– H df H df H df H df

FWT 16** 2 17.7** 2 5.7* 2 17** 2 SS 7.9* 1 3.47 1 9.2* 1 2 1 AL 6.7 3 9* 3 3 3 9* 3 FH 10* 1 13** 1 4* 1 12** 1 HT 21** 2 22** 2 14** 2 21** 2 D 14* 3 15* 3 7 3 13* 3

Vessel element Ray Multiseriate Percentage of length density ray height solitary vessels ––––––––––– ––––––––––– ––––––––––– ––––––––––– H df H df H df H df

FWT 9.7** 2 2.74 2 0.94 2 4.28 2 SS 4.1* 1 0.01 1 1.23 1 3.3 2 AL 12* 3 4 3 10* 3 5.9 3 MAP 6.2* 2 0.82 2 9.4* 2 4.6 2 DMP 4.57 2 4.5 2 5.65 2 0.74 2 FT 2 1 5* 1 2 1 0.16 1 HT 8* 2 1 2 0 2 3.39 2 D 6 3 1 3 1 3 7.25* 3

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Table 8. Results of Chi2 test between qualitative wood anatomical features (FWT and SS) and qualitative environmental and biological parameters. FWT = fibre wall thickness, SS = storied structure, AL = altitude, CMMT = cold month mean temperature, MAP = mean annual temperature, FT = forest type, HT = habit type, Statistically significant values in black. * = (p< 0.05); ** = (p < 0.001). Non significant results in grey.

Fibre wall thickness Storied structure –––––––––––––––––– ––––––––––––––––– Chi² df Chi² df FWT – – 8.47* 2 SS 8.475* 2 – – AL 31.79** 6 4.5784 3 CMMT 10.18* 4 6.63* 2 MAP 12.06* 4 2.0529 2 FT 7.26* 2 2.0995 1 HT 15.45* 4 4.88 2

Wood anatomical features present several significant correlations to one another (Table 6). Features relative to the “size” of vessels, MD, MIND, MAXD and VEL are positively correlated to one another. VD is negatively correlated to the previous group of variables (Table 6). Thus, the wider and longer the vessel elements, the lower the vessel density. FWT is positively correlated with vessel density and negatively with vessel size (MD, MIND, MAXD and VEL) (Kruskal-Wallis test, Table 7). FWT is also significantly associated to SS (χ² test, Table 8): individuals with storied wood structure or a tendency to storied structure have thin-walled fibres.VD is higher while MD, MIND, MAXD and VEL are lower in individuals with only thick-walled fibres (multiple comparison tests, data not shown). VD is lower while MD, MIND, and VEL are higher (Kruskal-Wallis test, Table 7 and multiple comparison tests, data not shown) in individuals with a tendency to storied structure. Characters related to vessel size (VD, MD, MIND, MAXD and VEL) differ also significantly in relation to the diameter of the sampled trunk or stem and the habit types. VD is lower in trees and in the largest diameter trunk, whereas MD, MIND, MAXD and VEL are higher (Kruskal-Wallis, Table 7 and multiple comparison tests, data not shown). PSV is lower in the largest trunks than in the smaller stems (Kruskal-Wallis, Table 7 and multiple comparison tests, data not shown). Furthermore, fibre walls are thinner in trees than in subtrees and shrubs (χ² test with p < 0.01). MD, MAXD, VEL and MRH values are significantly lower in individuals located at lower altitudes (Kruskal-Wallis test, Table 7 and multiple comparison test, data not shown). Individuals from lower altitudes have more thick-walled fibres (χ² test with p < 0.001, Table 8). FWT and SS were the only wood anatomical traits associated with temperature variables (CMMT) (χ² test with p < 0.05, Table 8). Specimens located in the coolest areas have thin-walled fibres and more storied structure. Wood anatomical characters were more intimately associated with variables related to water availability (H and MAP): VD is lower in hygrophilous habitats whereas MD and MIND are higher. Vessel elements and multiseriate rays are shorter in areas with low MAP (Kruskal-Wallis

Downloaded from Brill.com10/03/2021 11:11:59PM via free access 514 IAWA Journal, Vol. 32 (4), 2011 test Table 7 and multiple comparison tests, data not shown); fibre walls are thicker in the same areas (χ² test, Table 8). FT is the only environmental parameter related to RD: RD is lower in megathermal forest whereas FWT is lower in mesothermal forests (Kruskal-Wallis test, Table 7 and χ² test with p < 0.05, Table 8, and multiple comparison tests, data not shown).

DISCUSSION Our first observations of Mascarene Dombeyoideae are in accordance with the description proposed by Détienne and Jacquet (1993): presence of solitary and radially/ tangentially grouped vessels; presence of apotracheal and paratracheal parenchyma; occurrence of storied structure and heterocellular rays with sheath cells. All these wood anatomical characters were also outlined by Seyani (1991) in his observation of twelve Dombeya species from continental Africa. Détienne and Jacquet (1993) described the wood found in a group of species morphologically close to D. pilosa, with high MD, MAXD, MIND, VEL, low VD, thin-walled fibres and a low specific gravity. The higher number of species observed in our study enables a more accurate perception of the wood of the Mascarene Dombeyoideae. In addition to this first type of wood outlined by Détienne and Jacquet (1993) and found in seven species (D. blattiolens, D. ciliata, D. elegans, D. ficulnea, D. pilosa, D. punctata, D. reclinata), we found a second type of wood with a tendency to low MD, MAXD, MIND, VEL, high VD and thick-walled fibres, present in nine other species (D. acutangula, D. delislei, D. ferruginea, D. populnea, D. rodriguesiana, D. sevathianii, Ruizia cordata, Trochetia blackburniana, and T. granulata). Friedmann (1987) perceived this dichotomy within Mascarene Dombeya wood structure and distinguished the wood of the xerophilous species (D. populnea and D. rodriguesiana) from the light wood of the other Dombeya species. Local uses of Mascarene Dombeya concord with the descriptions proposed by the different authors: the dense wood of D. rodriguesiana and D. ferruginea, locally called “bois pipe” (pipe wood) (Baker 1877; Friedmann 1987) was traditionally used for making pipes, whereas the light wood of D. umbellata was used to make “gingades” or “guinguades” (Friedmann 1987), a sort of raft made of pieces of light wood, which was used to gather corals (Bollée 1993). Seyani (1991) noticed that the wood anatomical features have little taxonomical significance within African species ofDombeya . As for Mascarene Dombeyoideae, the evolutionary hypotheses of the storied structure and the thick-walled fibres show that these two characters are homoplastic and do not support molecular clades. Clade C radiated in La Réunion and represents one of the most diverse groups, endemic to this island. The presence of both thin-walled and thick-walled fibres in this group could have been an advantage to colonize contrasting habitats. Indeed this anatomical feature seems to be linked to the acquisition of wider vessels and thus to a better conductive efficiency allowing plants to be more competitive in wet localities. Both storied structure and the presence of both thin- and thick-walled fibres are widely present in Dombey- oideae (e.g. Pterospermum, Nesogordonia) suggesting evolutionary convergences and parallel developments. Conflicts between traditional (non-molecular) and molecular phylogenies are common. In many groups, tracing the evolutionary hypotheses for

Downloaded from Brill.com10/03/2021 11:11:59PM via free access Boura, Le Péchon & Thomas — Mascarene Dombeyoideae 515 morphological and anatomical characters on molecular phylogenetic trees results in a high degree of homoplasy (Lens et al. 2008; Serdar et al. 2008). However, several characters appear to be concordant in both traditional and molecular analyses. In our analysis, the presence of pseudo-scalariform elongated pits in Trochetia granulata and T. blackburniana supports a sister group relationship between these two species. Furthermore, this character may be a new synapomorphy of Trochetia. However, this result needs to be confirmed. Indeed, Trochetia is represented in our study only by two species when this genus actually includes six taxa. Morphologically, this genus diverges clearly from the other species included in Clade A (e.g. Dombeya ferruginea, D. populnea and Ruizia cordata). Then, Trochetia appears to be monophyletic in both molecular (Fig. 5, Clade A2) and morphological analyses (Le Péchon et al. 2009). However, the position of Trochetia within Dombeyoideae remains uncertain. The description and quantitative analysis of the wood anatomy of the 51 examined individuals, with different stem sizes, highlighted a trend between juvenile and mature structure that resembles the tendencies found between shrubs and trees. Many reports in the literature correlate vessel features with plant size and habit (Carlquist 1966; Wal- lace 1986). In spite of the difficulty to define clear habits, trees generally have a high efficiency in water transport and a greater mean diameter than other habits (Carlquist 1958; Cumbie & Mertz 1962; Walsh 1975; Baas & Schweingruber 1987; De Micco et al. 2008). VEL pattern is also most likely linked to plant habit: the geographic region that has the highest incidence of short vessel elements is actually the one that also has the highest percentage of shrubs (Wheeler et al. 2007). However, shrubs are highly important growth forms in arid areas. This observation outlines the existence of strong intercorrelations among parameters related to geography, temperature, precipitation and plant habit, making the contributions of each of them difficult to establish. Fibre wall thickness also seems to be linked to plant habits: Dombeyoid shrubs have thicker fibre walls than trees or subtrees as previously found from herbs to trees (Carlquist 1958; Cumbie 1960; Cumbie & Mertz 1962). The occurrence of high degrees of vessel grouping is usually more common in herbaceous forms or shrubs than in trees (Cumbie & Mertz 1962). Nevertheless, in Mascarene Dombeyoideae, the greatest percentage of solitary pores occurs in the smallest stems. As for ray diversity and occurrence of storied structure, although many authors showed that they are highly linked to the stem diameter (Cumbie & Mertz 1962; Walsh 1975; Noshiro & Suzuki 2001), neither the stem diameter nor the habit type influence them in Mascarene Dombeyoideae. While the influence of ontogenetic stage and habit on the variations of Mascarene Dombeyoideae wood is established, it seems that other parameters, such as environmental ones, are also related to it. Secondary xylem is known to be strongly correlated (i.e. through sap conduction and other functional adaptations) with environmental conditions (Fritts 1976; Carlquist 2001; Tyree & Zimmermann 2002; Baas et al. 2004). Equilibrium between efficiency and safety of the hydraulic system depends on several wood anatomical characters mainly linked to vessel elements (size, frequency, intervessel pitting) but also to cell wall, fibre and tracheid features (McCulloh & Sperry 2005; Wheeler et al. 2005, Hacke et al. 2006; Sperry et al. 2006; Jacobsen et al. 2007). The Mascarene Dombeyoideae actually show variations in their wood anatomical features

Downloaded from Brill.com10/03/2021 11:11:59PM via free access 516 IAWA Journal, Vol. 32 (4), 2011 along a humidity gradient: lower VD and higher MD, MIND and MAXD are related with more mesic conditions. These observations are in accordance with numerous other studies made either at the interspecific (Baas & Carlquist 1985; Carlquist 2001; Wheeler et al. 2007; De Micco et al. 2008) or intraspecific level (Zhang et al. 1988; Noshiro et al. 1994; Giantomasi et al. 2009). These results outline the predominance of “conductive efficiency” in areas where water availability is constant through the year and of “conductive safety” in areas where water is scarce during a more or less important part of it. Nevertheless, VEL and FWT are the only wood anatomical features directly linked to MAP. Thus, as Lindorf (1994) and Wickremasinghe and Herat (2006) have already described, VEL increases with increasing precipitation. More generally, VEL was shown to decrease along latitudinal and altitudinal gradients (Noshiro & Baas 2000; Wheeler et al. 2007). Shortening of vessel elements was hypothesized to reduce the impact of embolism during sap conduction in confining embolism in reduced portions of the wood (Carlquist 2001) that are easier to circumvent through pathway redundancy (Zimmermann 1983; Tyree et al. 1994). Adaptation to drought also relies on vessel resistance to implosion which is partly linked to the fibre wall features (Hacke et al. 2001; Sperry 2003; Baas et al. 2004). Thick-walled fibres increase mechanical strength of the wood, induce high density and reinforce the vessels (Jacobsen et al. 2005). In Mascarene Dombeyoideae, FWT presents numerous correlations with altitude but also to variables related to temperature, precipitation and also as previously mentioned to plant habit. FWT increases with increasing cold month mean temperatures and decreasing AL and MAP and thus taxa with only thick-walled fibres characterize the driest places. High specific gravity woods were shown to be common in arid regions: they represent nearly half of the wood of tropical and of South Africa (Chudnoff 1976). Authors often consider vessel grouping as another wood anatomical feature which participates to the hydraulic safety of the tree by providing redundancy (Carlquist 1984). If a vessel in a group fills with gas and induces the breaking of the water column, rerouting of the sap can occur through its intact neighbours (Carlquist 2001). In Mascarene Dombeyoideae, PSV is however not correlated to any of the environmental features but only to variables related to the size of the stems. As for ray features, contrarily to the study of Noshiro and Suzuki (2001) on Nepalese Rhododendron, we did not find any correlation with stem diameter nor with plant habit, but with environmental variables. MRH decreases with decreasing altitude and precipitation. RD values are higher in mesothermal forests than in megathermal ones.

AcknowledgEments

This study has been financially supported by the UMR 7207 (CNRS/MNHN/UPMC). We would like to thank the Mauritius National Parks and Conservation Service and the Forestry Service for permission to work in the forests and for general assistance. We warmly thank J.B. Pausé for providing wood samples, J.C. Sevathian (Mauritius Wild Life Foundation), E. and M.F. Grangaud and Dr. C. Lavergne (Conservatoire Botanique National de Mascarin, CBNM) for assistance in the fields. Finally, we are grateful to N. Salel (UPMC) for the preparation of wood sections; to Dr. S. Couette (MNHN), Dr. D. De Franceschi (MNHN), Dr. C. Gill (UPMC), Dr. D. Pons (UPMC), Dr. J. Anquetin, Dr. L. Gigord (CBNM), Prof. J.Y. Dubuisson (UPMC), and to two anonymous re- viewers for their helpful advices and comments.

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