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Wood Anatomy of the Mascarene Dombeyoideae: Systematic and Ecological Implications

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Wood Anatomy of the Mascarene Dombeyoideae: Systematic and Ecological Implications 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 par- atracheal 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. Downloaded from Brill.com10/03/2021 11:11:59PM via free access 494 IAWA Journal, Vol. 32 (4), 2011 1 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
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