IAWA Journal, Vol. 30 (3), 2009: 231–245 WOOD ANATOMY OF THE GENUS ABIES A REVIEW Luis García Esteban*, Paloma de Palacios, Francisco García Fernández and Ruth Moreno Universidad Politécnica de Madrid. Escuela Técnica Superior de Ingenieros de Montes, Departamento de Ingeniería Forestal, Ciudad Universitaria, 28040 Madrid, Spain *Corresponding author [E-mail: [email protected]] SUMMARY The literature on the wood anatomy of the genus Abies is reviewed and discussed, and complemented with a detailed study of 33 species, 1 sub- species and 4 varieties. In general, the species studied do not show diag- nostic interspecific differences, although it is possible to establish differences between groups of species using certain quantitative and quali- tative features. The marginal axial parenchyma consisting of single cells and the ray parenchyma cells with distinctly pitted horizontal walls, nodular end walls and presence of indentures are constant for the genus, although these features also occur in the other genera of the Abietoideae. The absence of ray tracheids in Abies can be used to distinguish it from Cedrus and Tsuga, and the irregularly shaped parenchymatous marginal ray cells are only shared with Cedrus. The absence of resin canals enables Abies to be distinguished from very closely related genera such as Keteleeria and Nothotsuga. The crystals in the ray cells, taxodioid cross-field pitting and the warty layer in the tracheids can be regarded as diagnostic generic features. Key words: Abies, Abietoideae, anatomy, wood. INTRODUCTION The family Pinaceae, with 11 genera and 225 species, is the largest conifer family. The genus Abies, with 48 species and 24 varieties, has the second highest number of species after the genus Pinus (Farjon 2001). Distribution is exclusive to the northern hemisphere. In Eurasia the southernmost species are located in the west in Morocco (A. pinsapo var. marocana and A. pinsapo var. tazaotana) and in the east in the Taiwanese mountains (A. kawakamii), while the northernmost species is in Siberia (A. sibirica). In America, the northernmost species is A. lasiocarpa and the southernmost is A. guatemalensis (Liu 1971). In general, the wood structure of the genus Abies is very homogeneous (Jacquiot 1955; Peraza 1964) and its features are very similar to the genus Pseudolarix (Phil- lips 1948). In fact, together with Pseudolarix and Keteleeria, Abies has the simplest structure of the Pinaceae (Greguss 1955). Downloaded from Brill.com10/04/2021 03:49:05AM via free access 232 IAWA Journal, Vol. 30 (3), 2009 The wood anatomy of Abies was studied by several researchers in the late 19th century (Castellarnau 1880; Kleeberg 1885) and early 20th century (Penhallow 1904; Jeffrey 1905; Thompson 1910; Vierhapper 1910). Studies refer to individual species (Castellarnau 1880), to species grouped according to provenance (Penhallow 1907; Wiesehuegel 1932), to their phylogeny or evolution (Jeffrey 1905) or to specific struc- tural aspects (Thompson 1912; Bannan 1936). In the mid-20th century, more complete studies were undertaken, such as by Phillips (1948), who described the characteristics of the genus and 5 species, and Greguss (1955), who also described the specific features of the genus in addition to 30 species and 2 varieties, including the most complete biometry made until that time. Subsequently, Greguss (1972) added to this work with 9 more species. Jacquiot (1955), Peraza (1964), Jane (1970), Panshin & DeZeeuw (1970), Esteban & Guindeo (1988), Schweingruber (1990) and Esteban et al. (1996) included general descriptions of the specific features of the genus Abies in their studies, as well as species descriptions. Esteban et al. (2002) described 30 species. Lastly, in its Softwood List, the IAWA Committee (2004) made many references to characteristic wood anatomical features of Abies. The objective of this work was to review and discuss existing studies on the wood anatomy of the genus Abies, and to complement this with a detailed study of 33 species, 1 subspecies and 4 varieties. MATERIAL AND METHODS The material used in this study came from the wood collections of several research centres. Table 1 lists the 33 species, 1 subspecies and 4 varieties studied, with refer- ences to the collections from which the samples were obtained (Stern 1988). In the case of Abies alba (ETSIMw X2174) and A. pinsapo (ETSIMw X2175, X2176, X2177, X2267 and X2268) the samples were collected by us in their natural areas in Spain and Morocco, from 5 mature trees felled in each area. Microscopic slides were prepared in accordance with the usual methods of softening, sectioning, staining and mounting, and the anatomical descriptions were made using the standard terminology of the IAWA Committee (2004). The samples were observed using light microscopy, without staining and after staining any resin with safranin and Sudan 4 (Jane 1970). The SEM samples were prepared as recommended by Heady and Evans (2000), and observed in a JEOL JSM-6380 scanning electron microscope. RESULTS AND DISCUSSION Table 1 summarises the features observed in the species, subspecies and varieties studied. Differentiation of species within the genus and from other genera In general terms, there are no qualitative differences that can be used consistently to separate species of the genus Abies. Below some examples are given of specific identification keys published in the past, that appear not to be valid on closer scrutiny. Castellarnau (1880) showed that Downloaded from Brill.com10/04/2021 03:49:05AM via free access Esteban et al. — Review of Abies wood anatomy 233 A. alba wood can easily be distinguished from that of A. pinsapo by the occurrence of marginal axial parenchyma with nodular transverse end walls in the former and its absence in the latter. Our observations show that this feature occurs in both species (Table 1). Krauss (1864) in Viguié & Gaussen (1928) proposed an identification key based on the rays, which distinguished A. cephalonica, A. sibirica, A. fraseri, A. alba, A. pinsapo, A. nordmanniana, A. cilicica and A. balsamea. The key was based on the number of cross-field pits and their form (oval or rounded) and the height and shape (oval or rounded) of ray parenchyma cells in the tangential section. In the present study no major differences were observed in the morphology of the ray cells which would enable the species of Abies to be distinguished. Penhallow (1896) proposed a classifica- tion of North American conifer wood in which he attributed the genus Abies with the following features: resin canals rare, recorded in A. nobilis and A. bracteata; absence of fusiform rays; absence of ray tracheids, except in the case of A. balsamea; end walls of ray parenchyma cells markedly nodular; and absence of helical thickenings in tracheids. However, resin canals and ray tracheids, used by Penhallow (1896), do not support any classification, as both features appear to be a response to wounding. Wiesehuegel (1932) also proposed an identification key for the American firs which included the presence or absence of crystals in the rays and the occurrence of partially biseriate rays. These two features are very common in all the species studied and cannot be used to separate species within Abies. The same author also distinguished A. balsamea, A. grandis, A. venusta and A. magnifica on the basis of biseriate tracheid pitting in comparison with the other American firs, in which the arrangement is uniseriate. Because the presence of biseriate pitting is closely associated with the tracheid diameter of earlywood and the diameter is closely related to climate conditions, this feature does not support any classification either. Lastly, Wiesehuegel (1932) used the number of pits per cross-field as a diagnostic feature for A. amabilis in the American firs based on the predominance of a single pit per cross-field. His results differ from those obtained in the present study, where the number of pits ranges from 2 to 4. Saint-Laurent (1932) established differences between the wood of A. pinsapo var. marocana and A. numidica on the basis of the more abundant axial parenchyma tissue in the former. In the present study no differences between these taxa were observed, as in both species the parenchyma is restricted to single marginal cells. Brown et al. (1949) proposed a classification to distinguish A. balsamea and A. fraseri from A. concolor, A. procera, A. grandis and A. magnificaon the basis of the tangential section of the ray parenchyma cells, which is oblong in A. balsamea and A. fraseri and circular to oval in the others. This feature has often been used to distinguish very similar species (Phillips 1948; Greguss 1955; Core et al. 1979; Roig 1992), but the observations in the present study show that the use of this feature is highly subjective. Similarly, Kukachka (1960) established a division of the genus based on the colour of the ray cell content, but this is also a subjective classification and is highly influenced by factors such as whether the wood is juvenile or mature, or the degree of heartwood formation. Peraza (1964) separated A. alba from the group A. pinsapo and A. pinsapo var. tazaotana on the basis of the higher frequency of rays more than 30 cells high in A. alba. However, Esteban et al. (2007), using mature wood from trunks of trees collected in their area of origin, Downloaded from Brill.com10/04/2021 03:49:05AM via free access 234 IAWA Journal, Vol. 30 (3), 2009 Table 1. Features of the species, subspecies and varieties of the genus Abies studied. NB. Only features that vary below the genus level are included. For anatomical features that are constant see text. Legends: +: present; –: absent. – Growth rings: a = abrupt, g = gradual early/latewood transition. – Tracheid pitting: u = uniseriate, b = biseriate, u (b) = uniseriate and occasionally biseriate. – Axial parenchyma: m = marginal, d = diffuse. – Ray width 2-seriate in part: + = number of biseriate rays greater than 1%, (+)1 = number of biseriate rays lower than 1%.