Salix ×marchettii (Salicaceae), a new nothospecies from the Apuan Alps (Northern Tuscany, Central Italy)

Abstract Merli, M. & F. Martini (2017). Salix ×marchettii (Salicaceae), a new nothospecies from the Apuan Alps (Northern Tuscany, Central Italy). Candollea 72: 341–345. In English, English abstract. A new nothospecies, Salix ×marchettii M. Merli & F. Mart. (Salix crataegifolia Bertol. × Salix eleagnos Scop. subsp. eleagnos) (Salicaceae), is described from the Apuan Alps. This new nothotaxon grows on screes between 1010 and 1105 m. The main morphological characters which distinguish Salix ×marchettii are the indumentum with mixed, silky and tomentose trichomes on the young twigs ; the leaf shape, elliptic-oblong or oblanceolate-oblong to narrowly oblanceolate ; the leaf margin, which may vary from flat to weakly revolute in the same specimen; the male catkins, cylindrical and arcuate-pendant, which have flowers with connate filaments for at least half their length or more. Received: March 28, 2017; Accepted: July 6, 2017; First published online: July 20, 2017


Introduction
Natural hybridization within the genus Salix L. (Salicaceae) is relatively common and is likely to involve about 5 % of the individuals in a population according to estimates by Neumann (1981). Willows, however, despite possessing inherently high capacity to cross even among taxonomically very distant taxa (e.g. between fruticose chamaephytes of Salix sect. Chamaetia Dumort. and nanophanerophytes of Salix sect. Arbuscella Ser. : Salix ×ganderi Huter ex K. Richt. = S. reticulata L. × S. waldsteiniana Willd.), do not form hybrids everywhere, but only under specific circumstances. According to Skvortsov (1999), the phenomenon would seem to be favoured by phylogenetic relationships among species and individual needs of adaptation to climatic or edaphic disturbances, typical of naturally unstable environments (i.e. moraines, screes) or degraded by human action (i.e. disused quarries, escarpments, etc.). On the other hand, within stable populations, the presence of hybrids is a substantially rare occurrence, despite the contiguity between individuals of different species. For further details see also Martini & Paiero (1988, in press), Lautenschlager (1989, Hörandl et al. (2002) and Gramlich & Hörandl (2016).
In addition, the attribution of a specimen to a hybrid form may often be due to inadequate knowledge of the specific variability which, in many cases, may be considerable, as in critical groups such as S. nigricans Sm. or S. caprea L.
Therefore, the morphological study of Salix hybrids requires extensive comparison of material, taking into consideration the variability of their respective parents. The diacritical characteristics of hybrids are expressed, in most cases, in the shape of the leaf blade and in the type of indumentum.

Materials and methods
The morphological and morphometric analysis was performed both on fresh material and on herbarium specimens deposited in ROV, MFU (also containing specimens of the herbarium Marchetti, Massa), TSB (acronyms follow Index Herbariorum, 2017) and of the private collections of MM [herb. Merli] and C. Argenti (Belluno). The taxa investigated (in brackets the number of studied specimens) were : S. crataegifolia Bertol. (20), S. eleagnos subsp. eleagnos (30) and the new hybrid (12). A Leica EZ4 HD microscope was used for observations and details of indumentum and catkins and for taking photographs. The scientific nomenclature follows Conti et al. (2005).

Salix ×marchettii shows intermediate characters between
S. crataegifolia and S. eleagnos subsp. eleagnos, but differs from both by the young twigs' indumentum (mixed silky and tomentose trichomes), the presence in the same specimen of both elliptic-oblong or oblanceolate-oblong (S. crataegifolia) and narrowly oblanceolate (S. eleagnos subsp. eleagnos) leaves.
Phenology. -Catkins appear with the leaves in May-June.
Etymology. -The new hybrid is dedicated to Prof. Dino Marchetti (Massa), researcher and expert on the flora of the Apuan Alps.
Notes. -The diacritical characters which distinguish S. ×marchettii from its parents are summarized in Table 1. In S. ×marchettii the young twig indumentum shows mixed trichomes, silky (typical of S. crataegifolia) and matted (tomentose), characteristic of S. eleagnos ( Fig. 2A). Size and shape of the leaf, elliptic-oblong or oblanceolate-oblong to narrowly oblanceolate, are also intermediate between those of the parents, as well as the margin, which may vary from flat (S. crataegifolia) to weakly revolute (S. eleagnos subsp. eleagnos) in the same specimen. Also the number of lateral veins, which varies between (9-)17 and 25 pairs, is intermediate between those of the parents, but in S. ×marchettii the veins appear prominent only below, as in S. eleagnos. The silky indumentum of the lower leaf blade and petiole recalls mainly S. crataegifolia and, as in the latter, is very dense in immature leaves. This indumentum persists on leaves when mature only on the midrib or is completely deciduous. The leaf petiole is usually more developed than in S. eleagnos. The heritage of S. eleagnos appears better defined in male catkins, whose sizes are intermediate between those of the parents, but the filaments are connate, as in S. eleagnos, for at least half their length or more (Fig. 2B). The development of female catkins recalls, in form and posture, that of S. crataegifolia, but, in the observations so far, the capsules were sterile. The floral bracts are similar in shape to those of S. crataegifolia, usually completely brownish, at first silky then glabrous or nearly so.
Salix crataegifolia, described by Bertoloni (1813), represents one of the most interesting willow species in Europe. The arcuate-pendant posture of the catkins, always observable in the males while in the females noticeable only until flowering, represents an ancestral character. Moreover, the morphology of the vascular bundles of the leaves, the small size of the pollen grains and low chromosome number are typical characters of this palaeoendemic species, phytogeographically isolated (Bechi et al., 1996 ;Garbari et al., 2007) and with a limited individual variability. For these same reasons Skvortsov (1999) hypothesizes a link between several species of the Chinese Himalaya (S. sikkimensis Anderss., S. daltoniana Anderss.) and S. ernesti C.K. Schneider from Sichuan. Salix crataegifolia is a steno-endemic chasmophyte of the Apuan Alps, with a distribution area of only 261.6 km 2 , which has convinced Roma-Marzio et al. (2015) to propose its inclusion among the "Near Threatened" [NT] species. Considering the above discussed features and the absence of other hybrid forms with S. crataegifolia as a parent, the discovery of S. ×marchettii has generated considerable surprise. Hence, among all Salix hybrids, S. ×marchettii represents a unique case because it is the result of a cross with S. crataegifolia and therefore an endemic nothospecies of the Apuan Alps. From an ecological point of view, it behaves as a genuine gravel species like S. eleagnos, while S. crataegifolia prefers rocks (limestone and metamorphic) and is rarely growing on gravels (Roma-Marzio et al., 2015). Salix ×marchettii is known so far from a single population on the northwestern slope of M. Pelato, where, however, it is fairly widespread on the road side gravel and on the underlying rocky slope, together with S. crataegifolia (abundant), S. eleagnos subsp. eleagnos and S. apennina (more rare), S. purpurea and S. caprea (both occasional). According to the local distribution of the two parents as reported by Ferrarini & Marchetti (1994) and Roma-Marzio et al. (2015), it can be hypothesized that S.